Groundwater temperature and flow studies in the Great Basin

A monitoring network in the alpine Brighton Basin was established to examine the relationship between air, ground, and noble gas groundwater recharge temperatures. Maximum noble gas groundwater recharge temperatures from 25 samples collected over 2 years averaged 2.9±1.2 °C, within the experimental error of the mean ground temperature of 2.3 °C , and vary from 0 to 7 °C, also comparable to ground temperature variations. Mean ground temperatures in the upper 1 m of soil over the 2 years were 1 °C cooler than mean air temperatures. This offset is explained by modeling a snow effect on ground temperature. This study indicates that interpretation of groundwater recharge temperatures derived from noble gases should be attentive to the local ground temperature effects in recharge areas. Two-dimensional modeling of fluid flow and heat transport are used to quantify effects of groundwater flow on the subsurface thermal regime and determine the lower limit of recharge rates that will produce an observable perturbation such that groundwater temperatures can be used to constrain them. The greatest temperature perturbations occur in the deepest portion of the recharge area. At recharge rates of 10 mm yr"1 or less, the hydrologic disturbance to the subsurface thermal regime is almost completely dependent on the recharge rate. At recharge rates higher than this, the hydrologic disturbance is dependent on both the recharge rate and permeability. At recharge rates of 50 mm yr"1 and greater, the plume of colder water persists towards the discharge area and could be easily measured and used to constrain recharge rates to the system. The Snake Valley area groundwater system was simulated using a threedimensional model incorporating groundwater flow and heat transport. This study represents one of the first regional modeling efforts to include calibration to groundwater temperatures. The inclusion of temperature observations reduced parameter uncertainties over using just water-level altitude and discharge observations. The distribution of simulated transmissivity includes areas of high transmissivity within and between hydrographic areas. Increased well withdrawals within these areas will likely affect a large portion of the study area, resulting in decreasing groundwater levels and discharge to springs and evapotranspiration.

GROUNDWATER TEMPERATURE AND FLOW STUDIES IN THE GREAT BASIN by Melissa Dawn Masbruch A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geology Department of Geology and Geophysics The University of Utah December 2013 Copyright © Melissa Dawn Masbruch 2013 All Rights Reserved The Unive r si t y of Utah Gradua te School STATEMENT OF DISSERTATION APPROVAL The dissertation of Melissa Dawn Masbruch has been approved by the following supervisory committee members: D. Kip Solomon Co-Chair 10/3/2013 Date Approved David Chapman Co-Chair 10/3/2013 Date Approved Victor Heilweil Member 10/3/2013 Date Approved John Bowman Member 10/3/2013 Date Approved William Parry Member 10/3/2013 Date Approved and by John Bartley Chair/Dean of the Department/College/School of Geology and Geophysics and by David B. Kieda, Dean of The Graduate School. ABSTRACT A monitoring network in the alpine Brighton Basin was established to examine the relationship between air, ground, and noble gas groundwater recharge temperatures. Maximum noble gas groundwater recharge temperatures from 25 samples collected over 2 years averaged 2.9±1.2 °C, within the experimental error of the mean ground temperature of 2.3 °C , and vary from 0 to 7 °C, also comparable to ground temperature variations. Mean ground temperatures in the upper 1 m of soil over the 2 years were 1 °C cooler than mean air temperatures. This offset is explained by modeling a snow effect on ground temperature. This study indicates that interpretation of groundwater recharge temperatures derived from noble gases should be attentive to the local ground temperature effects in recharge areas. Two-dimensional modeling of fluid flow and heat transport are used to quantify effects of groundwater flow on the subsurface thermal regime and determine the lower limit of recharge rates that will produce an observable perturbation such that groundwater temperatures can be used to constrain them. The greatest temperature perturbations occur in the deepest portion of the recharge area. At recharge rates of 10 mm yr"1 or less, the hydrologic disturbance to the subsurface thermal regime is almost completely dependent on the recharge rate. At recharge rates higher than this, the hydrologic disturbance is dependent on both the recharge rate and permeability. At recharge rates of 50 mm yr"1 and greater, the plume of colder water persists towards the discharge area and could be easily measured and used to constrain recharge rates to the system. The Snake Valley area groundwater system was simulated using a three­dimensional model incorporating groundwater flow and heat transport. This study represents one of the first regional modeling efforts to include calibration to groundwater temperatures. The inclusion of temperature observations reduced parameter uncertainties over using just water-level altitude and discharge observations. The distribution of simulated transmissivity includes areas of high transmissivity within and between hydrographic areas. Increased well withdrawals within these areas will likely affect a large portion of the study area, resulting in decreasing groundwater levels and discharge to springs and evapotranspiration. iv ABSTRACT........................................................................................................................... iii LIST OF TABLES............................................................................................................... viii LIST OF FIGURES..................................................................................................................x PREFACE............................................................................................................................ xvi CHAPTER 1. AIR, GROUND, AND GROUNDWATER TEMPERATURES IN AN ALPINE SETTING, BRIGHTON BASIN, UTAH.................................................................. 1 1.1 Abstract......................................................................................................1 1.2 Introduction................................................................................................2 1.3 Site Description and Monitoring Network.............................................. 8 1.3.1 Site Description..........................................................................8 1.3.2 Monitoring Network................................................................. 9 1.4 Data.......................................................................................................... 11 1.4.1 SNOTEL Meteorological Station Data..................................11 1.4.2 Ground Temperature D ata......................................................11 1.4.3 Groundwater Temperature Data............................................ 16 1.4.4 Noble Gas Groundwater Recharge Temperature Data and Age Data..................................................................................16 1.5 Results/Discussion..................................................................................20 1.5.1 Temperature Data.................................................................... 20 1.5.2 Relation of Air and Ground Temperatures to Temperature at the Water Table....................................................................... 23 1.5.3 Snow Effects............................................................................25 1.6 Summary and Conclusions.................................................................... 29 1.7 Acknowledgments................................................................................... 32 1.8 References................................................................................................32 2. USING GROUNDWATER TEMPERATURES TO CONSTRAIN RECHARGE RATES IN ARID INTERMONTANE BASINS.................................................... 36 TABLE OF CONTENTS 2.1 Abstract 36 2.2 Introduction..............................................................................................37 2.3 Conceptual Model of Groundwater Flow/Thermal Regime in the Great Basin...............................................................................................40 2.4 Modeling Approach................................................................................44 2.4.1 Mesh Design............................................................................45 2.4.2 Boundary Conditions..............................................................45 2.4.3 Model Parameters................................................................... 47 2.5 Model Results and Discussion...............................................................47 2.5.1 Purely Conductive Case..........................................................49 2.5.2 Influence of Bedrock Permeability and Recharge Rate....... 49 2.6 Conclusions..............................................................................................60 2.7 References................................................................................................62 3. HYDROLOGY AND NUMERICAL SIMULATION OF GROUNDWATER MOVEMENT AND HEAT TRANSPORT IN SNAKE VALLEY AND SURROUNDING AREAS, JUAB, MILLARD, AND BEAVER COUNTIES, UTAH, AND WHITE PINE AND LINCOLN COUNTIES, NEVADA............. 67 3.1 Abstract....................................................................................................67 3.2 Introduction..............................................................................................70 3.3 Previous Studies...................................................................................... 74 3.4 Hydrogeologic Setting............................................................................77 3.4.1 Hydrogeologic Framework.................................................... 78 3.4.2 Hydrogeologic Unit Hydraulic Properties............................ 83 3.4.3 Occurrence and Movement of Groundwater......................... 84 3.4.4 Conceptual Groundwater Budget...........................................88 3.4.5 Water-Level Fluctuations......................................................106 3.4.6 Groundwater Temperatures and Heat Flow........................109 3.5 Numerical Simulation of Groundwater Flow and Heat Transport....114 3.5.1 Model Construction..............................................................114 3.5.2 Observations Used in Model Calibration............................ 151 3.5.3 Model Calibration................................................................. 164 3.5.4 Model Evaluation.................................................................. 199 3.5.5 Implications............................................................................228 3.5.6 Model Limitations................................................................. 236 3.6 Summary................................................................................................242 3.7 References..............................................................................................249 APPENDIX A. EQUATIONS AND CALCULATIONS OF THERMAL PROPERTIES USED FOR MODEL INPUT..............................................................................................259 B. WATER-LEVEL OBSERVATION UNCERTAINTY CALCULATIONS...... 268 vi C. GROUNDWATER TEMPERATURE OBSERVATION UNCERTAINTY CALCULATIONS...................................................................................................277 vii LIST OF TABLES Table 1-1. Summary of air (SAT), ground (GT) and groundwater (GWT) temperature data............................................................................................................................. 15 1-2. Summary of noble gas maximum recharge temperatures (Tr), groundwater ages, and measured noble gas and tritium concentrations......................................19 2-1. Parameter values used for fluid and thermal properties..........................................48 3-1. Hydraulic properties of hydrogeologic units from the Great Basin carbonate and alluvial aquifer system study area..................................................................... 85 3-2. Summary of estimates of aquifer properties from results of aquifer tests in Spring and Snake Valleys......................................................................................... 86 3-3. Current study conceptual and ranges of previously reported groundwater budget estimates for hydrographic areas and the Snake Valley study area.......... 91 3-4. Thickness and depth to top of each layer in the Snake Valley area groundwater model..................................................................................................120 3-5. Summary of discharge data and uncertainty statistics for springs used as observations in the Snake Valley area groundwater model..................................158 3-6. Summary of discharge data and uncertainty statistics for streams used as observations in the Snake Valley area groundwater model..................................161 3-7. Horizontal hydraulic-conductivity estimates and statistics of hydrogeologic units in the DeathValley regional groundwater flow system and relation to hydrogeologic units used in the Snake Valley area groundwater model............ 170 3-8. Calibrated horizontal hydraulic-conductivity parameter values and statistics of parameters used in the Snake Valley area groundwater model.......................186 3-9. Calibrated horizontal-to-vertical anisotropy parameter values and statistics of parameters used in the Snake Valley area groundwater model.......................198 3-10. Summary statistics for measure of model fit for the Snake Valley area groundwater model..................................................................................................201 3-11. Summary of observed and simulated water-level altitudes for the Snake Valley area groundwater model..............................................................................203 3-12. Summary of observed and simulated discharge for the Snake Valley area groundwater model..................................................................................................213 3-13. Summary of observed and simulated groundwater temperatures for the Snake Valley area groundwater model..............................................................................216 3-14. Comparison of simulated, conceptual, and previously reported groundwater budget components for hydrographic areas and the Snake Valley study area....221 3-15. Summary statistics of simulated subsurface flow between hydrographic areas and out of the study area in the Snake Valley groundwater model and comparison to previous estimates...........................................................................227 A-1. Summary statistics for measured solid thermal conductivity samples from the Snake Valley study area.......................................................................................... 262 ix LIST OF FIGURES Figure 1-1. Lapse rates of temperature versus elevation in northern and central Utah............. 5 1-2. Map of land-surface topography of the Brighton Basin, Utah, and locations of monitoring sites.......................................................................................................7 1-3. Relation of ground temperatures at Site 1 (recharge area) to air temperatures and snow depth.......................................................................................................... 