Geochemistry of Hydric Soil Salinity in a Recharge‐Throughflow‐Discharge Prairie‐Pothole Wetland System

Abstract

AbstractThe saturation‐paste extract (SPE) chemistry of samples collected in the wet‐meadow and shallow‐marsh zones of seven North Dakota wetlands was related to SPE electrical conductivity to investigate the development of hydric‐soil salinity. Study wetlands represent a local, depression‐focused groundwater‐flow system. Recharge wetlands recharge the groundwater whereas discharge wetlands receive the majority of their water as groundwater discharge. Throughflow wetlands receive water from as well as yield water to the system. Development of soil salinity generally followed the Hardie and Eugster model of closed‐basin brine evolution, which considers the composition of solutions undergoing evaporation to be the result of chemical changes imposed by the successive formation of evaporite minerals. Hydric soils of recharge wetlands were nonsaline and free of calcite (CaCO3) and gypsum (CaSO4·2H2O). The chemistry of these soils results from evapotranspiration, recharge hydrology, ionic mobility, and exchange relationships. Increases in SPE Mg2+, Na+, and SO2‐4 dominance in more saline throughflow and discharge wetlands are caused by calcite and gypsum precipitation, with the former controlling alkalinity and the latter Ca2+ concentrations. At high salinities produced by concentration through freezing, mirabilite (Na2SO4·10H2O) crystallizes and controls Na+ levels, resulting in hypersaline solutions enriched in Mg2+ and SO2‐4. Additional variation in the patterns of salinity development can be explained by dominance of recharge over discharge, mixing with fresh or chemically discrete water, and valence dilution effects.