Soil solution and exchange complex response to repeated wetting-drying with modestly saline-sodic water

Coal bed natural gas (CBNG) extraction in the Powder River (PR) Basin of Wyoming and Montana produces modestly saline-sodic wastewater, which may have electrical conductivity (EC) and sodium adsorption ratios (SAR) exceeding accepted thresholds for irrigation (EC = 3 dS m(-1), SAR = 12 (mmol(c) l(-1))(1/2).As an approach to managing large volumes of CBNG-produced water, treatment processes have been developed to adjust produced water salinity and sodicity to published irrigation guidelines and legislated in-stream standards. The objective of this laboratory study was to assess acute and chronic soil solution EC and SAR responses to various wetting regimes simulating repeated flood irrigation with treated CBNG product water, followed by single rainfall events. Fifty-four soil samples from irrigated fields in southeast Montana were subjected to simulated PR water or CBNG water treated to EC and SAR values accepted as thresholds for designation of saline x sodic water, in a single wetting event, five wetting-drying events, or five wetting-drying events, followed by leaching with distilled water. Resultant saturated paste extract EC (ECe) and SAR of soils having < 33% clay did not differ from one another, but resulting ECe and SAR were all less than those for soil having > 33% clay. Repeated wetting with PR water having EC of 1.56 dS m(-1) and SAR of 4.54 led to SAR < 12, but brought ECe near 3 dS m(-1). Repeated wetting with water having salinity = 3.12 dS m(-1) and SAR = 13.09 led to ECe > 3 dS m(-1) and SAR near 12. Subsequent inundation and drainage with distilled water, simulating rainfall-quality leaching, reduced ECe and SAR more often in coarse-textured, high salt content soils than in finer-textured, lower salt content soils. Decreases in ECe upon leaching with distilled water were of greater magnitude than corresponding decreases in SAR, reinforcing supposition of sodium-induced dispersion of fine-textured soils as a consequence of rainfall following irrigation with water having salinity and sodicity levels equal to previously published thresholds.