Assessment of managed aquifer recharge at Sand Hollow Reservoir, Washington County, Utah, updated to conditions through 2016

Sand Hollow Reservoir in Washington County, Utah, was completed in March 2002 and is operated primarily for managed aquifer recharge by the Washington County Water Conservancy District. From 2002 through 2016, surface-water diversions of about 256,000 acre-feet (acre-ft) to Sand Hollow Reservoir have allowed the reservoir to remain nearly full since 2006. Groundwater levels in monitoring wells near the reservoir rose through 2006 but have fluctuated more recently because of variations in reservoir stage and nearby pumping from production wells. Between 2004 and 2016, about 37,000 acre-ft of groundwater was withdrawn by these wells for municipal supply. In addition, about 37,000 acre-ft of shallow seepage was captured by French drains adjacent to the North and West Dams and used for municipal supply, irrigation, or returned to the reservoir. From 2002 through 2016, about 141,000 acre-ft of water seeped beneath the reservoir to recharge the underlying Navajo Sandstone aquifer, which includes about 14,200 acre-ft of recharge during the 2015–16 time period since the last report published in 2016.

Water quality continued to be monitored at various wells in Sand Hollow during 2015–16 to evaluate the timing and location of reservoir recharge as it moved through the aquifer. Changing geochemical conditions at monitoring wells WD 4 and WD 12 indicate rising groundwater levels and mobilization of vadose-zone salts, which could be a precursor to the arrival of reservoir recharge.