| Although the production of Coalbed Natural Gas (CBNG) is variable, a large amount of produced water continues to be brought to the surface. The produced water can be very useful in the water-limited regions of Wyoming, but beneficial uses may be hindered by potential water quality problems. To assess these problems a water quality monitoring study began in 1999. Nine years of water data from CBNG outfalls and discharge ponds was measured over the last eleven years and used for trend analyses. The CBNG produced water was measured on-site for pH, electrical conductivity (EC), oxidation-reduction potential, dissolved oxygen, and temperature. The water samples were later analyzed for alkalinity, Ca, Na, Mg, K, Fe, Al, Cr, Mn, Pb, Cu, Zn, As, Se, Mo, Cd, Ba, B, Cl, SO4, NO3, and PO4. In addition to the trend analyses of CBNG produced water, pond sediment samples were collected over the last four years. Sediment samples were analyzed with TCLP (Toxicity Characteristic Leaching Procedure) for As, Ba, B, Cr, Cu, Mn, Mo, and Se.Trends in CBNG outfalls are not always the same in CBNG discharge ponds: environmental factors play an important role in the water quality of these produced waters. pH is not significantly changing in outfalls of the Belle Fourche River, Cheyenne River, Little Powder River, or Powder River watersheds, but the pH in the Tongue River watershed outfalls is increasing by 0.18 per year. pH in the discharge ponds is increasing by 0.13 per year in all watersheds. Sodium adsorption ratio (SAR) showed no significant increasing or decreasing trends in outfalls or discharge ponds. Alkalinity is significantly decreasing in the outfalls and discharge ponds of the Belle Fourche River, Cheyenne River, and Little Powder River watersheds, and is increasing significantly in the Powder River and Tongue River watersheds. Iron, copper, and chromium concentrations are decreasing in all outfalls of all watersheds. Iron concentrations in the discharge ponds are increasing in the Belle Fourche River, Cheyenne River and Tongue River watersheds, and are decreasing in the Little Powder and Powder River watersheds. Copper and chromium concentrations are decreasing in all watersheds.Overall trend analyses suggest that CBNG outfall produced water in all watersheds of the Powder River Basin meet beneficial use criteria for aquatic life, livestock and wildlife watering, and irrigation except for SAR and to some extent EC. The discharge ponds across all watersheds meet all criteria for all uses except for pH, SAR, and to some extent EC for irrigation, aquatic life, and livestock and wildlife watering. Arsenic concentrations in discharge ponds also exceed aquatic life standard of 7 mug/L in the Little Powder River and Powder River watersheds. All other water quality components (e.g. Ca, Mg, Na, K, NO3, PO4, SO4, Cl) and trace metals (e.g. Fe, Al, Cr, Mn, Pb, Cu, Zn, Se, Mo, Cd, Ba, B) meet criteria for all common beneficial uses such as, livestock and wildlife watering, aquatic life and irrigation. The results of trend analysis and sediment TCLP analysis discussed in this study could help the CBNG industry, ranchers and landowners, and the state and federal agencies manage CBNG produced water. |
