The southern segment of the Edwards aquifer in south-central Texas is one of the most productive subsurface reservoirs of potable water in the world, providing water of excellent quality to more than a million people in the San Antonio region, where the Environmental Protection Agency (EPA) has declared it to be a sole-source aquifer (van der Leeden and others, 1990). Depending on the depositional province within which the associated carbonate rocks originated (Maclay and Small, 1984), the Edwards aquifer is composed of several geologic formations (primarily limestone and dolostone) of Early Cretaceous age. Most water pumped from the Edwards aquifer comes form the Person and Kainer Formations, which were deposited over the San Marcos Platform. The principal source of ground water in study area is the Devils River Formation, which was deposited in the Devils River trend. The Devils River Formation provides large quantities of irrigation water to fertile bottomland areas of Medina and Uvalde Counties, where the success of farming and ranching activities has long depended upon water from the Edwards aquifer. The study area includes all of the Edwards aquifer recharge zone between the Sabinal River (on the west) and the Medina River (on the east) plus an updip fringe of the confined zone in east-central Uvalde and central Medina Counties. Over about ninety percent of the study area--within the Devils River trend--the Edwards aquifer is composed of the Georgetown Formation plus the underlying Devils River Formation. Over the remaining area--over the southwestern margin of the San Marcos platform--the Edwards aquifer consists of the Georgetown Formation plus the underlying Edwards Group (Rose, 1972), which comprises the Kainer and Person Formations.