Role of water table dynamics on stream nitrate export and concentration in agricultural headwater catchment (France)

Abstract

The objective of this work is to identify hydrological processes controlling nitrate export and base flow concentration at the year scale in agricultural headwater catchment streams fed by an unconfined aquifer. The study is based on the hydrological and hydrochemical monitoring of the stream and shallow groundwater of three headwater catchments (0.1­5 km2) in Western France over three to five water years. Results show that at the year scale nitrate export from the three catchments is a transport-limited process. The stream nitrate flux depends on how much water flows in the stream and not on the distribution of the flow over the year. Seasonal variations of concentration over the water year were complex. Variations were different for a given year from one catchment to another, and also different for a given catchment from one year to another. We show that the seasonal variations are controlled by water table depth dynamics along hillslope associated with spatially distributed nitrate concentration in the groundwater. The groundwater displays high nitrate concentrations, from 6 to 22 mg, in upland and in the deeper zones of bottom lands. Persistence of such high concentrations results from the geochemical and mineralogical properties of the aquifers that consist of old, very oxidised and strongly weathered material, such that any primary electron donors have been leached. In riparian zones, concentrations are close to zero due to denitrification with oxidation of organic matter. In winter, stream nitrate concentration is controlled by the nitrate rich groundwater flow from upland. In summer, groundwater flow from upland decreases and stream concentration is controlled by bottom land hydrological and biogeochemical processes. The shift between winter and summer control depends on the water table dynamics along hillslopes. As long as the water table remains deep in upland, summer controls prevail. As soon as water table rises in upland, hydraulic gradient and groundwater flow from this zone increase, leading to an increase in the stream nitrate concentration. The shift from summer to winter control can be considered as the result of a connection between the upland nitrate rich groundwater and the stream, connection triggered by upland water table rise.