Oil & gas (O&G) wastewater has been treated and discharged to fresh water streams in western Pennsylvania for several decades. These high-salinity (TDS> 150,000 mg/L) discharges not only lead to increased salinity of the streams, but also increased concentrations of radionuclides and alkaline earth metals in sediment. Here, we analyzed three sediment cores in a river-lake to investigate the potential accumulation and long-term impact of surface water disposal on contaminant concentrations in river and lake sediments downstream of O&G wastewater discharge facility. High sedimentation rates in the lake (>8 cm/year) provide a high-resolution record of changes in wastewater disposal in the watershed from ~1982-present. Elemental concentrations measured in both sediments and porewater identify an interval with higher TDS, Cl, Sr, Ba, and Ra. Elevated activities of radium (3-4 times background) are present with high 226Ra/228Ra ratios that likely reflect disposal of Marcellus formation produced waters. The sediments also preserve a unique inorganic chemical signature of O&G wastewater (e.g., Ba/Ca, Br/Cl, 87Sr/86Sr, 226Ra/228Ra) that can be used as a forensic tool to record discharges of effluent over time. The sediment cores also indicate that the interval of elevated concentrations with the diagnostic geochemical fingerprint corresponds to a period of time when the maximum volume of unconventional wastewater was discharged from facilities located upstream (2009-2011). A subsequent voluntary reduction in surface water disposal that began in 2011 has apparently helped reduce the overall load of contaminants in sediments deposited more recently (2012-2015). Sediment records such as these will help quantify risks associated with O&G wastewater disposal and guide future management strategies in Pennsylvania and elsewhere.
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