Monitoring geochemistry of CBNG produced water outfalls, disposal ponds, and sediments in Powder River Basin, Wyoming

Coalbed natural gas (CBNG) extraction in the western United States has expanded in the last decade to help supplement the increasing demand for energy. Wyoming's Powder River Basin (PRB), Wind River Basin, and Green River Basin hold approximately one-third of the recoverable CBNG reserves in the US. The PRB is estimated to hold approximately 31.7 trillion cubic feet (TCF) of recoverable CBNG and is most heavily developed (De Bruin et al., 2004). The byproduct of extraction of CBNG is large quantities of produced water, which can be disposed of as discharge into stream channels or disposal ponds, atomized, treated, or reinjected.;The present research is built upon previous CBNG produced water studies and continued with further monitoring of produced water geochemistry changes in the PRB. Long-term monitoring studies are useful for developing optimum uses for CBNG produced water and protecting natural recourses (e.g., soils, plants, and surface and groundwater) in the PRB. The specific objectives of this study were to: (1) continue to monitor CBNG outfall and disposal pond water in the PRB established by McBeth et al., (2003a) and Jackson and Reddy (2007a) for two more years; (2) determine geochemistry of CBNG outfalls and disposal pond water; (3) investigate changes in pH, SAR (sodium adsorption ratio), and trace elements concentrations over time in CBNG disposal ponds; (4) determine possible leaching of trace elements from the CBNG disposal pond sediments; and (5) evaluate potential beneficial uses of CBNG disposal pond water in the PRB, Wyoming.;The CBNG outfall water, disposal pond water, and sediment samples were collected from five sub basins of PRB including Cheyenne River (CR), Belle Fourche River (BFR), Little Powder River (LPR), Powder River (PR), and Tongue River (TR) in 2006 and 2007. Water samples were analyzed for pH, EC (electrical conductivity), TDS (total dissolved solids), major cations, anions, and trace elements. Sediment samples were analyzed for trace elements using toxicity characteristic leaching procedure (TCLP). Geochemical data analysis was performed using MINTEQA2 model to determine speciation, complexation, and mineral saturation processes. Statistical analysis was conducted to determine differences among sediments in watersheds and generate potential descriptive models.;Research results suggest the pH of the CBNG produced water outfalls ranged between 7 and 8 across the PRB. However, the pH of the corresponding CBNG disposal ponds was higher, between 8.4 and 9.4. This increase in pH was attributed to degassing of CO2 from disposal ponds and the calcareous nature of the PRB soils. The EC also increased from outfalls to disposal ponds due to the dissolution and introduction of soluble salts from surrounding soils and by environmental processes such as evaporation. The CBNG produced water in outfalls was dominated by Na+ and alkalinity concentrations. Most of the major elements examined (Ca, Mg, Na, and K) in this study increased from outfalls to disposal ponds, except Ca2+. In some of the disposal ponds (BFR, LPR) Ca2+ concentrations decreased substantially, which is attributed to the precipitation of calcite (CaCO3). Most of the trace element concentrations (both cationic and anionic) in CBNG outfalls and corresponding disposal ponds were low. However, anionic trace element concentrations (e.g., As, B, F, Mo, and Se), even though were low, were higher in disposal ponds compared to CBNG outfalls, which is due to high pH and desorption from oxide and hydroxides. Barium and Mn concentration in CBNG disposal ponds decreased substantially by precipitating as carbonate minerals into the sediments. In contrast, Al and Cu concentrations was higher in disposal ponds compared to CBNG outfalls in the PRB due to the formation of Al and Cu anionic complexes in the relatively high pH environments of the disposal ponds.;Sodium Adsorption Ratio (SAR) were higher in 2007 than in 2006 in outfalls of CR and PR watersheds, whereas SAR remained constant or slightly increased in outfalls in 2007 compared to 2006 in BFR and LPR watersheds. The SAR values in CBNG disposal ponds decreased in CR and LPR watersheds in 2007 compared to 2006. The BFR and PR watersheds also showed similar results for SAR values in the disposal ponds. Overall, results suggest a decreasing trend in SAR between 2006 and 2007) in all watersheds, except for TR. Overall higher SAR values in CBNG disposal pond compared to outfalls were attributed to a combination of factors such as increases in Na and decreases in Ca concentrations due to the precipitation of calcite.;Among As, B, Ba,Cr, Cu, Mn, and Se, trace elements tested, only Ba and Mn appear to leach from pond sediments. Barium leachate concentrations did not differ among PRB watersheds in either year. Conversely, Mn leachate concentrations did not differ among PRB watersheds in 2006 but did differ among watersheds in 2007. Barium concentrations in sediment leachates can be described from the dissolved Ba concentration, temperature, and pH of the disposal pond water as well as year the samples were taken. The Mn concentrations in sediment leachates were determined from dissolved Mn and alkalinity concentrations in the disposal pond water. The CBNG disposal pond sediment leachate trace element concentrations studied in this research were well below the TCLP contaminant limits set by the EPA.;A review of 7 years of water quality monitoring data including this study for CBNG produced water in disposal ponds suggests that in all watersheds pH commonly exceeded the limit for irrigation water use, wildlife and livestock watering, and aquatic life use. However, over time we expect a decrease in pH due to reentry of atmospheric CO2 (g) into the disposal ponds. In some watersheds, such as PR and TR, SAR exceeded the limit for irrigation water. In these watersheds treatment of CBNG produced water with clinoptilolite, a locally available zeolite, or electrodialysis reversal (EDR) process may help lower SAR and maximize the beneficial use of CBNG produced water for irrigation.;Major (e.g., Ca, Mg, K) or trace element (e.g., Cr, Fe, Ba, Cu, Mn, Zn, B, Cl, NO3, PO4, SO4, As, and Se) concentrations, except for Al and F, in CBNG produced water in disposal ponds meet water quality criteria for all uses (e.g., irrigation, wildlife and livestock, and aquatic life) in all watersheds in PRB. Further long-term monitoring of CBNG outfalls, disposal ponds, and sediment leachates in the PRB for geochemical processes will I help develop trend analysis to predict toxicity and leaching potential of trace elements from these disposal ponds as well as developing optimum uses for CBNG produced water in the PRB.