12 1-4. Relation of ground temperatures at Site 2 (discharge area) to air temperatures and snow depth.......................................................................................................... 13 1-5. Relation of groundwater temperatures from the well at Site 2 (discharge area) and groundwater recharge temperatures to air temperatures and snow depth...... 14 1-6. Age of groundwater samples (crosses) with 1 s.d. error bars collected from the well............................................................................................................................. 17 1-7. The snow effect- influence of snow event onset time and duration on mean annual surface ground temperatures relative to mean annual air temperatures 28 2-1. Location of the Great Basin, western United States............................................... 41 2-2. Conceptual diagram showing groundwater flow typical of the Great Basin, and how geothermal gradients and surface heat flow may be affected by groundwater flow.......................................................................................................42 2-3. Two-dimensional model grid used for simulations................................................ 46 2-4. Simulated surface heat flow for the purely conductive case (no groundwater flow)...........................................................................................................................50 2-5. Influence of bedrock permeability and recharge rate on the hydrologic disturbance to conductive heat flow.........................................................................51 2-6. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-13 m2 and recharge rate of 10 mm yr-1.............................................................53 2-7. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-12 m2 and recharge rate of 10 mm yr-1.............................................................54 2-8. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-13 m2 and recharge rate of 50 mm yr-1.............................................................55 2-9. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-12 m2 and recharge rate of 50 mm yr-1.............................................................56 2-10. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-13 m2 and recharge rate of 90 mm yr-1 ............................................................58 2-11. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-12 m2 and recharge rate of 90 mm yr-1.............................................................59 3-1. Location of the Snake Valley study area, Utah and Nevada...................................71 3-2. Surficial extent of hydrogeologic units and prominent structural geologic features in the Snake Valley study area................................................................... 79 3-3. Example cross section across the study area showing hydrogeologic units......... 80 3-4. Schematic diagram showing conceptualization of groundwater-budget components and budget calculation for the Snake Valley study area....................89 3-5. Conceptual rate of recharge from precipitation (in-place recharge+recharge from runoff (including unconsumed surface-water irrigation)+recharge from mountain stream baseflow) in the Snake Valley study area...................................99 3-6. Locations and types of discharge in the Snake Valley study area.......................102 3-7. Estimated total annual groundwater withdrawals from wells in Snake Valley (Utah side only), 1940-2010 ..................................................................................105 3-8. Location of three wells with multiple-year water-level records in the Snake Valley study area......................................................................................................107 3-9. Multiple-year water-level hydrographs from three wells in the Snake Valley study area..................................................................................................................108 3-10. Schematic diagram showing conceptualization of how geothermal gradients and surficial-heat flow are affected by groundwater flow....................................111 xi 3-11. Location of wells with thermal logs and associated estimated thermal gradients, and springs with temperature data in the Snake Valley study area....113 3-12. Location of the model grid for the Snake Valley study area............................... 118 3-13. Example cross section across the model domain showing hydrogeologic units and subsurface configuration of model layers.......................................................119 3-14. Conceptual rate and distribution of recharge from unconsumed irrigation from well withdrawals simulated in the Snake Valley area groundwater model.........................................................................................................................124 3-15. Conceptual rate of subsurface inflow simulated in the Snake Valley area groundwater model..................................................................................................126 3-16. Maximum groundwater evapotranspiration rate simulated in the Snake Valley area groundwater model..............................................................................128 3-17. Distribution of specified temperatures assigned to layer 1 of the Snake Valley area groundwater model..............................................................................134 3-18. Water-table temperature control points and natural neighbor interpolation temperature results...................................................................................................136 3-19. Derived lapse curves from (A) springs and shallow wells, and (B) noble gas recharge temperatures and altitudes from selected wells and springs in the Snake Valley study area.......................................................................................... 137 3-20. Simulated extent, thickness, and initial hydrogeologic unit zones of the upper basin-fill aquifer unit (UBFAU) in the Snake Valley area groundwater model..................................................................................................140 3-21. Simulated extent, thickness, and initial hydrogeologic unit zones of the lower basin-fill aquifer unit (LBFAU) in the Snake Valley area groundwater model..................................................................................................141 3-22. Simulated extent, thickness, and initial hydrogeologic unit zones of the volcanic unit (VU) in the Snake Valley area groundwater model.......................142 3-23. Simulated extent, thickness, and initial hydrogeologic unit zones of the upper carbonate aquifer unit (UCAU) in the Snake Valley area groundwater model..................................................................................................143 3-24. Simulated extent and thickness of the upper siliciclastic confining unit (USCU) in the Snake Valley area groundwater model.........................................144 xii 145 146 148 153 156 163 173 175 176 177 179 180 182 184 Simulated extent, thickness, and initial hydrogeologic unit zones of the lower carbonate aquifer unit (LCAU) in the Snake Valley area groundwater model.......................................................................................... Simulated extent, thickness, and initial hydrogeologic unit zones of the non-carbonate confining unit (NCCU) in the Snake Valley area groundwater model.......................................................................................... Map showing location of faults and simulated horizontal-flow barriers within the Snake Valley study area................................................................. Spatial distribution of water-level observations used in calibration of the Snake Valley area groundwater model........................................................... Spatial distribution of groundwater discharge observations used in calibration of the Snake Valley area groundwater model............................. Spatial distribution of groundwater temperature observations used in calibration of the Snake Valley area groundwater model............................. Composite scaled sensitivities of parameters used in the initial groundwater model definition of the Snake Valley study area..................... Composite scaled sensitivities for parameters used in the final calibrated groundwater model of the Snake Valley study area...................................... Values and linear 95-percent confidence intervals of parameters used in the final calibrated groundwater model of the Snake Valley study area..... Distribution of areal recharge parameters (multipliers) in the Snake Valley area groundwater model................................................................................... Total rate of recharge from precipitation, streams, and irrigation return flow simulated in the Snake Valley area groundwater model...................... Distribution of subsurface inflow recharge parameter (multiplier) and recharge rate simulated in the Snake Valley area groundwater model........ Distribution of evapotranspiration parameters (multipliers) in the Snake Valley area groundwater model...................................................................... Total rate of evapotranspiration simulated in the Snake Valley area groundwater model.......................................................................................... xiii 3-39. Distribution of simulated horizontal hydraulic conductivity of the non-carbonate confining unit (NCCU) in the Snake Valley area groundwater model..................................................................................................187 3-40. Distribution of simulated horizontal hydraulic conductivity of the lower carbonate aquifer unit (LCAU) in the Snake Valley area groundwater model..................................................................................................188 3-41. Distribution of simulated horizontal hydraulic conductivity of the upper siliciclastic confining unit (USCU) in the Snake Valley area groundwater model..................................................................................................189 3-42. Distribution of simulated horizontal hydraulic conductivity of the upper carbonate aquifer unit (UCAU) in the Snake Valley area groundwater model.........................................................................................................................190 3-43. Distribution of simulated horizontal hydraulic conductivity of the volcanic unit (VU) in the Snake Valley area groundwater model.......................................191 3-44. Distribution of simulated horizontal hydraulic conductivity of the lower basin-fill aquifer unit (LBFAU) in the Snake Valley area groundwater model.........................................................................................................................192 3-45. Distribution of simulated horizontal hydraulic conductivity of the upper basin-fill aquifer unit (UBFAU) in the Snake Valley area groundwater model.........................................................................................................................193 3-46. Weighted observations compared to weighted simulated values for (A) water-levels, (B) discharge, and (C) temperatures................................................ 208 3-47. Distribution of water-level residuals (observed minus simulated) in the Snake Valley area groundwater model.................................................................. 209 3-48. Weighted residuals and simulated values for (A) water-levels, (B) discharge, and (C) temperatures.............................................................................211 3-49. Distribution of simulated water-level altitudes in the Snake Valley area groundwater model..................................................................................................212 3-50. Simulated discharge as a percent of observed discharge in the Snake Valley area groundwater model.......................................................................................... 215 3-51. Calibrated model parameter values and 95-percent confidence intervals using only water-level observations, water-level plus discharge observations, and water-level plus discharge and temperature observations............................. 219 xiv 3-52. Position of a pumped well in relation to a spring with opposing directions of prepumping groundwater flow...........................................................................232 3-53. Simulated transmissivity in the Snake Valley area groundwater model............. 233 xv PREFACE This dissertation consists of three separate manuscripts that have either been published, been submitted for publication, or will be submitted for publication. All three chapters deal with the overarching theme of using groundwater temperatures to quantify various aspects of groundwater systems in the Great Basin. Chapter 1 is a paper entitled "Air, Ground, and Groundwater Recharge Temperatures in an Alpine Setting, Brighton Basin, Utah" by Melissa D. Masbruch, David S. Chapman, and D. Kip Solomon that was published in volume 48 of Water Resources Research in 2012. This chapter describes results from a detailed monitoring network that was used to examine the relationship between air, ground, and groundwater recharge temperatures in an alpine setting. It is reprinted here with permission from John Wiley and Sons. Chapter 2 is a paper entitled "Using Groundwater Temperatures to Constrain Recharge Rates in Arid Intermontane Basins" by Melissa D. Masbruch, D. Kip Solomon, and David S. Chapman that will be submitted for publication to a journal yet to be determined. This chapter describes the results of two-dimensional numerical modeling of the combined processes of fluid flow and heat transport which are used to quantify the effects of groundwater flow on the subsurface thermal regime and determine the lower limit of recharge rate that will produce an observable perturbation such that groundwater temperatures can be used to constrain recharge rates. Chapter 3 is a manuscript entitled "Hydrology and Numerical Simulation of Groundwater Movement and Heat Transport in Snake Valley and Surrounding Areas, Juab, Millard, and Beaver Counties, Utah, and White Pine and Lincoln Counties, Nevada" by Melissa D. Masbruch, Phillip M. Gardner, and Lynette E. Brooks. This manuscript was prepared for publication as an U.S. Geological Survey Scientific Investigations Report and is currently in review. This report describes the construction, calibration, and evaluation of a three-dimensional regional model incorporating groundwater flow and heat transport for Snake Valley and surrounding areas along the Utah-Nevada border. It is reprinted here courtesy of the U.S. Geological Survey. There are many people to thank that were instrumental in the completion of this work. First and foremost I would like to thank the members of my committee: Kip Solomon, Dave Chapman, Vic Heilweil, John Bowman, and Bill Parry. Their thoughtful guidance, encouragement, and help in solving problems helped keep me focused and moving forward. In preparing this dissertation, they offered invaluable feedback and suggestions for improvement. I am grateful to my fellow graduate students at the university and colleagues at the U.S. Geological Survey for their friendship, encouragement, humor, and advice. They were always willing to listen and offer helpful suggestions when elicited and helped keep me sane during the writing process. Finally, this dissertation is dedicated to my family, without whose love and support I would never have made it this far. To my parents, for always encouraging me to follow my dreams, no matter how far away they might take me, and always believing in me. To my brother and his family, and my best friend, for their love and good humor. xvii And to my dog, Annie, faithful companion of nearly 14 years, and my new dog, Sunshine, for keeping me grounded and making me get outside for fresh air every day. I love you all. xviii CHAPTER 1 AIR, GROUND, AND GROUNDWATER TEMPERATURES IN AN ALPINE SETTING, BRIGHTON BASIN, UTAH 1.1 Abstract Noble gases are useful tracers for constraining groundwater recharge temperature and elevation, critical in determining source areas of groundwater recharge in mountainous terrain. A monitoring network in the alpine Brighton Basin in the Wasatch Mountains of northern Utah, USA, was established to examine the relationship between air temperatures, ground temperatures, and noble gas groundwater recharge temperatures. Maximum noble gas groundwater recharge temperatures computed using the closed-system equilibration model from 25 samples collected over the 2 year period 2007 to 2009 averaged 2.96 ± 1.2 °C, within the experimental error of the mean ground temperature of 2.3 °C measured within the probable recharge area. Maximum noble gas recharge temperatures vary from 0 to 7 °C, also comparable to ground temperature variations in the region. Groundwater ages in the collected samples vary from 0 to 7 years, indicating changing flow paths to the collection site during the experiment. Mean ground temperature in the upper 1 m of soil over the 2 year time period is 2.3 °C, which is 1 °C cooler than the mean surface air temperature extrapolated from a nearby meteorological station. This comparison contradicts an earlier observation that mean annual ground temperatures in central Utah are generally warmer than air temperatures. The offset in the Brighton Basin is explained by modeling a snow effect on ground temperature. This detailed study suggests that interpretation of groundwater recharge temperatures derived from noble gases should be attentive to the complex local ground temperature effects in the recharge areas. 