Mechanisms of nitrate transfer from soil to stream in an agricultural watershed of French Brittany

In French Brittany, water pollution with nitrate due to intensive agriculture has become one of the major environmental concerns. In this article, the nitrate, sulfate and chloride concentrations from the groundwater and the stream of a first-order agricultural watershed, are analyzed to infer the mechanisms responsible for the distribution and transfer of nitrate within the watershed. The aquifer is constituted by three layers: the thin soil cover, the weathered shale and the fissured shale. The weathered shale groundwater appears to be a large reservoir of nitrate in the watershed. Indeed the amount of nitrate is estimated at about 450 kg N ha(-1), 5 to 9 times the total annual nitrate flux in the stream. In the upslope zones, this groundwater exhibited high nitrate concentrations (up to 138.4 +/- 10.5 mg NO(3)(-) L(-1)), which decreased along the flow paths towards the stream (77.1 +/- 13.8 mg NO(3)(-) L(-1)). Unlike nitrate, sulfate concentrations showed an increase from uphill to downhill (from 6.1 +/-0.8 to 12.5 +/-5.4 mg SO(4)(2-) L(-1)) with little change in chloride concentrations. These patterns are presumed to result from upward flows from fissured shale groundwater where denitrification by oxidation of pyrite occurs with sulfate as end product. A scheme of nitrate transfer is proposed where stream discharge would result from the mixing of three end members which are: uphill weathered groundwater, deep groundwater and water in the uppermost soil horizons of the bottomlands. Temporal variability of nitrate concentrations in base flow reflects changes in the relative contribution of each end member.