Characterizing the heterogeneity of the subsurface redox architecture in three dimensions using geophysics

Better characterization of the redox conditions in the subsurface is essential to improve our understanding of the distribution and behavior of contaminants in groundwater because most anthropogenic and geogenic contaminants are redox sensitive. The subsurface redox conditions are determined primarily through direct field observations, which provide point or 1-D profile information. The challenge is the upscaling of this information to the catchment scale in 3-D. Detailed knowledge of the hydrogeological structures, especially under spatially heterogeneous settings e.g., glacial deposits, is necessary to properly extrapolate the field observations. In this study, we attempted to determine the subsurface heterogeneity using a recently developed towed transient electromagnetic system (tTEM) in two agriculturally dominated Danish catchments underlain by glacial deposits. Modelled tTEM resistivities in 3D was used to visualize the geological structures and was then combined with redox information inferred from sediment color and groundwater chemistry. The tTEM resistivities successfully revealed complex geological structures of the near surface at the hectare scale. The redox conditions showed varying degrees of complexity related to the geological structures. Altogether, we defined three types of the subsurface redox conditions in 3-D, referred to as redox architecture, in our study catchments: 1) a planar redox architecture with a single redox interface; 2) a geological-window redox architecture with local complexity; and 3) a glaciotectonic-thrusted redox architecture with high complexity. Under the complex redox architectures, the groundwater chemistry was heterogeneous within short distances and did not evolve progressively vertically, implying complex flow pathways for contaminant transport as well.