COUPLING GEOELECTRICAL METHODS WITH GEOCHEMICAL MODELING TO UNDERSTAND SALT CYCLING IN PRAIRIE WETLANDS

Prairie wetlands are ubiquitous features of the Great Plains of North America, and provide important habitat for amphibians and migratory waterfowl. Complex hydrological controls maintain brackish surface pond salinities in closed-basin wetlands, which are sensitive to regional dry/wet climate cycles. In order to better understand hydrogeological controls on salt storage and release in these systems, we did a DC marine resistivity survey on a closed-basin prairie wetland (P1) in the Cottonwood Lake Study Area (Stutsman County, ND).

Inversions from the field survey matched well with laboratory tests on wetland sediment cores, and delineate ~ 8 m of clay-rich wetland sediments (bulk EC = 1.0 – 2.0 mS/cm) over less-conductive glacial till (bulk EC = 0.5 – 1.0 mS/cm). We imaged conductive “saline lenses” (bulk EC = 2.5 – 3.5 mS/cm) at 3 – 4 m depths below the sediment-water interface in the center of the wetland and near it’s perimeter within the currently ponded area. Geochemical modeling shows that a Mg-SO4 brine could be achieved by evaporating 90% of wetland surface water, which would result in a ~20 mS/cm fluid that could theoretically raise the conductivity of the soft sediments we sampled to that of the “saline lenses” we imaged in our survey. Such brines could be occluded during drying of the wetland during drought, when desiccation fractures and terrestrial plant roots greatly increase the vertical permeability of the wetland subsurface.