ECR Hydrology Research Tidbit: Freshwater? Deep in the Sea?

Much like groundwater discharges into rivers and streams, groundwater also discharges into the ocean. The discharge of terrestrially based water to the coastal ocean is called submarine groundwater discharge, or SGD, and has been observed globally, particularly in nearshore environments. SGD impacts both terrestrial and marine environments. It discharges terrestrially-based water that is frequently high in nutrients to the ocean and impacts terrestrial freshwater budgets. Whereas shallow SGD from unconfined aquifers has been extensively studied, deep SGD from confined aquifers is less well understood. This is due to the fact that deep SGD is difficult and costly to measure, frequently requiring ship time, and many of the methods used to measure shallow SGD such as seepage meters cannot be used offshore.

Figure1_PaldorTidbit

Figure 1. Study location (modified from Paldor et al., 2020a).

One debate regarding deep SGD is the mechanism driving offshore freshwater. On one side, offshore freshwater is thought to be a relic of glacial periods when sea level was lower – a “memory” of the past as Dr. Paldor describes it. But on the other side, some studies suggest that offshore SGD may also be a steady-state phenomenon driven by terrestrial recharge. This was shown in previous work by Dr. Paldor and collaborators using hydrological models (Paldor et al., 2020b).

 

Figure2_PaldorTidbit

Figure 2. Salinity anomaly from cross section in Achziv Canyon (modified from Paldor et al., 2020a).

In this study, Dr. Paldor and co-authors provide compelling evidence for offshore SGD from the coast of Israel into the Achziv Canyon in the Mediterranean Sea (Figure 1). Three different techniques are used to identify SGD: salinity and temperature profiles, and radium isotopes. Results from each technique point to the same conclusion: terrestrially based water that is fresher and colder than the seawater is being discharged through an outcrop of the Judea Group aquifer into the Mediterranean Sea at a depth of approximately 350 m (Figure 2). Not only does this corroborate Dr. Paldor’s previous modeling work in the same region, it also supports the hypothesis that deep SGD may also be driven by terrestrial recharge (Figure 3).

Figure3_PaldorTidbit

Figure 3. Conceptual diagram of active, modern deep SGD (from Paldor et al., 2020a).

The data and findings from this study provide important evidence that deep SGD is possible along any ocean or marginal sea and that this discharge may be driven by active, present-day processes (Figure 3). Furthermore, results from this study may be the missing link to balancing the groundwater budget of the hydrologic system in northern Israel.

Dr. Anner Paldor received his PhD in Hydrology and Water Resources Science from The Hebrew University of Jerusalem in 2019. Dr. Paldor is now a postdoctoral researcher at the University of Delaware working with Dr. Holly Michael. He is using coupled surface-subsurface hydrological models to understand mechanisms mediating coastal vulnerability to storm surge inundation, as well as conducting modeling studies on how storm surge events impact coastal stability via changes in groundwater fluxes and exchange. His long-term research interests include several aspects of hydrogeology, including saltwater intrusion, groundwater resources management, surface-groundwater interactions, and coastal geomechanics.

Reference: Paldor, A., Katz, O., Aharonov, E., Weinstein, Y., Roditi-Elasar, M., Lazar, A., & Lazar, B. (2020a). Deep Submarine Groundwater Discharge—Evidence From Achziv Submarine Canyon at the Exposure of the Judea Group Confined Aquifer, Eastern Mediterranean. Journal of Geophysical Research: Oceans, 125(1), 1–16. https://doi.org/10.1029/2019JC015435

In text citations: Paldor, A., Aharonov, E., & Katz, O. (2020b). Thermo-haline circulations in subsea confined aquifers produce saline, steady-state deep submarine groundwater discharge. Journal of Hydrology, 580 (July 2019), 124276. https://doi.org/10.1016/j.jhydrol.2019.124276

 

By Julia A. Guimond
University of Delaware

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