Land application with saline-sodic coalbed natural gas co-produced waters in Wyoming's Powder River Basin: Impacts to soil and biological properties

Coalbed natural gas (CBNG) development in the Powder River Basin (PRB) of Montana and Wyoming is driven by the Nation's growing demand for energy. Wyoming's PRB had over 13,000 CBNG producing wells in 2004 with more than 50,000 future wells projected. The large volumes of saline-sodic co-production water extracted with CBNG in the PRB are anticipated to exceed 162,000 hectare-meter by 2020 and will require development of suitable water handling strategies. Land application with sprinkler irrigation systems is a common method for managing these waters. Studies examined various soil and native vegetation impacts resulting from up to five seasons of land application with saline (EC = 1.6 to 4.8 dS m-1) and sodic (SAR = 17 to 56 mmol1/2 L-1/2) CBNG water. Treated (irrigated) and representative control (non-irrigated) areas were established at several study sites and examined through the 2003-2005 field seasons. Soil and plant types, water application rates, and water and soil treatment strategies were variable across study sites so parameters from each treated area were compared directly to those from representative control areas. Soil texture, pH, EC, SAR, ESP and Darcy flux rates were measured at six depth intervals to 120 cm. Bulk densities were measured at three depth intervals to 30 cm. Surface infiltration rates were also examined. Multiple year applications of CBNG water reduced surface infiltration rates, produced consistent trends of increased soil EC, SAR and ESP at depths up to 120 cm and reduced Darcy flux rates to 120 cm. Differences were significant (p ≤ 0.10) at most depths on most sites. CBNG water applications also resulted in significant (p ≤ 0.05) increases in native perennial grass biomass production and cover (treated vs. control areas). Species diversity, measured by evenness, was reduced. Biological effects were variable and complex, reflecting site specific conditions and management strategies. These findings indicate concern for effective Na + and soluble salt leaching success with current management and treatment strategies. Degraded soil physical and chemical properties will require mitigation during reclamation efforts following cessation of CBNG water applications.