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, saltwater 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 groundwater 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. Plain Language Summary Most coastal regions in the world rely on groundwater as their main source of fresh water. The availability of fresh water in many coastal communities is threatened by population growth, sea-level rise, and increases in the number and intensity of storm surges. This threat raises the importance of an optimal management of fresh groundwater in coastal areas to limit the salinization of fresh groundwater resources. However, the management of coastal groundwater is often complex and data is limited. In this research we have successfully used electrical resistivity tomography to measure effect of tides and storm surges on fresh groundwater at a pilot location on the Dutch coast (called the Sand Engine). In addition, most of the measured changes in groundwater head and salinity could be imitated with a numerical groundwater model. This proved that with a thorough understanding of the local system, groundwater models can be used to make accurate predictions of the impact of tides and storm surges on fresh groundwater resources.