SEAWARD CONTINUATION OF GROUNDWATER FLOW SYSTEMS ALONG THE COAST OF THE NETHERLANDS

Geophysical and hydrogeological methods were used to study the offshore continuation of groundwater flow conditions and submarine groundwater discharge along the coast of The Netherlands near the town of Zandvoort. A belt of dunes, several kilometers wide, borders the coast in the area. The dune system is used by the Amsterdam Water Supply for artificial recharge of surface water transported from the Rhine river and subsequent abstraction of the filtrated groundwater. The fresh water lens in the dune belt penetrates most of the underlying Pleistocene aquifer system to a depth of about 130 m. A phreatic and semi-confined aquifer can be distinguished, separated by a Holocene clay-rich unit at a depth of about -20 m MSL. First, onshore, vertical electrical sounding (VES) and time domain electromagnetic measurements (TDEM) were conducted along shore-perpendicular transects to characterize the lens geometry. Chloride data from observation wells were used to constrain the inversions. Additionally, hydraulic head monitoring allowed study of the tidal response of the semi-confined aquifer. Modeling yielded a storage coefficient S=6*10^-4, transmissivity T=1030 m²day^-1 and confining layer leakance L=1.5*10^-3 day^-1. Together with hydraulic head data, these findings suggest a sub-sea fresh water discharge from the confined aquifer of about 200 m²day^-1. Cone penetration tests for electrical conductivity (CPCT) and pore pressure were conducted on the beach down to the mean low water line. The measurement revealed a brackish phreatic aquifer and saline confining layer, underlain by a variable thickness (29-44 m) fresh-brackish water layer in the semi-confined aquifer.  Subsequently, offshore TDEM measurements were made using a floating, coincident, antennae/receiver loop, 50 m in diameter. Indications for fresh-water beneath the seafloor was found up to 1.5 km off the coast. Variable-density flow and transport modeling is being conducted to investigate if these data represent (quasi-)steady outflow conditions, or whether the offshore fresh water distribution reflects transient conditions associated with recent coastline migration and onshore activities in the dune area.