Vadose-zone geochemistry of playa wetlands, High Plains, Texas




Romanak, Katherine Duncker

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Soil-gas beneath 3 playa wetlands was monitored June 1992-May 1995 and sampled from as deep as 45 feet within the >2OO-ft unsaturated zone. Playa soils, surface water, rainwater, and groundwater were also analyzed, along with subsurface temperature and pressure. The objective was to assess factors controlling vadose zone geochemistry in the playa basin slope, playa lake annulus (shoreline), and playa floor. CO₂ as high as 17%, CH₄ as high as 2.2%, and O₂ from 0 to 21% were measured during the study and indicate the playa subsurface is microbially active. Microbial CO₂ gas production is highest in the playa floor and in the annulus during high water levels where high water flux (indicated by low soil carbonate and chloride contents) and high soil organic carbon exist. Methanogensis occurs locally on the playa floor in the "transition zone" and may result from locally high carbon flux and subsequent increased oxygen demand that exceeds oxygen replenishment to the subsurface. Soil-gas CO₂ is distributed throughout the subsurface via dissolution in infiltrating water and through gas transport. Floor and annulus areas have high PCO₂s (> 2.5%) that react with soil carbonate to cause extensive CO₂ and calcite dissolution as indicated by CO₂:0₂ mole ratios < 1 and N₂ > 78%. Soil breathing also lowers subsurface CO₂ concentrations but supplies O₂ to subsurface microbes. Temporary increases in CO₂ and decreases in O₂ result from standing water in playas and wetting fronts produced by rainfall events. Subsurface pressure measurements indicate that infiltrating water lowers gas permeability and inhibits mixing of subsurface and atmospheric gases. Barriers to vertical atmospheric mixing and pressure gradients produced by CO₂ dissolution in the playa center also drive lateral gas transport back and forth between the playa slope and playa floor. These flux potentials are as large as diffusive flux potentials but at times act in different directions. Direction of advective transport fluctuate with changes in barometric pressure favoring transport from the slope into the playa during high barometric pressures and transport out of the playa toward the slope during low barometric pressures.