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

Abstract

Encroachment of urban development 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 crossing the recharge zone and by direct infiltration of precipitation on the outcrop. In 1996, the U.S. Geological Survey, in cooperation with the Edwards Aquifer Authority and the Nature Conservancy of Texas, began a study to compare stormwater runoff from two sites in the recharge zone in watersheds of similar size and different land use. One site is located on Lorence Creek, a tributary to Salado Creek in northern Bexar County, and the other site is on an unnamed tributary of the Frio River in northern Uvalde County (fig. 1). Flow at the two sites results only from storm runoff. 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.