Trace elements and macroinvertebrate community structure across a gradient of shale gas extraction

Natural gas is an important energy source in the United States and its importance is expected to increase through 2035. Shale gas is becoming the predominant form of natural gas. Extraction of shale gas by hydraulic fracturing could deliver trace elements to draining streams by sediment disturbance during pad formation, spilled or leaked hydraulic fracturing fluids and produced waters, and increased road traffic. Base-flow water column trace elements and macroinvertebrates were collected in spring 2011, 2012, and 2013 across streams in the Fayetteville Shale. Additionally, sediments and three different macroinvertebrate taxa representing three different functional feeding groups were collected in spring 2013 for trace element analysis. Trace elements related to natural gas activity were of most concern in 2011, when infrastructure development and traffic peaked. Aluminum, chloride, lead, nickel, sodium, and potassium were all positively related to natural gas activity. Nickel and lead exceeded aquatic life criteria at all high gas activity sites. Chloride, potassium, and sodium were positively related to natural gas well density across repeated sampling dates, suggesting hydraulic fracturing fluids may be entering draining streams. Macroinvertebrates were negatively related to increased trace elements, but benthic substrate composition was a more important factor influencing macroinvertebrate community composition. Macroinvertebrate relationships with natural gas activity were most notable in 2011, and less so in 2012 and 2013 when infrastructure development slowed. In 2013, trace element concentrations were low in the water column, sediments, and macroinvertebrates. Despite the low concentrations, differences were seen in trace element concentrations among a scraper, collector-gatherer, and predator. The scraper and/or collector-gatherer had greater concentrations than the predator for 11 of the 19 trace elements measured. Trace element concentrations in the sediment and macroinvertebrates were generally not related to trace element concentrations in the water column. These results suggest natural gas extraction has the potential to increase trace elements in draining streams and negatively affect macroinvertebrate communities.