3D characterization of the subsurface redox architecture in complex geological settings

Nitrogen (N) leaching caused by agricultural activities is one of the major threats to the aquatic ecosystems and public health. Moving from the agricultural soils through the subsurface and reemerging to the surface water, N undergoes various biogeochemical reactions along pathways in the subsurface, which occur heterogeneously in space and time. Thus to improve our understanding on the fate and distribution of N in the aquatic environment, detailed knowledge about the subsurface hydrogeological and biogeochemical conditions, especially the redox conditions, are essential. In this study, 3D information of the redox conditions termed the redox architecture was investigated in two Danish catchments with intensive agriculture underlain by glacial deposits. Towed transient electromagnetic (tTEM) resistivity was interpreted which reveals the subsurface geological structures at a few hectare scale. These geophysical data were integrated with sediment and water chemistry for the redox architecture interpretations. The top soils of both catchments are characterized as clay-till, but the tTEM showed that the subsurface hydrogeological structures are distinctively different. We identified three types of redox architectures in the studied 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. The baseflow N load at the catchment outlets reflect the contributions of N via oxic pathways through the complex redox architectures of the subsurface. We conclude that in some landscapes, the redox architecture cannot be simplified as a single interface that roughly follows the terrain; hence, thorough investigations of the structural heterogeneity of the local redox architectures will be necessary to improve simulations of N evolution along pathways and quantifications of N attenuation under various mitigation scenarios.