Soil chemical and physical changes resulting from irrigation with coalbed natural gas co-produced water: Effects of soil amendments and water treatments

Abstract

Significant quantities of water are being produced and discharged as a by-product of coalbed natural gas (CBNG) development in the Powder River Basin (PRB) of Wyoming and Montana. Elevated salinity and sodicity in CBNG water has the potential to increase soil salinity and sodicity. Soil salinization and sodification may adversely affect soil physical properties such as, structure, infiltration, permeability, and aeration, resulting in changes in nutrient supply, modification of the soil exchange complex with dispersion, and pH effects. A sodic soil has been shown to maintain good soil structure if the salinity level is maintained above the threshold electrolyte concentration (TEC). In this study, cropland soil and CBNG water were treated with gypsum and sulfur. Plots were monitored to evaluate the effects of gypsum and sulfur on pH, electrical conductivity (EC), sodium adsorption ratio (SAR), and sulfate (SO₄^2-) concentrations. Changes in soil chemistry due to the addition of different qualities of irrigation waters and different soil amendments were monitored using a split plot experiment. The CBNG water used for irrigation had an EC of 1.3 dS m^-1 and SAR of 24.3 mmol^1/2 L^-1/2. Baseline and post treatment soil samples were collected to a depth of 60 cm within each study plot, analyzed, and characterized for chemical parameters. Comparisons between baseline and post irrigation soil chemistry data after one season indicated treatment of irrigation water statistically increased (P<0.05) the rate at which Na ⁺ moved through the profile. The addition of gypsum and sulfur as a water treatment and a soil amendment was the most effective in maintaining low SAR at the soil surface. Both EC and SAR statistically increased with all treatments in the top two sampling depths (A and Bt₁ horizons). Additional soil samples were taken two years after CBNG water irrigation and compared to baseline and post irrigation data to evaluate changes in soil physical and chemical properties. Single ring infiltration experiments were also conducted within each plot to determine if infiltration rates were affected by water type and/or water and soil treatments. Decreases in SAR were observed in the A horizons for all water treatment and soil amendment combinations.