Delineation of a saline ground-water boundary from borehole geophysical data

This study uses borehole geophysical data to calculate the total dissolved solids (TDS) concentration of ground water in Devonian and Silurian carbonate aquifers in northern and southwestern Indiana and specifically, to delineate the 10,000 mg/l (milligrams per liter) TDS boundary. The 10,000 mg/l TDS boundary as used here refers to the transitional zone in the Devonian rocks where the TDS concentration of saline ground water changes from less than 10,000 mg/l to 10,000 mg/l or greater. The 10,000 mg/l boundary not only defines the maximum allowable TDS concentration recommended by the U.S. Environmental Protection Agency for potentially potable water, but also establishes an internal "limit" for the Silurian and Devonian carbonate aquifers in Indiana where density differences may be important for ground-water flow modeling. The study demonstrated that geophysical log data can provide a useful means of estimating TDS concentration in areas where water-quality data are lacking, found an excellent correlation between the resistivity-porosity method and measured TDS concentrations, and found several anomalous areas in Indiana where TDS concentration in Devonian and Silurian aquifers were higher or lower than expected. The TDS content of ground water in Devonian and some Silurian units was estimated by using three independent geophysical methods: static spontaneous-potential, mud-filtrate-resistivity, and resistivity-porosity. Each method yields values for formation-water resistivity (Rw), from which TDS concentrations can be estimated. Calculated concentrations from each method were compared to available water-quality analyses. Use of the resistivity-porosity method produced the most accurate results. To help quantify the values used in the resistivity-porosity method, 19 samples cut from cores were evaluated by laboratory analyses. The static spontaneous-potential method was used only where data were not available for the resistivity-porosity method. The mud-filtrate-resistivity method yielded poor results and was not used. Chemical analyses of ground water and TDS concentrations estimated from borehole geophysical log data (resistivity-porosity and static spontaneous potential methods) were used to establish areas where TDS concentrations in ground water are less than or greater than 10,000 mg/l. The concentrations of TDS of water from the Silurian and Devonian carbonates were generally found to increase in a consistent trend downdip with depth into the Illinois and Michigan Basins; however, this trend was not observed in several anomalous areas where the 10,000 mg/l TDS boundary becomes more irregular in shape. There are also areas where the underlying Silurian carbonates have TDS concentrations that are less saline than the overlying Devonian carbonates. These anomalous areas may be the result of nonuniform recharge during Pleistocene time or more recent times, or from differences in permeability that control flow paths and rates.