Simulation of Managed Aquifer Recharge from Sand Hollow Reservoir, Southwestern Utah

Sand Hollow Reservoir, located in the Hurricane Bench study area in southwestern Utah, is operated primarily as a managed aquifer recharge project by the Washington County Water Conservancy District.  Sand Hollow Reservoir is located on a thick sequence of the Jurassic-age Navajo Sandstone and Kayenta Formation. Within the study area, unconfined conditions generally exist throughout the Navajo aquifer. The total recharge from the reservoir to the underlying Navajo Sandstone aquifer from 2002 to 2009 was about 86,000 acre-feet. Natural recharge as infiltration of precipitation is approximately 2,100 acre-feet per year. Discharge occurs as seepage to the Virgin River, well withdrawals, and to drains at the base of reservoir dams. The Navajo aquifer exhibits large variability in hydraulic conductivity (0.8 to 32 feet per day) due to bedrock fractures that trend north-northeast throughout the area. 

A numerical groundwater flow model was developed to simulate the groundwater flow system and movement of managed aquifer recharge through the Navajo aquifer. The model simulates recharge as natural infiltration of precipitation and by infiltration of managed aquifer recharge from Sand Hollow Reservoir.  Groundwater discharge is simulated as well withdrawals, shallow drains at the base of reservoir dams, and seepage to the Virgin River.  The model was calibrated to observed water levels, tracer-based groundwater travel times, drain discharges, and monthly estimated reservoir recharge.  The model provides information to make estimates of total additional groundwater storage available within the study area and to investigate travels times of recharged water to discharge points.  The model predicts an estimated 325,000 acre-ft of additional groundwater storage when steady state is reached with respect to reservoir recharge (approximately 300 years from 2009). The simulated earliest arrival time of reservoir water to the Virgin River occurs 500 years from 2009.