PORE WATERS CONTAMINATED WITH OIL FIELD BRINE, OSAGE COUNTY, OKLAHOMA:IMPLICATIONS FOR WATER QUALITY AND ECOSYSTEM HEALTH

Shallow ground water is contaminated with brine from past and present operations at an oil field production site in northeastern Oklahoma. The contaminated ground water provides a source of salts, metals, and hydrocarbons to sediment and water of adjacent Skiatook Lake. An additional source of contamination at the site is a former brine storage pit of 10 m diameter that is now submerged beneath the lake. Cores of the upper 30-40 cm of lake sediment were taken at an offshore saline seep, at the submerged pit, and at a location containing relatively uncontaminated lake sediment. Pore waters from each 2-cm interval were separated by centrifugation and analyzed for dissolved anions, cations, and trace elements. High concentrations of dissolved chloride in pore waters (200-5000 ppm) provide the most direct evidence of oil field brine contamination and compare to an average value of ~35 ppm in Skiatook Lake. Cl/Br mass ratios of 220-250 in contaminated pore waters are comparable to values from produced water and saline soil extracts collected onshore. Dissolved concentrations of Se, Pb, and Cu in Cl-rich pore waters exceed current USEPA criteria for toxicity to freshwater aquatic life. The offshore seep sustains high concentrations of dissolved Cl at the sediment/water interface. In the pit the dissolved Cl concentrations are highly anomalous only at depths below 10 cm, indicating a combination of recent burial, mixing, and exchange with lake water, and bioturbation in this near-shore setting. Annual contribution of Cl to the lake is estimated to be ~20 kg for the entire area of the submerged pit (diffusive flux dominant), and ~5 kg for an assumed circular seepage area of 1 m diameter (advective flow dominant). Such contributions have minimal impact on water quality in the 323,000 acre-ft lake.