Stream nitrate variations explained by ground water head fluctuations in a pyrite-bearing aquifer

In the context of agricultural nitrogen excesses in northwestern France, pyrite-bearing weathered schist aquifers represent important hydrological compartments due to their capacity to eliminate nitrate (NO3-). Under oxygen-free conditions, nitrate is reduced simultaneously with the oxidation of pyrite leading to the release of sulfate (SO4/2-). The aim of the present study is to identify the hydrological conditions under which the weathered schist ground water influences the stream water chemistry, leading to a decrease in NO3- concentration. We measured the ground water head on a small catchment over weathered schist, near the bank and under the streambed, and analyzed the chemical composition of the ground water as well as the stream water on both seasonal and storm-event timescales. Using SO4/2- as a tracer of the weathered schist ground water, we showed that ground water inflow caused a decrease of NO3- concentration in the stream during the autumn as well as during storm events in spring and summer. In summer, the NO3- concentration was controlled by the sources of the stream, and in winter by the shallow ground water inflow. The effect of the weathered schist ground water on the NO3- depletion remained relatively limited in time. This effect persisted into late autumn as long as the NO3(-) -rich shallow ground water did not feed the stream. The duration and intensity of the effect would be extended by decreasing the shallow ground water inflow, which depends on climate as well as the presence of landscape features such as hedges and buffer zones.