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Computer simulation of subsurface drip irrigation using coalbed methane produced waters

Development of coalbed methane (CBM) as an energy resource typically requires extraction of large volumes of water from underground coal seams. In the semiarid Powder River Basin of Wyoming and Montana, some of the CBM water is put to beneficial use through use for irrigation. Careful management is necessary due to elevated conductivites (2-3 mS/cm), high sodium adsorption ratios (SAR = 20-50) and sodium-bicarbonate composition of the water. One management strategy is deep subsurface drip irrigation (SDI), where drip tapes are placed ~92 cm below the surface in conjunction with a deep-rooted crop like alfalfa. CBM water is acidified to reduce alkalinity prior to application and is applied to fields year-round.

We are using computer simulations to help understand and predict effects of chemical and physical processes as native and introduced salts mix in soils of SDI fields. Spatial and temporal distributions of water and solutes in the soil are being simulated by using the computer program VS2DT. Geochemical modeling with the program PHREEQC suggests dissolution of native gypsum (0 to 4 wt. %) provides a short-term source of Ca to mitigate adverse effects of Na in soil solution. Detrital and pedogenic carbonate minerals (1 to 6 wt. %) should provide longer-term sources of Ca and Mg. Soil pCO2 influences the concentration of Ca via calcite solubility and will influence the SAR of soil solution and resulting soil structure. Model predictions are being evaluated using field data from moisture and conductivity sensor arrays and soil pCO2 measurements, as well as laboratory data on soil chemistry, mineralogy, and texture. Computer simulations can help predict the long-term fate of introduced solutes and the functioning of similar SDI systems in other settings or climates.