EVOLUTION OF THE RECHARGE ZONE OF THE BALCONES FAULT ZONE EDWARDS AQUIFER, SOUTH CENTRAL TEXAS USA

The recharge zone of the Balcones Fault Zone Segment of the Edwards Aquifer in south central Texas USA exhibits a number of different recharge morphologies. Recharge to the Edwards Aquifer occurs from streams originating in the contributing zone and entering the recharge zone, from direct precipitation upon the recharge zone between stream segments, and through interformational flow between the Trinity Aquifer and the Edwards Aquifer. Surface recharge to the San Antonio segment of the Edwards Aquifer ranges from 43,000 acre-feet per year to more than 2,000,000 acre-feet per year.

Intrastream recharge occurs from direct precipitation upon the Edwards limestone outcrop and may equal up to 50 percent of recharge in some smaller basins. An evaluation of a number of caves and cave maps from the Texas Speleological Survey indicates that many caves in the recharge zone are not related to the existing topography. Internal structures in many caves indicate that they were formed by upward ascending water under artesian conditions and include multi story cave levels, ramiform cave patterns, cupolas, vents, and ascending flow structures on cave walls. As the deeper portions of the artesian zone became more efficient at transmitting water and the upper confining unit above the Edwards limestone was removed through erosional processes, the upper artesian zone transitioned from artesian to water table conditions. Cave morphology indicates that hypogenic caves are now being overprinted by epigenic processes.