THE OSAGE-SKIATOOK PETROLEUM ENVIRONMENTAL RESEARCH PROJECT

James K. Otton*
U.S. Geological Survey
MS 939 Box 25046
Lakewood, CO 80225
Voice: 303-236-8020
Fax: 303-236-0459
E-mail: jkotton@usgs.gov

Yousif K. Kharaka
U.S. Geological Survey
Meneo Park, CA

The Osage-Skiatook petroleum environmental research (OSPER) project is examining the impacts of oil production at two long-term research sites (A and B) adjacent to Skiatook Lake in Osage County, Oklahoma. About 1.5 and 1.0 hectare of land at the A site (an inactive oil lease) and the B site (an active oil lease), respectively, are affected by salt scarring, tree kills, soil salinization, and brine and petroleum contamination resulting from the leakage of produced water and associated hydrocarbons from brine pits and accidental releases from active and inactive pipes and tank batteries. To date, the studies have included aerial-photo interpretation and highly accurate GPS mapping of site features and topography, organic and inorganic geochemistry of ground waters, tracer test and long-term water-level monitoring, hydrologic modeling, surficial and bedrock geologic mapping, surface soil and bedrock geochemistry and mineralogy, microbial population and hydrocarbon degradation characterization, plant vegetation surveys, oak-tree-ring dating, and biogeochemistry of oak-tree leaves. In a collaborative effort, EPA scientists have characterized soil total petroleum hydrocarbon (TPH), nutrients, and salinity, and have planted remedial plots comprised of trees and grasses, and sampled lake bottom sediments for contaminants. Both sites are in a dissected area of modest relief underlain by interbedded shale, siltstone, and sandstone. Thicker resistant sandstone units typically form the hillcrests. Shale, mudstone, siltstone, and thin sandstone beds underlie hillslopes. A layer of eolian sand of varying thickness blankets much of the terrane. Oak forests cover the hillslopes and mixed grasslands and oak forests cover ridge crests. The following findings might provide insights for assessment and remediation of similar sites elsewhere: 1) 3-dimensional plumes of high-salinity water (as much as about 30,000 mg/L TDS) with chemical and isotopic characteristics similar to those of the produced-water source have been mapped at both sites. Currently, the depth and boundaries of these plumes are not well defined, because all completed wells, regardless of depth, show variable impacts of produced water and/or associated hydrocarbons; 2) highly saline produced waters (150,000 mg/L TDS) have been diluted by precipitation and ground water mixing, and are modified geochemically by interaction with bacteria and bedrock; 3) a surface layer of fine, permeable sand about 50 cm or more thick provides protection to the root zone of oaks, grasses, and forbs from salts in the underlying strata--the fine sand inhibits wicking of salt up to the root zone; 4) reducing, chloride- and organic-rich waters mobilize iron as Fe²⁺ which causes distinctive bleaching and iron staining of sandstone outcrops; 5) the altered physical and chemical features of an impacted area retard colonization by oaks, however, selected parts of an impacted area can become colonized for reasons not yet understood; 6) hydrocarbons at and near the ground surface degrade readily to asphaltic residues with iron- reducing bacteria playing a significant role, but locally, BTEX compounds can persist in the subsurface; the abundant soluble iron present at the sites is acting as the terminal electron acceptor for the bacteria; 7) nitrogen levels are low in local soils, natural and impacted; 8) a chloride to conductivity ratio of 1:10 is diagnostic of local background waters or leachates of bedrock whereas a 1:4 ratio is diagnostic of produced-water-salt impacts; the chloride to bromide ratios are also distinctively different; 9) sodium found in salt-impacted soils and bedrock at the two sites is present both in soluble form and on exchangeable sites on clays; 10) large amounts of salts and organic compounds remain in the local groundwater at A site after more than 65 years of natural attenuation; 11) erosion developed early in the 70+ year history of the A site salt scar but also stabilized early probably because the channel eroded down to hard sandstone bedrock layers; and 12) oak trees near the A site salt scar are taking in chloride from produced-water salts through their roots in the shallow subsurface as much as 45 m from its margin, confirming the presence of such salts as indicated by electromagnetic geophysical signatures, shallow coring, and ultimately drilling.