| OCR Text |
Show 3.1 Background Information on Salt Lake County Applications Salt Lake County is surrounded by the Wasatch Mountain range, the Oquirrh Mountain range and the Great Salt Lake from east to north in a clockwise direction. Salt Lake Valley overlies a two- layer aquifer system that has an upper shallow unconfined aquifer and a deep confined aquifer ( Figure 3.1). The confined aquifer is the principal aquifer for large scale water supply development. It is overlain for the most part by relatively impermeable deposits which act as a confining bed with thickness ranging from 40 to 100 feet. In the upper mountain recharge areas the principal aquifer is unconfined. The shallow unconfined aquifer overlying the confined aquifer has a maximum thickness less than 50 feet. The principal aquifer generally yields water readily to wells. The most productive wells in the aquifer are located near the recharge areas bounded by the Wasatch Mountain range in the south eastern section of the county. Most of the recharge to the confined aquifer passes through the deep, unconfined aquifer ( Waddell et al, 1987) The shallow unconfined aquifer is recharged by upward leakage from the confined aquifer as well as downward infiltration at the edges of the valley. Because of poor water quality and low yield, the shallow, unconfined aquifer is not significantly used for municipal water supply. A study of groundwater management alternatives using simulation for Salt Lake County was done by Waddell et al. ( 1987). A three- dimensional groundwater flow model was calibrated to simulate the aquifer for 1969 to 1982 historical pumping. They selected 122 wells in Salt Lake County, that had pumping rates greater than 0.3 cfs and ran simulations that increased pumping by up to 65,000 acre- feet/ year. Assessments of the impact on the aquifer quality near the Great Salt Lake and near other contaminated areas and peak drawdowns were made. They found that increasing the rate of pumpage could increase the drawdown by 40 to 60 feet in the area of Sandy where most of increase of withdrawals was simulated. The finite difference grid used by Waddell et al.( 1987) was utilized to generate response matrices for a 10 year period for the set of wells with existing pumpage greater than 0.6 cubic feet/ second. Candidate well and existing well location were reviewed to select 39 nodes for pumping representing a total of 65 wells. All the pumpage data and records used for the ten water agencies identified were supplied by U. S. G. S. ( Table 3.1). The U. S. G. S. 3- dimensional finite difference model of McDonald and Harbaugh ( 1984) was used with a 38 row by 28 column grid system superimposed over Salt Lake County ( Figure 3.2). The grid spacing ranged from 0.7 mile to 1.0 mile depending on the steepness of the hydraulic gradient, the rate of change in transmissivity or the number of wells in the locale. The location of each well node, used for the optimization study, the area it belongs to, the number of associated physical wells, and the historical average pumping rate from 1973 to 1982 for the node are presented in Table 3.3. These data were synthesized from Waddell et al. ( 1987) and from information provided by Mr. K. M. Waddell of the U. S. G. S.. The land surface elevations and initial aquifer head were obtained at each well node from maps and from values tabulated ( Table 3.4) by Waddell et al. ( 1987). Constant head boundaries were specified along the north and northwest boundaries of the county to represent the Great Salt Lake. Table 3.1 also shows a fairly high degree of variability in annual pumping for most of the water supply agencies considered. For example, Murray's annual pumping varies 33 |
