NATURAL ATTENUATION AND REMOVAL OF CONTAMINANT SALT FROM PRODUCED WATER RELEASES BY RUNOFF: OSPER SITE A, OSAGE COUNTY, OKLAHOMA

The Osage-Skiatook Petroleum Environmental Research (OSPER) site A is part of a depleted oil field, where 15-30 million kg of salt was discharged to the surface during 1912-1937 in high salinity (180,000 mg/L TDS) produced water associated with ~100,000 bbl of oil production. Salt contamination from the Na-Ca-Cl type brine has visibly impacted a ~1.2 hectare area that is characterized by a salt scar, excessive soil and rock erosion, and dead vegetation, downslope from two pits that were the primary source for produced water releases. Extensive geologic, hydologic, and geophysical characterization of the site, including repeated geochemical sampling of groundwaters from 44 monitoring wells, show that: 1) contaminant salts have been mostly leached from soil and surficial rocks, except on the salt scar, where salt efflorescence is common during the hot/dry seasons; and 2) a plume of high salinity groundwater (2,000-30,000 mg/L TDS), up to 10 m in vertical thickness and at least 2.5 hectare in lateral extent, extends predominantly to the north, downslope and downdip from the brine pits and salt scar, terminating at Skiatook Lake, a 4250-hectare reservoir that provides drinking water to the local communities. The plume is estimated to contain 0.1-0.3 million kg of salt. Site A is ideally suited to study the natural overland transport of salt, because the salt impacted area lies within a small 1.7 hectare watershed, which drains into a narrow ephemeral stream at the base of the salt scar and just above the conservation water level for Skiatook Lake. A weir, installed at the site in March 2003, has enabled the periodic monitoring of discharge and chemical composition of surface runoff during precipitation events. Results to date show that the initial runoff that leaches the previously precipitated surficial salt can have a relatively high salinity (up to 3,000 mg/L TDS), but that relatively small amount of total salt (~500 kg/yr) is removed by runoff from normal precipitation rates (90-100 cm/yr) in the area. This result suggests that it will require hundreds of years to remove the salt from the impacted area under current conditions, supporting the conclusion that significant salt from produced water releases still remains in the rocks of the impacted area after more than 65 years of natural attenuation.