The aims of the 3-year Swiss-Romanian project are to develop tools that allow the representation and efficient use of 3D geological and hydrogeological information in active or abandoned salt mining areas. The tools will form the basis for process models that are able to evaluate different scenarios to face problems related to land subsidence and aquifer contamination, and to develop sustainable development strategies for mining areas. The Ocna Mures mining area in Transylvania, which is continuously facing environmental problems such as surface collapse, land subsidence, and surface water and aquifer contamination from brines, is ideally suited to serve as an example for our approach. The rock salt of the salt mines has been explored since the Roman ages in open excavations and galleries until the 1970ies, and is still extracted up to this date within solution mining wells. A comprehensive database of the subsurface structure of sediments and exploration cavities, their geo-mechanical and geo-hydraulic properties, and hydraulic boundary conditions are prerequisite to develop innovative concepts for water resource protection and to manage landscapes in ancient salt mining areas. Our conceptual approach includes the following tasks:

1. To develop an adaptive data-management system: the Geodata_System will incorporate the different geological, geophysical, hydrogeological, hydrochemical and geotechnical information. The GIS-based system will provide the necessary data for modeling and scenario development, will assist the visualization of results, will evaluate hazard areas and will, therefore help to find strategies for future development of surface and subsurface resource.
2. To collect geophysical and hydrogeological data in the Ocna Mures mining perimeter: geoelectrical resistivity surveys at the scale of the mining field will be performed to identify the morphology of the salt-sterile contact and the underground voids up to a depth of about 30 m. The presence of voids is to be verified through the analysis of existing survey drills. 10 observation wells will be tested by experimental pumping tests to derive geohydraulic parameters of the aquifer. Water samples will be taken for hydro-chemical analyses from the wells and the surface waters of the River Mures and the saline lakes.
3. To develop a geological 3D model of the mining area: the regional 3D model includes all relevant available geological horizons in the area and can be refined to local geological 3D models covering the planning scale of subprojects. The geological models can be transformed to coupled 3D groundwater flow and transport models, as well as to geomechanical models to investigate specific processes.
4. To simulate flow and transport modeling for scenario development: the 3D numerical code FEFLOW will be used for the regional model of the mining area, which simulates flow and transport and accounts for density effects due to salinity variations. In model parts where the density contrast is very high (up to saturation), we suggest to use Mixed Finite Elements for the fluid flow problem and a combination of Discontinuous Galerkin and Multi-Point Flux Approximation methods for the transport.
5. To evaluate the hazard: the results from the proposed tasks will lead to an integrative evaluation of the hazards in the Ocna Mures mining area. They include the identification of areas where instability phenomena are likely to occur. Hazard maps will be embedded into strategies and planning procedures for future sustainable land development.

The project team consists of 8 scientists and two PhD students from both the Department of Geological Engineering at the University of Bucharest and the Applied and Environmental Geology at Basel University. The Romanian partner benefits from the experience of the Basel group on data management, field data assessment, and development of 3D geological and hydrogeological models in 2D and 3D. The Swiss partner benefits from the Romanian expertise in assessing geotechnical hazards, application of geophysical methods, and the evaluation of the feasibility of technical solutions.