119 Recent Investigations of the Hydrogeology Edwards (Balcones Fault Zone) Aquifer, the Trinity (Hill Country) Aquifer, and Their Interactions, South-Central Texas

Recent research into the hydrogeological characteristics of the Edwards (Balcones Fault Zone) and/or Trinity (Hill Country) aquifers, south-central Texas, including their inter-aquifer connections, vulnerability, and prospects for long-term sustainability given changes in population, land use, and climate of the region.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 209

Importance of Allogenic versus Autogenic Recharge to the Karstic Edwards and Hill Country Trinity Aquifer System, Texas

Paper number: 119-2
Program: Technical Sessions
Time: 1:50 PM – 2:10 PM

Authors

Abstract

The Edwards Aquifer and Hill Country Trinity Aquifers are regional karst systems which derive both allogenic and autogenic recharge from large geographic areas. Autogenic recharge can be thought of as occurring directly in the “recharge zone” of an aquifer, while allogenic recharge relates to surface flows derived from watersheds outside the recharge zone — commonly referred to as the “contributing zone”. The conventional concept of recharge to the Edwards Aquifer is that precipitation falls on its contributing zone, flows as surface water until arriving at the recharge zone, then enters the aquifer as allogenic recharge. Additional recharge would be experienced from precipitation that falls directly onto the recharge zone as autogenic recharge. This concept treats the contributing zone as a surface of relatively low permeability where recharge is minimal. In reality, the Edwards Aquifer contributing zone consists of both recharge (autogenic) and contributing zones (allogenic) of the Trinity Aquifer which then also recharge the Edwards Aquifer via cross-formational flow in areas where the Edwards and Trinity Aquifer are hydrologically linked in the subsurface. From the perspective of the Edwards Aquifer, significant recharge may be realized in the Trinity Aquifer before surface flow arrives at the Edwards Aquifer recharge zone. All of this renders the conventional assessment of recharge to either aquifer as inaccurate if not misleading. The revised conceptualization suggests that the regional Trinity and Edwards aquifer system is composed of heterogeneous allogenic and autogenic recharge areas. Evidence supporting this conceptualization includes water-chemistry data, water-elevation measurements, water-balance measurements, gain-loss surveys, dye trace studies, cross-formational flow assessments, and groundwater hydraulics. This characterization of the complex nature of allogenic and autogenic recharge and the hydrologic connectivity of these two major karst aquifers justifies greater protections to this critical groundwater resource.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412680

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