1.2 Introduction Determining sources of recharge to aquifers is becoming increasingly important as demands on groundwater continue to increase. One such area where water demands are increasing at a rapid rate is the intermountain west of the U.S. Intermountain basin-fill aquifers and underlying permeable bedrock aquifers are a significant source of groundwater in these arid and semiarid regions. Existing studies [.Anderson andFreethey, 1996; Gates, 1995; Manning and Solomon, 2003; Mason, 1998; Prudic and Herman, 1996] have shown that water sourced in the adjacent mountain blocks accounts for one third to nearly all of the groundwater recharge to these basins. Accurate estimations of the amount of mountain-block recharge to these aquifers are important for water resource management planning. Several studies [Aeschbach-Hertig et al., 1999; Ballentine and Hall, 1999; Manning and Caine, 2007; Manning and Solomon, 2003; Mazor, 1991; Rauber et al., 1991; Zuber et al., 1995] have shown noble gases to be useful traces for examining groundwater recharge temperature (Tr) and elevation (H), which in turn can be used to determine source areas of groundwater recharge to the intermountain aquifers. In order to constrain both recharge temperatures and elevations, however, a recharge temperature versus elevation curve (Tr lapse curve) must be developed for the area in question [Aeschbach-Hertig et al., 1999; Manning and Solomon, 2003]. 2 The variation of air temperature with elevation is well known from atmospheric science. An average environmental lapse rate is -6.5 °C km 1, intermediate between a dry adiabatic lapse rate of -9.9 °C km-1 and a saturated adiabatic lapse rate of -5.0 °C km-1. Studies by Aeschbach-Hertig et al. [1999] and Zuber et al. [1995] used Tr lapse curves that were developed assuming a consistent relation between Tr and the mean annual air temperature (Ta) at all elevations, either Tr = Ta at all elevations [Aeschbach-Hertig et al., 1999], or Tr = Ta - 1 °C at all elevations [Zuber et al., 1995]. In these studies, the noble gas data were used to derive a set of best-fit pairs of H and Tr for each sample by specifying different values of assumed H and then solving for Tr and excess air. The most probable values of H and Tr for each sample were then determined by finding the point of intersection between the suite of best-fit solutions and the assumed recharge lapse curve. This technique was applied to a small number of samples with mixed results; for some samples the derived H values were reasonable, while for others the derived H values were inexplicably too high or too low. Manning and Solomon [2003] took a more rigorous approach to derive a local Tr lapse curve for the Wasatch Mountains of central Utah. In their study, dissolved noble gases were sampled in 16 springs and mine tunnels at various elevations within the mountain block, and a derived maximum and minimum Tr for each sample was determined using constrained minimum and maximum H values particular to each sampling site. Manning and Solomon [2003] then used the derived Tr and H data to develop a Tr lapse curve for the Wasatch Mountains using a least squares linear regression. Their Tr lapse curve has a similar slope (-7.3 °C km-1) to the atmospheric lapse rate (for adiabatic cooling) in the Wasatch Mountains (-6.4 °C km-1); however, it is 3 approximately 2 to 4 °C cooler than the atmospheric lapse curve (Figure 1-1). Based on all derived minimum and maximum values of Tr, Manning and Solomon [2003] concluded that, on average, Tr was about 2 °C cooler than Ta within the Wasatch Mountains. Due to the lack of wells in high alpine recharge areas within the Wasatch Mountains, however, Manning and Solomon's [2003] derived recharge lapse curve was never ground-truthed; that is, Manning and Solomon [2003] did not measure ground (water table) temperatures in recharge areas to confirm that they were in agreement with the noble gas derived recharge temperatures. The observation that Manning and Solomon's [2003] derived Tr lapse curve is cooler than the atmospheric lapse curve for the Wasatch Mountains is significant in many respects. Generally, shallow water table (10-20 m depth) temperatures are approximately 1 to 2 °C warmer than Ta [Anderson, 2005; Domenico and Schwartz, 1998], and mean annual soil temperatures can be biased 1 to 4 °C higher than Ta [Powell et al., 1988; Putnam and Chapman, 1996] (Figure 1-1). Studies by Bartlett et al. [2004, 2005], Cey [2009], and Smith et al. [1964] have also shown that in areas of prolonged snow cover mean annual ground temperatures may be warmer than Ta as the snow insulates the ground from colder winter air temperatures. Cey [2009] has furthermore used numerical simulations to explore the effects of precipitation, water table depth and air temperature variations on mean water table temperatures during groundwater recharge. Alternatively, under some circumstances snow cover and snow melt may produce cooler mean annual ground temperatures than mean annual air temperatures. Bartlett et al. [2004, 2005] show that while snow may insulate the ground from cooler air temperatures during the winter, persistent snow cover in late spring may pin the ground 4 5 Oo CD D -i-< TO 0C L Eo Elevation (m) Figure 1-1. Lapse rates of temperature versus elevation in northern and central Utah. Shown are the mean annual atmospheric lapse curve for the Wasatch Mountains (solid line) derived from SNOTEL (red crosses) and Western Regional Climate Center (blue crosses) meteorological station data; groundwater recharge lapse curve from Manning and Solomon [2003] for the Wasatch Mountains (dashed line); and mean annual ground temperature lapse curve for sites in central Utah from Powell et al. [1988] (dotted line). Modified from: Masbruch et al. [2012]. temperature near 0 °C while air temperatures warm during the spring and early summer, producing mean annual ground temperatures that are cooler than Ta. Additionally, as snow melt is often the main source of recharge in mountainous terrain, large volumes of snow melt infiltrating fractured rock may keep temperatures in the unsaturated zone and water table near 0 °C, especially as water table depths may decrease to less than 3 m depth during spring snow melt events [Buttle, 1989; Hill, 1990; Klump et al., 2006]. Consequently, in many high alpine areas, Tr could be considerably lower than Ta. To determine why recharge temperatures within the Wasatch Mountains are apparently cooler than mean annual air temperatures, this study investigates the relation between air, ground and groundwater recharge temperatures within the Brighton Basin, a high alpine basin located within the Wasatch Mountains. The area chosen within the Brighton Basin is considered to be an ideal location for several reasons: (1) installation of a shallow monitoring well at a local discharge site where groundwater levels are near land surface was possible; (2) recharge areas within the basin are constrained by the topography of the basin, and span only about a 100 m difference in elevation; they cannot be lower than the elevation of the discharge site at 2,770 m and cannot be much higher than about 2,890 m where there is a break in slope between the basin and the peaks surrounding the basin as it is highly unlikely that groundwater recharge is occurring at the top of the peaks surrounding the basin; and (3) as the topographic map shows, the selected sites exist in a small subbasin, which further limits the location of the probable recharge area contributing water to the discharge site (Figure 1-2). Other subbasins exist northeast and southwest, likely with groundwater flow regimes separate from the sampling sites. 6 7 111°36'0"W 111°35'0"W 111°34'0"W Distance (meters) Figure 1-2. (top) Map of land-surface topography of the Brighton Basin, Utah, and locations of monitoring sites. Dotted lines delineate subbasins, and hatchured area represents the probable recharge area for the monitoring sites. (bottom) Two-dimensional cross section of land-surface topography and conceptualization of possible groundwater flow paths (dashed lines) within the basin. Modified from: Masbruch et al. [2012]. A monitoring network within the basin was used to compare air, ground, and groundwater recharge temperatures and groundwater ages over a period of more than 2 years. Air temperature and snow depth data were drawn from a meteorological station within the basin that is part of the SNOTEL (Snow Telemetry, Natural Resources Conservation Service National Water and Climate Center) network. Ground temperatures at multiple depths were continuously monitored using temperature probes that were installed at local recharge and discharge areas within the basin. Groundwater temperatures within a shallow well in the discharge area were also continuously monitored. Noble gas and tritium samples from the well were generally collected every 2 to 8 weeks to determine groundwater recharge temperatures and groundwater ages. This study had three objectives. First, the data collected from the monitoring network were used to examine how the noble gas recharge temperatures relate to ground and air temperatures. Second, the data were used to identify possible seasonal effects in the groundwater recharge temperatures, ages, and flow regime within the basin. Third, the data were used to validate, at least at one point, the derived recharge lapse curve developed by Manning and Solomon [2003]. Validation of this lapse curve has implications for using noble gases collected from discharge areas within mountainous terrain to develop a recharge lapse curve, and application of this approach in a variety of high alpine terrains. 1.3 Site Description and Monitoring Network 1.3.1 Site Description The connection between air, ground, and groundwater recharge temperatures was investigated by establishing a monitoring network within the Brighton Basin, a high 8 alpine basin located at the head of Big Cottonwood Canyon within the Wasatch Mountains (Figure 1-2). The Wasatch Mountains are located to the east of the Salt Lake Valley in northern Utah and form the eastern margin of the Basin and Range physiographic province. The Brighton Basin ranges in elevation from 2,650 m (8,700 ft) to over 3,200 m (10,500 ft). The peaks surrounding the basin to the north and east are Tertiary igneous intrusions of the Alta and Clayton stocks, and form the divide between the headwaters of Big Cottonwood Creek on the west and Pine Creek on the east [Stokes, 1986]. The basin was carved by glaciation, and as a result the basin contains many small glacial moraines. Mean annual precipitation in the Wasatch Mountains ranges between 50 to 130 cm [Manning and Solomon, 2003]; most of this precipitation falls as snow. The Brighton Basin receives an average of 1,270 cm (500 inches) of snow per year. Groundwater recharge in the basin is mainly derived from snowmelt that infiltrates into fractures in the bedrock of the Alta and Clayton stocks, or through the unconsolidated glacial deposits. Groundwater discharge in the basin is to small springs, streams, lakes, and evapotranspiration. 1.3.2 Monitoring Network Air temperature and snow depth data were drawn from a preexisting SNOTEL meteorological station (SNOTEL site: Brighton, Utah; Site number: 366), located at an elevation of 2,667 m (8,750 ft) within the basin. Multidepth ground temperature probes were installed in two locations within the basin. The first probe was installed in a small glacial moraine (Site 1) at an elevation of approximately 2,790 m within the probable recharge area. The second probe was installed in a wetland/bog type discharge area (Site 9 2) approximately 230 m downgradient from the glacial moraine, at an elevation of approximately 2,770 m. These probes (also known as MRC (Measurement Research Corporation) probes, constructed by Geneq) consist of a string of precision thermistors epoxied into a single, 109 cm long rod. Five thermistors placed at 7, 12, 22, 52, and 102 cm from the top of the probe were used to measure ground temperatures. Water temperatures were continuously monitored at a shallow well installed in the wetland area near the probe at Site 2 using a HOBOWater Temp Pro v2 Logger (developed by Onset). The well was constructed using 2-inch diameter PVC tubing with a 30-inch length screen. The bottom of the well is located approximately 1.6 m below land surface. The logger was suspended from the well cap so that it was positioned at approximately the middle of the well screen. Groundwater recharge temperature and age were determined using noble gas and tritium samples that were collected periodically at the well. Noble gas samples were collected using passive diffusion samplers similar to those shown in Sanford et al. [1996]. The samplers were allowed to equilibrate within the well water for at least 24 h. The gases were then measured using a quadrapole mass spectrometer at the University of Utah noble gas laboratory, and from these measured gases a groundwater recharge temperature was determined (see section 1.4.4 below). Tritium samples were collected in 1 L plastic bottles, and were used to determine the apparent groundwater age using the tritium/helium-3 (3 H/3 He) dating method [Solomon and Cook, 2000]. 10 1.4 Data 1.4.1 SNOTEL Meteorological Station Data Air temperatures and snow depth were measured at the Brighton meteorological station that is part of the SNOTEL network. Data from this station are archived on the SNOTEL website which can be accessed at http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=366&state=ut. The station has been in operation since 1 October 1985, and has been recording hourly air temperatures since 31 January 1996; before this date, air temperatures were recorded only one to four times per day. Snow depth at the station has been measured hourly since 7 October 1997. Data from the period of 1 March 2007 to 1 March 2009, which encompasses the period of noble gas sampling and ground temperature monitoring, are shown in Figures 1 -3 through 1-5 and summarized in Table 1-1. 