Mapping the fresh-saltwater interface in the coastal zone using high-resolution airborne electromagnetics

Abstract

A major part of the drinking water in the Netherlands is derived from groundwater stored in coastal and inland aquifers. The water occurs through natural processes, but in some areas artificial infiltration of lake and river water in the coastal sand dunes is necessary to support the demand for clean drinking water in the densely populated country. In fact, artificial infiltration along the Dutch coast has been common practice for decades and is an essential part of the drinking water supply. The sustainability of this approach demands detailed knowledge of the fresh–saltwater balance to avoid salinization of the aquifers. The availability and quality of the groundwater are typically estimated using groundwater models, which can be used to forecast the behaviour of groundwater systems to external stresses, such as climate change. The reliability of groundwater models hinges on measurements such as hydraulic heads, stream flow rates, permeability of the subsurface and in coastal regions the boundary conditions for the coastal edge of the model. Here, a limiting factor is often the lack of offshore data since it is logistically difficult and costly to drill in the seabed. Airborne electromagnetics are increasingly being used to support groundwater management through high-resolution largescale mapping of aquifer properties. The method provides strong sensitivity to important hydrogeological units such as the fresh-saltwater interface and clay layers (which often constitute the base of the aquifers). Hence, AEM has been widely used to address issues such as mapping saltwater intrusion (Gunnink etal., 2012; Jørgensen et al., 2012) and aquifer delineation (Chandra et al., 2016; Schamper et al., 2013). One of the key advantages of the method is that it is airborne, allowing areas, which would otherwise be difficult to access, to be mapped in a cost-effective manner. As demonstrated in the present study, the fresh-saltwater interface and coastal boundary conditions of the groundwater model can be determined by combining onshore and offshore airborne electromagnetics.