The Edwards-Trinity aquifer system underlies about 42,000 square miles of west-central Texas, where mostly gently dipping Lower Cretaceous strata comprise three regional aquifers and two regional confining units. The aquifers are the Edwards Aquifer of the Balcones fault zone, the Trinity Aquifer of the Balcones fault zone and Hill County, and the Edwards-Trinity Aquifer of the Edwards Plateau and Trans-Pecos. The Navarro-Del Rio confining unit confines the downdip part of the Edwards Aquifer, and the Hammett confining unit confines the updip, basal part of the Trinity Aquifer and a small southeastern fringe of the Edwards-Trinity Aquifer. Transmissivity averages less than 10,000 feet squared per day throughout more than 90 percent of the study area as the result of widespread cementation and secondary mineral growth. However, in fractured and leached rocks in the Balcones fault zone, transmissivity averages about 750,000 feet squared per day in the Edwards aquifer, which occupies less than 10 percent of the area.