Recharge to the Navajo Sandstone aquifer of southwestern Utah

The Navajo Sandstone is a regionally important fractured bedrock aquifer that is being increasingly utilized to supply water to the rapidly growing communities of the Colorado Plateau region of the southwestern United States. Prior to this investigation, little work had been done examining fractured sandstone recharge processes in arid environments. Detailed investigations of both natural and artificial recharge at Sand Hollow basin in southwestern Utah were conducted to better understand parameters controlling vadose-zone infiltration and ground-water recharge. Environmental tracers from excavations and boreholes indicate that recharge to the Navajo aquifer is spatially and temporally variable. Important factors causing this variability include trapped gas, surficial soil coarseness, outcrop runoff, and precipitation intensity and duration. The effect of gas entrapment is important when ponding occurs during both artificial recharge and natural recharge. Results from a gas-partitioning tracer test during an infiltration experiment at Sand Hollow basin show that trapped gas can reduce sandstone permeability by an order of magnitude. Net infiltration at Sand Hollow primarily occurs through coarse-grained soils in the upland parts of the basin, particularly in areas receiving runoff from exposed sandstone.; Recharge rates to the Navajo Sandstone aquifer at Sand Hollow basin were evaluated at one-dimensional, two-dimensional, and three-dimensional scales using a variety of methods. The estimated mean basin-wide recharge rate is about 10 mm/yr, indicating that recharge is about 5 percent of annual precipitation. This is higher than reported values for clastic sediments in similar desert environments, likely because of the inability of plant roots to penetrate and extract water from shallowly buried non-fractured sandstone.; Ground-water dissolved-gas concentrations indicate centennial- to millennial-scale changes in recharge rates. Compared to the average water table temperature at Sand Hollow basin, cooler dissolved-gas recharge temperatures were calculated from water samples with ages between 50 and a few thousand years. This indicates that recharge rates were higher than present and caused the water table in parts of the basin to approach land surface, allowing the rapid transport of cooler-season infiltration to the water table. Darcy's Law calculations show that recharge rates may have been at least 60 percent higher than present.