Salt Water Meets Fresh Water – Scientific Approach Meets Societal Needs

Coastal aquifer systems provide encompassing challenges when it comes to the sustainable use of water including the complex hydrological-hydrogeological interface between surface-subsurface regions and the complex interaction with the bordering sea. The understanding of water flow and solutes transport within and across this hydraulic-dynamic interface zones is essential for water resource management. The quantification of all relevant water cycle components in the context of fresh water availability studies and the evaluation of impacts of climate change and sea level rise are current scientific challenges. Sea level rise is creating additional pressures by increasing salt water intrusion, an increase in the likelihood and adverse impacts of flood events and the artificial drainage of freshwater in low-lying areas. In addition, rising freshwater demand combined with the effort of designing a nutrient migration policy augments the pressure to act in many coastal regions.

This paper presents a regional groundwater availability study located in the northwestern part of Germany on the East Frisian Peninsula, the Sandelermöns region, which covers an area of about 1,000 km², including pumping wells of waterworks, area of saline groundwater and intensive agriculture. The project goal is to fill the existing gap between process understanding by high complex physically based modeling and water resource management by improved communication between relevant stakeholders.

The assessment of the freshwater availability is covered by two numerical 3d regional density-driven groundwater flow models (d³f++, FEFLOW), suitable for coastal lowlands and heterogeneous aquifers, with an open interface to the deterministic hydrological water budget model (PANTA RHEI). The paper discusses first the design configuration of the hydrological model and the numerical groundwater models, including the reconstruction of the regional hydrogeological model, the parametrization of the model and especially the determination of initial and boundary conditions with aero electromagnetic data sets. The paper gives an overview of the 3d density-driven model calibration and validation results and emphasizes the highly complex effects of climate change scenarios in combination with demand scenarios.

In the second part, the paper presents the development and application of the dialog platform CAM (Coastal Aquifer Management) which uses multi-criteria decision analysis techniques (MCDA) to strengthen transparency and objectivity in decision-making processes and encourage communication between decision-makers. The idea of CAM in combination with numerical modeling foregrounds the development of “best management practices” based on physical process understanding and the implementation of action plans through institutional capacity and stakeholder mobilization.