Abstract

The Trinity Aquifer is a Texas Water Development Board (TWDB) designated major aquifer and underlies all or parts of 70 counties in Texas as well as extending into portions of Oklahoma and Arkansas. In Texas, the aquifer extends uninterrupted from the Oklahoma border to south-central Texas. We define the Northern Trinity Aquifer as that portion of the Trinity Group in Texas which lies north of the Colorado River. We utilized the hydrostratigraphic zonation from the groundwater availability model (GAM) developed for the TWDB (Kelly et al., 2014) which subdivided the Trinity Group into five distinct hydrostratigraphic units; Hosston, Pearsall, Hensell, Glen Rose, and Paluxy. These Lower Cretaceous units are predominantly composed of interbedded sands, shales, and carbonates with minor evaporate beds present. The combined saturated thickness of the Northern Trinity Aquifer averages between 600 and 1900 feet.

In 2009, the 81st Texas Legislature provided funding to the TWDB to establish the Brackish Resources Aquifer Characterization System (BRACS). The goal of the program is to map and characterize the brackish portions of the aquifers in Texas in sufficient detail to provide useful information and data to regional water planning groups and other entities interested in using brackish groundwater as a water supply. House Bill 30, passed by the 84th Texas Legislative Session in 2015, requires the TWDB to identify and designate brackish groundwater production zones in the aquifers within the state. The Trinity Aquifer was one of the aquifers selected for study in House Bill 30.

In this study we combined the hydrostratigraphic units of the Northern Trinity Aquifer GAM with groundwater salinity values measured from groundwater samples and calculated from geophysical well logs. The geophysical well log measurements used in calculating groundwater salinity were mainly induction resistivity run primarily in oil and gas wells. Salinity zones were then mapped based upon the combined sampled and calculated total dissolved solids concentrations of the groundwater for each of the five hydrostratigraphic units. The four salinity zones mapped for each hydrostratigraphic unit are based upon total dissolved solids concentrations: these are (1) fresh (0 to 1000 milligrams per liter); (2) slightly saline (1000 to 3000 milligrams per liter); (3) moderately saline (3000 to 10,000 milligrams per liter); and (4) very saline (10,000 to 35,000 milligrams per liter). Groundwater volumes were calculated for each mapped salinity zone for each of the five hydrostratigraphic units. All data and analysis techniques were documented and will be made publicly available.