In-stream measurements at low-flow reveals transport and denitrification patterns in the sub-surface

Excess nitrogen in surface and groundwater, mainly in the form of nitrates, is a major concern for all stakeholders, because it leads to the degradation of drinking water resources and to the eutrophication of ecosystems. The export of nitrogen from inland to the coast is strongly determined by the transport and denitrification processes occurring in headwater catchments. Yet, most regulatory frameworks, such as the EU Water Framework Directive, impose the monitoring of medium-to-large rivers (> 100 km2) while headwaters, too numerous to be systematically monitored, are poorly understood. Headwater catchments are often characterized by strong connections between surface water and shallow aquifers, leading to a high impact of sub-surface processes on river water quality. Understanding the processes occurring in the sub-surface is thus necessary to predict river water quality, but it remains a major challenge because of the difficult access to groundwater. Here we propose a new approach to infer sub-surface processes in headwater catchments from in-stream measurements. In an agricultural catchment, we measured nitrate and silica along headwater streams fed by a crystalline shallow aquifer, during low-flow period. Silica was used as a proxy for water residence time. We observed several trends between the nitrate concentrations and the water residence times, interpreted as the result of distinct patterns of transport and denitrification in the sub-surface. Based on this case study, we propose a general framework to infer the processes occurring in the sub-surface from the resulting chemical trends observed in low-flow streams. Regarding the simplicity of the measurement method, this framework appears as a powerful tool for water management practices. Further studies in several areas in the world will allow to validate its broad applicability.