Risk assessment of human exposure to Ra-226 in oil produced water from the Bakken Shale

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Unconventional oil production in North Dakota (ND) and other states in the United States uses large amounts of water for hydraulic fracturing to stimulate oil flow. Most of the water used returns to the surface as produced water (PW) containing different constituents. Some of these contents are total dissolved solids and radionuclides. The most predominant radionuclide in PW is radium-226 (Ra-226) of which level depends on several factors including the content of certain cations. A multivariate regression model was developed to predict Ra-226 in PW from the Bakken Shale based on the levels of barium, strontium, and calcium. The simulated Ra-226 activity concentration in PW was 535 pCi/L supporting extremely limited actual data based on three PW samples from the Bakken (527, 816, and 1210 pCi/L). The simulated activity concentration was further analyzed by studying its impact in the event of a PW spill reaching a surface water body that provides drinking water, irrigation water for crops, and recreational fishing. Using food transfer factors found in the literature, the final annual effective dose rate for an adult in ND was estimated. The global average annual effective dose rate via food and drinking water is 0.30 mSv, while the predicted dose rate in this study was 0.49 mSv indicating that there is potential risk to human health in ND due to Ra-226 in PW spills. This predicted dose rate is considered the best case scenario as it is based on the simulated Ra-226 activity concentration in PW of 535 pCi/L which is close to the low end actual activity concentration of 527 pCi/L.