Results from Measurements and Modelling

A further model variation worked with adjusted pH. In detail pH is calculated according to Eq. (8) using ion concentrations, which partially stem from model simulation and are partially fixed. For example NH concentrations changing along the 

Redox fronts exist in aquifers and can be compared to the occurrence of redox fronts in marine and limnic sediments. However the spatial scale is quite different in groundwater compared with lake or river sediments. While sedimentologists usually report fronts in the scale of cm, in groundwater these move much further and are more extended. In the Oderbruch aquifer a steady peak of Mn could be observed in a travel distance of 360 m from the river. Fe increases along the entire cross-section. A front for Fe could exist even beyond the final position in about 1000 m linear distance away from the river. 

Transport and redox reactions can be modelled simultaneously in a numerical simulation. In order to abtain realistic results it is necessary to include geochemical expertise. In order to model the geochemistry in the aquifer a reactive transport model was set up. Starting from a preliminary state, in which only redox and precipitation were considered, finally inorganic carbon and acid based chemistry were included. 

From a theoretical point of view this means that kinetic as well as equilibriiun reactions had to be implemented in addition to the genuine transport processes as advection and dispersion. 

There still remain some open questions which could be mentioned partially only in this paper. One is to deal with the number of imknown parameters. The number of parameters in geochemical and biogeochemical models is often very high, so that even modem parameter estimation tools, like PEST, have problems to handle the problem. There are problems with the execution time of these models and with their convergence towards an optimal solution in the parameter space  

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