Date of this Version
Science of the Total Environment 472 (2014) 1152–1162
Water (brine) co-producedwith oil in theWilliston Basin is some of themost saline in the nation. The Prairie Pothole Region (PPR), characterized by glacial sediments and numerous wetlands, covers the northern and eastern portion of the Williston Basin. Sheridan County, Montana, lies within the PPR and has a documented history of brine contamination. Surface water and shallow groundwater in the PPR are saline and sulfate dominated while the deeper brines are much more saline and chloride dominated. A Contamination Index (CI), defined as the ratio of chloride concentration to specific conductance in a water sample, was developed by the Montana Bureau of Mines and Geology to delineate the magnitude of brine contamination in Sheridan County. Values >0.035 indicate contamination. Recently, the U.S. Geological Survey completed a county level geographic information system (GIS)-based vulnerability assessment of brine contamination to aquatic resources in the PPR of the Williston Basin based on the age and density of oilwells, number ofwetlands, and streamlength per county. To validate and better define this assessment, a similar approach was applied in eastern Sheridan County at a greater level of detail (the 2.59 km2 Public Land Survey System section grid) and included surficial geology. Vulnerability assessment scores were calculated for the 780 modeled sections and these scores were divided into ten equal interval bins representing similar probabilities of contamination. Two surface water and two groundwater samples were collected fromthe sectionwith the greatest acreage of Federal land in each bin. Nineteen of the fortywater samples, and at least one water sample from seven of the ten selected sections, had CI values indicating contamination. Additionally, CI values generally increasedwith increasing vulnerability assessment score,with a stronger correlation for groundwater samples (R2 = 0.78) than surface water samples (R2 = 0.53).