GEOCHEMICAL EFFECTS OF SUBSURFACE DRIP IRRIGATION WITH COAL-BED METHANE PRODUCED WATERS

Coal-bed methane (CBM) accounts for 7% of natural gas production in the United States, but co-produces larger volumes of water than traditional natural gas development. In the semiarid Powder River Basin (PRB) of Wyoming and Montana, some CBM water is put to beneficial use through irrigation. CBM waters in the PRB typically have specific conductances of 2-3 mS cm^-1, high sodium adsorption ratios (SAR = 20-50), and are Na-HCO₃ dominated. This composition necessitates careful management of irrigation to avoid soil degradation. One approach is to distribute the water via subsurface drip irrigation (SDI) tapes that are placed ~92 cm beneath the soil surface and grow deep-rooted, perennial crops like alfalfa. CBM water currently used in these systems is acidified to reduce alkalinity prior to application and fields are irrigated year-round.

This investigation focuses on the effects of SDI in the PRB to predict the short- and long-term fate of native and introduced solutes. Computer simulations using the models VS2DT and PHREEQC were used to assess effects of unsaturated flow and geochemical reactions. Soil data from fields irrigated with CBM water for six years, and adjacent non-irrigated rangeland, were used to calibrate and assess the computer simulations. Native gypsum dissolution, calcite precipitation, and concentration of solutes by evapotranspiration are prominent geochemical processes in irrigated soils. Gypsum dissolution provides calcium, which lowers SAR values and reduces deleterious effects of sodium. Mass balance calculations based upon local precipitation, irrigation rates, and evapotranspiration estimates suggest that the unsaturated zone between the ground surface and ~13 m depth, or shallower water table, is at field capacity. Nonetheless, preliminary data indicate that 60% of sodium added by irrigation is retained between 60 and 300 cm depth and only 5% above 60 cm. Unsaturated flow simulations suggest that summer drying of surface soil, intense root uptake close to the drip tape, and continued irrigation during winter may reduce upward migration of sodium. As increasing demand strains freshwater supplies, SDI and similar technologies may provide beneficial use from marginal quality waters co-produced by energy resource development or other sources.