The Edwards and Trinity aquifers are significant sources of water for industrial and agricultural use, for ecological resources in Central Texas, and are the sole sources of water for many people in the area. In the Barton Springs / Edwards Aquifer Conservation District, demand for groundwater has increased considerably in recent years, to the point that usage has either reached or nearly reached the sustainable yield of these aquifers. The Trinity Aquifer has increasingly become a source of water as limits have been placed on the Edwards Aquifer. Stratigraphically, the Trinity Aquifer underlies the Edwards Aquifer. However, along the Balcones Fault Zone, normal faulting has juxtaposed the two aquifers horizontally, with units of the Trinity to the west of the Edwards in the study area (Figs. 1 and 2).

Proper management of these aquifers requires an understanding of factors affecting the hydraulic relationship between the two aquifers. Although previous hydrologic studies of groundwater resources suggest a hydraulic connection between the Trinity and Edwards aquifers, that relationship is poorly understood. The hydraulic connection is crucial to water budgets and predictions of groundwater availability and quality in the Trinity and Edwards aquifers and flow at Barton Springs. Until recently, there has been insufficient head data and water-quality data to assess the hydrologic connection and potential for flow between the Edwards and Trinity aquifers.

Methodology

Because demand for groundwater in Central Texas is rapidly increasing, groundwater scientists are now studying its aquifers in far greater detail. Most aquifer parameters are determined from wells that penetrate the entire Edwards section or wells that are completed over considerable thicknesses of the Trinity. Monitoring of more discrete intervals can provide data that reflect the true complexity of these aquifers. To address these issues, the Barton Springs / Edwards Aquifer Conservation District collects data from three well pairs and has installed a multiport well with 14 monitoring zones in the Edwards, Upper Trinity, and Middle Trinity aquifers (Fig. 1).

Geology and Hydrogeology

The Edwards Aquifer, located in south-central Texas, is one of the most prolific karst aquifers in the United States. The Edwards Aquifer is a karst aquifer developed in