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Stream Nitrate Variations Explained by Ground Water Head Fluctuations in a Pyrite-Bearing Aquifer

UMR INRA-ENSA, Sol-Agronomie-Spatialisation, 65 rue de Saint-Brieuc, CS 84215, 35042 Rennes Cedex, France

View larger version (37K): [in a new window]   Fig. 1. Sketched map showing location of Le Moulinet catchment and study site. The term D denotes the drain outlet, while S denotes the sources of the stream. (a) Ground water level survey and sampling site with wells under the stream bed (WS1 and WS2) and on the bank (WB). (b) Details of the ground water level device.

View larger version (50K): [in a new window]   Fig. 2. (a) Daily rainfall. Variations of stream level and weathered schist ground water head in (b) the well under the stream bed (WS1) and (c) the well on the bank (WB).

View larger version (36K): [in a new window]   Fig. 3. (a) Daily variations of stream levels (thick, solid line) and water-table levels (thin, solid line for under the stream bed [WS1 and WS2] and dashed line for on the bank [WB]) during winter high-flow events. (b) Daily variations of water-table level under the stream bed (WS1) during summer.

View larger version (34K): [in a new window]   Fig. 4. Bimonthly variations of NO3– and SO42– concentrations in stream water (--) and weathered schist ground water in the three wells (WB, --; WS1, --; WS2, --).

View larger version (17K): [in a new window]   Fig. 5. Increase of H4SiO4, Ca2+, Mg2+, K+, and Na+ concentrations and decrease of pH versus enrichment of SO42– in the weathered schist ground water at a drain outlet at different dates in late 1996.

View larger version (46K): [in a new window]   Fig. 6. Seasonal rainfall and potential evapotranspiration (PET) with variations of stream discharge, NO3–, and SO42– contents in stream water based on twice-monthly monitoring between 1995 and 2000.

View larger version (26K): [in a new window]   Fig. 7. Daily rainfall and Cl–, NO3–, and SO42– variations in stream water.

View larger version (27K): [in a new window]   Fig. 8. Plot of NO3– versus SO42– showing variation in stream water samples based on twice-monthly monitoring between 1995 and 2000, and some source water samples in April, June, and September 1995.

View larger version (56K): [in a new window]   Fig. 9. Schematic diagrams showing the different hydrological contributions to stream flow (weathered schist ground water, shallow ground water, and sources) and their effects on nitrate and sulfate levels in the stream water at each season. The dotted line represents water table elevation. The variable relative contributions of weathered schist and shallow ground waters to the stream flow are symbolized by variable thicknesses of black arrows. The size of NO3– and SO42– symbols varies with nitrate and sulfate levels in the stream water at each season.