Nitrate Degradation in the Aquifer of an Agricultural Catchment – An Integrative Modelling Approach

Abstract:

Agricultural activity has increased nitrate concentration in aquifers worldwide, which represents one of the major environmental challenges of our generation. Nitrate is highly mobile in groundwater and if transported to denitrifying environments (i.e. anaerobic areas with the presence of bioavailable organic carbon (basis for heterotrophic denitrification) or pyrite (basis for autotrophic denitrification)) degraded to nitrogenous gas. These areas are often small, but account for a high percentage of nitrate removal. Consequential groundwater flow, a nitrate supplier to these hot spots, influence significantly the fate of nitrate.

A hydro-geochemical modeling approach is used to demonstrate the relation between nitrate inputs and denitrifying services provided by catchment structure and flow dynamics. A developed three-dimensional numerical groundwater flow model is capable to map groundwater flow and visualize preferential nitrate flow paths in a 35 km² agricultural catchment, western France. Environmental proxies for microbial processes (natural isotopic abundance of nitrogen and oxygen) are used to identify denitrification processes in the aquifer. These information are combined with the flow paths obtained by the groundwater model in a post-processing step. An overall understanding of groundwater flow patterns and therefore nitrate input to denitrifying environments yield to better management decisions and predictions for nitrate attenuation.