Fresh, Brackish, and Saline Groundwater Resources in the Carrizo-Wilcox Aquifer in Groundwater Management Area 13—Location, Quantification, Producibility, and Impacts

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2016

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Brackish groundwater is becoming increasingly important as fresh groundwater resources diminish. Brackish groundwater is defined as water containing between 1000 milligrams per liter (mg/L) and 10,000 mg/L total dissolved solids (TDS) (LBG-Guyton Associates, 2003). The Texas Water Development Board (TWDB) divides groundwater salinity into five categories: fresh (<1000 mg/L TDS), slightly saline (1000–3000 mg/L TDS), moderately saline (3000–10,000 mg/L TDS), very saline (10,000-35,000 mg/L TDS), and brine (>35,000 mg/L TDS) (Winslow and Kister, 1956). Reliable maps and models of brackish and saline groundwater resources are needed for planning purposes to meet rising water demands. Brackish groundwater is usable with minimal treatment for many purposes in agricultural and oil field operations and may be better suited than seawater (35,000 mg/L TDS) for desalination. For example, in Groundwater Management Area (GMA) 13 in South Texas, brackish groundwater in the Carrizo–Wilcox aquifer is a potential source of water for hydraulic fracturing in the Eagle Ford Shale play (Scanlon et al., 2014).

Brackish groundwater is difficult to distinguish and quantify because few direct salinity measurements are available. Most chemical analyses of formation water samples are either from freshwater aquifers or from oil field brines. Geophysical logs can help fill the gap between fresh groundwater and formation brine. Geophysical log interpretation spans the entire groundwater flow regime from outcrop to deep subsurface and from fresh groundwater to brine. Geophysical logs provide continuous vertical records of the electrical properties of both rocks and fluids in wells, whereas groundwater sample analysis provides only point-sourced data. However, hydrochemistry data from groundwater sampling are needed to calibrate geophysical log interpretations. This study characterizes brackish groundwater distribution and quantification using four integrated approaches: (1) groundwater quality and hydrochemistry as context for salinity mapping and to better understand salinity sources, (2) geophysical log (electric log) interpretation of groundwater salinity to map brackish groundwater, (3) calculation of volumes of fresh, brackish, and saline groundwater to quantify the resource, and (4) groundwater modeling to help predict the impacts of brackish groundwater production. This report covers the first half of our study of brackish groundwater resources in GMA 13—the Carrizo—Wilcox Aquifer. The second half of the study will cover the Queen City—Sparta Aquifer.

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