Hydrogeology of the Edwards and Trinity Aquifers within Hays County

Science Center Objects

The Texas Water Development Board classifies the karstic Edwards and Trinity aquifers as major sources of water in south-central Texas. To effectively manage the water resources in the area, detailed maps and descriptions of the geologic framework and hydrostratigraphic units of the aquifers in Hays County, Texas are needed. In 2016 and 2018, the Texas Water Science Center (TXWSC), in cooperation with the Edwards Aquifer Authority, mapped geologic framework and hydrostratigraphy of the Edwards and Trinity aquifers within Hays County, Texas at 1:24,000 scale.

The Edwards and Trinity aquifers are major sources of water for agriculture, industry, and communities in south-central Texas. Groundwater flow and storage in the Edwards and Trinity aquifers are largely controlled by the aquifers’ hydrostratigraphy and structure, therefore, understanding these features will aid in anticipating and mitigating issues related to groundwater, environment, and land use. In order for water-resource managers, such as the Edwards Aquifer Authority (EAA), to better manage these resources, a refined understanding of the spatial distribution of the geologic framework and hydrostratigraphic units of the aquifers is needed. This study will re-examine the geology, geologic framework, and hydrostratigraphic characteristics of these areas and refine the previous mapping using better access to geologic properties and more accurate mapping tools such as global positioning systems. 

This project is part of a multi-phase effort to refine existing and develop new geologic and hydrostratigraphic maps of the Edwards and Trinity aquifers at the 1:24,000 scale in parts of Bexar, Comal, Hays, Blanco, Kendall, Bandera, Medina, Uvalde, Real, Edwards, and Kinney Counties.

Phase l of this project focused on making these refinements and mapping efforts in Bexar and Comal Counties. This Phase 2 effort focused on mapping in Hays County.

Task 1 - Data Compilation:

The TXWSC will review existing reports, geologic literature, maps, and field mapping investigations of the geology in Hays County to aid in mapping of the hydrostratigraphic subdivisions. The Survey will compile pre-existing geophysical borehole data and correlate that data with surface mapping, especially in areas where access is restricted or obscured. Geophysical borehole data also will be used to compare similar hydrostratigraphic units identified in the subsurface with the outcrop.

Digital orthophoto quadrangles (DOQs), satellite imagery, and other aerial photographs may be used to identify human activities and/or subaerial geologic expressions. Human activities may include road cuts, quarries or gravel pits, land clearings for development, new roads or widening of roads forming new land developments, and excavation of trenches for water lines or pipelines. Subaerial geologic expressions include areas of bare rock or rock ledges. These features are not apparent on topographic maps but might be indicated on remotely sensed imagery.

Task 2 - Field Mapping:

The TXWSC will remap the Edwards and Trinity Aquifers outcropping in the study area.  Mapping will include the identification of physical features such as faults. Faults will be located and their displacements approximated by identification of juxtaposed rock units. Faults and fractures, which represent a primary means of recharge to the Edwards and Trinity Aquifers, will be located when possible.

The TXWSC will use pre-existing geophysical borehole data identified in Task l to determine the thickness of the hydrostratigraphic units and to assist in determining fault displacement in complexly faulted areas or where rocks are poorly exposed or inaccessible.  

Task 3 - Analysis and Reporting:

The TXWSC will produce a report and county scale hydrostratigraphic map showing the hydrostratigraphy, faulting, and significant geologic characteristics within the Edwards and Trinity Aquifer outcrops in Hays County, Texas.