MAPPING BRACKISH GROUNDWATER IN THE WOODBINE AQUIFER, TEXAS

The Brackish Resources Aquifer Characterization System (BRACS) is a program of the Texas Water Development Board (TWDB) tasked with mapping the distribution and quality of brackish groundwater in the state of Texas to aid in the exploration and development of this resource for inland groundwater desalination.

The objectives of a BRACS study are to collect, analyze, and interpret publicly available well data and geophysical well logs to map the relevant geologic boundaries, net sand of the aquifer, distribution of total dissolved solids, and other aquifer properties that may be of interest to stakeholders. The results are then utilized to estimate the volume and distribution of brackish groundwater in the aquifer and to delineate areas best suited for brackish groundwater production.

For the Woodbine Aquifer study, we mapped groundwater salinity using historical measured water quality samples combined with estimated total dissolved solids (TDS) calculated from resistivity logs using the R_wa (Resistivity Water Apparent) Minimum Method. The R_wa Minimum Method is based on Archie’s equation, which relates the resistivity of water to the overall resistivity measured for the formation. To utilize this method, we established key study-specific input parameters including a deep resistivity reading (R_t) from each well log, formation porosity (φ), and a cementation exponent (m). Using measured water quality data, we determined the relationship of conductivity to TDS for a wide range of values to convert the calculated resistivity of the water (R_w) to TDS.

Once stratigraphy, lithology, and salinity classes were mapped, we calculated the estimated in-place volumes of brackish groundwater in the range of 1,000 mg/L to 10,000 mg/L TDS. Upon the completion of the project, a report detailing the methodology and results, GIS datasets, and study specific database tables (including log interpretations) will be available at the TWDB website (http://www.twdb.texas.gov/groundwater/bracs/index.asp).