1.4.2 Ground Temperature Data Ground temperatures were measured at multiple depths up to 1 m at two locations within the basin, using the MRC probes in conjunction with Campbell Scientific CR-10 data loggers. At both locations, thermistors on the probes were sampled every 60 s, and the mean of 30 measurements were stored every 30 min. At Site 1 the MRC temperature probe was installed on 3 February 2007 in a glacial moraine within the probable recharge area. Ubiquitous subsurface cobbles prevented full penetration of the probe at this site; only two thermistors, therefore, were located below ground at 22 and 72 cm depth, respectively. The water table at this site was not intersected during installation of the probe; however, the soil near the bottom of the hole into which the probe was inserted was very moist, suggesting that the water table at this site was only slightly deeper than 11 12 Site 1 - recharge area (March 1, 2007 through March 1, 2009) 30 -10 -15 -20 -25 15 C i 5 e ° Q c o S 10 O o(er utrate Ci Ee 1 * Ground temperature / tT1 22 cm depth 1 72 cm depth -5 Mar07 Jun07 Sep07 Dec07 Mar08 Jun08 Sep08 Dec08 Mar09 Date 5 0 Figure 1-3. Relation of ground temperatures at Site 1 (recharge area) to air temperatures and snow depth. (top) Hourly mean air temperature and snow depth data from the SNOTEL meteorological station, and 30-min mean ground temperature data at Site 1, Brighton Basin, Utah; (bottom) enlargement of the 30-min mean ground temperature data at Site 1. Modified from: Masbruch et al. [2012]. 13 O <o CD i_ -i-< iT_O 0Ci E0 25 20 15 10 5 0 -5 -10 -15 -20 -25 30 Site 2 - discharge area (March 1, 2007 through March 1, 2009) 5 o <o CD i_ -i-< iT_O 0Ci E0 Mar07 Jun07 Sep07 Dec07 Mar08 Jun08 Sep08 Dec08 Mar09 Date Figure 1-4. Relation of ground temperatures at Site 2 (discharge area) to air temperatures and snow depth. (top) Hourly mean air temperature and snow depth data from the SNOTEL meteorological station, and 30-min mean ground temperature data at Site 2, Brighton Basin, Utah; (bottom) enlargement of the 30-min mean ground temperature data at Site 2. Modified from: Masbruch et al. [2012]. 14 Well Site - discharge area (March 1, 2007 through March 1, 2009) 30 25 20 15 O 1° Air temperature Groundwater temperature 5 4.5 4 3.5 3 Mar07 Jun07 Sep07 Dec07 Mar08 Jun08 Sep08 Dec08 Mar09 Date Figure 1-5. Relation of groundwater temperatures from the well at Site 2 (discharge area) and groundwater recharge temperatures to air temperatures and snow depth. Shown are hourly mean air temperature and snow depth data from the SNOTEL meteorological station, 30-min groundwater temperature data from the well, and maximum groundwater recharge temperatures derived from noble gas samples collected from the well, Brighton Basin, Utah. Modified from: Masbruch et al. [2012]. Table 1-1. Summary of air (SAT), ground (GT) and groundwater (GWT) temperature data. Modified from: Mcisbruch et al. [2012], 2007-2008 SNOTEL Site 1 Site 2 Well Max. SAT Min. Mean GT 22 cm Mean GT 72 cm Mean GT 2 cm Mean GT 7 cm Mean GT17 cm Mean GT 47 cm Mean GT 97 cm Mean GWT Mean Mar 14.0 -18.8 0.7 -0.39 0.43 1.00 0.99 0.96 1.24 1.70 2.291 Apr 17.4 -9.2 2.7 -0.29 0.42 1.01 1.04 1.03 1.23 1.58 1.98 May 18.6 -6.0 7.5 -0.28 0.28 0.93 0.85 0.72 0.84 1.24 1.76 Jun 23.3 -2.3 13.4 4.14 2.22 4.03 3.75 3.24 2.63 2.26 3.45 Jul 25.4 8.7 17.2 9.43 6.23 6.32 6.21 6.00 5.45 4.47 5.62 Aug 24.1 6.3 15.6 10.20 8.00 6.40 6.30 6.16 5.86 5.04 6.59 Sep 21.7 -6.4 9.8 7.23 7.21 4.86 4.97 5.08 5.27 4.87 5.99 Oct 16.5 -9.2 4.5 2.48 4.07 2.82 2.88 2.89 3.30 3.76 4.46 Nov 12.1 -13.6 0.8 0.25 1.96 0.71 0.94 1.16 1.92 2.80 3.59 Dec 8.8 -20.5 -7.2 -0.55 0.81 1.06 1.30 1.28 1.54 2.26 2.91 Jan 6.3 -22.4 -7.4 -0.47 0.56 1.17 1.23 1.23 1.51 2.09 2.61 Feb 9.4 -17.3 -5.3 -0.47 0.43 1.21 1.21 1.13 1.34 1.87 2.51 annual mean - - 4.4 2.62 2.73 2.64 2.65 2.58 2.68 2.83 3.652 SNOTEL Site 1 Site 2 Well SAT GT 22 cm GT 72 cm GT 2 cm GT 7 cm GT17 cm GT 47 cm GT 97 cm GWT 2008-2009 Max. Min. Mean Mean Mean Mean Mean Mean Mean Mean Mean Mar 7.7 -16.1 -3.6 -0.42 0.38 1.13 1.09 0.98 1.16 1.69 2.31 Apr 13.2 -15.7 -0.9 -0.38 0.34 1.02 0.99 0.88 1.05 1.59 2.06 May 18.0 -9.0 4.2 -0.34 0.29 0.74 0.76 0.72 1.00 1.60 1.85 Jun 22.5 -2.2 10.2 -0.35 0.18 0.52 0.57 0.57 0.85 1.47 1.60 Jul 25.0 6.7 16.1 5.21 2.55 4.46 4.27 3.90 2.99 2.29 3.81 Aug 25.1 3.2 14.8 8.41 6.01 4.85 4.88 4.88 4.81 4.19 5.50 Sep 19.5 -1.3 9.3 5.36 5.24 3.80 3.83 3.83 3.98 3.86 4.42 Oct 16.8 -10.7 4.6 2.45 3.68 2.59 2.64 2.69 3.01 3.24 3.83 Nov 12.6 -12.7 0.5 1.01 2.03 1.54 1.59 1.60 1.91 2.39 3.00 Dec 7.9 -20.2 -5.8 0.19 1.26 1.19 1.25 1.25 1.52 2.04 2.68 Jan 11.1 -22.1 -3.6 -0.02 0.85 0.79 0.86 0.87 1.21 1.83 2.47 Feb 7.8 -16.1 -3.8 0.00 0.73 0.90 0.92 0.85 1.10 1.69 2.29 annual mean 3.5 1.78 1.97 1.98 1.99 1.94 2.06 2.33 2.99 1Value interpolated from differences in monthly data for 2008-2009 \®. 2007-2008 2Value calculated as mean of monthly mean data for 2007-2008 79 cm at the time of installation. At Site 2 the MRC temperature probe was installed on 24 February 2007 in a bog/wetland (discharge) area approximately 230 m downgradient from the glacial moraine and Site 1. At this site, the land surface was constantly saturated suggesting that the water table was at, or slightly above, land surface. At Site 2, it was possible to install the probe to a depth of 104 cm, so all five thermistors were below ground at 2, 7, 17, 47, and 97 cm depth. Data for the two probes from the period of 1 March 2007 to 1 March 2009 are shown in Figures 1-3 and 1-4, and summarized in Table 1-1. 1.4.3 Groundwater Temperature Data In addition to ground temperatures, groundwater temperatures within a shallow well installed in the discharge area (Site 2) near the MRC probe were measured using a HOBO temperature logger. The sensor was suspended from the well cap to a depth of approximately 1.2 m (middle of the well screen); temperatures were logged at 30-min intervals and periodically downloaded throughout the study. The logger in the well was deployed on 31 March 2007. Groundwater temperatures for the period 31 March 2007 to 1 March 2009 are shown in Figure 1-5 and are summarized in Table 1-1. 1.4.4 Noble Gas Groundwater Recharge Temperature Data and Age Data Noble gas samples for the determination of groundwater recharge temperature and tritium samples for the determination of groundwater age were generally collected every 2 to 8 weeks from 6 February 2007 to 25 May 2009 from the well. Groundwater recharge temperature and age data from the well are shown in Figures 1-5 and 1-6, respectively, 16 Apparent Groundwater Age (years) 17 Mar07 Jun07 Sep07 Dec07 Mar08 Jun08 Sep08 Dec08 Mar09 Date Figure 1-6. Age of groundwater samples (crosses) with 1 s.d. error bars collected from the well. Also shown are hourly mean snow depth data (gray line) from the SNOTEL meteorological station, Brighton Basin, Utah. Modified from: Masbruch et al. [2012]. Snow Depth (m) and are summarized in Table 1-2. Currently, there are several models that are used in the determination of recharge temperatures from noble gas data, which differ in the way in which the ‘‘excess air'' component is treated; these include the total dissolution (TD) model [Andrews and Lee, 1979; Stute andSchlosser, 1993], the partial re-equilibration (PR) model [Stute et al., 1995], the closed-system equilibration (CE) model [Aeschbach-Hertig et al., 2000; Ballentine and Hall, 1999], the multistep partial re-equilibration (MR) model [Kipfer et al., 2002], the partial degassing (rism diopters (PD)) model [Lippmann et al., 2003], the negative pressure (NP) model [Mercury et al., 2004], the oxygen depletion (OD) model [Hall et al., 2005], and the gas diffusion relaxation (GR) model [Sun et al., 2008]. This study uses the CE model for the determination of recharge temperatures from the noble gas data. The purpose of this study was not about comparing results from the different excess air models, but rather about comparing noble gas derived groundwater recharge temperatures with groundwater table temperatures. The consistency between the mean model results and the mean groundwater table temperatures measured within the Brighton Basin suggests that the CE model adequately represents conditions within the basin. Measured noble gas and tritium concentrations are given in Table 1-2. Using inverse modeling techniques as described by Aeschbach-Hertig et al. [1999], these gas concentrations were then used to determine the unknown parameters of recharge temperature, excess air, and the fractionation of the excess air; salinity and recharge elevation (pressure) were prescribed a priori as 0 and 2,768 m, respectively. The inverse modeling technique uses a nonlinear least squares method that finds those values of the 18 Table 1-2. Summary of noble gas maximum recharge temperatures (Tr), groundwater ages, and measured noble gas and tritium concentrations. Modified from: Masbruch et al. [2012], Sample ID Collection date Maximum Tr (°C) Apparent groundwater age (years) n2 (ccSTP/g) x 10'2 error: +5.0% 40Ar (ccSTP/g) x 10'4 error: +3.0% 84Kr 20Ne 129Xe 4He (ccSTP/g) (ccSTP/g) (ccSTP/g) (ccSTP/g) x 10'8 x 10'7 x 10'9 x 10'8 error: error: error: error: 14.0% +2.0% +5.0% +1.0% R/Ra1 error: +1.0% 3H (TU) error: +5.0% WA03 20070306 1.611.1 3.910.4 1.2 3.4 4.8 1.4 3.5 3.5 1.1 6.9 WA04 20070320 4.211.4 6.810.4 1.2 3.4 4.6 1.5 3.5 3.6 1.2 7.8 WA05 20070403 0.010.8 0.710.5 1.3 3.5 4.8 1.5 4.0 3.5 1.0 6.9 WA06 20070417 1.211.1 3.210.5 1.3 3.6 4.8 1.5 3.6 3.8 1.0 6.8 WA07 20070430 1.111.1 1.010.5 1.4 3.7 5.0 1.6 3.6 3.7 1.0 7.4 WA08 20070513 2.611.2 0.010.5 1.3 3.6 4.8 1.5 3.5 3.6 1.0 7.1 WA09 20070528 1.411.1 0.210.5 1.4 3.8 5.1 1.5 3.6 3.6 1.0 7.6 WB10 20070610 3.111.2 0.110.5 1.3 3.5 4.8 1.5 3.4 3.6 1.0 7.3 WB11 20070708 7.211.3 0.010.5 1.2 3.2 4.2 1.4 2.9 3.5 1.0 7.9 WA12 20070729 2.511.3 1.310.5 1.3 3.4 4.6 1.5 3.4 3.8 1.0 7.4 WA13 20070819 6.811.2 2.110.4 1.2 3.2 4.2 1.5 2.9 3.6 1.1 7.5 WA14 20070826 2.811.2 1.610.4 1.3 3.5 4.6 1.6 3.5 3.8 1.1 7.7 WB15 20070909 4.711.3 2.310.4 1.3 3.5 4.5 1.6 3.1 3.8 1.1 7.8 WA16 20071003 3.211.7 3.010.4 1.3 3.4 4.6 1.6 3.4 3.9 1.1 4.5 WA17 20071024 2.211.4 2.710.5 1.4 3.6 4.9 1.6 3.4 3.8 1.1 7.2 WA18 20071118 3.111.2 1.310.5 1.3 3.4 4.7 1.6 3.4 3.7 1.1 7.4 WA19 20071205 4.011.5 4.910.4 1.3 3.4 4.7 1.6 3.1 3.9 1.1 7.0 WA20 20080116 0.011.6 7.810.4 1.5 3.3 5.5 1.8 3.8 4.5 1.1 7.6 WA21 20080210 7.411.4 0.610.5 1.2 3.7 4.3 1.5 3.2 3.6 1.0 7.4 WA22 20080406 3.911.2 3.910.4 1.3 3.5 4.5 1.5 3.2 3.6 1.1 7.4 WA23 20080519 0.111.1 4.610.4 1.4 3.6 4.9 1.6 4.1 3.8 1.1 7.8 WA24 20080706 2.611.4 0.610.4 1.3 3.5 4.7 1.6 3.3 3.8 1.0 8.5 WA25 20080908 1.311.0 7.210.4 1.3 3.5 5.0 1.5 3.4 4.0 1.1 7.7 WB26 20081019 2.111.0 4.810.4 1.3 3.5 4.9 1.5 3.3 3.8 1.1 8.1 WB27 20090204 2.510.9 3.110.4 1.3 3.4 4.7 1.4 3.1 3.5 1.1 8.0 !R is the3He/4He ratio of the sample; Ra is the 3He/4He ratio of air (1.384xl0'6) model parameters that minimize %, which is the sum of the squared deviations between the modeled and measured concentrations, normalized to the respective experimental uncertainties [Aeschbach-Hertig et al., 2002]. The reported 1o (i.e., 1 standard deviation) uncertainties in the recharge temperatures and ages (Table 1-2 and Figures 1-5 and 1-6) were determined using Monte Carlo simulations, whereby the measurement errors of the noble gas and tritium concentrations were varied. 1.5 Results/Discussion 1.5.1 T emperature Data The data collected from the monitoring network were used to examine how the air, ground, and noble gas groundwater recharge temperatures relate to one another. Additionally, the data were used to identify possible seasonal variations in the groundwater recharge temperatures and ages, which may point to seasonal changes in the groundwater flow regime within the basin. The data were also used to investigate the effects of snow cover on ground temperatures within the basin. Air temperatures for the period March 2007 to March 2009 varied between -22.4 °C and 25.4 °C (Table 1-1). Maximum temperatures occurred in July and August, while minimum temperatures occurred in January. Monthly mean air temperatures for March to September 2007 were 0.5 to 4.3 °C warmer than monthly mean temperatures for March to September 2008. Conversely, monthly mean air temperatures for December to February 2007 to 2008 were 1.4 to 3.8 °C colder than monthly mean temperatures for December to February 2008 to 2009. Monthly mean temperatures for October and November 2007 and 2008 were fairly similar, with differences of only 0.1 and 0.3 °C. Annual mean temperature for the 2 years was 4.4 and 3.5 °C, respectively. Because Sites 20 1 and 2 are ~100 m higher in elevation than the meteorological site where air temperatures are measured, there is about a 0.6 °C offset in mean annual air temperatures (cooler) at Sites 1 and 2. Ground temperatures at Site 1 for the period March 2007 to March 2009 varied between -0.84 and 11.45 °C at 22 cm depth, and between -0.21 and 8.91 °C at 72 cm depth (Figure 1-3), while at Site 2 ground temperatures varied between -2.77 and 7.72 °C at the shallowest depth (2 cm), and between 1.04 and 5.22 °C at the deepest depth (97 cm) (Figure 1-4). As expected, ground temperatures at both of these sites show less variation in minimum and maximum temperatures than air temperatures, with greater attenuation at greater depths. Additionally, ground temperatures at Site 2 show less variation than ground temperatures at Site 1. This difference is likely due to Site 2 lying within a discharge area and groundwater flow through this site further dampens the annual variation in temperatures. At both Site 1 and Site 2, maximum ground temperatures generally occurred in July or August, lagging behind the timing of maximum air temperatures, with longer lag times occurring at deeper depths. For instance, at Site 1, the deeper thermistor at 72 cm depth reaches its maximum temperature slightly later than the thermistor at 22 cm depth (Figure 1-3); the same can be seen at Site 2 where maximum ground temperatures generally occurred in July for depths of 2, 7, and 17 cm, and in August at 47 and 97 cm depth (Figure 1-4). Minimum ground temperatures at the shallower depths at both sites (22 cm depth at Site 1 and 2, 7, and 17 cm depth at Site 2) generally occurred in late fall, just before the onset of snow cover (Figures 1-3 and 1-4). Minimum temperatures at the deeper depths 21 (72 cm depth at Site 1; and 47 and 97 cm depth at Site 2) generally occurred in late spring/early summer during the annual snowmelt event. Additionally, ground temperatures at the shallower depths at both sites were warmer than temperatures at deeper depths from just after the disappearance of the snow cover through the summer months and into early fall; conversely, ground temperatures at the shallower depths were cooler than temperatures at deeper depths during the fall until just after the disappearance of the snow cover (Figures 1-3 and 1-4). Both the difference in the timing of the occurrence of minimum temperatures between the shallower and deeper depths, as well as the relative difference in temperatures between the shallower and deeper depths throughout the year shows that the shallower ground temperatures are more directly influenced by air temperatures, while the deeper ground temperatures are more directly influenced by groundwater flow. Annual mean ground temperatures at Site 1 were up to 1.18 °C colder than annual mean air temperatures (adjusted for elevation of Site 1) for 2007 to 2008, and 1.12 °C colder than mean annual air temperatures (adjusted for elevation of Site 1) for 2008 to 2009 (Table 1-1). Similarly, annual mean ground temperatures at Site 2 were up to 1.22 °C colder than annual mean air temperatures (adjusted for elevation of Site 2) for 2007 to 2008, and 0.96 °C colder than annual mean air temperatures (adjusted for elevation of Site 2 for 2008 to 2009). These results are consistent with the 2 °C offset between mean annual air temperatures and groundwater recharge temperatures derived by Manning and Solomon [2003] for the Wasatch Mountains. Groundwater temperatures at the well for the period March 2007 to March 2009 varied between 1.10 and 6.89 °C (Figure 1-5). Maximum temperatures generally occurred 22 in August, attenuated and lagged slightly longer than 1 month after maximum air temperatures. Minimum temperatures occurred in either May 2007 or June 2008 during the annual snow melt event. Annual mean groundwater temperatures for 2007 to 2008 and 2008 to 2009 were 3.65 °C and 2.99 °C, respectively; this is slightly warmer than annual mean ground temperatures at both Site 1 and Site 2, and 0.75 and 0.51 °C colder than the annual mean air temperature (adjusted for elevation of Site 2) for the 2 years. The warmer temperatures at the well versus ground temperatures are likely due to the well measuring deeper temperatures (about 1.2 m depth), and/or from warm water moving up from depth that is typical of discharge areas. 