GIS-BASED METHODS FOR MAPPING REGIONAL BEDROCK RECHARGE

Direct infiltration of precipitation to permeable bedrock aquifers, often the most important form of ground-water recharge, has been difficult to quantify at the regional scale, particularly in arid climates. GIS techniques for regional estimates of recharge to desert sandstone are currently being developed. These GIS methods are based on net infiltration rates determined by using vadose-zone environmental tracers from both boreholes and basin-scale trenches. Tracer studies show that infiltration is extremely variable and highest beneath coarse-grained soils and areas receiving run-on from exposed outcrops during rain events. Multivariate analysis using linear least squares inversion techniques resulted in the development of an empirical relation between net infiltration as the percent of precipitation and five potential controlling parameters (soil coarseness, soil thickness, topographic slope, distance to upgradient outcrop, and sandstone fracture density). This percent of precipitation is multiplied by the average annual precipitation at each location to determine actual recharge rates. Statistical analysis indicates that the accuracy of recharge estimates is not substantially diminished when only the three surficial parameters (soil coarseness, topographic slope, and distance to upgradient outcrop) are used. Because these surficial data are available from geologic, soils, and topographic maps, regional-scale recharge maps can be readily constructed. These techniques were used to create a recharge map of the Navajo Sandstone Aquifer in Washington County, Utah, with rates ranging from 1 to 65 millimeters per year. This map will be used for water-resources evaluation, recharge area source-protection, and land-use planning and zoning.