Geoelectrical Analyses of Sulfurous Wetland Sediments and Weathered Glacial Till in the Prairie Pothole Region

Abstract:

Millions of prairie wetlands occur in topographic depressions throughout the Prairie Pothole Region (PPR) of North America, an important ecoregion for amphibians and migratory birds. Climate is known to drive complex critical zone processes determining sulfur fate and transport in the PPR, but the specific mechanisms controlling the storage and release of salinity beneath the wetlands remain poorly understood. To help clarify this, we conducted a DC resistivity field survey of a closed-basin groundwater discharge wetland at the Cottonwood Lake Study Area, North Dakota; and collected wetland cores along one of the survey transects for laboratory analyses of resistivity, porewater/solid-phase geochemistry, and other physical properties.

Inversions of our field survey delineate two primary geoelectrical layers beneath the wetland: the top ~8 m of wetland sediments and weathered glacial till (ρ₂₅ = 4 – 5 Ω-m) overlying more resistive glacial till at depth (ρ₂₅ = 7 – 12 Ω-m). Conductive lenses (ρ₂₅ = 1 – 2 Ω-m) occur within the upper layer at 2 – 3 m depths in the center of the wetland and along a concentric band within the current ponded area, which corresponds to the location of the pond shoreline before extremely wet conditions in the 1990’s expanded the wetland. The resistivities of wetland core segments (ρ₂₅ = 2 – 7 Ω-m) match well with the upper layer inferred from the field survey, and show an inverse trend of bulk core to porewater resistivity for clay-rich intervals due to variations in moisture content. Our results demonstrate that geospatial patterns of subsurface salinity relate to wetland hydrodynamics during dry-wet climate cycles and should be considered when using geoelectrical methods to upscale geochemical measurements in PPR wetlands.