Radium isotopes as submarine groundwater discharge (SGD) tracers: review and recommendations

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Garcia-Orellana, J., Rodellas, V., Tamborski, J., Diego-Feliu, M., van Beek, P., Weinstein, Y., Charette, M., Alorda-Kleinglass, A., Michael, H. A., Stieglitz, T., & Scholten, J. Radium isotopes as submarine groundwater discharge (SGD) tracers: review and recommendations. Earth-Science Reviews, 220, (2021): 103681, https://doi.org/10.1016/j.earscirev.2021.103681.Submarine groundwater discharge (SGD) is now recognized as an important process of the hydrological cycle worldwide and plays a major role as a conveyor of dissolved compounds to the ocean. Naturally occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra) are widely employed geochemical tracers in marine environments. Whilst Ra isotopes were initially predominantly applied to study open ocean processes and fluxes across the continental margins, their most common application in the marine environment has undoubtedly become the identification and quantification of SGD. This review focuses on the application of Ra isotopes as tracers of SGD and associated inputs of water and solutes to the coastal ocean. In addition, we review i) the processes controlling Ra enrichment and depletion in coastal groundwater and seawater; ii) the systematics applied to estimate SGD using Ra isotopes and iii) we summarize additional applications of Ra isotopes in groundwater and marine studies. We also provide some considerations that will help refine SGD estimates and identify the critical knowledge gaps and research needs related to the current use of Ra isotopes as SGD tracers.J.Garcia-Orellana acknowledges the financial support of the Spanish Ministry of Science, Innovation and Universities, through the “Maria de Maeztu” programme for Units of Excellence (CEX2019-000940-M), the Generalitat de Catalunya (MERS; 2017 SGR – 1588) and the project OPAL (PID2019-110311RB-C21). V. Rodellas acknowledges financial support from the Beatriu de Pinós postdoctoral program of the Generalitat de Catalunya (2017-BP-00334 and 2019-BP-00241). M. Charette received support from the U.S. National Science Foundation (OCE-1736277). J. Scholten acknowledges the support through the SEAMOUNT BONUS project (art. 185), which is funded jointly by the EU and the Federal Ministry of Education and Research of Germany (BMBF, grant no. 03F0771B). P. van Beek and T. Stieglitz acknowledge support from the French ANR project MED-SGD (ANR-15-01CE-0004) and chair @RAction MED-LOC (ANR-14-ACHN-0007-01). A. Alorda-Kleinglass acknowledges financial support from ICTA “Unit of Excellence” (MinECo, MDM2015-0552-17-1) and PhD fellowship, BES-2017-080740. H. Michael acknowledges support from the U.S. National Science Foundation (EAR-1759879). M. Diego-Feliu acknowledges the financial support from the FI-2017 fellowships of the Generalitat de Catalunya (2017-FIB-00365). Fig. 3, Fig. 4, Fig. 7, Fig. 12 were designed by Gemma Solà (www.gemmasola.com)

The Interrelations between a Multi-Layered Coastal Aquifer, a Surface Reservoir (Fish Ponds), and the Sea

This research examines the interrelations in a complex hydrogeological system, consisting of a multi-layered coastal aquifer, the sea, and a surface reservoir (fish ponds) and the importance of the specific connection between the aquifer and the sea. The paper combines offshore geophysical surveys (CHIRP) and on land TDEM (Time Domain Electro Magnetic), together with hydrological measurements and numerical simulation. The Quaternary aquifer at the southern Carmel plain is sub-divided into three units, a sandy phreatic unit, and two calcareous sandstone (‘Kurkar’) confined units. The salinity in the different units is affected by their connection with the sea. We show that differences in the seaward extent of its clayey roof, as illustrated in the CHIRP survey, result in a varying extent of seawater intrusion due to pumping from the confined units. FEFLOW simulations indicate that the FSI (Fresh Saline water Interface) reached the coastline just a few years after pumping has begun, where the roof terminates ~100 m from shore, while no seawater intrusion occurred in an area where the roof is continuous farther offshore. This was found to be consistent with borehole observations and TDEM data from our study sites. The water level in the coastal aquifer was generally stable with surprisingly no indication for significant seawater intrusion although the aquifer is extensively pumped very close to shore. This is explained by contribution from the underlying Late Cretaceous aquifer, which increased with the pumping rate, as is also indicated by the numerical simulations

