Deep Subsurface Drip Irrigation with Sodic Coal-Bed Natural Gas Water in Northeast Wyoming.

A deep (92 cm) subsurface irrigation system is being used to derive beneficial use from water produced as a byproduct with coal-bed natural gas in the semi-arid Powder River Basin of Wyoming.   The produced water commonly has relatively low salinity and high sodium adsorption ratios, characteristics that can lead to degradation of soil permeability. Large volumes of water are produced year-round and the search for economical and environmentally safe disposal methods has been difficult. We studied soils irrigated by this system to understand resulting changes in water and soluble salt distribution.  The water is treated with sulfuric acid and then applied to hay crops year-round. We examined fields irrigated for six years and used computer simulations to better understand water movement and identify geochemical changes. Combined irrigation and precipitation exceed potential evapotranspiration by 300 to 480 mm annually. Excess water initially contributes to increased storage in the unsaturated zone, and eventually causes seasonal rises in the water table. Soil below the depth of the drip tubing contains relatively abundant native gypsum (2.4 ± 1.7 wt. %). Modeling shows that dissolution of the gypsum lowers the sodium adsorption ratio of acidified SDI water from 54 to 7 and increases the electrical conductivity from 2.1 mS cm^-1 to 4.1 mS cm^-1, thus decreasing the risk of clay dispersion and permeability loss. With sustained irrigation, gypsum can become depleted in the vicinity of the drip tubing, leading to increased soil-water sodium adsorption ratios (>14) and decreased electrical conductivity (3.2 mS cm^-1). Sodium in the irrigation water accumulates in the soil, but little is detectable at depths less than 30 cm, and only 8 to 15% of added sodium has accumulated above the drip tubing. Thus, deep SDI provides a means of deriving beneficial use from sodic waters with limited impact on soil properties.