Using Borehole Geophysics to Design a Well Field for Targeted Water Quality

The Colorado River Municipal Water District retained Daniel B. Stephens & Associates, Inc. to design a 15 million gallon per day expansion to their well field in Ward County, Texas. Groundwater is present in the Cenozoic Pecos Valley Alluvium in hydraulic connection with the underlying Triassic Dockum Formation and water quality varies substantially. Chloride and total dissolved solids (TDS) increase with depth and also vary laterally. Multi-level sampling from drill stem tests at three sites showed chloride ranging from less than 100 mg/L to nearly 5,000 mg/L. The project goal was to install 21 new wells with an average chloride concentration of less than 600 mg/L. An extended geophysical logging program was employed to provide data from which varying water quality could be characterized. Correlations between resistivity and TDS and between TDS and chloride were established and resistivity logs were corrected for borehole and clay effects.  A combination of Archie’s equation and the Resistivity Ratio Method was applied to the logging suite to estimate varying chloride levels throughout the saturated zone in 24 test borings. These data were then used to optimize well designs that balanced the desire to maximize production with meeting the targeted average water quality goal. During this process, chloride levels were estimated for each screened zone and well. The accuracy of the chloride predictions for each well varied widely, but about half of the predictions were accurate to within 33%. Errors are attributed to the breakdown of the applied assumptions in clay-rich zones and the fact that average chloride estimates were length-weighted and did not account for likely higher water production rates associated with the cleaner sand intervals. On a macroscopic basis the process was successful in that the pumping-weighted average chloride for the new wells was 575 mg/L, exceeding the project goal.