Abstract
The long-term fate of agricultural nitrate depends on rapid subsurface transfer, denitrification and storage in aquifers. Quantifying these processes remains an issue due to time varying subsurface contribution, unknown aquifer storage and heterogeneous denitrification potential. Here, we develop a parsimonious modelling approach that uses long-term discharge and river nitrate concentration time-series combined with groundwater age data determined from chlorofluorocarbons in springs and boreholes. To leverage their informational content, we use a Boussinesq-type equivalent hillslope model to capture the dynamics of aquifer flows and evolving surface and subsurface contribution to rivers. Nitrate transport was modelled with a depth-resolved high-order finite-difference method and denitrification by a first-order law. We applied the method to three heavily nitrate loaded catchments of a crystalline temperate region of France (Brittany). We found that mean water transit time ranged 10–32 years and Damköhler ratio (transit time/denitrification time) ranged 0.12–0.55, leading to limited denitrification in the aquifer (10–20%). The long-term trajectory of nitrate concentration in rivers appears determined by flows stratification in the aquifer. The results suggest that autotrophic denitrification is controlled by the accessibility of reduced minerals which occurs at the base of the aquifer where flows decrease. One interpretation is that denitrification might be an interfacial process in zones that are weathered enough to transmit flows and not too weathered to have remaining accessible reduced minerals. Consequently, denitrification would not be controlled by the total aquifer volume and related mean transit time but by the proximity of the active weathered interface with the water table. This should be confirmed by complementary studies to which the developed methodology might be further deployed.
Highlights
Intensive agriculture which has developed since the 1960's has caused eutrophication in aquatic environments (Steffen et al, 2015; Withers et al, 2014)
We argue that dealing with these issues requires (1) data constraining the partitioning of groundwater transit times, (2) to define the most important catchment properties and (3) time scales that they control in the aim to (4) reproduce catchment nitrate dynamics and be able to predict concentration trends
Based on the validated models, we identify the main aquifer properties controlling the nitrate dynamics in terms of hydrogeological behavior, reactivity and geological structure
Summary
Intensive agriculture which has developed since the 1960's has caused eutrophication in aquatic environments (Steffen et al, 2015; Withers et al, 2014). Nitrogen excess led to dramatic green algae proliferations having ecosystem, sanitary and economic repercussions (Galloway et al, 2008; Kronvang et al, 2005; Ménesguen and Salommon, 1988). These problems have raised public awareness and led to regulations to reduce the nutrient load in water bodies (Boers, 1996; European Comission, 1991). Since the 1990's, the agricultural inputs of nitrogen have decreased in many European regions (Abbott et al, 2018; Aquilina et al, 2012; Kronvang et al, 2008; Poisvert et al, 2017). The fate of the missing nitrogen (input minus river export), which is either stored or removed, is uncertain and blurred by other uncertainties such as data uncertainties and the imbrications of both spatial and time scales (Breemen et al, 2002)
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