P.S. Pauw

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.

Complex conductivity of soils

The complex conductivity of soils remains poorly known despite the growing importance of this method in hydrogeophysics. In order to fill this gap of knowledge, we investigate the complex conductivity of 71 soils samples (including four peat samples) and one clean sand in the frequency range 0.1 Hz to 45 kHz. The soil samples are saturated with six different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m21, NaCl, 258C) in order to determine their intrinsic formation factor and surface conductivity. This data set is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in-phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This data set confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent.

Fresh Water Lens Persistence and Root Zone Salinization Hazard Under Temperate Climate

In low lying deltaic areas in temperate climates, groundwater can be brackish to saline at shallow depth, even with a yearly rainfall excess. For primary production in horticulture, agriculture, and terrestrial nature areas, the fresh water availability may be restricted to so-called fresh water lenses: relatively thin pockets of fresh groundwater floating on top of saline groundwater. The persistence of such fresh water lenses, as well as the quantity and quality of surface water is expected to be under pressure due to climate change, as summer droughts may intensify in North-West Europe. Better understanding through modelling of these fresh water resources may help anticipate the impact of salinity on primary production. We use a simple model to determine in which circumstances fresh water lenses may disappear during summer droughts, as that could give rise to enhanced root zone salinity. With a more involved combination of expert judgement and numerical simulations, it is possible to give an appraisal of the hazard that fresh water lenses disappear for the Dutch coastal regions. For such situations, we derive an analytical tool for anticipating the resulting salinization of the root zone, which agrees well with numerical simulations. The provided tools give a basis to quantify which lenses are in hazard of disappearing periodically, as well as an impression in which coastal areas this hazard is largest. Accordingly, these results and the followed procedure may assist water management decisions and prioritization strategies leading to a secure/robust fresh water supply on a national to regional scale.

Saltwater Upconing Due to Cyclic Pumping by Horizontal Wells in Freshwater Lenses.

This article deals with the quantification of saltwater upconing below horizontal wells in freshwater lenses using analytical solutions as a computationally fast alternative to numerical simulations. Comparisons between analytical calculations and numerical simulations are presented regarding three aspects: (1) cyclic pumping; (2) dispersion; and (3) finite horizontal wells in a finite domain (a freshwater lens). Various hydrogeological conditions and pumping regimes within a dry half year are considered. The results show that the influence of elastic and phreatic storage (which are not taken into account in the analytical solutions) on the upconing of the interface is minimal. Furthermore, the analytical calculations based on the interface approach compare well with numerical simulations as long as the dimensionless interface upconing is below 1/3, which is in line with previous studies on steady pumping. Superimposing an analytical solution for mixing by dispersion below the well over an analytical solution based on the interface approach is appropriate in case the vertical flow velocity around the interface is nearly constant but should not be used for estimating the salinity of the pumped groundwater. The analytical calculations of interface upconing below a finite horizontal well compare well with the numerical simulations in case the distance between the horizontal well and the initial interface does not vary significantly along the well and in case the natural fluctuation of the freshwater lens is small. In order to maintain a low level of salinity in the well during a dry half year, the dimensionless analytically calculated interface upconing should stay below 0.25.

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.