HYDROCHEMICAL VARIATION IN A GROUNDWATER DISCHARGE FEN

In a ground-water discharge fen that supports species-rich herbaceous vegetation, the chemistry of the shallow ground water was monitored over two distinct periods. The fen is situated on a thick sandy aquifer in a region that is known for its acid soils. An irrigation ditch supplies the aquifer with alkaline water just upstream of the fen. Three water types of different origin contribute to the hydrochemical pattern in the en. Recently infiltrated acid ground water is present on higher grounds, neutral-to-alkaline irrigation water discharges at the upper fen margin, and deep ground water that is rich in iron discharges in the lower parts. The soil chemistry and species distribution are in accordance with this pattern: rich fen at the upper margin and poor fen in lower parts. However, we detected considerable hydrochemical variability due to varying influence of water types and temporal acidification of the shallow ground water. The latter leads to anionic dominance shifts from HCO3- to SO42- and the release of base cations in the soil solution. This hydrochemical variability is an additional source of environmental heterogeneity and may thus be partly responsible for the species richness.