Effects of coalbed natural gas development on fish assemblages in tributary streams in the Powder River Basin, Montana and Wyoming

Abstract

The Powder River Basin in Wyoming and Montana is undergoing the world's largest development of coalbed natural gas (CBNG) extraction. Potential exists for substantial effects on aquatic ecosystems because CBNG development involves production and disposal of large quantities of CBNG product water that differs from surface waters and alters natural flow regimes. In 2005 and 2006, I compared fish assemblages in streams with (treatment) and without (control) CBNG development, determined fish presence, growth, and survival in streams composed entirely of product water, and compared fish assemblages at multiple points above and below development to determine the effects of coalbed natural gas development on fish assemblages in the Powder River Basin. Some evidence suggested CBNG development had little or no effect on fish. For example, species richness and index of biotic integrity (IBI) scores were similar between developed and undeveloped sites, and no strong relationships existed between overall IBI scores or most IBI metric scores and the number or density of CBNG wells in a drainage area. Streams composed largely or entirely of product water were inhabited by reproducing populations of several species of fish. Other evidence suggested that CBNG may negatively affect fish assemblages over time. Conductivity was on average higher in treatment streams and was negatively related to biotic integrity. Bicarbonate, one of the primary salts in product water, appeared to be harmful to some species of fish. One salt-tolerant non-native species, northern plains killifish, was observed almost exclusively in treatment streams. The study was limited by a lack of pre-development data, unquantifiable product-water discharges, and because it was conducted during dry years. Potential effects of CBNG development may be more apparent during wet years when more sensitive fish assemblages are present. Monitoring efforts, development of a bicarbonate water quality standard, and efforts towards requiring complete product-water discharge reporting should continue.