1.5.2 Relation of Air and Ground Temperatures to Temperature at the Water Table Noble gas samples collected from the well at Site 2 were used to calculate groundwater recharge temperatures, which essentially record the temperature at the water table. The noble gas recharge temperatures reported in this study were calculated at the altitude of the well screen, so they represent the maximum recharge temperatures possible, as it is unlikely that the well is receiving groundwater recharge at a lower elevation than the well screen. Groundwater recharge temperatures from noble gas samples collected between March 2007 and March 2009 ranged between 0.0 ± 1.6 °C (16 January 2008) and 7.4 ± 1.4 °C (10 February 2008), and averaged 2.9 ± 1.2 °C (Figure 1-5 and Table 1-2), consistent with ground temperatures measured within the basin. Average maximum groundwater recharge temperatures were approximately 0.3 °C warmer to 2.2 °C cooler than annual mean air temperatures (adjusted for elevation of Site 1) from 2007 to 2008, 23 and were 0.0 to 1.3 °C cooler than annual mean air temperatures (adjusted for elevation of Site 1) from 2008 to 2009. These differences are comparable to the 2 °C difference between groundwater recharge temperatures and mean annual air temperatures inferred by Manning and Solomon [2003] for the Wasatch Mountains. Groundwater recharge temperatures calculated from noble gas samples collected between March and December 2007, appear to track the groundwater temperatures measured at the well, following an attenuated and lagged annual temperature variation. This pattern is pronounced in 2007 with a range of 7 °C between summer and winter samples. Apparent groundwater ages (Figure 1-6 and Table 1-2) from these same samples, however, varied between 0 and 7 years. This seasonal pattern in the noble gas recharge temperatures did not continue into 2008 and 2009 (Figure 1-5), with samples collected after December 2007 showing much more scatter, and no definitive seasonal trends. These data thus show general agreement between noble gas recharge temperatures and groundwater temperatures albeit with some complexity. Apparent groundwater ages from water collected between March 2007 and March 2009 at the well ranged between 0.0 ± 0.5 years and 7.8 ± 0.4 years (Figure 1-6 and Table 1-2). From March through December 2007, the apparent ages followed a seasonal pattern, with winter samples being 2 to almost 7 years older than late spring/early summer samples. This seasonal age variation points to possible variations in the groundwater flow regime throughout 2007. During times when there is little to no groundwater recharge (i.e., fall/winter) the well is capturing older groundwater. During high recharge times of the year (i.e., the annual snowmelt event during late spring/early summer) these older flow paths are pushed deeper within the aquifer, and are no longer 24 being captured by the well; the well is capturing flow paths carrying younger water instead. It does not take much change in the depth of the flow paths to change which paths are being captured by the well; in fact, changes in depth as little as 20 cm may produce the seasonal pattern seen in the apparent age data in 2007. Like the noble gas recharge temperatures, the seasonal pattern in apparent age data did not continue into 2008 and 2009. The high scatter in apparent ages and noble gas recharge temperatures suggests that the groundwater flow regime within the Brighton Basin is quite complex, and warrants further study to explain the scatter within the data. Because the apparent age data suggest groundwater ranging up to 7.8 years, the air temperatures from 2000 to 2007 were also examined to determine differences between groundwater recharge temperatures and air temperatures for these older age samples. Annual mean air temperatures from 2000 to 2007 ranged between 2.9 °C (2002 and 2004) and 4.6 °C (2007), and averaged 3.5 °C (data accessed from SNOTEL website at http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=366&state =ut). Mean maximum groundwater recharge temperatures for the groundwater samples that show ages of being recharged before March 2007 were approximately 0.0 ± 1.2 °C to 1.1 ± 1.2 °C cooler than mean annual air temperatures from 2000 to 2007. Again, this is comparable to, to slightly less than, the 2 °C difference between groundwater recharge temperatures and mean annual air temperatures inferred by Manning and Solomon [2003] for the Wasatch Mountains. 1.5.3 Snow Effects Comparison of changes in monthly mean ground and groundwater temperatures versus changes in monthly mean air temperatures over the 2 year study period illustrate 25 the effects of snow cover on the ground temperatures within the basin. For example, monthly mean ground and groundwater temperatures for March through May 2007 are comparable to monthly means for March to May 2008 (differences of only 0.00 to 0.36 °C), despite monthly mean air temperatures for March through May 2007 being approximately 3.3 to 4.3 °C warmer than March through May 2008 (Table 1-1). This consistency in ground temperatures is likely due to snow cover insulating the ground from the air temperatures during these times of both years (Figures 1-3 through 1-5). Monthly mean ground and groundwater temperatures for June 2007 were 0.79 to 4.49 °C warmer than monthly mean temperatures for June 2008 (Table 1-1); monthly mean air temperatures for June 2007 also were 3.2 °C warmer than June 2008. The cooler ground temperatures in June 2008 may be attributed to the fact that (1) snow cover persisted one month longer in 2008 than in 2007 (Figures 1-3 through 1-5), resulting in insulating the ground from the warmer air temperatures for a longer period of time in 2008; and/or (2) air temperatures in June 2008 were cooler than air temperatures in 2007. While monthly mean air temperatures from July through September 2007 are only 0.5 to 1.1 °C warmer than July through September 2008, monthly mean ground and groundwater temperatures from July through September 2007 are 0.85 to 4.22 °C warmer than July through September 2008, with the largest differences occurring in July (Table 1-1). Again, this difference may be partly attributed to the snow cover in 2008 persisting longer in the spring and summer months, thereby preventing the ground from warming as much as in 2007 (Figures 1-3 through 1-5). Finally, monthly mean shallow ground temperatures for November to December 2007 are 0.13 to 0.83 °C colder than monthly mean ground temperatures for November to December 2008 (Table 1-1). This is likely due to the later 26 onset of snow in 2007 than 2008; in 2008, the onset of snow occurred nearly a month earlier than in 2007, thereby insulating the ground from the colder air temperatures for a longer period of time (Figures 1-3 through 1-5). The snow effects on mean annual ground temperatures were quantified using a numerical model of snow-ground thermal interactions developed by Bartlett et al. [2004, 2005]. Bartlett et al. [2004] found that snow onset time and duration were the two greatest controlling factors in determining whether the mean annual ground temperature is warmer or cooler than the mean annual air temperature. This temperature difference, called the ‘‘snow effect'' [Bartlett et al., 2004], is plotted in terms of the controlling factors in Figure 1-7 for Brighton Basin, Utah. A snow season can either raise or lower the mean annual ground temperature relative to the air over an annual cycle. Warming of the mean annual ground temperature relative to air occurs when snow onset coincides roughly with the daily mean air temperature falling below 0 °C, and lasting until daily mean air temperatures rise above the freezing point. During this time the ground is insulated by snow from the cold winter temperatures. Depending on the annual surface air temperature (SAT) cycle, this warming can be 2 °C or greater. Alternatively, cooling of the mean annual ground temperature relative to air occurs when the snow onset is late and the duration is long, meaning that snow keeps the ground temperature pinned near 0 °C, long after the daily mean temperature has risen above freezing. The Bartlett et al. [2004] snow model uses inputs of both the annual and diurnal temperature cycles, as well as the diffusivity of the snow pack; however, the model assumes that the thermal properties (diffusivity) of the snow are homogenous and constant in both space and time. In actuality, the snowpack undergoes compaction due to 27 28 Figure 1-7. The snow effect-influence of snow event onset time and duration on mean annual surface ground temperatures relative to mean annual air temperatures. Contours illustrate the difference in °C between the mean annual surface ground temperature and the driving function (labeled SAT above). The top left panel shows results using an "air-filled" snow thermal diffusivity of 2x10-8 m2/s; the top right panel shows results using an "ice-like" snow thermal diffusivity of 1x10-6 m2/s. The points represent snow onset and duration of annual snow events observed at the Brighton SNOTEL meteorological station from 1997 to 2011.The bottom panel shows the annual driving function (solid line) and the limits of the diurnal fluctuations (dashed lines). Modified from: Masbruch et al. [2012]. melting and refreezing, which effectively changes the density and thermal properties of the snow as a function of time [Bartlett et al., 2004]. Therefore, in order to capture the end members of the evolving snowpack and provide a constraint on the snow effect within the Brighton Basin, two simulations of the snow model were run: one with a thermal diffusivity of 2 x 10-8 m2 s-1 which represents a ‘‘fluffy, air-filled'' snow, and one with a thermal diffusivity of 1 x 10-6 m2 s-1 which is representative of a more ‘‘icelike'' snow. Results from the snow model simulations are shown in Figure 1-7. Figure 1-7 (top left) shows model results for the thermal diffusivity of air-filled snow, and the top right panel shows the model results for the thermal diffusivity of more ice-like snow. Solid dots on Figure 1-7 indicate the onset time and duration for all annual snow events between 1997 and 2011 at the Brighton SNOTEL meteorological station, and indicate that the snow effect at Brighton is between +1.0 °C and -2.0 °C, with a mean snow effect of -1.0 °C. This cooling is consistent with the measured ground, groundwater, and groundwater recharge temperatures within the basin. 1.6 Summary and Conclusions Although this study did not set out to evaluate noble gas thermometry comprehensively, it does provide details of the thermal regime of both groundwater recharge and discharge areas in an alpine setting. The thermal effects of snow cover in this setting are also studied. Using noble gas temperatures collected from groundwater samples within a discharge area that originates from a highly constrained recharge area, it is concluded that the noble gas recharge temperatures are consistent with surface ground temperatures within the probable recharge area, and that surface ground temperatures are 29 cooler than mean annual air temperatures. To determine why groundwater recharge temperatures within the Wasatch Mountains are cooler than mean annual air temperatures, this study investigates the relation between air, ground, and groundwater recharge temperatures within the Brighton Basin, a high alpine basin located within the Wasatch Mountains. Hydrogeologic considerations of this site provide a tight constraint on the location and elevation of recharge areas. A pre-existing meteorological station from the SNOTEL network provided measurements of air temperatures and snow depth. Ground temperature probes were installed in both a local recharge and a local discharge area within the basin to determine the relation between air and shallow ground temperatures at these sites. Additionally, a well was installed in the discharge area that allowed for sampling of noble gases and tritium used to determine groundwater recharge temperature and age. Detailed monitoring over a 2 year period allowed identification of possible seasonal and annual signals in groundwater recharge temperatures and ages. Based on this monitoring, the following conclusions can be drawn: 1. Maximum noble gas groundwater recharge temperatures computed using the CE model from 25 samples collected from March 2007 to March 2009 in the Brighton Basin, Utah, at an elevation of approximately 2,770 m, average 2.9 ± 1.2 °C. This average is within the experimental error of the mean ground temperature of 2.28 °C measured in the probable recharge area over the same time period. 2. The variation in noble gas recharge temperatures is from 0 to 7 °C. This range is also comparable to ground temperature variations in the region throughout the annual cycle. In the first year of monitoring, the noble gas temperatures appear to 30 follow an attenuated and lagged annual temperature variation similar to the ground temperatures, although the pattern is not replicated in the second year. Because apparent groundwater ages in the collected samples vary from 0 to 7 years, the groundwater flow pattern within the basin is likely complex and warrants further study. 3. Mean ground temperature in the upper 1 m of soil at measurement Sites 1 and 2 over the 2 year time period is 2.32 °C. The ground temperature is 1.05 °C colder than the mean SAT (adjusted for elevation of Sites 1 and 2) of 3.37 °C over the same period. This offset contradicts the trend of surface temperature variation with elevation (lapse rate) in central Utah, whereby ground temperatures are warmer than air temperatures; the offset, however, is explained by a snow effect where late spring and early summer snow cover cools the ground relative to air. Interpretation of groundwater recharge temperatures derived from noble gases, therefore, must be attentive to local ground temperature effects in the probable recharge zones. These conclusions indicate that in a snow dominated, high alpine area, such as the Brighton Basin, ground temperatures are cooler than air temperatures. The noble gas recharge data corroborate this fact, and the results are consistent with the 2 °C difference between groundwater recharge temperatures and mean annual air temperatures inferred by Manning and Solomon [2003] for the Wasatch Mountains. This observation implies that in high alpine areas, the assumption that Tr = Ta may not be valid. It appears that by utilizing noble gas recharge data from discharge points within the mountain block, a more appropriate Tr lapse curve can be derived for the area in question, thereby permitting a 31 more correct interpretation of recharge altitude and, therefore, sources of recharge to the groundwater system. 