Ground-water flow and numerical simulation of recharge from streamflow infiltration near Pine Nut Creek, Douglas County, Nevada

Ground-water flow and recharge from infiltration near Pine Nut Creek, east of Gardnerville, Nevada, were simulated using a single-layer numerical finite-difference model as part of a study made by the U.S. Geological Survey in cooperation with the Carson Water Subconservancy District. The model was calibrated to 190 water-level measurements made in 27 wells in December 2000, and in 9 wells from August 1999 through April 2001. The purpose of this study was to estimate reasonable limits for the approximate volume of water that may be stored by recharge through infiltration basins, and the rate at which recharged water would dissipate or move towards the valley floor. Measured water levels in the study area show that infiltration from the Allerman Canal and reservoir has created a water-table mound beneath them that decreases the hydraulic gradient east of the canal and increases the gradient west of the canal. North of Pine Nut Creek, the mound causes ground water to flow toward the northern end of the reservoir. South of Pine Nut Creek, relatively high water levels probably are maintained by the mound beneath the Allerman Canal and possibly by greater rates of recharge from the southeast. Water-level declines near Pine Nut Creek from August 1999 through April 2001 probably are caused by dissipation of recharge from infiltration of Pine Nut Creek streamflow in the springs of 1998 and 1999. Using the calibrated model, a simulation of recharge through a hypothetical infiltration basin covering 12.4 acres near Pine Nut Creek applied 700 acre-feet per year of recharge over a six-month period, for a total of 3,500 acre-feet after 5 consecutive years. This recharge requires a diversion rate of about 2 cubic feet per second and an infiltration rate of 0.3 foot per day. The simulations showed that recharge of 3,500 acre-feet caused water levels near the basin to rise over 70 feet, approaching land surface, indicating 3,500 acre-feet is the maximum that may be stored in a 5-year period, given the basin location and surface area used in the simulations. Greater amounts probably could be stored if separate infiltration basins were installed at different locations along the Pine Nut Creek alluvial fan, applying the recharge over a larger area. The water-table mound resulting from recharge extended 7,000 feet north, west, and south of the infiltration basin. After recharge ceased, water levels near the center of the mound declined rapidly to within 20 feet of initial levels after 2 years, and within 10 feet of initial levels after 7 years. The recharge mound dissipates laterally across the modeled area at decreasing rates over time. A water-level rise of 1 foot moved westward towards the valley floor 660 feet from peak conditions after 1 year, and averaged 550 feet, 440 feet, and 330 feet per year for the periods 1-4, 4-7, and 7-10 years, respectively, after recharge ceased. Simulations of subsequent pumping from hypothetical wells near the infiltration basin were made by applying pumping near the basin beginning 1 year after recharge of 3,500 acre-feet ceased. Pumping was applied over a 6-month period for 4 years from one well at 400 acre-feet per year, withdrawing 1,600 acre-feet or 45 percent of that recharged, and from two wells totaling 800 acre-feet per year, withdrawing 3,200 acre-feet or 90 percent of that recharged. Pumping of 1,600 acre-feet caused water-levels near the infiltration basin to decline only slightly below initial levels. Pumping of 3,200 acre-feet caused water-levels near the infiltration basin to decline a maximum of 30 feet below initial levels, with smaller declines extending laterally in all directions for 4,000 feet from the pumping wells. Water-level declines are a result of pumping at a rate sufficient to withdraw the majority of the water recharged through the infiltration basin. Although the declines may affect water levels in nearby domestic wells, the simulations show that water levels recover quickly after