Hydrogeochemistry of the unsaturated zone of a salt flat in Hudspeth County, Texas
Chapman, Jeannette Elise Burgen
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The playas of the Salt Basin in Trans-Pecos Texas are natural laboratories for the study of the hydrodynamic, hydrochemical, and sedimentologic properties of the unsaturated zone under the conditions of evaporation from a shallow water table. Water beneath the salt-flat surface moves upward from the saturated zone, through a thick capillary fringe, to the unsaturated zone where it is removed by evaporation. Daily temperature fluctuations change soil suction values and seasonal variations in temperature alter the thickness of the capillary fringe. There is little change in the chemical composition of the pore water as it moves from below the water table to the capillary fringe because the filled pore spaces of the capillary fringe prevent evaporation from taking place. However, an enrichment in the heavy isotopes of hydrogen and oxygen in the groundwater, as compared to area precipitation, suggests that evaporation may have occurred earlier along the flow path. As water moves from the top of the capillary fringe into the unsaturated zone, evaporation in the partially-filled pore spaces increases the total dissolved solids content. According to the increase in chlorinity, the brine has lost over 60% of its original volume by the time it has moved to within 20 cm of the surface. Evaporation in the unsaturated zone further enriches the brine in deuterium and oxygen-18. Gypsum precipitation in the unsaturated zone depletes the shallow pore water in calcium and sulfate, relative to chloride, and forms white patches, enterolithic bands, and discontinuous lenses of pure gypsum. The sediments are made almost entirely of gypsum and dolomite. The high magnesium-to-calcium molar ratio in the brines and the poorly ordered nature of the dolomite mud in the sediment column indicate that the salt-flat dolomite formed by the alteration of a calcium carbonate precursor. A lack of lateral continuity in sediment structures and a change in sediment character from massive below the water table to laminated above indicate that the shallow salt-flat sediments were formed by vadose-zone processes rather than by sedimentation in an ancient lake.