1.7 Acknowledgments We would like to thank Paul Gettings, Tom Marston, Derrick Hasterok, and Angie Vincent for help in the set up and installation of the ground temperature probes and the well. Thanks also go to Marshall Bartlett for the use and running of simulations of his snow model, and to Alan Rigby for help with laboratory analysis of the noble gas samples. 1.8 References Aeschbach-Hertig, W., F. Peeters, U. Beyerle, and R. Kipfer (1999), Interpretation of dissolved atmospheric noble gases in natural waters, WaterResour. Res., 35, 2779-2792. Aeschbach-Hertig, W., F. Peeters, U. Beyerle, and R. Kipfer (2000), Paleotemperature reconstruction from noble gases in ground water taking into account equilibration with entrapped air, Nature, 405, 1040-1044. Aeschbach-Hertig, W., M. Stute, J. F. Clark, R. F. Reuter, and P. Schlosser (2002), A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA), Geochim. Cosmochim. Acta, 66, 797-817. Anderson, M. P. (2005), Heat as a groundwater tracer, Ground Water, 43, 951-968. Anderson, T. W., and G. W. Freethey (1996), Simulation of groundwater flow in alluvial basins in south-central Arizona and parts of adjacent states, US Geol. Surv. Prof. Pap. 1406-D, 78 pp. Andrews, J. N., and D. J. Lee (1979), Inert gases in groundwater from the Bunter Sandstone of England as indicators of age and paleoclimatic trends, J. Hydrol., 41, 233-252. Ballentine, C. J., and C. M. Hall (1999), Determining paleotemperatures and other variables by using an error-weighted, nonlinear inversion of noble gas concentrations in water, Geochim. Cosmochim. Acta, 63, 2315-2336. 32 33 Bartlett, M. G., D. S. Chapman, and R. N. Harris (2004), Snow and the ground temperature record of climate change, J. Geophys. Res., 109, F04008, doi :10.1029/2004JF000224. Bartlett, M. G., D. S. Chapman, and R. N. Harris (2005), Snow effect on North American ground temperatures, 1950-2002, J. Geophys. Res., 110, F03008, doi :10.1029/2005JF000293. Buttle, J. M. (1989), Soil moisture and groundwater responses to snowmelt on a drumlin sideslope, J. Hydrol., 105, 335-355. Cey, B. D. (2009), On the accuracy of noble gas recharge temperatures as a paleoclimate proxy, J. Geophys. Res., 114, D04107, doi:10.1029/ 2008JD010438. Domenico, P. A., and F. W. Schwartz (1998), Physical and Chemical Hydrology, John Wiley and Sons, New York. Gates, J. S. (1995), Description and quantification of the groundwater basins of the Wasatch Front, Utah, 1904-1994, in Environmental and Engineering Geology o f the Wasatch Front Region, Utah, Utah Geol. Surv. Publ., vol. 24, edited by W. R. Lund, Utah Geological Society, Salt Lake City, Utah, 221-248. Hall, C. M., M. C. Castro, K. C. Lohmann, and L. Ma (2005), Noble gas and stable isotopes in a shallow aquifer in southern Michigan: Implications for noble gas paleotemperature reconstructions for cool climates, Geophys. Res. Lett., 32, L18404, doi:10.1029/2005GL023582. Hill, B. R. (1990), Groundwater discharge to a headwater valley, northwestern Nevada, USA, J. Hydrol., 113, 265-283. Kipfer, R., W. Aeschbach-Hertig, F. Peeters, and M. Stute (2002), Noble gases in lakes and ground waters, in Noble Gases in Geochemistry and Cosmochemistry, Rev. Mineral. Geochem., vol. 47, edited by D. Porcelli, C. Ballentine, and R. Wieler, 615-700. Klump, S., M. S. Brennwald, and R. Kipfer (2006), Comment on ‘‘Noble gas and stable isotopes in a shallow aquifer in southern Michigan: Implications for noble gas paleotemperature reconstructions for cool climates'' by Chris M. Hall et al., Geophys. Res. Lett., 33, L24403, doi:10.1029/2006GL027496. Lipmann, J., M. Stute, T. Torgersen, D. P. Moser, J. A. Hall, L. Lin, M. Borcsik, R. E. S. Bellamy, and T. C. Onstott (2003), Dating ultra-deep mine waters with noble gases and 36Cl, Witwatersrand Basin, South Africa, Geochim. Cosmochim. Acta, 67, 4597-4619. 34 Manning, A. H., and J. S. Caine (2007), Groundwater noble gas, age, and temperature signatures in an Alpine watershed: Valuable tools in conceptual model development, WaterResour. Res., 43, W04404, doi :10.1029/2006WR005349. Manning, A. H., and D. K. Solomon (2003), Using noble gases to investigate mountain-front recharge, J. Hydrol., 275, 194-207. Masbruch, M.D., D.S. Chapman, and D.K. Solomon (2012), Air, ground, and groundwater recharge temperatures in an alpine setting, Brighton Basin, Utah, Water Resour. Res., 48, W10530, doi:10.1029/2012WR012100. Mason, J. L. (1998), Ground-water hydrology and simulated effects of development in the Milford area, an arid basin in southwestern Utah, US Geol. Surv. Prof. Pap. 1409-G, 69 pp. Mazor, E. (1991), Applied Chemical and Isotopic Groundwater Hydrology, Halsted Press, New York. Mercury, L., D. L. Pinti, and H. Zeyen (2004), The effect of the negative pressure of capillary water on atmospheric noble gas solubility in ground water and paleotemperature reconstruction, Earth Planet. Sci. Lett., 223, 147-161. Powell, W. G., D. S. Chapman, N. Balling, and A. E. Beck (1988), Continental heat-flow density, in Handbook o f Terrestrial Heat-Flow Density Determination With Guidelines and Recommendations o f the International Heat Flow Commission, edited by R. Haenel, L. Rybach, and L. Stegena, Kluwer Acad., Boston, Mass., 167-222. Prudic, D. E., and M. E. Herman (1996), Ground-water flow and simulated effects of development in Paradise Valley, a basin tributary to the Humboldt River in Humboldt County, Nevada, uS Geol. Surv. Prof. Pap. 1409-F, 92 pp. Putnam, S. N., and D. S. Chapman (1996), A geothermal climate change observatory: First year results from Emigrant Pass in northwest Utah, J. Geophys. Res., 101, 21, 877-21, 890. Rauber, D., H. H. Loosli, H. Schmassmann, and J. N. Andrew (1991), Noble gases in groundwater, in Applied Isotope Hydrology: A Case Study in Northern Switzerland, edited by, F. J. Pearson, Jr., et al., Elsevier, Amsterdam, 116-152. Sanford, W. E., R. G. Shropshire, and D. K. Solomon (1996), Dissolved gas tracers in groundwater: Simplified injection, sampling, and analysis, Water Resour. Res., 32, 1635-1642. 35 Smith, G. D., F. Newhall, L. H. Robinson, and D. Swanson (1964), Soil temperature regimes: Their characteristics and predictability, US Soil Conserv. Serv. Rep., SCS-TP-144. Solomon, D. K., and P. G. Cook (2000), 3H and 3He, in Environmental Tracers in Subsurface Hydrology, edited by P. G. Cook and A. L. Herczeg, Kluwer Acad., Boston, Mass., 233-260. Stokes, W. L. (1986), Geology of Utah, Occas. Pap., 6, Utah Museum Nat. History, Salt Lake City, Utah. Stute, M., M. Forster, H. Frischkorn, A. Serejo, J. F. Clark, P. Schlosser, W. S. Broecker, and G. Bonani (1995), Cooling of tropical Brazil (5 °C) during the Last Glacial Maximum, Science, 269, 379-383. Stute, M., and P. Schlosser (1993), Principles and applications of the noble gas paleothermometer, in Climate Change in Continental Isotopic Records, Geophys. Monogr. Ser., vol. 78, edited by P. K. Swart et al., AGU, Washington, D.C., 89­100. Sun, T., C. M. Hall, M. C. Castro, K. C. Lohmann, and P. Goblet (2008), Excess air in the noble gas groundwater paleothermometer: A new model based on diffusion in the gas phase, Geophys. Res. Lett., 35, L19401, doi:10.1029/2008GL035018. Zuber, A., S. M. Weise, K. Osenbruck, J. Grabczak, and W. Ciezkowski (1995), Age and recharge area of thermal waters in Ladek Spa (Sudeten, Poland) deduced from environmental isotope and noble gas data, J. Hydrol., 167, 237-349. CHAPTER 2 USING GROUNDWATER TEMPERATURES TO CONSTRAIN RECHARGE RATES IN ARID INTERMONTANE BASINS 2.1 Abstract Two-dimensional numerical modeling of the combined processes of fluid flow and heat transport are used to quantify the effects of groundwater flow on the subsurface thermal regime and determine the lower limit of recharge rate that will produce an observable perturbation such that groundwater temperatures can be used to constrain recharge rates. Simulations were executed for a vertical section through a basin with a geometry and moderate to high permeabilities representative of aquifers within the Great Basin. Groundwater recharge rates were varied between 1 and 100 mm yr-1. For all recharge rates and bedrock permeabilities, the greatest temperature perturbations (up to greater than 60 °C) occur in the deepest portion of the recharge area. At lower recharge rates (10 mm yr-1 or less), the hydrologic disturbance to the subsurface thermal regime is almost completely dependent on the recharge rate. At recharge rates higher than this, the hydrologic disturbance is dependent on both the recharge rate and the permeability. Bedrock permeabilities appear to control the distance to which the temperature perturbation will extend from the recharge area. At recharge rates of 50 mm yr-1 and above, the plume of colder water extends past the recharge area and persists throughout and under the basin-fill deposits towards the discharge area, at fairly shallow depths. This plume of cooler water could be easily measured and used to constrain recharge rates to the system as a whole. The lower limit of recharge rates needed to produce a thermal perturbation large enough such that groundwater temperatures can be used to constrain recharge rates, therefore, is about 50 mm yr-1. 2.2 Introduction One of the most difficult hydrologic budget components to determine is groundwater recharge from the infiltration of precipitation, especially in mountainous terrain where hydrologic data may be sparse due to the scarcity of wells. In the Great Basin in the western U.S., groundwater recharge from the infiltration of precipitation primarily occurs in the mountain blocks and is the main source of groundwater to both the mountain-block and adjacent basin aquifers. In recent years, groundwater development within the Great Basin targeting permeable consolidated rock aquifers beneath the basin-fill deposits and in the surrounding mountains has increased [Masbruch et al., 2011]. Accurate estimates of groundwater recharge to these aquifers, therefore, are essential for water resources planning. Early groundwater studies in the Great Basin, beginning with Maxey andEakin [1949] generally focused on the basin-fill (valley) aquifers, and recharge estimates were calibrated to groundwater discharge in the basin-fill aquifer. These earlier methods did not consider groundwater discharge within the mountain block or recharge to underlying consolidated rock aquifers and, therefore, only considered "net" recharge to the unconsolidated basin-fill aquifer [Masbruch et al., 2011]. More recently, a new class of spatially distributed recharge estimation techniques based on water-balance methods has 37 been developed for the Great Basin [Flint and Flint, 2007a; 2007b; Flint et al., 2011; Hevesi et al., 2003; Leavesley et al., 1983; Markstrom et al., 2008]. These techniques take into account all groundwater recharge and discharge processes within the mountain block and subsequent recharge of a portion of infiltration of runoff to the basin-fill aquifer and, therefore, provide estimates for "total" recharge from precipitation. Uncertainties in these estimates, however, may be as high as ± 50 percent [Flint et al., 2011]. Because of this high uncertainty, finding other methods of constraining groundwater recharge estimates from precipitation is of importance to hydrologic studies within the Great Basin. Manning and Solomon [2005] showed that groundwater temperatures measured within the Salt Lake Valley, located along the eastern margin of the Great Basin, could be used to constrain recharge rates, as well as subsurface mountain-block to basin-fill groundwater flow. Groundwater temperatures collected from over 50 wells within the Salt Lake basin-fill aquifer showed a cold-water plume extending from the adjacent Wasatch Mountains, which is the recharge area for groundwater in the Salt Lake Valley. Manning and Solomon [2005] were able to constrain the absolute magnitude of subsurface inflow of mountain-block groundwater into the basin-fill aquifer, and ultimately groundwater recharge, by using groundwater temperature and age data collected from the basin-fill aquifer in conjunction with a three-dimensional finite element flow and transport model. Results from Manning and Solomon [2005] showed that groundwater age data could be used to constrain the lower end of recharge, while groundwater temperature data could be used to constrain the higher end of recharge. This study focuses on whether the approach used by Manning and Solomon 38 [2005] could be applied to other drier/warmer climatic settings within the Great Basin. A "generic" coupled groundwater flow/thermal model was constructed using hydrologic and thermal characteristics that are typically found in the southern Great Basin, which is a much drier and warmer environment compared to the Salt Lake Valley. This model was used to investigate the relative magnitude of thermal perturbations caused by groundwater flow that may occur within a warmer/drier climate, and to determine recharge rates needed to produce a significant thermal perturbation such that groundwater temperatures might be used to determine or constrain recharge rates. Previous studies such as Smith and Chapman [1983] and Forster and Smith [1988; 1989] have used similar modeling techniques to determine the effects of groundwater flow on the subsurface thermal regime by varying parameters such as permeability, anisotropy, water table topography or position, aquifer geometry and properties, and regional heat flow. Smith and Chapman [1983] used fully saturated models and specified the position and geometry of the water table; Forster and Smith [1988; 1989] included the unsaturated zone, and used infiltration of recharge with a free surface method to let the water table position and geometry vary. Forster and Smith [1989] examined the effects of lowering recharge, and concluded that in systems with a deep water table (greater than 50 m) the rate of groundwater recharge best characterizes the potential for an advective disturbance of the subsurface thermal regime. The current study takes this concept one step further in quantifying the lower limit of the amount of recharge needed to produce a significant thermal perturbation such that groundwater temperatures can be used to constrain recharge rates. 39 2.3 Conceptual Model of Groundwater Flow/Thermal Regime in the Great Basin The Great Basin is a region of internal drainage in the western United States that covers much of Nevada, western Utah, and parts of California, Oregon, Idaho, and Arizona (Figure 2-1). It is bounded on the east by the Wasatch Range and the Colorado Plateau and on the west by the Sierra Nevada Range. The dominant topography of the area consists of north-south trending valleys and adjacent mountain ranges characteristic of the Basin and Range province. Groundwater within the Great Basin primarily occurs in basin-fill aquifers composed of unconsolidated deposits that occupy the intermontane valleys, and permeable bedrock aquifers which exist at depth beneath the basin-fill aquifers and are exposed at the surface in the intervening mountain ranges. The bedrock aquifers are predominantly part of a large, regionally extensive set of Paleozoic and early Mesozoic carbonate rocks that underlie most of eastern Nevada, western Utah, and parts of southeastern California and southern Idaho and make up what is known as the carbonate-rock aquifer system [Prudic et al., 1995]. Figure 2-2 shows a conceptualized groundwater flow system between a mountain-block aquifer and an adjacent basin-fill aquifer that is characteristic of the Basin and Range province of the western United States. Groundwater recharge occurs mainly in the mountain blocks and upland areas from precipitation. Natural groundwater discharge occurs to evapotranspiration, springs, and streams/lakes/reservoirs. Because of the connectivity of the underlying consolidated bedrock aquifers, some basins may also receive recharge as subsurface inflow from upgradient basins, or discharge groundwater 40 41 1 2 0 ° 1 1 7 ° 1 1 4 ° 1 1 1 ° Figure 2-1. Location of the Great Basin, western United States. State boundary data from: U.S. Census Bureau [2000]. 