GEOCHEMICAL ANALYSIS OF THE TONGUE RIVER AND ASSOCIATED DRAINAGES IN THE POWDER RIVER BASIN: ANALYZING THE INFLUENCE OF NATURAL PROCESSES AND COALBED NATURAL GAS PRODUCTION

The Tongue River flows north out of Wyoming’s Big Horn Mountains and Powder River Geologic Basin into Montana. It is a major water source for agriculture and several small communities in Wyoming and Montana. The Tongue River and associated drainages pass through areas of coalbed natural gas (CBNG) production in both states. There are concerns that water discharged during CBNG production in Wyoming is degrading Tongue River water quality. Specifically, elevated total dissolved solid concentrations (TDS) have been observed in February and March north of the Wyoming state line. This research used stable isotopes of carbon, as well as oxygen and hydrogen and water chemistry analysis from environmental and produced water samples, to characterize the geochemical properties, identify the origins of the early spring TDS spike, and to assess the effects of CBNG production on water quality in the Tongue River drainage.

Water samples were collected in the fall and the spring from the Tongue River drainage, CBNG wells, CBNG outfalls and reservoirs, and a spring. Tongue River samples are Ca-HCO₃-type water, whereas associated intrabasinal drainages are Na-SO₄-type and Ca-HCO₃-type water. Low TDS (179 to 580 mg/L) are measured in drainages that have montane headwaters, while intrabasinal sourced waters exhibit a high TDS (1,698-3,205 mg/L). During high flows in the spring, intrabasinal drainages recorded higher levels of TDS along the entire sampled section. Isotopes of oxygen and hydrogen recorded relatively strong evaporative effects in the fall samples in most surface water samples.

All CBNG produced water is Na-HCO₃-type. Produced water samples have TDS concentrations that range from 595 to 2,584 mg/L. Isotopes of dissolved organic carbon (DIC) of most of the produced water samples are enriched, indicating the influence of bacterial methanogenesis. DIC was most enriched in samples near the state line and east of Sheridan.

Water chemistry analysis shows that natural geochemical processes in intrabasinal drainages are responsible for the elevated TDS during high flow months as they overprint montane drainages. No environmental samples show DIC enrichment, indicating that CBNG production is not impacting Tongue River water quality.