| Description |
Previous watershed assessments have relied on annual baseflow estimates to provide an improved, albeit limited, understanding of the groundwater contribution to surface water bodies. In order to quantify the volume of groundwater in storage, additional information such as groundwater mean transit time (MTT) is needed. In this study, several approaches were evaluated for determining the groundwater MTT in the West Fork Duchesne River watershed in northeastern Utah. The most promising result was obtained with lumped-parameter modeling (LPM) of environmental tracers (SFR6R, CFCs, and P3PH/P3PHe) from 21 springs within the catchment. Approximately 30% of the springs exhibited an exponential transit time distribution (TTD); the remaining ~ 70% were best characterized by a piston-flow TTD. The flow-weighted groundwater MTT for the West Fork watershed is about 40 years with approximately 20 years spent in the unsaturated zone. A cumulative distribution of these ages revealed that a majority of the groundwater within the catchment is between 30 and 50 years old. As a result, the West Fork is considered a fairly stable catchment; it is hypothesized that short-term changes in recharge brought about by 5-10 year droughts are unlikely to have as profound effect on this watershed as compared to systems with shorter MTTs. A chemical hydrograph separation of West Fork stream flow estimated the average annual baseflow to be ~ 1.7 x 10P7P mP3P/year, which was assumed to be a proxy for groundwater discharge from the watershed. Using this MTT and baseflow estimation, the volume of mobile groundwater stored in the West Fork watershed was calculated to be ~ 6.5 x 10P8P mP3P. This volume translates to an average saturated zone thickness of ~ 20 m, or a recharge rate of ~ 0.09 m/year over the area of the watershed. In addition to spring sampling, several nonvolatile tracers (major ions, dissolved silica, and tritium) were evaluated in stream water. Over the scale of the watershed studied, there was no apparent correlation between major ions, or silica, and groundwater age. Furthermore, the usefulness of baseflow tritium (P3PH) was significantly limited given the fact that atmospheric P3PH records in the region have only recently stabilized. Future watershed-scale assessments should evaluate groundwater MTT, in addition to annual baseflow, in order to quantify groundwater storage and more accurately assess watershed susceptibility to development and climate variability. |
