Geochemical changes along a river-groundwater infiltration flow path: Glattfelden, Switzerland

Seasonal changes in the river concentrations of key components (carbonate alkalinity, total inorganic carbon, O 2, NO 3 −, NH 4 +, Ca, Mg and SiO 2), trace metals (Mn, Fe, Cu, Zn and Cd), and microbial activity are reflected in systematic changes in mass transfer along a saturated river-groundwater infiltration flow path. Organic matter degradation is the predominant process during the early stages of groundwater recharge: further changes along the flow path can be explained by equilibration with calcite, CO 2 gas exchange, and mixing between freshly infiltrated water and deep groundwater. Mn exhibits enhanced mobility during periods of denitrification, presumably due to decreased redox potential and associated Mn oxyhydroxide reduction. Fe concentrations decrease across the river-groundwater boundary, probably due to retention of Fe-rich colloids. The reductive dissolution of Fe oxides is not observed. Copper concentrations increase in concert with organic matter degradation in the early stages of groundwater recharge, and persist at elevated levels for at least the first 100 m of groundwater flow. Zn concentrations are virtually unchanged during early recharge, but decrease further along the flow path, probably due to sorption on aquifer material. In contrast, Cd concentrations increase in concentration in the early stages of recharge and decrease further downflow.