NATURAL REMEDIATION OF GROUNDWATER IMPACTED BY PETROLEUM PRODUCTION OPERATIONS: THE OSPER A SITE IN OSAGE COUNTY, OK

James Thordsen*
Yousif K. Kharaka
Evangelos Kakouros
Gil Ambats

U.S. Geological Survey
Mail Stop 427
Menlo Park, CA 94025
Voice: 650-329-4557
Fax: 650-329-4538
E-mail: jthordsn@usgs.gov

The Osage-Skiatook Petroleum Environmental Research (OSPER) A site, located within the depleted Lester lease, has an area of about 1.5 hectare that is impacted by produced water and hydrocarbon releases that occurred primarily 65-90 years ago. In addition to groundwater contamination, impacts include salt scars, excessive soil and rock erosion, brine and asphalt pits, degraded oil, dead trees and shrubs and other visible surface features. Eventually, the bulk of inorganic salts and dissolved organic species will reach the adjacent Skiatook Lake, a 4250-hectare reservoir that provides drinking water to local communities and is a recreational fishery. Water samples from nearby oil wells indicate that the produced water source was a Na-Ca-Cl brine (~150,000 mg/L total dissolved solids), with relatively high concentrations of Mg, Sr, Fe, Mn and NH₄, but low concentrations of SO₄, H₂S and dissolved organics.

Results of detailed analysis of water samples, periodically obtained from 35 wells (1-36 m deep), were used to map a 3-D plume of high salinity groundwater (5,000-30,000 mg/L TDS) with chemical and isotopic characteristics that are similar to those of the produced water. These and future results will be used also to investigate the natural processes that modify the chemical composition of the plume-groundwater, including mixing with pristine groundwater and percolating water from precipitation, sorption, mineral precipitation/dissolution, evapotranspiration, volatilization and bacterially mediated oxidation/reduction reactions. The rate of salt removal from the site by surface runoff is being determined by measuring the volume and chemical composition of water flowing over a weir installed close to the Skiatook reservoir in a location that captures most of the surface and base flow from this site following precipitation. Results to date show that the natural salt-removal processes at this site are slow, thus providing a valid explanation for the fact that large amounts of salts from produced-water releases still remain in the groundwater plume after more than 65 years of natural attenuation.