A 3D hydrogeochemistry model of nitrate transport and fate in a glacial sediment catchment: A first step toward a numerical model

Diffuse pollution of nitrate from agricultural fields is a critical environmental problem around the world. Sources and sinks of nitrate are heterogeneously distributed over various spatial scales, and the connectivity and transport pathways between them also change at different temporal scales. Therefore, understanding the impact of these variabilities in nitrate transport to the aquatic environment is fundamental for a correct numerical modelling of nitrate transport within a catchment. This study, hence, investigated controls on the spatiotemporal variability of nitrate in a glacial landscape and upscaled it to catchment scale by synthesizing geological, hydrogeochemical, and geophysical information. We found that different parts of the sedimentary succession define the locations of nitrate sinks in this catchment. Denitrification mainly may occur mainly around small patches of postglacial sediments on the outwash plain, which is the youngest formation covering the top layer of the catchment. In contrast, in older geological elements, which constitute the hill and the layers below the outwash plain, oxic, nitrate-containing groundwater was found, probably because of depletion of reduced compounds over the long exposure time. We also found that the boundary between these two formations may govern the seasonal shift of this oxic groundwater's connectivity to the stream consequently nitrate export from the catchment. This conceptual understanding of nitrate transport and sinks then was transformed into a 3D hydrogeochemistry model based on a high-resolution resistivity model of the catchment. We propose that such a basic understanding of how a catchment hydrogeochemically operates should be the first step toward setting up a catchment scale hydrological model with reactive N transport.