Deep Submarine Groundwater Discharge—Evidence From Achziv Submarine Canyon at the Exposure of the Judea Group Confined Aquifer, Eastern Mediterranean

Abstract

Deep submarine groundwater discharge (DSGD) of fresh-brackish water was suggested to occur globally, yet its driving mechanism is not well constrained. Specifically, it is unclear whether the phenomenon may represent a steady-state condition of a hydrological system, in which the terrestrial recharge area is hydraulically connected with the submarine discharge area (by a confined aquifer, for example). Recently, our hydrogeological modeling suggested that such a system exists in northern Israel. The model predicted the location of a brackish water seepage within the Achziv Submarine Canyon,~10 km offshore, where the Cretaceous Judea Group confined aquifer is carved by the canyon. The present work is a field study that attempts to identify these brackish seeps in the water column of the canyon at the aquifer exposure. The field observations include salinity and temperature profiling of the water column along the canyon during three cruises and 224 Ra measurements of water samples from the canyon. The results point to a significant near-bottom low-salinity and low-temperature anomaly right where the aquifer submarine outcrop is mapped that coincide with a high 224 Ra activity anomaly. Such near-bottom anomalies are absent in an adjacent submarine canyon, in which the confined aquifer is not exposed. The anomalies in the water column of the Achziv Submarine Canyon corroborate the results of the hydrogeological modeling that predicted brackish DSGD from the sea bottom at the aquifer outcrop. The possibility of DSGD in specific geo-hydrological settings is relevant for various biological, geochemical, hydrological, and geotechnical studies of the marginal ocean.

Electrical Resistivity Anomalies Offshore a Carbonate Coastline: Evidence for Freshened Groundwater?

“…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). 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). 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).…”

Marine Electromagnetic Imaging and Volumetric Estimation of Freshwater Plumes Offshore Hawai'i

“…Therefore, combined with a suite of biochemical parameters (e.g., Rodellas et al., 2015), CSEM can help assess SGD effects on marine biogeochemistry at local and regional scales. However, to ideally evaluate the impact of nearshore/offshore SGD at different geologic settings [e.g., karstic submarine canyon (Paldor et al., 2020), carbonate rock aquifers (Holmden et al., 2012)], studies should also include CTD, nutrient, metal, and carbonate chemistry time‐series measurements to derive freshwater fluxes, residence time, and seasonal variability.…”

“…The fresh portion of SGD is critical, as it potentially buffers ocean acidification with groundwater alkalinity (Cyronak et al., 2013; Slomp & Van Cappellen, 2004). While coastal SGD has been documented globally at various geological settings (Kim & Kim, 2011; Knee et al., 2016; Paldor et al., 2020; Prakash et al., 2018; Stieglitz, 2005), reports on deep offshore SGD sources (disconnected from coastal SGD, usually appear at water depths >50 m) are scarce but of potential importance. Fresh SGD is commonly associated with oceanographic, hydrogeological, and environmental processes that affect chemical weathering, ocean eutrophication, and climate change by emanating solutes and gases to the ocean (e.g., Moore, 2010; Kim & Kim, 2011; Taniguchi et al., 2019; Luijendijk et al., 2020).…”

Ideas and perspectives: Land-ocean connectivity through groundwater

“…OFG systems may be coupled with FSGD (e.g. Paldor et al, 2020;Attias et al, 2021), and modelling shows that FSGD and OFG can occur in equilibrium with present-day sea level for a range of different stratigraphic configurations (Michael et al, 2016). However, OFG can also be decoupled from interaction with the water column (see e.g.…”

“…Likewise, OFG is a potential freshwater resource, or a resource of water that can be treated with desalination with comparably small energy consumption (Bakken et al, 2012), and has therefore gained increased attention over the past decade (Post et al, 2013;Micallef et al, 2021). Although OFG is generally a relic of past sea-level low stand (fossil groundwater), some reservoirs are likely hydraulically connected to the terrestrial aquifers groundwater system, as shown for the U.S. Atlantic coast (Gustafson et al, 2019;, Canterbury Bight, New Zealand Weymer et al, 2020), and the Achziv submarine canyon in northern Israel (Paldor et al, 2020). Here, we emphasize the importance of improving our understanding of connected OFG, since its extraction as an unconventional resource for mitigating temporal water scarcity in coastal communities might cause seawater intrusion (Yu and Michael, 2019a) and distant land subsidence (Chen et al, 2007;Yu and Michael, 2019b).…”