FLUSHING OF CHLORIDE AND NITRATE FROM SEDIMENTS UNDERLYING A COAL-BED METHANE PRODUCED-WATER IMPOUNDMENT, POWDER RIVER BASIN, WYOMING

Development of coal-bed methane in the Powder River Basin has increased substantially in recent years. Water produced with the gas is commonly placed in surface impoundments where it is allowed to evaporate and infiltrate. As of January 2007, permits had been issued for more than 4000 impoundments in the Wyoming portion of the Basin. A study was conducted on changes in water and sediment chemistry as water from an impoundment infiltrated the subsurface. Sediment cores were collected prior to operation of the impoundment and after its closure and reclamation. Suction lysimeters at three depths below the base of the impoundment were used to collect water samples. Pre-impoundment profiles of chloride and nitrate in sediments displayed patterns typical of those found in arid and semi-arid regions of the western U.S., with bulges of high concentration at depths of 2 – 4 m. Chloride storage in the 8 m of unconsolidated sediments ranged up to 6,500 kg/ha; nitrate storage ranged up to 8,500 kg (as N)/ha. Chloride and nitrate concentrations in lysimeter water samples indicated a flushing over time: initial increases in concentrations were followed by declines and ultimately by concentrations that were relatively steady over time. Post-impoundment profiles indicated that, contrary to inferences based on lysimeter samples, large amounts of chloride (61% of original mass) and nitrate (15% of original mass) remained in the sediments after 10-months of impoundment operation. These data and water samples from observation wells indicate that nitrate is more mobile and more readily flushed from the sediments than chloride. An estimated 12,300 kg of chloride (9,600 kg from the sediments and 2,700 kg from the impoundment water) and 13,500 kg nitrate (as N) (all from the sediments) were flushed into ground water at the study site. If sediments at the 4,000 permitted impoundment locations within the Powder River Basin have similar amounts of stored chloride and nitrate, over 48 million kg of chloride and 52 million kg of nitrate (as N) could be released to ground water in the Basin. These results demonstrate that changing land use (or changing climate) in arid and semi-arid regions can produce substantial loadings of chloride and nitrate to ground water. The dynamics of the flushing of these constituents from the sediments are not yet fully understood.