Accumulation of Marcellus Formation Oil and Gas Wastewater Metals in Freshwater Mussel Shells

For several decades, high-salinity water brought to the surface during oil and gas (O&G) production has been treated and discharged to waterways under National Pollutant Discharge Elimination System (NPDES) permits. In Pennsylvania, USA, a portion of the treated O&G wastewater discharged to streams from 2008 to 2011 originated from unconventional (Marcellus) wells. We collected freshwater mussels, Elliptio dilatata and Elliptio complanata, both upstream and downstream of a NPDES-permitted facility, and for comparison, we also collected mussels from the Juniata and Delaware Rivers that have no reported O&G discharge. We observed changes in both the Sr/Cashell and 87Sr/86Srshell in shell samples collected downstream of the facility that corresponded to the time period of greatest Marcellus wastewater disposal (2009–2011). Importantly, the changes in Sr/Cashell and 87Sr/86Srshell shifted toward values characteristic of O&G wastewater produced from the Marcellus Formation. Conversely, shells collected upstream of the discharge and from waterways without treatment facilities showed lower variability and no trend in either Sr/Cashell or 87Sr/86Srshell with time (2008–2015). These findings suggest that (1) freshwater mussels may be used to monitor changes in water chemistry through time and help identify specific pollutant sources and (2) O&G contaminants likely bioaccumulated in areas of surface water disposal.

数十年来，油气（O&G）生产过程中被带出地表的高盐度废水，始终按照《国家污染物排放消除系统（National Pollutant Discharge Elimination System, NPDES）》的许可要求进行处理后排入河道。在美国宾夕法尼亚州，2008至2011年间排入溪流的经处理油气废水中，有一部分源自非常规开采的马塞勒斯（Marcellus）组页岩气井。我们在一家持有NPDES许可的设施的上下游区域，采集了两种淡水蚌类——尖锄蚌（Elliptio dilatata）与扁平蚌（Elliptio complanata）；为设置对照，我们还从未有油气废水排放记录的朱尼亚塔河（Juniata River）与特拉华河（Delaware River）采集了蚌类样本。我们观测到，在该设施下游采集的蚌壳样本中，壳体锶钙比（Sr/Ca_shell）与壳体87锶/86锶同位素比值（87Sr/86Sr_shell）均发生了变化，且该变化与马塞勒斯组（Marcellus Formation）废水处置量最大的时期（2009–2011年）相对应。重要的是，壳体Sr/Ca比值与87Sr/86Sr比值的变化趋势向马塞勒斯组产出的油气废水的特征值方向偏移。与之相反，在排污口上游以及无处理设施的水体中采集的蚌壳样本，其壳体Sr/Ca比值与87Sr/86Sr比值在2008–2015年间波动幅度更低，且未呈现随时间变化的趋势。上述研究结果表明：其一，淡水蚌类可用于长期监测水体化学组成的动态变化，助力识别特定污染物来源；其二，油气污染物极有可能在地表水排放区域发生生物富集。