Marcellus and mercury: Assessing potential impacts of unconventional natural gas extraction on aquatic ecosystems in northwestern Pennsylvania.

Mercury (Hg) is a persistent element in the environment that has the ability to bioaccumulate and biomagnify up the food chain with potentially harmful effects on ecosystems and human health. Twenty-four streams remotely located in forested watersheds in northwestern PA containing naturally reproducing Salvelinus fontinalis (brook trout), were targeted to gain a better understanding of how Marcellus shale natural gas exploration may be impacting water quality, aquatic biodiversity, and Hg bioaccumulation in aquatic ecosystems. During the summer of 2012, stream water, stream bed sediments, aquatic mosses, macroinvertebrates, crayfish, brook trout, and microbial samples were collected. All streams either had experienced hydraulic fracturing (fracked, n = 14) or not yet experienced hydraulic fracturing (non-fracked, n = 10) within their watersheds at the time of sampling. Analysis of watershed characteristics (GIS) for fracked vs non-fracked sites showed no significant differences (P > 0.05), justifying comparisons between groups. Results showed significantly higher dissolved total mercury (FTHg) in stream water (P = 0.007), lower pH (P = 0.033), and higher dissolved organic matter (P = 0.001) at fracked sites. Total mercury (THg) concentrations in crayfish (P = 0.01), macroinvertebrates (P = 0.089), and predatory macroinvertebrates (P = 0.039) were observed to be higher for fracked sites. A number of positive correlations between amount of well pads within a watershed and THg in crayfish (r = 0.76, P < 0.001), THg in predatory macroinvertebrates (r = 0.71, P < 0.001), and THg in brook trout (r = 0.52, P < 0.01) were observed. Stream-water microbial communities within the Deltaproteobacteria also shared a positive correlation with FTHg and to the number of well pads, while stream pH (r = -0.71, P < 0.001), fish biodiversity (r = -0.60, P = 0.02), and macroinvertebrate taxa richness (r = -0.60, P = 0.01) were negatively correlated with the number of well pads within a watershed. Further investigation is needed to better elucidate relationships and pathways of observed differences in stream water chemistry, biodiversity, and Hg bioaccumulation, however, initial findings suggest Marcellus shale natural gas exploration is having an effect on aquatic ecosystems.