Radium attenuation and mobilization in stream sediments following oil and gas wastewater disposal in western Pennsylvania

Centralized waste treatment facilities (CWTs) in Pennsylvania discharged wastewater from conventional and unconventional oil and gas (O&G) wells into surface waters until 2011, when a voluntary request from the Pennsylvania Department of Environmental Protection (PA DEP) encouraged recycling rather than treating and discharging unconventional O&G wastewater. To determine the effect of this request on the occurrence of radium in streams, we sampled sediments at five CWTs that processed conventional O&G wastewater from 2011 to 2017 and compared results to published data. Despite the policy change in 2011 that reduced disposal of unconventional wastes (i.e., Marcellus) to surface water in Pennsylvania, the continued disposal of conventional O&G wastewater led to elevated radium activities in sediments at the point of discharge that were often hundreds of times higher than background. While these elevated activities were also present in downstream sediments (1.5× higher than background), the elimination of unconventional O&G wastewater disposal through the CWTs since 2011 decreased radium loading to the stream by approximately 95%. Sequential extractions and geochemical modeling using PHREEQC indicate that radium likely co-precipitates with barite or barite-celestite solid solutions and accumulates in the sediment as treated O&G effluent enters the stream. Adsorption of “exchangeable” radium, barium, and strontium on hydrous iron and manganese oxide coatings on fine-grained stream sediments is an important radium sequestration mechanism further downstream that can decrease the cation concentrations and potential for radio-barite co-precipitation. Radium downstream of CWTs was more abundant and more available for dissolution and desorption than radium in upstream sediments.