Bush, P.W., Ardis, A.F., Fahlquist, Lynne, Ging, P.B., Hornig, C.E., and Lanning-Rush, Jennifer, 2000, Water Quality in South-Central Texas, Texas, 1996–98, Denver CO: U.S. Geological Survey Circular 1212, 32 p.; The upper part of the South-Central Texas Study Unit (hereinafter, Study Unit) encompasses parts of the topographically rugged and picturesque Edwards Plateau and the comparatively flat, gently coastward-sloping Gulf Coastal Plain physiographic regions. An abrupt topographic break, the Balcones escarpment, separates the two landforms.
San Antonio is the principal urban area in the Study Unit. The agricultural areas of primary interest are west of San Antonio. Rangeland predominates in the Edwards Plateau.
The Study Unit contains the Edwards aquifer in the Balcones fault zone, a zone of northeastward-trending parallel faults that straddles the Balcones escarpment. The Study Unit also contains the Trinity aquifer in the “Hill Country,” the local name for the eastern part of the Edwards Plateau in the region.