TRACE AND MAJOR ELEMENT RATIOS IN ELLIPTIO DILATATUS SHELLS AS INDICATORS OF SURFACE WATER QUALITY IN WESTERN PENNSYLVANIA

Centralized waste treatment facilities in western Pennsylvania accept, treat, and discharge conventional oil and gas wastewaters. This wastewater contains elevated Cl, Br, Ba, Sr, and Ra among other trace elements. Prior to 2012, these facilities also accepted unconventional flowback and produced water. While unit processes in these treatment plants are designed to remove many contaminants, some elevated loads of Cl, Br, Sr, and Ra are discharged into waterways. Previous research suggests that discharge from these facilities has led to accumulation of radium in sediments and has possibly contributed to habitat loss and further decline of multiple endangered mussel species. Further work has shown that mussels record environmental signals in their shells from the water in which they live. This work analyzes metals and metal isotopes in shells of mussels as indicators of changing water quality downstream of the discharge of brine treatment facilities. We measured major and trace element ratios by laser ablation inductively coupled plasma mass spectroscopy (LA-ICPMS) in shells of Elliptio dilatatus, a member of a common genus of mussel. Combining element ratios with ¹⁸O/¹⁶O ratios (δ¹⁸O) we can observe seasonal trends along the growth bands of the shells and more accurately date changes in concentration within the shells. We also micromilled along the outermost edge of the cross section of the shells to determine ⁸⁷Sr/⁸⁶Sr ratios measured by multi-collector ICP-MS (MC ICP-MS) to indicate the likely origin of the strontium (e.g., mine drainage, oil and gas wastewater). Elevated ratios of Sr/Ca and characteristic ⁸⁷Sr/⁸⁶Sr ratios in the shells of Elliptio dilatatus collected downstream of one wastewater treatment facility correspond to periods of the highest reported discharge from that facility (2009-2012). Large deviations in ⁸⁷Sr/⁸⁶Sr through time in downstream shells suggest a change in the major source of Sr in that waterway, while a shell collected upstream of the same facility as well as shells collected in non O&G impacted waterways show no such change. This indicates that mussel shells have accumulated and recorded an anthropogenic signal of O&G discharge through time in a waterway receiving treated O&G wastewater.