Groundwater flow and transport modeling will be carried out for all case study areas. An innovative modeling approach of applying models with gradual complexity and analyzing them as a model ensemble will be used. A multi-model ensemble approach is commonly used in climate models and in some hydrological modeling applications but is less common in groundwater modeling although the same set of conditions is given: high uncertainty of input data and, potential systematic errors by an inadequate conceptualization of the modeling problem. Therefore, this approach will be elaborated and proposed for GWS modeling as well in the MEDSAL Project.

The first modeling approach is based on analytical elements solutions of the differential equation (DE) for groundwater flow. Analytical elements are solutions of the DE for simplified boundary conditions and are available, and can be used to predict groundwater flow and transport for these simplified cases. We propose to use them as the first step of the analysis to define sensitive and suitable monitoring points and to define “red lines” for groundwater abstraction or to identify existing over-abstraction that will or could lead to GWS.

As a second model type compartment models or mixing cell models will be used. Compartment models, or mixing cell models (MCMs) have been developed and extended to non-steady conditions and are suited for regions with lack of physical data and limited hydrogeological conceptualization but available hydrogeochemical and environmental tracer data. Hydrogeochemical data are classified into end-members and the flows between these end-members solving the mass balance equations for water and solutes (including reactions and radioactive decay, if necessary) are identified by inverse modeling.

In cases where enough physical, conceptual and hydrogeochemical data are available, numerical groundwater flow models and transport modules will be applied. In line with the multi-model ensemble approach, numerical models will be compared to and validated jointly with AEM and compartmental models. Groundwater modeling will be carried out with an open-source, variable density, finite difference or finite element model depending on geological conditions. Variable density flow numerical models will be used to approach the physical phenomenon of salinization in more detail compared to simple constant density flow models, and furthermore, they will be used to assess the concentration of salts in pumped water and consequently to assign the use of water in each well.

Adar, E., & Kuells, C. (2002). MCMsf - Mixing-cell model for a steady flow MIG. Mixing-cell input generator: A short manual for installation and operation of MCMsf using the MIG mixing-cell input generator. In Use of Isotopes for Analyses of Flow and Transport Dynamics in Groundwater Systems (pp. 1–29). IAEA.