Denitrification and mixing in a schist aquifer: influence on water chemistry and isotopes

Nitrate concentrations in groundwater close to the water table in the upper weathered zone of the Coët-Dan catchment (Brittany, France) reach 200 mg/l. With intensive agriculture covering 86.5% of the catchment area, the pollution results principally from the spreading of livestock manure. The rapid decrease of NO3− concentrations with depth in the fractured part of the aquifer is partly the result of dilution by older groundwater that was probably never polluted, as deduced from the isotopic data of water (3H, δ2H, δ18O) and sulphate (δ34S, δ18O) molecules. However, the extent of denitrification is demonstrated by N2 concentrations, as well as by mass balance calculations showing that denitrification may contribute to a loss of at least 50–70 mg l−1 of NO3− just below the upper zone of the aquifer. Several electron donors may be involved in the denitrification reactions; for example, ferrous iron and sulphides of the pyrite and organic matter, with at least the last two processes being aided by the presence of bacteria. Autotrophic denitrification is considered to be the predominant process in the unweathered schist, whereas heterotrophic processes may occur in the overlying weathered zone. Autotrophic reactions tend to liberate SO4, but concentrations of sulphate are probably controlled by precipitation of sulphate amorphous phases and also minerals such as jarosite.