AN IMPROVED METHOD FOR MODELING THE INFILTRATION POTENTIAL OF THE EDWARDS AQUIFER RECHARGE AREA, COMAL COUNTY, SOUTH-CENTRAL TEXAS

One of the areas for recharge to the Edwards aquifer occurs in Comal County, south-central Texas. Comal Springs sustains federally-listed endangered species and provides a major water supply to downstream users in Comal and adjacent counties. Residential and commercial development atop the recharge area have increased the risk for ground-water contamination from point and non-point pollution sources. Previous ARC GIS analyses of readily-available spatial datasets of Bexar County have produced vulnerability maps of the Edwards contributing and recharge zones. The five digital datasets used in these models included: (1) hydraulic properties of Edwards Group hydrostratigraphic units, (2) faults/fractures, (3) caves/sinkholes, (4) slope analyses from 30-meter DEM surfaces, and (5) soil characteristics.

Using the 5 parameters listed above, infiltration-potential modeling of the Edwards recharge area in Comal County produced composite values that range from 0-85. These ratings are based on the effect each parameter has on precipitation/runoff entering the Edwards ground-water system. This modeling shows that the southern part of the county has the greatest infiltration potential. Of the eight Edwards Group hydrostratigraphic units present, the areas containing the kirschberg evaporite and leached and collapsed members are more likely candidates to infiltration.

When a 10-meter DEM grid and new hydrostratigraphic digital datasets are added to the calculations, the infiltration-potential models change significantly. The 10-meter grid allows smaller cells in the matrix to appear whereas the same polygons are missing in the 30-meter grid. Faults and fractures tend to be concentrated in the more brittle hydrostratigraphic units, particularly the dolomitic and grainstone members, and the addition of a fracture density parameter shows the northern part of the county to have high infiltration potential. And because fracture intensities increase on the up-thrown sides of the faults, the adjacent units to the north will exhibit higher composite values than their down-thrown counterparts. Finally, incorporation of basin accumulation indices, which calculates the infiltration potential of contributing (drainage) areas to slope, and up-to-date vegetation coverages, is ongoing.