Seasonal biogeochemical cycles in riverborne groundwater

Abstract

The behavior of dissolved (<0.45 μm) inorganic species and changes in relevant properties of anthropogenically polluted river water were investigated during infiltration and movement in a hydraulically connected saturated aquifer. Water from the river and from several sampling wells was analyzed over 5 years for temporal and spatial chemical changes.At the early stage of infiltration, a drastic decrease in pH and in the concentrations of O2 and NO3− occurs. The gradients in these properties are most significant in the interstitial water of the river sediments. They are the result of the degradation of aquatic biota. The amount and grain morphology of river sediments varies seasonally and depends also on water flow conditions.Temperature-related variations in microbiological activity in the river water and in the sediments of the riverbed induce pronounced annual cycles in the aquifer for several of the investigated properties. Each summer, under anoxic conditions, manganese (hydr) oxides dissolve, and trace metals (Cu, Zn, Cd) are mobilized. The main sources for manganese and other trace elements were found within the river sediments; degradation of algae and other aquatic biota and dissolution of calcite contribute to the observed effects. Higher winter concentrations of O2 and NO3− result in precipitation of Mn and other redox sensitive elements in the aquifer. Zinc and Cd are retarded by interactions with the aquifer material, whereas Cu is mobile, probably as an organic complex.