Longitudinal changes in potential toxicity of coalbed natural gas produced water along Beaver Creek in the Powder River Basin, Wyoming

In the Powder River Basin (PRB) in northeast Wyoming, coalbed natural gas (CBNG) is extracted from coal seams at depths of up to 500 m. Some concurrently produced waters are discharged into ephemeral drainages like Beaver Creek, a tributary to the Powder River. Total ammonia, CO 2, Na+ and HCO3- concentrations are high in waters from some of the deeper coal seams. I evaluated the fate and effects of ammonia in CBNG produced water discharged into Beaver Creek from August 2006 through March 2007. The study included (1) observation of fish and amphibians, (2) in-stream toxicity tests at 6 sites using caged fathead minnow larvae (FHM; Pimephales promelas), (3) concurrent ambient-pH and CO2 pH-controlled laboratory toxicity tests using the same site waters, (4) analyses of water quality parameters in the effluent and the receiving water, and (5) evaluation of ammonification processes occurring during transport from the stream to the laboratory. Although 96-h survival of FHM in-stream and in the laboratory was comparable to controls, 144-h survival was lower in some ambient-pH exposures in the laboratory as pH increased due to CO2 degassing. In Beaver Creek, pH gradually increased along a downstream gradient from initially circumneutral values, primarily due to CaCO3 buffering. Because ammonia decreased along the same gradient, presumably assimilated by plants and microbes, the potential toxicity also decreased. I conclude that CO2 pH-controlled toxicity test methods are preferred laboratory toxicity tests because this method prevents the degassing and artifactual increase in pH. Yet, based on the evaluation of ammonia changes in unpreserved CBNG effluent transported from the discharge point to the laboratory, I also conclude laboratory toxicity tests may be biased.