Numerical model schematization of a complex hydrostructural Cretaceous groundwater basin for the purpose of protection zone evaluation

One of the major projects in regional hydrogeological investigations in Poland is a multi-annual cycle of studies on the implementation of programs and documentation in relation to the establishment of protection areas for the Major Groundwater Basins (MGBs). Depending on the size of the area and complexity of the selected aquifer system, the work on the numerical model becomes adequately demanding. The model must in this case be designed as a three-dimensional, multi-layered, taking into account the role of a near the surface aquifer in the potential rate of migration of contaminants. It is particularly difficult to design models of the basins located in the older Mesozoic hydrogeological structures of dual-porosity characteristic, covered by Neogenic and Quaternary sediments. One of these basins is MGB No. 317 (ed. Kleczkowski 1990) identified within the northern Sudetic trough. The boundaries of the MGB are associated with the occurrence of structural aquifer limits, including the relatively large area of outcrops of the upper Cretaceous sediments with a narrow zone of Triassic outcrops. The paper presents the main assumptions of the model schematization, especially regarding the parameterization of hydrostructural conditions in integration with geoinformatic tools and MODFLOW modules. This study is focused on the problems associated with the proper schematization of the multilayer groundwater system on a regional scale, particularly with regard to the first aquifer and its hydraulic contact with the Mesozoic water-bearing horizons of the MGB. Model simulations, together with a semi-analytical analysis of the rate of flow in the vadose zone, finally allowed to determine the protection area of the MGB that is presented