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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.

In a setting of predominantly saline surface waters in Zeeland, the Netherlands, The only available shallow fresh groundwater resource is present in the form of freshwater lenses floating on top of saline groundwater. This fresh water is vital for agricultural, industrial, ecological, water conservation and drinking water functions. An essential first step for managing the usable water properly is to know the location of the fresh?saline groundwater interface. Traditional salinity mapping with ground-based vertical electrical soundings, electrical cone penetration tests or chloride measurements from groundwater samples is time-consuming and, therefore, expensive to cover large areas. Airborne electromagnetics, which is fast and can cover large areas in short time, is an efficient alternative. Therefore, a consortium of BGR, Deltares and TNO conducted the project FRESHEM Zeeland during 2014?2017. An area of more than 2000 km2 was surveyed using BGR?s helicopter-borne geophysical system totalling 9640 line-km. The helicopter-borne electromagnetic data, after inversion to resistivity?depth models, served as baseline information for further interpretation. Without information on lithology, however, an accurate discrimination between fresh and saline groundwater applying fixed resistivity thresholds would fail if clayey sediments exist. Therefore, a probabilistic Monte Carlo approach was developed within the FRESHEM project. This approach combines helicopter-borne electromagnetic resistivities, 3D geological model (GeoTOP), laboratory results (formation factor and surface conductivity) and local in situ groundwater measurements for the translation of resistivity data to chloride concentration. As such detailed information is generally not available, another approach, which uses only helicopter-borne electromagnetic results to derive the thicknesses of the freshwater lenses from smooth inversion models, is presented in this paper. The corresponding fresh?saline groundwater interfaces are derived from steepest resistivity?depth (log?log) gradients appearing within a certain resistivity range. The bounds of this range are defined by resistivity values, which predominantly correlate with fresh or saline water, nearly independent of the lithology type. The results of this approach are checked using both synthetic and field data. The latter are compared with electrical cone penetration test measurements, the common threshold approach and the 3D chloride distribution of the FRESHEM results, particularly in an evaluation area where the transition of fresh to saline groundwater is relatively sharp. The fresh?saline groundwater interfaces derived by all methods are quite similar on average with deviations in the order of a metre. Locally, however, greater deviations occur, particularly close to the coast and along creek ridges or dunes, where the elevation of the fresh?saline groundwater interface varies notably. ? 2018 European Association of Geoscientists & Engineers.

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.

The forecast of climate change effects on the groundwater system in coastal areas is of key importance for policy makers. The Dutch water system has been deeply studied because of its complex system of low-lying areas, dunes, land won to the sea and dikes, but nowadays large efforts are still being done to find out the best techniques to describe complex fresh-brackish-saline groundwater dynamic systems. In this paper, we describe a methodology consisting of high-resolution airborne electromagnetic (EM) measurements used in a 3-D variable-density transient groundwater model for a coastal area in the Netherlands. We used the airborne EM measurements in combination with borehole-logging data, electrical conductivity cone penetration tests and groundwater samples to create a 3-D fresh-brackish-saline groundwater distribution of the study area. The EM measurements proved to be an improvement compared to older techniques and provided quality input for the model. With the help of the built 3-D variable-density groundwater model, we removed the remaining inaccuracies of the 3-D chloride field and predicted the effects of three climate scenarios on the groundwater and surface water system. Results showed significant changes in the groundwater system, and gave direction for future water policy. Future research should provide more insight in the improvement of data collection for fresh-brackish-saline groundwater systems as it is of high importance to further improve the quality of the model.

In deltaic areas with saline seepage, freshwater availability is often limited to shallow rainwater lenses lying on top of saline groundwater. Here we describe the characteristics and spatial variability of such lenses in areas with saline seepage and the mechanisms that control their occurrence and size. Our findings are based on different types of field measurements and detailed numerical groundwater models applied in the south-western delta of the Netherlands. By combining the applied techniques we could extrapolate measurements at point scale (groundwater sampling, temperature and electrical soil conductivity (TEC)-probe measurements, electrical cone penetration tests (ECPT)) to field scale (continuous vertical electrical soundings (CVES), electromagnetic survey with EM31), and even to regional scale using helicopter-borne electromagnetic measurements (HEM). The measurements show a gradual mixing zone between infiltrating fresh rainwater and upward flowing saline groundwater. The mixing zone is best characterized by the depth of the centre of the mixing zone Dmix, where the salinity is half that of seepage water, and the bottom of the mixing zone Bmix, with a salinity equal to that of the seepage water (Cl-conc. 10 to 16 g l−1). Dmix is found at very shallow depth in the confining top layer, on average at 1.7 m below ground level (b.g.l.), while Bmix lies about 2.5 m b.g.l. The model results show that the constantly alternating upward and downward flow at low velocities in the confining layer is the main mechanism of mixing between rainwater and saline seepage and determines the position and extent of the mixing zone (Dmix and Bmix). Recharge, seepage flux, and drainage depth are the controlling factors.