Several studies have demonstrated the effectiveness of controlled-source electromagnetic (CSEM) methods to image the electrical resistivity distributions associated with OFG along continental margins and volcanic islands (e.g., Attias et al., 2020, 2021 Gustafson et al., 2019 Haroon, Lippert, et al., 2018 Micallef et al., 2020). In both cases, the characteristics of OFG are poorly constrained, and questions regarding their geometry and distribution, as well as the geological controls and timing of emplacement remain unanswered (Micallef et al., 2021). OFG systems today are either recharging through an active connection to their terrestrial counterparts, or disconnected, relict systems. In the case of the latter, emplaced groundwater migrated landwards more slowly than the ensuing sea-level rise, and remnants of these groundwater bodies are still found offshore (Cronin, 2012 Harrar et al., 2001 Person et al., 2003 Post et al., 2013). The key emplacement mechanisms for OFG include present-day meteoric recharge from the onshore portion of the aquifer (Michael et al., 2016 Paldor et al., 2020) and/or meteoric recharge of shelf areas that were exposed during sea-level lowstands (Meisler et al., 1984). To mitigate the increasing demand on groundwater systems, offshore freshened groundwater (OFG) has been proposed as an alternative source of potable water (Bakken et al., 2012). Problems are especially critical in arid regions, where groundwater is often the only source of freshwater, and where periods of highest consumption coincide with periods of lowest meteoric recharge. Groundwater resources in coastal regions are globally deteriorating due to population growth, pollution, and climate change (Aeshbach-Hertig & Gleeson, 2012 Michael et al., 2017 Richey et al., 2015). However, resource potential is low due to its relict nature and low permeability host environment. Hydrogeological modeling suggests that freshened pore fluids, emplaced during sea-level lowstands and preserved in low permeability units, are potentially still found within carbonate shelves. The anomalies hosted in the former are likely associated with low porosities, whereas the anomaly within the latter is indicative of pore fluid freshening. Here, we use controlled-source electromagnetic, seismic reflection, and core sample data to derive a lithological model for the eastern margin of the Maltese Islands and identify four distinct resistivity anomalies within the Upper Coralline and Globigerina Limestone formations. In addition, carbonates constitute substantial amounts of the global coastlines, yet it is not known if and how they can sustain freshened groundwater offshore. Carbonate lithologies host considerable quantities of the Earth's freshwater resources and partially supply a quarter of the global population with drinkable water.
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