G.H.P. Oude Essink

Saltwater intrusion in 3D large-scale aquifers: a dutch case

Abstract Saltwater intrusion in a three-dimensional large-scale coastal aquifer in the northern part of the province Noord-Holland, The Netherlands, is investigated. The computer code MOCDENS3D, which is the code MOC3D of Konikow et al. (1996) but adapted for differences in density, is used to model the displacement of fresh, brackish and saline ground-water in this hydrogeologic system. It appears that a severe salinisation already occurs. This process is initiated by the reclamation of the (low-lying) polders during the past centuries. Though seepage quantities decreases in many polder araes due to an increase in salinity, the salt load increases significantly.

Groundwater salinisation in the Wadden Sea area of the Netherlands: quantifying the effects of climate change, sea level rise and anthropogenic interferences

Hydrogeological research in coastal areas has gained considerable attention over the last decades due to increasing stresses on fresh groundwater resources. Fundamental groundwater flow and solute transport analyses remain essential for a concise understanding of the governing processes that lead to salinisation of fresh groundwater resources. However, the challenge of modern research is the application and quantification of these processes in real world cases. In this context, deltaic areas are amongst the most difficult study areas as they often have a complex groundwater salinity distribution. The Wadden Sea area in the northern part of the Netherlands is an example of such an area. We quantified salt water intrusion and salinisation of groundwater flow systems in two representative case studies in the Wadden Sea area, using the density dependent groundwater flow and transport code MOCDENS3D. The results indicate that sea-level rise and autonomous processes will cause severe salinisation in the future, especially in the low polder areas close to the sea. In addition, we show that enhanced land subsidence due to salt exploitation accelerates this process. Salinisation can be mitigated to some extent by raising surface water levels in polders and by creating saline groundwater collection areas that maintain a low controlled water level.

Monitoring and simulation of salinity changes in response to tide and storm surges in a sandy coastal aquifer system

Tidal dynamics and especially storm surges can have an extensive impact on coastal fresh groundwater resources. Combined with the prospect of sea-level rise and the reliance of many people on these resources, this demonstrates the need to assess the vulnerability of coastal areas to these threats. In this study, we investigated the impact of tides and storm surges on coastal groundwater at a pilot location on the Dutch coast (viz., the Sand Engine). To monitor changes in groundwater salinity under a variety of conditions, we performed automated measurements with electrical resistivity tomography for a period of 2 months between November 2014 and January 2015. The obtained resistivity images were converted to salinity images, and these images served effectively as observations of the impact of tidal fluctuations, salt- water overwash during storm surges, and the recovery of the freshwater lens after land-surface inundations. Most of the observed changes in groundwater head and salinity could be reproduced with a two- dimensional variable-density groundwater flow and salt transport model. This shows that groundwater models can be used to make accurate predictions of the impact of tides and storm surges on fresh ground- water resources, given a thorough understanding of the (local) system. Comparisons of measurements and model simulations also showed that morphological changes and wave run-up can have a strong impact on the extent of land-surface inundations in (low-elevation) dynamic coastal environments, and can therefore substantially affect coastal fresh groundwater resources

Frequency-domain Helicopter-borne EM Survey for Delineation of the 3D Chloride Distribution in Zeeland, the Netherlands

In the project FRESHEM Zeeland the entire Province of Zeeland in the south-western part of the Netherlands was surveyed using frequency-domain helicopter-borne electromagnetics (FDHEM). The airborne survey of more than 9000 line-km was conducted in 2014-15 by the German Federal Institute for Geosciences and Natural Resources (BGR). Together with the Dutch partners Deltares and TNO, an approach has been developed to translate the FDHEM data into a full 3D mapping of the chloride concentration of an area of about 1800 km². Verification with an independent dataset showed that groundwater salinity can be accurately calculated based on FDHEM measurements and a stochastic lithological model. Using indicator kriging as interpolation method turns out to be suitable to construct a 3D voxel model, revealing high resolution spatial patterns of groundwater salinity.