Abstract

Coal bed natural gas (CBNG) product water is usually disposed into nearby constructed disposal ponds. Geochemistry of product water is not clearly understood. The objective of this study was to collect product water samples at outfalls and corresponding disposal ponds and monitor pH, electrical conductivity (EC), calcium (Ca), magnesium (Mg), sodium (Na), alkalinity, iron (Fe), aluminum (Al), chromium (Cr), manganese (Mn), lead (Pb), copper (Cu), zinc (Zn), arsenic (As), boron (B), selenium (Se), molybdenum (Mo), cadmium (Cd), and barium (Ba). From Na, Ca, and Mg measurements, sodium adsorption ratios (SAR) were calculated, and used in a regression model. Additionally, the mobility of trace elements in the disposal ponds was examined based on sediment fractionation studies. Sediment samples were separated into exchangeable, carbonate bound, iron-manganese oxide bound, organically bound, and residual mineral fractions to determinate the fate of arsenic (As), selenium (Se), barium (Ba), and iron (Fe). Outfalls and corresponding disposal ponds were sampled from five different watersheds including Cheyenne River (CHR), Belle Fourche River (BFR), Little Powder River (LPR), Powder River (PR), and Tongue River (TR) within the Powder River Basin (PRB), Wyoming from 2003 to 2005. Results suggest that outfalls are chemically different from corresponding disposal ponds. Sodium, alkalinity, and pH all tend to increase, possibly due to environmental factors such as evaporation, while calcium decreased from outfalls to associated discharge ponds due to calcite precipitation. Watersheds examined in this study were chemically different form each other and most discharge ponds within individual watersheds tended to increase in Na and SAR from 2003 to 2005. Most trace metal concentrations in the produced water increased from outfall to disposal pond except for Ba. In disposal ponds, Ba, As, and B concentrations increased from 2003 to 2005. Geochemical modeling predicted precipitation and dissolution reactions as controlling processes for Al, Cu, and Ba concentrations in CBNG produced water. Adsorption and desorption reactions appear to control As, Mo, and B concentrations in CBNG water in disposal ponds. Since discharge pond water was chemically changing as a function of watershed chemistry, I predicted SAR of discharge pond water using a regression model. The predicted discharge pond water results suggested a high correlation (R² = 0.83) to outfall SAR. Within the pond sediment, Fe and Ba concentrations did not appear to have an overall increase in any fraction among years. However, As and Se concentrations, though low, increased between years in the exchangeable and carbonate bound fractions. In the near future, As and Se concentrations may increase to levels that would induce chronic toxicity in wildlife and livestock. Coalbed natural gas pond sediments should be monitored for As and Se concentrations in the water soluble fraction periodically and at the termination of production to prevent potential damages to livestock and wildlife. Overall, results of this study will be useful for landowners, water quality managers, and industry in properly managing product water from natural gas extraction.