Shallow groundwater and soil chemistry response to 3 years of subsurface drip irrigation using coalbed-methane-produced waterRéponse de la chimie de la nappe superficielle et du sol à 3 ans d’irrigation souterraine au goutte à goutte avec de l’eau produite lors de l’extraction de méthane de charbonRespuesta del agua subterránea somera y de la química del suelo a 3 años de riego subsuperficial por goteo usando agua extraída para el gas metano de mantos de carbón浅层地下水和土壤化学对 采用煤层—甲烷产出水三年滴灌的响应Resposta química do solo e da água subterrânea a 3 anos de rega gota a gota de subsuperfície usando água produzida em camadas de carvão e metano

Disposal of produced waters, pumped to the surface as part of coalbed methane (CBM) development, is a significant environmental issue in the Wyoming portion of the Powder River Basin, USA. High sodium adsorption ratios (SAR) of the waters could degrade agricultural land, especially if directly applied to the soil surface. One method of disposing of CBM water, while deriving beneficial use, is subsurface drip irrigation (SDI), where acidified CBM waters are applied to alfalfa fields year-round via tubing buried 0.92 m deep. Effects of the method were studied on an alluvial terrace with a relatively shallow depth to water table (∼3 m). Excess irrigation water caused the water table to rise, even temporarily reaching the depth of drip tubing. The rise corresponded to increased salinity in some monitoring wells. Three factors appeared to drive increased groundwater salinity: (1) CBM solutes, concentrated by evapotranspiration; (2) gypsum dissolution, apparently enhanced by cation exchange; and (3) dissolution of native Na–Mg–SO4 salts more soluble than gypsum. Irrigation with high SAR (∼24) water has increased soil saturated paste SAR up to 15 near the drip tubing. Importantly though, little change in SAR has occurred at the surface.