Characterization of heterogeneous subsurface environments

Abstract

The demand for energy efficient and sustainable management of the water balance in mining areas is high. Groundwater recharge is one component of the water balance that is especially important for the forecast of the vertical position of the groundwater table, which is needed for the assessment of different mining water management strategies. The aim of the EGSIM project is to couple models of the unsaturated and saturated soil zone to simulate groundwater recharge in a process-oriented way. The common method of using temporal average values for groundwater recharge cannot be applied to the task of modeling future climate changes because it does not consider the intensity and variability of precipitation. However, since the development of the water balance for mining regions has to be investigated over several decades, changing climatic conditions and their influence on the groundwater recharge have to be regarded. Basis of the coupled model is the 3D groundwater simulation tool PCGeofim® (Mueller et al., 2003) which uses the finite volume method to simulate groundwater flow and transport processes in particular in mining regions. It is coupled with PCSiWaPro® (Graeber et al., 2006) which provides a transient upper boundary condition for PCGeofim®. PCSiWaPro® is a 2D model that simulates the water balance by using the Richard's equation for transient flow and solute transport processes in the unsaturated zone with regard to non-steady atmospheric boundary conditions. These atmospheric conditions are provided by applying a weather generator that uses Markov chain approximations to produce synthetic climate time series. In order to couple both models and to take into account the interactions between the unsaturated and the saturated zone an interface and exchange parameters as well as the processes that drive this exchange have to be defined. As the natural interface between both soil water zones is the groundwater table, it is adopted as the modeling interface into the coupled model and therefore has to be represented by both individual models. PCGeofim® provides values of the changing groundwater table as exchange parameters which are used for the lower boundary condition of PCSiWaPro®. In return the flux over this groundwater table is passed to PCGeofim®. The differing model dimensions (2D and 3D) and the different spatial and temporal scales are specific problems that have to be regarded for the coupling. A methodology is currently being developed that tackles these difficulties. Its goal is to couple 2D models of the unsaturated zone with intersections of PCGeofim®. Evaluations using lysimeter data are conducted to verify the modeled groundwater recharge fluxes with measured data.