42 Figure 2-2. Conceptual diagram showing groundwater flow typical of the Great Basin, and how geothermal gradients and surface heat flow may be affected by groundwater flow. Letters in bottom figures correspond to wells A, B, and C in top figure. Also shown are the conductive geothermal gradient and surface heat flow (dashed lines) that would exist if no groundwater flow was occurring. Top panel modified from: Masbruch et al. [2011]. through subsurface outflow to downgradient basins. In most basins, however, the range crest and the evapotranspiration/playa or basin-fill stream are considered to be groundwater divides. It has long been recognized that advective transport of heat by groundwater in the shallow subsurface poses the greatest obstacle in determining heat flow at depth from surface observations [Lachenbruch andSass, 1977]. If groundwater flows are large enough, they will redistribute heat within the subsurface, and alter the natural conductive geotherm (temperature vs. depth) of the area. Figure 2-2 shows a conceptualization of how groundwater flow may redistribute heat. In areas of groundwater recharge, temperatures tend to be lower and will depress the natural conductive geotherm as cold water enters the subsurface. This produces an area of lower than expected heat flow at the surface. Correspondingly, in areas of discharge, groundwater discharge will raise the natural conductive geotherm as warm water at depth is brought to the surface. Changes in the geothermal gradient and distribution of surface heat flow, as well as the associated groundwater temperatures, can be used to assess the magnitude of groundwater flow in an area. Numerous past studies have demonstrated the influence of fluid flow on the subsurface temperature distribution [Bredehoeft andPapadopulos, 1965; Cartwright, 1971; Domenico and Palciauskas, 1973; Donaldson, 1962; Forster and Smith, 1989; Parsons, 1970; Smith and Chapman, 1983; Stallman, 1963, 1965], and utilized this dependence as an aid in delineating the flow field [Bredehoeft and Papadopulos, 1965; Cartwright, 1970; Keys and Brown, 1978; Manning and Solomon, 2005; Salem et al., 2004; Sorey, 1971]. Climatically and hydrogeologically, the Salt Lake basin represents a much 43 cooler/wetter environment compared to areas within the southern Great Basin. Recharge rates within the southern Great Basin are one to three orders of magnitude lower than recharge rates within the Salt Lake basin, and generally range between 0.05 and 34 mm yr-1, with an average rate of 7 mm yr-1 [Masbruch et al., 2011]. Average annual air temperatures within the southern Great Basin valleys are about 15 °C, which is 3 °C warmer than those in the Salt Lake Valley (based on 18-yr average daily temperatures from Daymet data from Daymet web page, http://www.daymet.org, accessed on September 20, 2010). The atmospheric lapse rate, calculated using historical data from 36 meteorological stations located within the southern Great Basin (data from Western Regional Climate Center web page, http://www.wrcc.dri.edu, accessed on September 20, 2010) is -8.0 °C km-1. The main groundwater discharge mechanism is evapotranspiration. 2.4 Modeling Approach Groundwater flow and energy (thermal) transport were modeled using the U.S. Geological Survey code, SUTRA [Foss and Provost, 2002]. SUTRA is a two­dimensional/ three-dimensional, finite-element/finite-difference, saturated/unsaturated code that simulates both flow and either solute transport or thermal energy transport in porous media. A beta version of SUTRA that includes drains [A. Provost, U.S. Geological Survey, written commun., August 9, 2007] was used to allow for the simulation of evapotranspiration as a head-dependent process. A two-dimensional, cross­sectional model based on a topographic basin typical of the Basin and Range was developed and executed using the pre- and postprocessor graphical user interface, Argus ONE (Argus Holdings, Ltd.). 44 2.4.1 Mesh Design The two-dimensional, cross-sectional mesh (Figure 2-3) was generated using the FishNet mesh (deformable grid of quadrilaterals) option in SUTRA. The mesh was constructed using six superblocks to account for changes in slope as one moves from the valley to the mountain block. The mesh is 25 km long, and the top surface (land surface) ranges in elevation from 1,000 m to 2,700 m. Each superblock was divided into 20 elements in the z-direction, while the number of elements in the x-direction for each superblock was chosen so that the elements were ~250 m long (Figure 2-3). The mesh is 1 m thick in the y-direction. The modeled topographic basin was assumed to be symmetrical; only half of the basin, therefore, was modeled. 2.4.2 Boundary Conditions The bottom and sides of the model domain are no-flow boundaries with respect to groundwater flow. The top boundary is a mix of a specified-flux boundary, allowing for recharge over the mountain block, and a head-dependent flux boundary, allowing for discharge in the valley through evapotranspiration (Figure 2-3). With respect to thermal energy, the sides of the model domain are no-flux boundaries as heat flow is assumed to parallel these boundaries. The bottom boundary is a specified-flux boundary to allow basal heat flow to enter the model, while the top boundary is a specified-temperature boundary. Temperatures along the top boundary range between 15 °C on the left hand side of the model to 1.4 °C on the right hand side of the model, and are defined on the basis of elevation using an atmospheric lapse rate of -8 °C km-1 with a valley reference temperature of 15 °C. The temperature of the recharging water was assumed to equal the temperatures of the top boundary of the model where the 45 46 0 5 10 15 20 25 Distance (km) Figure 2-3. Two-dimensional model grid used for simulations. Elevation (m) 47 recharge is being applied. 2.4.3 Model Parameters Model parameters of bedrock permeability, basal heat flux, thermal conductivity of solids, and recharge rates were varied throughout the modeling process to investigate the sensitivity of groundwater thermal perturbations to each of these parameters. Table 2­1 lists these and other model parameters, the range over which the parameters were varied, and the references used to define these parameters. The ranges of the parameters are considered typical of those of groundwater aquifers in the Great Basin. 2.5 Model Results and Discussion Initial sensitivity analyses showed that the two parameters that had the greatest effect in producing a hydrologic disturbance to the conductive thermal regime were recharge rate and bedrock permeability. This is highly similar to hydrologic studies that have determined that the position of the water table is either topographically controlled or recharge controlled [Haitjema and Mitchell-Bruker, 2005; Gleeson and Manning, 2008], especially in mountainous terrain. The thermal parameters of basal heat flux and thermal conductivity of aquifer solids, while affecting the absolute temperatures, had little effect on changing temperatures relative to the conductive case. Thermal parameters were only important in simulations with very low recharge rates (less than 5 mm yr-1) compared to effects produced by greater groundwater flow due to recharge rates higher than this. It is likely that at lower permeabilities than were investigated in this study, thermal parameters would also have a larger effect [Smith and Chapman, 1983]. This study, however, was focused on determining the hydrologic disturbance of the thermal regime Table 2-1. Parameter values used for fluid and thermal properties. Model parameter Parameter values Source Permeability of bedrock1 10-13-10-U m2 Belcher etal. [2001, 2002]; San Juan et al. [2004] Permeability of basin-fill deposits2 1.7xl0'12 m2 Belcher et al. [2001, 2002]; San Juan et al. [2004] Porosity of bedrock1 0.05 Harrill andPrudic [1998] Porosity of basin-fill deposits2 0.3 Domenico and Schwartz [1990] Thermal conductivity of aquifer solids1 2-4W m'1 K"1 Langevin etal. [2008] Thermal conductivity of fluid2 0.6 W m'1 K'1 Langevin etal. [2008] Reference density of fluid2 1,000 kg m'3 Domenico and Schwartz [1990] Specific heat of fluid2 4,1861kg1 K'1 Smith and Chapman [1983] Basal heat flux1 60-100 mW m'2 Sassetal. [2005] Varied through range for sensitivity analyses 2Held constant for all simulations in aquifers within the Great Basin, which generally have moderate to high permeability. The discussion below focuses on how the recharge rate and bedrock permeability influence the hydrologic disturbance to conductive heat flow. Thermal parameter values used in the following simulations were 80 mW m-2 for the basal heat flux (middle of the range typical for the Basin and Range [Sass et al., 2005]), and 3 W m-1 K-1 for the thermal conductivity of the aquifer solids (middle of the range typical for thermal conductivity of rocks [Langevin et al., 2008]). 2.5.1 Purely Conductive Case Figure 2-4 shows the simulated surface heat flow and temperature distribution for the purely conductive case in the absence of groundwater flow. Surface heat flow across the top boundary of the model was computed using temperatures at the top two nodes of the model grid, and is the same as the basal heat flux (80 mW m-2). Due to the topography and application of the lapse rate temperatures across the top boundary of the model, the simulated temperature contours are subparallel to the ground surface. Temperatures in this simulation range between 1.4 °C and 84.6 °C. 2.5.2 Influence of Bedrock Permeability and Recharge Rate The influence of bedrock permeability and recharge rate on the hydrologic disturbance to the conductive heat flow field is shown in Figure 2-5. Recharge rates were varied between 1 and 100 mm yr-1 and bedrock permeabilities were varied between 1x10-13 and 1x10-11 m2, typical of carbonate rock aquifers in the Basin and Range (Table 2-1). The contours in Figure 2-5 represent the root mean square deviation of temperatures from the purely conductive case calculated at all model grid nodes, expressed in percent. 49 50 r 100 - 90 - 80 - 70 - 60 - 50 - 40 - 30 - 20 - 10 - 0 ra x<a 25 20 15 10 25 20 15 10 Distance (km) i!> LU 5 5 0 Figure 2-4. Top panel: Simulated surface heat flow for the purely conductive case (no groundwater flow). Heat flow profile calculated by using the upper two nodes in the model grid. Bottom panel: Simulated temperature distribution for the purely conductive case. Contours are in degrees Celsius. 51 Recharge Rate (mm yr"1) Figure 2-5. Influence of bedrock permeability and recharge rate on the hydrologic disturbance to conductive heat flow. Contours are the root mean square deviation of temperatures from the purely conductive case calculated at all model grid nodes, expressed in percent. At recharge rates of 10 mm yr-1 or less, the hydrologic disturbance is almost completely dependent on the recharge rate. Figures 2-6 and 2-7 show the results of simulations with recharge rates of 10 mm yr-1, and bedrock permeabilities of 5x10-13 and 5x10-12 m2, respectively. Temperatures in the lower permeability simulation range between 1.4 °C and 75.1 °C, and temperatures in the higher permeability simulation range between 1.4 °C and 76.8 °C. At this recharge rate, the largest temperature perturbations occur in the deepest portions of the recharge area, and directly beneath the nodes where discharge is occurring. The temperature difference is near zero for the majority of the area within and beneath the basin fill. Surface heat flow rates are approximately 25 mW m-2 in the recharge area, and rapidly approach basal heat flux rates within 2 to 3 km of the recharge area. Surface heat flow rates in the discharge area are much higher, with values ranging between about 400 and 500 mW m-2. At recharge rates of 10 mm yr-1 groundwater temperatures would be need to be measured in very specific locations in the recharge or discharge areas to detect these thermal anomalies. Figures 2-8 and 2-9 show the results of simulations with recharge rates of 50 mm yr-1, and bedrock permeabilities of 5x10-13 and 5x10-12 m2, respectively. Temperatures in the lower permeability simulation range between 1.4 °C and 62.9 °C, and temperatures in the higher permeability simulation range between 1.4 °C and 51.0 °C. At this recharge rate, the largest temperature perturbations are in the deeper portions of the recharge area. This plume of cooler water, however, persists throughout and under the basin-fill deposits towards the discharge area, and at fairly shallow (less than 500 m) depths unlike simulations with recharge rates of less than 50 mm yr-1. Likewise, the surface heat flux -2 also follows this pattern. Surface heat flow rates are approximately 5 mW m or less in 52 .Surface heat flow Basal heat flow 25 20 15 -I- 10 500 - 400 - 300 - 200 - 100 - 0 0 25 20 15 10 Distance (km) 53 ScCT -L2l_ -C X ewm >_<D LU E Recharge Rate (mm yr-1) 5 0 Figure 2-6. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10"13 m2 and recharge rate of 10 mm yr-1. Top panel: Simulated surface heat flow. Top middle panel: Simulated temperature distribution across the model domain (contours are in degrees Celsius). Bottom middle panel: Temperature difference distribution from the purely conductive case across the model domain (contours are in degrees Celsius). Bottom panel: Contours are the root mean square deviation of temperatures from the purely conductive case calculated at all model grid nodes, expressed in percent; cross represents the location of the simulation in bedrock permeability-recharge rate space. 54 Surface heat flow 25 20 25 20 15 -I- 10 Distance (km) " l- 15 l- 10 Basal heat flow 500 400 300 200 100 0 3.000 2.500 2.000 1.500 1,000 500 0 -500 xe ■-m > LU Distance (km) Figure 2-7. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10"12 m2 and recharge rate of 10 mm yr-1. Top panel: Simulated surface heat flow. Top middle panel: Simulated temperature distribution across the model domain (contours are in degrees Celsius). Bottom middle panel: Temperature difference distribution from the purely conductive case across the model domain (contours are in degrees Celsius). Bottom panel: Contours are the root mean square deviation of temperatures from the purely conductive case calculated at all model grid nodes, expressed in percent; cross represents the location of the simulation in bedrock permeability-recharge rate space. 0 55 Surface heat flow 25 25 20 Basal heat flow I 15 ‘ T * • * T * 10 Distance (km) 20 15 10 Distance (km) 1,600 1,200 800 400 0 £'£ x*e ■I-m 3.000 2.500 2.000 1.500 1,000 500 0 -500 0 5 0 Recharge Rate (mm yr-1) Figure 2-8. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-13 m2 and recharge rate of 50 mm yr-1. Top panel: Simulated surface heat flow. Top middle panel: Simulated temperature distribution across the model domain (contours are in degrees Celsius). Bottom middle panel: Temperature difference distribution from the purely conductive case across the model domain (contours are in degrees Celsius). Bottom panel: Contours are the root mean square deviation of temperatures from the purely conductive case calculated at all model grid nodes, expressed in percent; cross represents the location of the simulation in bedrock permeability-recharge rate space. 