Excess nutrients from agriculture and urbanization have created a cascade of ecological crises around the globe. Nitrogen and phosphorus pollution have triggered eutrophication in ~80% of freshwater and coastal ecosystems, contributing to the freshwater biodiversity crisis and costing the global economy trillions every year. Much of the research on nutrient transport and retention has focused on surface environments, which are easier to access and biologically active. However, surface characteristics of watersheds such as land use and network configuration do not adequately explain variation in nutrient state observed in rivers, lakes, and estuaries. Recent research suggests that subsurface processes and characteristics may be more important in determining watershed-level nutrient fluxes and removal than previously understood. Here, we apply a recent ecohydrological approach called the HotDam framework to compare surface and subsurface removal of nitrate, a common inorganic form of nitrogen.
Results/Conclusions
Using a rich biogeochemical dataset from 13 wells and 12 streams in small watershed in western France, we tested the importance of residence time, biogeochemical reaction times, and flowpath in determining nitrate concentration and flux. Measurements were taken at various depths in the groundwater, as well as on the surface, to understand how microbial activity transforms nitrate. We saw evidence of denitrification as the dominant process in nitrate removal in the subsurface water environments. Our findings align with previous research that suggest each aquifer’s ability to remove nitrogen is highly variable, because of extreme variation in residence time and moderate variation in reaction time. Our data adds to this knowledge and suggests groundwater is biogeochemically more active and spatially variable but surface water is more physicochemically variable. Linking these components is critical to meeting water quality targets and ensuring water security in the Anthropocene.