Soil-Water Responses to Land Application of Coalbed Natural Gas Co-Produced Water as Measured by Chemical Constituent and Stable Isotope Analysis

Production of Coalbed Natural Gas (CBNG, which is technically methane gas) involves removal of water to reduce the pressure within coal seams that allows methane gas migration. Water coproduced from coal seams during natural gas extraction can be an environmental concern in the Powder River Basin (PRB) of Wyoming. Past research has shown that application of CBNG water to irrigating crop and forage lands changed soil chemical and physical properties and exceeded salinity and sodicity limits for sensitive plants. In order to better understand how PRB soils and natural waters are impacted from land application and/or discharge of CBNG waters, chemical constituent and stable isotope analyses were conducted to determine soil-water responses. The concentration and ¹³C isotopes of dissolved inorganic carbon (DIC) in CBNG water were significantly different with natural stream waters. After reacting CBNG waters and natural stream waters with PRB soils, results of solution concentrations and ¹³C isotopes of DIC showed a clear decreasing trend in ¹³C of DIC values over time. Based on carbon dioxide evolution, the mechanisms proposed for decreasing DIC are addition of biogenic CO₂ and the dissolution of carbonate minerals. These are the two major sources of carbon contributing to DIC in natural ground water. Both processes result in lower ¹³C of DIC values when compared to CBNG water alone. These results suggest that CBNG waters are chemically and isotopically different than natural surface waters and that water chemistry and carbon isotopes change over time when reacting with PRB soils and atmosphere. Current work is focusing on the mechanisms causing solution isotopic changes.