Quality of stormwater runoff from an urbanizing watershed and a rangeland watershed in the Edwards aquifer recharge zone, Bexar and Uvalde counties, Texas, 1996-98

on the outcrop of the Edwards aquifer (the recharge zone), particularly in Bexar County, has raised the issue of possible contamination of water that enters the aquifer. Increasing residential and commercial development on the recharge zone increases the potential for runoff containing toxic substances, oil spills, or leakage of hazardous materials to contaminate the regional drinking water supply. The Edwards aquifer is a dipping sequence of extensively faulted, fractured, and dissolutioned limestone and dolostone that yields large quantities of water to wells and springs. The recharge zone is essentially coincident with the area in which the aquifer crops out. Recharge to the aquifer is derived mainly from seepage from streams Both watersheds have similar climate, topography, soils, and vegetation. Land use in the Lorence Creek watershed is primarily single-family residential, commercial, and transportation. Land use in the watershed of the Frio River tributary is primarily rangeland. The drainage area of each watershed is less than 2 square miles.