Large-scale, probabilistic salinity mapping using airborne electromagnetics for groundwater management in Zeeland, the Netherlands

Abstract

Seawater intrusion has often resulted in scarce fresh groundwater resources in coastal lowlands. Careful management is essential to avoid the overexploitation of these vulnerable fresh groundwater resources, requiring detailed information on their spatial occurrence. Airborne electromagnetics (EM) has proved a valuable tool for efficient mapping of ground conductivity, as a proxy for fresh groundwater resources. Stakeholders are, however, interested in groundwater salinity, necessitating a translation of ground conductivity to groundwater salinity. This paper presents a methodology to construct a high-resolution (50 × 50 × 0.5 m 3 ) 3D voxel model of groundwater chloride concentration probability, based on a large-scale (1800 km 2 , 9640 flight line kilometres) airborne EM survey in the province of Zeeland, the Netherlands. Groundwater chloride concentration was obtained by combining pedotransfer functions with detailed lithological information. The methodology includes a Monte Carlo based forward uncertainty propagation approach to quantify the inherent uncertainty in the different steps. Validation showed good correspondence both with available groundwater chloride analyses, and with ground-based hydrogeophysical measurements. Our results show the limited occurrence of fresh groundwater in Zeeland, as 75% of the area lacks fresh groundwater within 15 m below ground surface. Fresh groundwater is mainly limited to the dune area and sandy creek ridges. In addition, significant fresh groundwater resources were shown to exist below saline groundwater, where infiltration of seawater during marine transgressions was hindered by the presence of clayey aquitards. The considerable uncertainty in our results highlights the importance of applying uncertainty analysis in airborne EM surveys. Uncertainty in our results mainly originated from the inversion and the 3D interpolation, and was largest at transition zones between fresh and saline groundwater. Reporting groundwater salinity instead of ground conductivity facilitated the rapid uptake of our results by relevant stakeholders, thereby supporting the necessary management of fresh groundwater resources in the region.

Origin and Dynamics of Saltwater Intrusion in a Regional Aquifer: Combining 3‐D Saltwater Modeling With Geophysical and Geochemical Data

“…Unfortunately, this is challenged by high uncertainties and ambiguity associated with the translation of AEM data into chloride concentrations, especially in highly heterogeneous clay‐rich environments (Siemon et al, ). Delsman et al () demonstrate a probabilistic approach to derive categories of salt concentration from AEM data, which could in the future be of value for constraining saltwater intrusion models. Additionally, the long computer run times for simulating regional saltwater intrusions (each run in this study took ≈3 days on a 12 core Intel Xeon CPU E5‐1650 3.5GHz (64GB RAM) PC) are a challenge.…”

Quantifying Geophysical Inversion Uncertainty Using Airborne Frequency Domain Electromagnetic Data—Applied at the Province of Zeeland, the Netherlands

“…Standard processing of the HEM data was undertaken by BGR and, for this survey, is summarized in Delsman et al () and detailed in section . The following postprocessing steps were undertaken for this study: (1) measurements where bird altitudes above 60 m were removed due to low signal to noise ratios, and (2) every second downline measurement was removed, resulting in a measurement every 8 m to facilitate faster inversion times.…”

“…Standard processing of the HEM data was undertaken by BGR and, for this survey, is summarized in Delsman et al (2018) and detailed in section 2.4.1. The following postprocessing steps were undertaken for this study:…”

“…These methods have since been successfully applied for groundwater mapping campaigns across the world (e.g., Auken et al, ; Chongo et al, ; Fitterman & Deszcz‐Pan, ; Siemon et al, ). In the Netherlands, the technique was used in 2009 over the island of Schouwen‐Duivenland (Faneca Sànchez et al, ) and again in 2014–2015 covering the entire province of Zeeland (Delsman et al, ).…”

A three-dimensional palaeohydrogeological reconstruction of the groundwater salinity distribution in the Nile Delta Aquifer

“…The Nile Delta is vital for Egypt. It is an important bread basket, as it has been through history (Dermody et al, 2014), since it is the main area suitable for agriculture in an otherwise water-scarce region (WRI, 2008). Furthermore, the area is densely populated (Higgins, 2016), and the population is only expected to increase over the coming decades (World Bank, 2018).…”