56 J \ Surface heat flow 25 25 25 20 20 20 Basal heat flow I 15 '■V V V Distance (km) 10 15 10 Distance (km) 15 10 1,600 1,200 800 400 0 -3,000 -2,500 -2,000 -1,500 -1,000 - 500 - 0 - -500 -3,000 -2,500 -2,000 -1,500 -1,000 - 500 - 0 - -500 xe ■-m Distance (km) 0 5 0 5 0 Recharge Rate (mm yr-1) Figure 2-9. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-12 m2 and recharge rate of 50 mm yr-1. Top panel: Simulated surface heat flow. Top middle panel: Simulated temperature distribution across the model domain (contours are in degrees Celsius). Bottom middle panel: Temperature difference distribution from the purely conductive case across the model domain (contours are in degrees Celsius). Bottom panel: Contours are the root mean square deviation of temperatures from the purely conductive case calculated at all model grid nodes, expressed in percent; cross represents the location of the simulation in bedrock permeability-recharge rate space. the recharge area, and only reach values of 50 to 55 mW m-2 away from the recharge area before reaching the discharge area. Surface heat flow rates in the discharge area are much higher, with values ranging between about 1,000 and 1,600 mW m-2. This plume of cooler water, therefore, could be easily measured using wells placed in the basin-fill deposits and used to constrain recharge rates to the system as a whole. Under these conditions the root mean difference in temperatures from the conductive case is slightly more dependent on bedrock permeability. Differences in the bedrock permeability at this recharge rate, however, produce only very slight differences in the distribution of the simulated temperatures and temperature perturbations. In the higher permeability simulation, temperature perturbations persist about 3 to 5 km further away from the recharge area than in the lower permeability simulation. It appears, therefore, that the permeability controls how far the perturbation will extend from the recharge area. Figures 2-10 and 2-11 show the results of simulations with recharge rates of 90 mm yr-1, and bedrock permeabilities of 5x10-13 and 5x10-12 m2, respectively. Temperatures in the lower permeability simulation range between 1.4 °C and 51.8 °C, and temperatures in the higher permeability simulation range between 1.4 °C and 38.8 °C. Similarly to the other simulations, the largest temperature perturbations are in the deeper portions of the recharge area. Also similarly to the simulations with a recharge rate of 50 mm yr-1, the plume of cooler water persists throughout and under the basin-fill deposits towards the discharge area, and at even shallower depths than the simulations using a recharge rate of 50 mm yr-1. Surface heat flow values range between about 0 and -2 20 mW m everywhere except the discharge area, where surface heat flow values range -2 between about 1,500 and 3,000 mW m" . Under these conditions the root mean difference 57 58 25 20 Distance (km) 15 10 Distance (km) 25 20 15 10 -ii2 -c xe ■-m 3.000 2.500 2.000 1.500 1,000 500 0 -500 3.000 2.500 2.000 1.500 1,000 500 0 -500 Distance (km) 5 0 5 0 Recharge Rate (mm yr-1) Figure 2-10. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-13 m2 and recharge rate of 90 mm yr-1. Top panel: Simulated surface heat flow. Top middle panel: Simulated temperature distribution across the model domain (contours are in degrees Celsius). Bottom middle panel: Temperature difference distribution from the purely conductive case across the model domain (contours are in degrees Celsius). Bottom panel: Contours are the root mean square deviation of temperatures from the purely conductive case calculated at all model grid nodes, expressed in percent; cross represents the location of the simulation in bedrock permeability-recharge rate space. 59 25 20 Distance (km) Distance (km) 15 10 -ii2 -c xe ■-m 3.000 2.500 2.000 1.500 1,000 500 0 -500 3.000 2.500 2.000 1.500 1,000 500 0 -500 Distance (km) 5 0 Recharge Rate (mm yr-1) Figure 2-11. Thermal effects of groundwater flow in a basin with bedrock permeability of 5x10-12 m2 and recharge rate of 90 mm yr-1. Top panel: Simulated surface heat flow. Top middle panel: Simulated temperature distribution across the model domain (contours are in degrees Celsius). Bottom middle panel: Temperature difference distribution from the purely conductive case across the model domain (contours are in degrees Celsius). Bottom panel: Contours are the root mean square deviation of temperatures from the purely conductive case calculated at all model grid nodes, expressed in percent; cross represents the location of the simulation in bedrock permeability-recharge rate space. in temperatures from the conductive case is even more dependent on bedrock permeability than the previous simulations at lower recharge rates. Differences in the bedrock permeability at this recharge rate produce a greater difference in the distribution of the simulated temperatures and temperature perturbations than the previous simulations. In the higher permeability simulation, temperature perturbations persist at least 5 km further away from the recharge area than in the lower permeability simulation. These simulations definitely show that the permeability controls how far the temperature perturbation will extend from the recharge area. Additionally, groundwater temperatures are as much as 10 °C cooler in the discharge area in the higher permeability simulation versus the lower permeability simulation. Surface heat flow values are also slightly lower at the discharge area for the higher permeability simulation versus the lower permeability simulation. 2.6 Conclusions Two-dimensional numerical modeling of the combined effects of fluid flow and heat transport were used to quantify the effects of groundwater flow on the subsurface thermal regime. The numerical simulations could also be used to determine the lower limit of recharge rate that will produce an observable perturbation such that groundwater temperatures can be used to constrain recharge rates. Simulations of a basin 25 km wide to a depth of -500 m with 1,700 m of topographical relief, representative of basins within the Great Basin, and with moderate to high permeabilities representative of aquifers within the Great Basin, lead to the following conclusions: 1. Higher recharge rates and bedrock permeabilities produce greater thermal perturbations than lower recharge rates and permeabilities. 60 2. For all recharge rates and bedrock permeabilities, the greatest temperature perturbations occur in the deepest portion of the recharge area. 3. At lower recharge rates (10 mm yr-1 or less) the hydrologic disturbance is almost completely dependent on the recharge rate. At these recharge rates the temperature perturbation throughout the majority of the simulated area is nearly undetectable. Groundwater temperatures would need to be measured in very specific locations in the recharge or discharge areas to detect the hydrologic disturbance. 4. At more moderate recharge rates (50 mm yr-1) the hydrologic disturbance is slightly more dependent on the bedrock permeability. Differences in the bedrock permeability at this recharge rate, however, produce only very slight differences in the distribution of the simulated temperatures and temperature perturbations. 5. At high recharge rates (90 mm yr-1) the hydrologic disturbance is even more dependent on the bedrock permeability. Differences in the bedrock permeability at this recharge rate produce a greater difference in the distribution of the simulated temperatures and temperature perturbations versus simulations at lower recharge rates. Groundwater temperatures are as much as 10 °C cooler in the discharge area in the higher permeability simulation versus the lower permeability simulation. 6. Bedrock permeabilities appear to control the distance to which the temperature perturbation will extend from the recharge area. For moderate recharge rates (around 50 mm yr-1) temperature perturbations at the higher permeabilities extend at least 3 km further from the recharge area than the perturbations at lower 61 permeabilities; at higher recharge rates (around 90 mm yr-1) the temperature perturbations at higher permeabilities extend at least 5 km further from the recharge area. 7. Variations in the surface heat flux are quite different depending on the recharge rate. At low recharge rates, the highest variations only exist in the recharge and discharge areas. At higher recharge rates, the differences are larger in the recharge and discharge areas, and also persist in areas away from the recharge and discharge areas. Measurement of the surface heat flux is a robust indicator of the hydrologic disturbance caused by a specific recharge rate. 8. At recharge rates of 50 mm yr-1 and above, the plume of colder water extends past the recharge area and persists throughout and under the basin-fill deposits towards the discharge area, at fairly shallow (less than 500 m) depths and can be detected by measuring surface heat flux unlike simulations with recharge rates of less than 50 mm yr-1. This plume of cooler water could be easily measured using wells placed in the basin-fill deposits and used to constrain recharge rates to the system as a whole. The lower limit of recharge rates needed to produce a thermal perturbation large enough such that groundwater temperatures can be used to constrain recharge rates, therefore, is 50 mm yr-1. 2.7 References Belcher, W.R., P.E. Elliott, and A.L. Geldon (2001), Hydraulic-property estimates for use with a transient ground-water flow model of the Death Valley regional ground-water flow system, Nevada and California, US Geol. Surv. Water-Resour. Invest. Rep. 01-4120, 33 pp. 62 63 Belcher, W.R., D.S. Sweetkind, and P.E. Elliott (2002), Probability distributions of hydraulic conductivity for the hydrogeologic units of the Death Valley regional ground-water flow system, Nevada and California, US Geol. Surv. Water-Resour. Invest. Rep. 02-4212, 24 pp. Bredehoeft, J.D., and I.S. Papadopulos (1965), Rates of vertical groundwater movement estimated for the Earth's thermal profile, Water Resour. Res., 1, 325-328. Cartwright, K. (1970), Groundwater discharge in the Illinois Basin as suggested by temperature anomalies, Water Resour. Res., 6, 912-918. Cartwright, K. (1971), Redistribution of geothermal heat by a shallow aquifer, Geol. Soc. of Am. Bull., 82, 3197-3200. Domenico, P.A., and V.V. Palciauskas (1973), Theoretical analysis of forced convective heat transfer in regional ground-water flow, Geol. Soc. of Am. Bull., 84, 3803­3814. Domenico, P.A., and F.W. Schwartz, (1990), Physical and Chemical Hydrogeology, John Wiley and Sons, Inc., New York. Donaldson, J.G. (1962), Temperature gradients in the upper layers of the earth's crust due to convective water flows, J. Geophys. Res., 76, 3449-3459. Flint, A.L., and L.E. Flint, (2007a), Application of the Basin Characterization Model to estimate in-place recharge and runoff potential in the Basin and Range carbonate-rock aquifer system, White Pine County, Nevada, and adjacent areas in Nevada and Utah, US Geol. Surv. Scien. Invest. Rep. 2007-5099, 20 pp. Flint, A.L., L.E. Flint, and M.D. Masbruch (2011), Input, calibration, uncertainty, and limitations of the Basin Characterization Model, Appendix 3 in Conceptual model of the Great Basin carbonate and alluvial aquifer system, edited by V.M. Heilweil and L.E. Brooks, US Geol. Surv. Scien. Invest. Rep. 2010-5193, 149-164. Flint, L.E., and A.L. Flint (2007b), Regional analysis of ground-water recharge, in Ground-water recharge in the arid and semiarid southwestern United States edited by D.A. Stonestrom, J. Constantz, T.P.A. Ferre, and S.A. Leake, US Geol. Surv. Prof Pap. 1703, 29-59. Forster, C., and L. Smith (1988), Groundwater flow systems in mountainous terrain, 1. Numerical modeling technique, Water Resour. Res., 24, 999-1010. Forster, C., and L. Smith (1989), The influence of groundwater flow on thermal regimes in mountainous terrain: A model study, J. Geophys. Res., 94, 9439-9451. 64 Gleeson, T., and A.H. Manning (2008), Regional groundwater flow in mountainous terrain: Three-dimensional simulations of topographic and hydrogeologic controls, Water Resour. Res., 44, W10403, doi:10.1029/2008WR006848. Haitjema, H.M., and S. Mitchell-Bruker, (2005), Are water tables a subdued replica of the topography?, Ground Water, 43, 781-786. Harrill, J.R., and D.E. Prudic (1998), Aquifer systems in the Great Basin region of Nevada, Utah, and adjacent states-Summary report, US Geol. Surv. Prof. Pap. 1409-A, 66 pp. Heilweil, V.M., and L.E. Brooks, eds. (2011), Conceptual model of the Great Basin carbonate and alluvial aquifer system, US Geol. Surv. Scien. Invest. Rep. 2010­5193, 191 pp. Hevesi, J.A., A.L. Flint, and L.E. Flint (2003), Simulation of net infiltration and potential recharge using a distributed-parameter watershed model of the Death Valley region, Nevada and California, US Geol. Surv. Water-Resour. Invest. Rep. 2003­4090, 161 pp. Keys, W.S., and R.F. Brown (1978), The use of temperature logs to trace the movement of injected water, Ground Water, 16, 32-48. Lachenbruch, A.H., and J.H. Sass (1977), Heat flow in the United States, in The Earth's Crust edited by J.G. Heacock, Geophys. Mono. Ser., AGU, Washington, D.C., 626-675. Langevin, C.D., D.T. Thorne, Jr., A.M. Dausman, M.C. Sukop, and W. Guo (2008), SEAWAT version 4: A Computer program for simulation of multi-species solute and heat transport, US Geol. Surv. Tech. andMeth. Book 6, Chapter A22, 39 pp. Leavesley, G.H., R.W. Lichty, B.M. Troutman, and L.G. Saindon, (1983), Precipitation-runoff modeling system: User's manual, US Geol. Surv. Water-Resour. Invest. Rep. 83-4238, 207 pp. Manning, A.H., and D.K. Solomon (2005), An integrated environmental tracer approach to characterizing groundwater circulation in a mountain block, Water Resour. Res., 41, W12412, doi:10.1029/2005WR004178. Markstrom, S.L., R.G. Niswonger, R.S. Regan, D.E. Prudic, and P.M. Barlow (2008), GSFLOW-Coupled ground-water land surface-water flow model based on the integration of the precipitation-runoff modeling system (PRMS) and the modular ground-water flow model (M0DFL0W-2005), US Geol. Surv. Tech. and Meth. 6-D1, 240 pp. 65 Masbruch, M.D., V.M. Heilweil, S.G. Buto, L.E. Brooks, D.S. Susong, A.L. Flint, L.E. Flint, and P.M. Gardner (2011), Groundwater budgets, chap. D in Conceptual model of the Great Basin carbonate and alluvial aquifer system, edited by V.M. Heilweil, and L.E. Brooks, US Geol. Surv. Scien. Invest. Rep. 2010-5193, 73­126. Maxey, G.B., and T.E. Eakin (1949), Ground water in White River Valley, White Pine, Nye, and Lincoln Counties, Nevada, Nev. Off. St. Eng. Water Resour. Bull. no. 8, 59 pp. Parsons, M.L. (1970), Groundwater thermal regime in a glacial complex, Water Resour. Res., 6, 1701-1720. Prudic, D.E., J.R. Harrill, and T.J. Burbey (1995), Conceptual evaluation of regional ground-water flow in the Carbonate-Rock Province of the Great Basin, Nevada, Utah, and adjacent states, US Geol. Surv. Prof. Pap. 1409-D, 102 pp. Salem, Z.E., M. Taniguchi and Y. Sakura (2004), Use of temperature profiles and stable isotopes to trace flow lines: Nagaoka area, Japan, Ground Water, 42, 83­91. San Juan, C.A., W.R. Belcher, R.J. Laczniak, and H.M. Putnam (2004), Hydrologic components for model development, chap. C in Death Valley regional ground­water flow system, Nevada and California-Hydrogeologic framework and transient ground-water flow model, edited by W.R. Belcher, US Geol. Surv. Scien. Invest. Rep. 2004-5205, 103-136. Sass, J.H., S.S. Priest, A.H. Lachenbruch, S.P. Galanis, Jr., T.H. Moses, Jr., J.P. Kennelly, Jr., R.J. Munroe, E.P. Smith, F.V. Grubb, R.H. Husk, Jr., and C.W. Mase (2005), Summary of supporting data for USGS regional heat-flow studies of the Great Basin, 1970-1990, US Geol. Surv. Open-File Rep. 2005-1207, online version 1.0, URL: http://pubs.er.usgs.gov/publication/ofr20051207, accessed on