Airborne electromagnetic mapping of coastal groundwater salinity: Quantifying uncertainty and investigating methodological improvements

Low elevation coastal zones (LECZs), defined here as areas ≤10 m above mean sea-level, have attracted people for millennia. With their abundant resources and access to trading ports, today these areas host nearly 800 million people – a figure that is predicted to rise to 1.4 billion by 2060. Naturally, it follows that with population growth comes an increased demand for freshwater. Globally, about 50% of the world’s population rely on groundwater to satisfy basic requirements – and LECZs are no exception. However, owing to anthropogenic activity, these aquifers are highly stressed and vulnerable to saltwater intrusion – where freshwater can be displaced by saline groundwater. As a result, aquifers within LECZs require effective management, which in turn requires an excellent regional understanding of fresh-saline groundwater distributions. Airborne electromagnetic (AEM) surveys offer a rapid and cost- effective method to map this, and thus are increasingly used for these purposes. Despite increasing popularity, AEM is relatively poorly understand in terms of regional (provincial or country scale) groundwater salinity mapping. The primary objective of this thesis is therefore twofold: 1) to better understand the uncertainties involved and 2), use this understanding to develop novel mapping methods. Consequently, the following research questions were formulated: 1. What is the effect of using different inversion methods and parameters on mapping results? 2. How are results affected by different quantities of available data? 3. Based on the results of chapters 2 and 3, what further methodological improvements can we make? 4. Are groundwater salinity movements sensitive to repeated AEM surveys? To answer these research questions, I used data from the Province of Zeeland, The Netherlands. Zeeland experienced sea-level transgressions in the early Holocene, followed by the construction of by man-made coastal defences – which allowed the recent freshening of shallow aquifers. As a result, much of Zeeland comprises shallow rainwater lenses (often as little as 1 – 2m thick), and therefore offers a fascinating study area for applied groundwater research. Furthermore, a recently undertaken, high quality AEM survey and plentiful ground-based data provide an ideal testing ground. Throughout the thesis, these data were used either directly, or as basis for the construction of highly detailed synthetic models