ARTIFICIAL RECHARGE TO FRACTURED BEDROCK: THE NAVAJO SANDSTONE OF SOUTHWESTERN UTAH

As alluvial-fill aquifers in the Western United States become fully exploited and as communities expand into more-arid desert environments, permeable bedrock aquifers are increasingly being targeted for water-resources development. In order to avoid ground-water depletion, water-management strategies for these bedrock aquifers are beginning to include artificial recharge. The Navajo Sandstone of the Colorado Plateau and northern Mojave physiographic provinces is one such target aquifer. Laboratory-scale and aquifer testing indicates that its hydraulic conductivity and effective porosity is about 0.3 meters (m) per day and 20 percent, respectively. Because of this relatively high conductivity and porosity, surface-water recharge techniques are currently being tested and applied. These recharge methods include spreading basins, stream diversions along dry washes, and recharge trenches. Pilot- and full-scale studies utilizing these methods in southwestern Utah have resulted in infiltration rates ranging over two orders of magnitude. The infiltration rate beneath a large and deep surface-water reservoir (4-square kilometer area by about 10-m deep) covered by thin soils (less than 2 m) overlying weathered sandstone with caliche-filled fractures was about 0.005 m per day. Surface spreading of water on similar soil-covered sandstone during a pilot-scale test of a shallow infiltration pond (3,000-square m area by about 0.5-m deep) resulted in infiltration rates of about 0.05 m per day. A seepage study of surface water diverted to an exposed sandstone wash during snowmelt runoff indicated maximum potential infiltration rates of about 0.3 m per day. Finally, an infiltration test using a trench (100-m long by about 3-m deep) cut into non-weathered sandstone with open fractures and backfilled with gravel resulted in infiltration rates of about 0.5 m per day. In addition to surficial soils, caliche deposits and fracturing, other factors controlling infiltration rates are hydraulic connectivity between surface water and the water table, siltation/biofilm development, temperature-dependent water viscosity, and trapped air.