Concepts f or transboundary groundwater management in a region of extensive groundwater use and numerous contaminated sites

The Rhine gravel aquifer in the Basel area (northwestern Switzerland) extends to both France and Germany. Since Basel developed into a major center for the chemical and pharmaceutical industry in the 19 th century, vast areas in this region were or were likely to have been contaminated. In addition, there are abandoned sites of small enterprises and numerous areas (fillings of former gravel pits) on adjacent French and German territory which are probably also contaminated. The aquifer is used by numerous municipal and industrial water suppliers. Two case studies are presented that illustrate the need for transboundary groundwater management concepts in the region. Whereas the first case study illustrates short-term impacts on groundwater resources during a major suburban development project, the second case study shows long-term changes of groundwater flow regimes and the regional distribution of contaminants. For both case studies it is shown that river-groundwater interaction along the Rhine is an important element of the regional groundwater flow regime. It further can be demonstrated that considerable risk, with regards to the mobilization of contaminants, can be caused by changes in regional scale groundwater flow regimes together with changed hydraulic boundary conditions. The change in groundwater flow regimes and the reversal of flow lines may lead to the contamination of areas that were formerly not or only weakly polluted. These areas suddenly may lie within the capture zones of municipal or industrial groundwater wells or within the groundwater drainage of construction sites. Such risks of contamination require the development of concepts and methods for groundwater protection and management. A prerequisite for groundwater protection and management is a good knowledge of the spatiotemporal processes of regional scale groundwater flow regimes, which requires appropriate modeling and monitoring. This allows the evaluation of the impacts of planned changes at an early stage and to develop suitable groundwater management systems.