119-9 Assessing the vulnerability of the Edwards and Trinity aquifers in areas where they are hydraulically connected

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

Presenting Author:

Authors:

Abstract:

The Edwards and Trinity aquifers in south-central Texas, USA, are major karstic-carbonate aquifers that provide primary sources of water to nearly 3 million people and sustain flows to ecologically important spring systems. The aquifers are also hydraulically connected. For example, the Edwards Aquifer receives most of its recharge from losing streams that cross its outcrop (Edwards Aquifer recharge zone).  Flows in these streams originate via runoff and spring flows in the Texas Hill Country (Edwards Aquifer contributing zone). However, the Texas Hill Country is characterized by outcrops of Trinity Aquifer hydrostratigraphic units. Thus, the contributing zone of the Edwards Aquifer is also the recharge zone of the Trinity Aquifer. Moreover, regional faulting juxtaposes the hydrostratigraphic units of the Edwards and Trinity aquifers in the subsurface, providing pathways for interformational flow between the two systems.

Potential contaminant sources to the Edwards Aquifer, such as wastewater discharges, are strictly regulated over its recharge zone, but wastewater discharges are permitted over its contributing zone and thus may affect recharge to the Trinity Aquifer. This is especially important for Trinity Aquifer wells that may receive rapid recharge from the surface but also for areas where the two aquifers may hydraulically communicate. Unlike the Edwards Aquifer, the Trinity Aquifer hydrostratigraphy contains several aquitards that result in distinct hydrogeologic units known as the upper, middle, and lower Trinity aquifers. When coupled with structural complexities of the Balcones Fault Zone and limited well construction data for many domestic Trinity Aquifer wells, it is difficult to identify in advance which wells are most susceptible to contamination.

We have embarked on a robust water level and geochemical sampling program to better characterize the connections between the Trinity and Edwards aquifers. Geochemical results provide valuable information regarding the likely sources of water in Trinity Aquifer wells, while water level data reveal the distinct differences in the rate and style of recharge even for closely spaced wells completed in the same aquifer unit. As we learn more about factors that contribute to recharge in specific areas, we can identify ways to better protect both aquifers.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9826

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