Origin and fate of radium in flowback and produced water from Marcellus Shale gas exploration

Marcellus Shale is one of the world's largest unconventional gas resources. Recent developments in horizontal drilling and hydraulic fracturing enabled efficient and economical extraction of natural gas from unconventional (shale) resources and have led to rapid expansion of natural gas production in the United States. Hydrofracturing generates large volume of flowback and produced water that contains high concentrations of total dissolved solids (TDS), heavy metals, and naturally occurring radioactive materials (NORMs) resulting in significant environmental and public concerns and challenging waste management issues. Ra-226 is the dominant form of NORM and is one of the key challenges for sustainable management of Marcellus Shale wastewater. This study is focused on the life cycle of NORMs during natural gas extraction from Marcellus Shale. A rapid method for Ra-226 analysis by inductively coupled plasma mass spectrometry (ICP-MS) was developed to overcome some of the shortcomings of current analytical techniques (e.g., long detection time). The fate of Ra-226 under different scenarios associated with the shale gas extraction, including origin of Ra-226, partitioning in flowback water storage and treatment facilities, and associated solid waste disposal issues were evaluated in this study. This study showed that radium mainly originates from relative rapid shale leaching. High concentration of radium in the Marcellus Shale wastewater can be managed by proper treatment (e.g., sulfate precipitation). However, solid waste generated from treatment facilities or impoundments containing elevated radium concentrations far exceed the limits for disposal in the Resource Conservation and Recovery Act Subtitle D (RCRA-D) landfills. Current practice in landfill management allows the disposal of this solid waste by controlling the Allowed Source Term Loading (ALST) on annual basis. However, if the landfill capacity to accept all the NORM generated from Marcellus Shale gas extraction becomes insufficient, other disposal or beneficial use options for solid waste should be developed. Reuse of radium enriched barite as weighting agent in drilling mud might be a sustainable strategy to reduce the mass of NORM that has to be disposed in the landfills. Health risks associated with NORMs were evaluated for several typical scenarios associated with Marcellus Shale gas extraction. Total effective dose equivalent (TEDE) at drilling pads, storage impoundments and landfills are well below the Nuclear Regulatory Commission (NRC) limit for the general public of 100 mrem/yr even under the worst-case scenario assumptions. Workers in the centralized waste treatment facilities might receive excessive TEDE and appropriate measures recommended by NRC should be applied. For example, a safe distance of 5 m is recommended to reduce TEDE to acceptable level. Hence, the key environmental and public health risks associated with NORM brought to the surface by natural gas extraction from Marcellus Shale are from the spills that may contaminate surface and groundwater. Overall, this study contributes to the understanding of the fate of NORMs associated with Marcellus Shale gas wastewater management and expands the ability to resolve the environmental concerns associate with NORMs. A novel rapid analytical for Ra-226 measurement by ICP-MS offers an alternative for researchers to quickly analyze environmental samples. The fate of Ra-226 in centralized treatment facilities and storage facilities is important for operators to choose proper management strategy for liquid and solid waste disposal/reuse. The health risk associated with NORM that is assessed in this study will help to resolve the public concern stemming from the high NORM extracted from Marcellus Shale play and provides several options to further reduced its risks