Assessment of Artificial Recharge at Sand Hollow Reservoir, Washington County, Utah, Updated to Conditions through 2006
Scientific Investigations Report 2007-5023
Sand Hollow, Utah, is the site of a surface-water reservoir completed in March 2002 and operated by the Washington County Water Conservancy District (WCWCD) primarily as an aquifer storage and recovery project. The reservoir is an off-channel facility that receives water from the Virgin River, diverted near the town of Virgin, Utah. Hydrologic data collected are described and listed in this report, including ground-water levels, reservoir stage, reservoir-water temperature, meteorology, evaporation, and estimated ground-water recharge. Since the construction of the reservoir in 2002, diversions from the Virgin River have resulted in generally rising stage and surface area. Large spring run-off volumes during 2005-06 allowed the WCWCD to fill the reservoir to near capacity, with a surface area of about 1,300 acres in 2006. Reservoir stage reached a record altitude of about 3,060 feet in May 2006, resulting in a depth of nearly 90 feet and a reservoir storage of about 51,000 acre-feet. Water temperature in the reservoir shows large seasonal variation and has ranged from about 5 to 32°C. Estimated ground-water recharge rates have ranged from 0.01 to 0.43 feet per day. Estimated recharge volumes have ranged from about 200 to about 3,500 acre-feet per month. Total ground-water recharge from March 2002 through August 2006 is estimated to be about 51,000 acre-feet. Estimated evaporation rates have varied from 0.05 to 0.97 feet per month, resulting in evaporation losses of 20 to 1,200 acre-feet per month. Total evaporation from March 2002 through August 2006 is estimated to be about 17,000 acre-feet. The combination of generally declining recharge rates and increasing reservoir altitude and area explains the trend of an increasing ratio of evaporation to recharge volume over time, with the total volume of water lost through evaporation nearly as large as the volume of ground-water recharge during the first 8 months of 2006. With removal of the viscosity effects (caused by seasonal water temperature variations), the intrinsic permeability indicates a large seasonal variation in clogging, with large winter increases likely caused by a combination of both decreased biofilms and the reduced volume of trapped gas bubbles.