USING VS2DT TO SIMULATE YEAR-ROUND, DEEP SUBSURFACE DRIP IRRIGATION WITH COAL-BED METHANE PRODUCED WATERS

Coal-bed methane (CBM) is a rapidly expanding energy sector in the United States. In Wyoming's Powder River Basin, development of this resource produces large volumes of water with sodium-bicarbonate chemistry and relatively high total dissolved solids. Deep (~90 cm) subsurface drip irrigation (SDI) is an emerging technology and has been pursued as a disposal option for CBM waters for several years. SDI is attractive because it offers the possibility of deriving beneficial use by growing water-intensive crops like alfalfa in a semiarid landscape. Simultaneously, solutes associated with the CBM water are potentially stored in the unsaturated zone. A concern with using SDI to dispose of saline water is salinization of surface soil and/or groundwater.

Our goal is to better understand and predict the fate of native and introduced solutes in SDI fields. Computer simulation is a tool we use to assess complex interactions between the numerous parameters and processes controlling transport of water and solutes. The introduction of irrigation water and solutes is controlled by the depth and spacing of injection points, as well as timing and rate of water injection. Seasonality and amount of precipitation influence the availability of low-solute water in near-surface soil. Evapotranspiration is the major sink for precipitation and irrigation water and is influenced by seasonal and inter-annual climate variability as well as vertical and horizontal crop root distribution. Water flow in soil is largely a function of soil texture and interactions between degree of saturation and hydraulic conductivity.

Multi-year simulations of water and solute movement through a two-dimensional vertical slice of an SDI field are being run using the USGS program VS2DT. The simulations are refined by comparing model output to laboratory data on samples of SDI field soils and measurements from an array of underground sensors. The striking two-dimensional patterns of water and solute distribution in SDI field soils provide good qualitative targets for assessing simulation accuracy. The resulting models should unravel some of the complexity of deep SDI using saline waters. Predictions could be made as to how such systems might be managed in different climates or settings to reduce the risk of soil salinization.