GEOCHEMICAL INVESTIGATIONS OF WASTEWATER SPILLS RELATED TO UNCONVENTIONAL OIL DEVELOPMENT IN THE WILLISTON BASIN

Increasing demand for fossil fuels and depletion of traditional oil and gas reservoirs has driven the development of unconventional oil and gas (UOG) resources, e.g., shale gas and tight oil, using methods such as horizontal drilling and hydraulic fracturing. These technologies produce liquid (e.g., produced waters) and solid (e.g., drill cuttings) wastes, which pose unknown risks and potential consequences to the quality of atmospheric, groundwater, surface-water, soil, and other environmental resources. We are conducting a broad study of the potential environmental impact of wastes from UOG development. As part of this effort we are investigating accidental wastewater leaks and spills in the Williston basin. In January 2015 an estimated 3 million gallons of wastewater brine (TDS 138,000 mg/L) from UOG production in the Williston Basin spilled into Blacktail Creek, a small tributary of the Missouri River, in North Dakota. We sampled sediments and water upgradient and downgradient from the spill in February 2015, a few weeks following the spill. River water samples had elevated chloride (1,026 mg/L) and bromide (7.8 mg/L) downgradient from the spill site, compared to upgradient (10 mg/L and 0.4 mg/L respectively). Lithium (250 ug/L), boron (1.75 mg/L) and strontium (7.1 mg/L) were present at concentrations 5-10 times the upgradient concentrations. Hydrocarbons indicative of oil were not detected in the dissolved phase but low levels of aromatic hydrocarbons, such as tetramethylbenzenes and di-methylnaphthalenes, were detected in unfiltered river water samples near the spill. Dissolved organic carbon concentrations were greater than 20 mg/L, both upgradient and downgradient from the spill, complicating the identification of organic fingerprints of the wastewater. Water downgradient from the spill is supersaturated with respect to barite, indicating barite precipitation is a potential natural sink for aqueous and sediment-bound barium and radium in the river bed. Analyses for radium and strontium isotopes, as well as trace organic compounds, are currently underway in both water and sediment samples. Additional samples were collected in June 2015, and a longer-term study plan is underway, in order to assess the potential transport or natural attenuation of these constituents in the river.