COMPLEXITY OF THE DOCKUM AQUIFER IN SCURRY COUNTY, TEXAS
Sources of complexity of Dockum aquifer chemistry include differences in stratigraphic intervals in which water wells are constructed and oil-field activities. According to TWDB, all water wells in Scurry County are classified as Dockum aquifer wells; however, they range in depth from 100-500 ft. In addition, Ogallala Fm. sediments overlie the Dockum throughout most of Scurry County. In order to understand influence of stratigraphic variations on Dockum chemistry, we constructed structure contour maps of the Santa Rosa subunit of the Dockum Group. The Santa Rosa is the lowest member of the Dockum group (McGowan et al., 1979), which contains sand and gravel layers that produce the largest quantity of Dockum groundwater (Dutton and Simpkins, 1986). Groundwater in Ogallala wells located outside of SACROC is higher quality than in Dockum Santa Rosa wells. For example, median concentrations of sulfate in Ogallala wells outside of SACROC are ~33 mg/L versus ~230 mg/L sulfate in Dockum Santa Rosa wells.
We conclude on the basis of stratigraphy, water chemistry, and limited water level data that the 100 to 150 ft deep wells in Scurry County are completed in Ogallala sediments. The deepest wells are completed in Dockum Santa Rosa; these wells are constructed with open or gravel-packed annuli to take advantage of the larger quantity of groundwater in Dockum Santa Rosa and higher quality groundwater in overlying Ogallala sediments. Perhaps most of the water wells in Scurry County should be classified as Ogallala-Dockum wells?