Natural Mitigation of Chloride- Contaminated Surface Water by Base Flow, Permian Basin Region, Texas

The Upper Colorado River exceeds governmental drinking water standards for dissolved solids, chloride, and sulfate. The stream contacts Permian and Triassic aquifers and bypasses numerous Permian Basin hydrocarbon producing operations. The stream comprises Na-Cl hydrochemical facies throughout the area. Regional oilfield-brine Na/Cl values range 0.5 to 1.8, with mean and median values of 0.82 (n=975). In 18 stream samples charge-balance values of Na/Cl range 0.75 to1.107, with a mean of 0.94 and median of 0.96; and of (Ca+Mg)/(SO₄+HCO₃) range 0.86 to 1.37, with mean and median values of 1.08. These data suggest that 1) chlorides may originate mainly from dissolution of Upper Permian halite rather than from oilfield brines and 2) cation exchange cannot account for groundwater elevated Na/Cl values.

Upper Colorado River salinity decreases downstream overall (8,430 to 1,540 mg/L TDS; 3,950 to 411 mg/L Cl; 1,500 to 566 mg/L SO₄). Ratios of major ionic species vary systematically along flow and record base flow from sulfate- and carbonate-rich aquifers. Downstream increasing values for SO₄/Cl of 0.32 to 1.3 record incremental input of dissolved Permian-evaporite sulfate. Increasing values for HCO₃/SO₄ of 0.23 to 0.9 through the most downstream reaches record base flow from Permian carbonate aquifers. A highly localized SO₄/Cl-trend reversal marks enhanced chloride input near a small oilfield where water wells in the San Angelo aquifer show elevated chloride concentrations that are uncharacteristic of the aquifer elsewhere. Br/Cl and Na/Cl conservative mixing models indicate that stream hydrochemical characteristics in this location can be explained by a mixture of local oilfield brine with 98.6% to 99.5% local groundwater.