Soil water flow and irrigated soil water balance in response to Powder River Basin coalbed methane product water
Buchanan, Margaret MacNeill
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A repacked soil columns experiment and a series of computer soil water balance simulations were conducted to examine potential impacts of coalbed methane (CBM) water from Montanaαs Powder River Basin (PRB) on soil water flow and water balance in PRB soils. CBM water is often high in sodium, which may separate soil clay particles, particularly after soil exposure to low-salinity rainfall or snowmelt, and when soils contain expansible smectite clay minerals. Aggregates in soils exposed to sodic water may swell and slake, and clays and other fine particles may disperse, clogging soil pores and slowing or preventing soil water flow. In the soil columns experiment, A and B horizon materials from sandy loam, silt loam, and clay loam soils were pre-treated with water having salinity and sodicity typical of PRB CBM water or of Powder River (PR) water currently used for irrigation in the basin. Tension infiltrometer measurements were used to determine infiltration flux, first using pre-treatment water, and subsequently deionized (DI) water, simulating rainwater. Measurements were compared by pre-treatment water, horizon, and soil type. Under pre-treatment water testing, the sandy loam and clay loam soils pre-treated with CBM water exhibited smaller infiltration flux values than when pre-treated with PR water. Only the sandy loam soil showed a greater decrease in infiltration flux with DI water on soils pre-treated with CBM relative to PR water pre-treated soils. There was no difference in infiltration flux decrease with DI water between A and B horizon soils, or between smectite and non-smectite soils. The soil water balance numerical simulations modeled potential effects of sodic irrigation waters on sandy loam, silt loam, clay loam and silty clay PRB soils under sprinkler or flood irrigation, during one growing season. Baseline soil water retention functions were constructed for the five soils, and adjusted via trends identified in the literature to create five additional functions for each soil, simulating exposure to five increasingly sodic irrigation waters. Simulation results showed greater impact of sodic irrigation under flood than sprinkler irrigation. The fine sandy loam and silty clay loam soils exhibited the fewest changes in water balance partitioning, while the silt loam and silty clay soils showed the greatest changes, especially in increased runoff and reduced transpiration.