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Using Electric Logs to Estimate Groundwater Salinity and Map Brackish Groundwater Resources in the Carrizo-Wilcox Aquifer in South Texas


In South Texas, brackish groundwater is a potentially important resource for oil field and agricultural uses. However, it is difficult to distinguish and quantify because few direct salinity measurements are available. In this paper, methods are presented for using resistivity logs to estimate groundwater salinity and to map brackish groundwater resources in the Carrizo-Wilcox Aquifer in South Texas. Electric logs were used to correlate and map stratigraphy and to estimate groundwater salinity. Salinity estimations were based on two methods: (1) empirical relationship between deep resistivity (R0) and formation water salinity; and (2) calculation of formation water resistivity (Rw) using a modified version of the Archie equation. Both methods provide cutoff values of R0 to distinguish broad categories of groundwater salinity: fresh (<1000 mg/L total dissolved solids [TDS]), slightly saline (1000–3000 mg/L TDS), moderately saline (3000–10,000 mg/L TDS), and very saline (>10,000 mg/L TDS). Brackish groundwater includes both slightly saline and moderately saline waters. Each significant (>10 ft thick) sand in the Carrizo-Wilcox Aquifer was assigned to one of these categories, and then cross sections and thickness maps were constructed to locate and quantify the resource. The Carrizo–upper Wilcox interval contains most of the fresh groundwater in the aquifer, whereas the lower Wilcox interval contains mainly brackish and saline groundwater. Within Carrizo–upper Wilcox sands, fresh groundwater grades downdip into brackish groundwater without intervening flow barriers. Brackish groundwater in lower Wilcox sands, however, is hydraulically separated from fresh groundwater in overlying sands. Therefore, the lower Wilcox interval is the most favorable target for brackish groundwater production without impacting fresh groundwater resources. Lower Wilcox brackish groundwater sands are thickest in the north and northeast parts of the study area. The lower Wilcox interval contains roughly 423 million acre-ft of brackish groundwater in place in the confined part of the aquifer in South Texas.