Modeling Salt Redistribution in Response to Deep Subsurface Drip Irrigation with Coal-Bed Methane Produced Water.

Production of coal-bed methane (CBM) typically requires pumping large volumes of water out of underground coal seams.  In the Powder River Basin of Wyoming and Montana, some of the CBM water is being used beneficially for irrigation. Careful management of irrigation is necessary due to high solute content and the 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 prior to application, and applied to fields year-round. By injecting the water at greater depth than standard shallow (10 to 30 cm) SDI systems, the potential for evaporative salt accumulation near the surface is reduced. However, the redistribution of native and introduced salts is difficult to predict as the chemical and physical processes involved are complex. Geochemical modeling using PHREEQC suggests native sulfates (0 to 4 wt. %) are rapidly removed from zones close to irrigation tapes by dissolution. Detrital and pedogenic carbonate minerals (1 to 6 wt. %) dissolve and likely re-precipitate elsewhere in soil profiles. Soil pCO₂ limits the concentration of calcium in soil solution and should therefore influence soil exchangeable sodium percentage and soil structure. The spatial distribution of water and solutes in the soil zone is being modeled using the program VS2DTI. The models predict that root uptake is crucial for intercepting upward water movement that would transport solutes to near-surface horizons.  Transport to shallow depths is likely to result in evaporative concentration of solutes and deterioration of soil quality. Soil pCO₂ measurements and newly installed sensors for monitoring soil water content and electrical conductivity are providing data to evaluate model predictions. Validated simulations will aid in optimizing the use of SDI systems and forecasting their long term impact.