CHEMICAL PROCESSES IN INFILTRATED RIVERBED SEDIMENTS

Redox and other chemical processes were investigated in situ in the se diments of the River Glatt, Switzerland, and were qualitatively reprod uced in laboratory column experiments. At the study site, river water infiltrates continuously through the sediments of the riverbed into th e groundwater aquifer. In the summer, a temperature induced seasonal c ycle was observed with concentration minima of oxygen and nitrate and maxima of dissolved organic carbon (DC), phosphorus, manganese, and ir on. The seasonal changes in the concentrations of these elements and c ompounds were due to an enhanced oxidation of organic matter in the se diments, during the summer. This reaction released DOC and phosphorus and lowered the redox potential within the sediments of the riverbed. The lower redox potential enabled a bacteria-mediated dissolution of m anganese and iron (hydr) oxides within the first few centimeters of th e infiltrated sediments of the riverbed. The release of manganese led also to a significant increase of its dissolved concentrations in the groundwater close to the river bank. On the basis of the observed reac tions, a redox potential of less than or equal to-100 mV was estimated in the interstitial water of the sediments during summer conditions. Significant quantities of manganese and iron were also released in the spring during a period of enhanced calcite dissolution due to CO2 pro duction by higher biological activity.