Quantification of geochemical fluxes in a river-recharged aquifer
in the "Oderbruch" Region, eastern Germany
With an area of approximately 800 km², the "Oderbruch" represents Germany's largest polder region. The region has been artificially drained during the past 250 years. Today, a steep hydraulic gradient between the water table of the river Oder and the aquifer exists and a dyke prevents the agriculturally highly used area from flooding. The river water infil-trates laterally into the shallow, confined aquifer. A major part of the percolating water is discharged by the main drainage ditch which is running parallel to the dyke. As a result, the infiltration process is very complex and has a strong vertical component. As the groundwater travels through the anoxic aquifer, a number of redox-controlled chemical changes and reactions occur and the water becomes increasingly more reduced. The "Oderbruch" has been chosen by the German Research Foundation (DFG) for the study of redox processes in nature within the Priority Program "Geochemical processes with long-term consequences in anthropogenicly affected seepage water and groundwater".
The aim of the study is to develop a detailed understanding of both hydraulic and geo-chemical processes taking place during bank filtration. 2D and 3D transient models will serve as a base to supply precise knowledge of groundwater flow paths for high and low river water levels. Along 1D flow paths transport and reaction processes of the affected redox species will be modelled. Finally, turnover rates of the infiltrating water and its dis-solved solid content will be determined and quantified. In previous projects, only the very shallow groundwater has been considered (maximum depths of 7 m). Within the current study, mainly the deeper parts of the 20-30 m deep aquifer will be focused on. The 1 km² exemplary field site "Bahnbrücke", located adjacent to the river Oder, was cho-sen for detailed investigations. It consists of 24 conventional piezometers in varying depths plus 3 multi-level wells. The area is intensively surveyed and monitored. Relevant hydrau-lic and hydrochemical parameters are analysed regularly. In addition, investigations were carried out further inland up to a distance of 4 km from the Oder.
The sandy aquifer gets progressively coarser towards its base where it is margined by a boulder marl. The aquifer is topped by an alluvial loam. Due to the strong current, the river base is highly permeable and the hydraulic contact between river and groundwater is un-restrained. The base of the main drainage ditch is very heterogeneous. The sands are partially covered with up to 2 m of clay and silt. Depending on the permeability of the drain, between 20 and 80 % of the bank-filtered water are discharged. Of the electron acceptors present in the river water, O2 and NO32- are reduced within the first few dm of underground passage. As a result of the commencing Mn-oxide reduction, Mn2+ content of the water increases up to a distance of 150 m from the Oder, where reduction of Fe-oxides takes over. It appears that the increase of Fe2+ in the water is accompanied by a decrease of Mn2+, probably due to the enhanced precipitation of carbonates (MnCO3,rhodochrosite), which is encouraged when the dissolution of Fe-oxides produces a more alkaline and thus CO32- richer solution. Up to a distance of 1 km inland the water does not pass the ferrous zone and SO42-reduction does not commence.