The development of complex decision support model systems for the analysis of regional water policies for regions with intense socio-economic development effecting and being affected by the water resources system is of increasing importance. One of the most illustrative examples are regions with open-pit lignite mining. 

Such model systems have to be based on appropriate submodels e.g. for water quality processes. The paper describes submodel for groundwater and surface water quality with special regard to open-pit lignite mining regions. We consider the discharge of acid ferruginous water into rivers as having the most important impact on water quality in open-pit lignite mining areas. One goal of the model system is the choice of the necessary degree of purification for mine water treatment plants, taking into account self-purification in rivers and remaining pits as well as the water quality demand of down-stream water users. 

Based on comprehensive water quality models, the development of which is described in the paper, the possibilities for the derivation of reduced models are described. Those model have been elaborated for groundwater, as the source of pollution, mine water treatment plants as control units, river sections with an intake of acid ferruginous water, and remaining pits, which can also serve as effective control units. 

Related with each other, these models form the complex system model, a system of differential equations. They were numerically solved. The computer program is included in the paper.