Abstract

As part of the Texas Water Development Board (TWDB) Brackish Resources Aquifer Characterization System (BRACS), a study to delineate the extent of brackish groundwater in the Hill Country portion of the Trinity Aquifer in Texas was performed. In this study, we used geophysical well logs, submitted driller’s reports, aquifer tests, and measured water quality samples to map the stratigraphy and water quality for six Trinity Group hydrostratigraphic units as well as the Hammett shale aquitard.

We analyzed 1266 geophysical well logs and driller logs to map the stratigraphy of the Trinity Aquifer in all or parts of 24 counties. These logs were used to derive 4079 stratigraphic picks that we interpolated along with select known major faults to generate stratigraphic surfaces.

The stratigraphic surfaces were then used to assign aquifer codes to aquifer tests and measured water quality analyses compiled from various public data sources, such as the TWDB Groundwater Database, public water supply entities, and groundwater conservation districts.

To determine groundwater salinity in downdip portions of the aquifer, we used a variation of the Alger-Harrison resistivity ratio method to make 1487 salinity calculations from geophysical well logs. We then used the calculated salinity values, along with measured water quality samples, to produce salinity maps for each of the six mapped hydrostratigraphic units.

Upon the completion of the project, a report detailing the methodology and results, GIS datasets, and study specific database tables (which include log interpretations) will soon be available at the TWDB website (http://www.twdb.texas.gov/groundwater/bracs/index.asp).