Geochemical processes conceptual models for reactive transport in soil and groundwater / Концептуальные модели геохимических процессов для реактивного переноса в почве и подземных водах

In the field of groundwater numerical flow modelling has become a standard tool for scientific and also for most of the practical problems. The modelling software has matured during the past 10 years from being user-unfriendly with complex interfaces to a powerful, easy-to-use, windows-based system. For porous media flow in the saturated zone, the basic principle pertinent to all common programmes like MODLFOW, FEFLOW etc. is the solution of the linear DARCY- and continuity-equations. The numerical problems associated with the limitations in discretisation known in earlier days are not a serious burden any more because of the high-speed PCs with hundreds of Mbytes memory available nowadays. Therefore, groundwater flow modelling is not any more a technical challenge but – and this has not changed throughout the years – it remains a conceptual challenge. Furthermore the subsurface heterogeneity still represents a major challenge for the simulation of real world systems. Flow directions, flow velocities and consequently concentration values will depend on the adequate description of the subsurface heterogeneity.  Compared to flow models, geochemical reaction models can be more complex in terms of the processes and parameters involved. These models allow to describe quantitatively numerous reaction paths which occur in natural aquatic systems. The user may deal with a variety of dissolved substances in seepage and groundwater as well as numerous solid substances forming the aquifer material – and all these substances may react in various ways based on characteristic reaction kinetics or reaction rates. In short, one may have to deal with a complex system of interacting processes and a large number of parameters at the same time. The real challenge of geochemical modelling is therefore: how can one reduce the complexity of real world aquatic systems in such a way that it can be adequately described with the models available taking also into account our limited understanding of the system. The modelling software available today, e.g. the programme PHREEQC, is user-friendly and offers a large variety of reaction types as well as large data bases for the reaction constants. Consequently, the (experimental) data requirements needed for the understanding of a real system and also for the prediction of future system states are usually quite high – not to mention the fact that a considerable number of the geochemical processes are not yet fully understood. <...>