High Resolution Monitoring of Seawater Intrusion in a Multi-Aquifer System-Implementation of a New Downhole Geophysical Tool

Monitoring of seawater intrusion is extremely important for the management of coastal aquifers, and therefore requires reliable and high-frequency monitoring tools. This paper describes the use of a new near field and downhole geophysical tool that monitors seawater intrusion in boreholes with high vertical resolution. This sensor is further used to study the impact of pumping on water electrical conductivity profiles (ECP) at the fresh-saline water interface. The new device was installed in a confined calcareous sandstone aquifer along the northern Israeli coast. The site includes two monitoring wells and one pumping well located at distances of 50, 75 and 125 m from shoreline, respectively. The new geophysical tool, called the subsurface monitoring device (SMD), was examined and compared to water an electric conductivity profiler (ECP) and a conductivity temperature depth (CTD) driver’s data. All methods show similar salinity trends, and changes in pumping regime were clearly identified with both the SMD and CTD. The advantage of using the SMD tool is the high temporal and spatial resolution measurement, which is transferred via internet and can be analyzed and interpreted in real time. Another advantage of the SMD is that it measures the electrical resistivity of the aquifer mostly outside the well, while both water ECP and the CTD measure in-well electrical conductivity; therefore, are subjected to the artefact of vertical flow in the well. Accordingly, while the CTD shows an immediate and sharp response when pumping is stopped, the SMD provides a gradual electric conductivity (EC) change, demonstrating that stability is reached just after a few days, which illustrates, more precisely, the hydrological response of the aquifer

Submarine groundwater discharge as a major source of nutrients to the Mediterranean Sea

The Mediterranean Sea (MS) is a semienclosed basin that is considered one of the most oligotrophic seas in the world. In such an environment, inputs of allochthonous nutrients and micronutrients play an important role in sustaining primary productivity. Atmospheric deposition and riverine runoff have been traditionally considered the main external sources of nutrients to theMS, whereas the role of submarine groundwater discharge (SGD) has been largely ignored. However, given the large Mediterranean shore length relative to its surface area, SGD may be a major conveyor of dissolved compounds to the MS. Here, we used a 228Ra mass balance to demonstrate that the total SGD contributes up to (0.3-4.8)·1012 m3·y-1 to the MS, which appears to be equal or larger by a factor of 16 to the riverine discharge. SGD is also a major source of dissolved inorganic nutrients to the MS, with median annual fluxes of 190·109, 0.7·109, and 110·109 mol for nitrogen, phosphorous, and silica, respectively, which are comparable to riverine and atmospheric inputs. This corroborates the profound implications that SGD may have for the biogeochemical cycles of the MS. Inputs of other dissolved compounds (e.g., iron, carbon) via SGD could also be significant and should be investigated

Melt instabilities in an intraplate lithosphere and implications for volcanism in the Harrat Ash-Shaam volcanic field (NW Arabia)

We investigate melt generation in a slowly extending lithosphere with the aim of understanding the spatial and temporal relationships between magmatism and preexisting rift systems. We present numerical models that consider feedback between melt generation and lithospheric deformation, and we incorporate three different damage mechanisms: brittle damage, creep damage, and melt damage. Melt conditions are calculated with a Helmholtz free energy minimization method, and the energy equation is solved self-consistently for latent heat and shear heating effects. Using a case of a slowly extending (1-1.5mm/yr) continental lithosphere with a relatively low surface heat flow (similar to 50mW/m(2)), we show that melt-rich shear bands are nucleated at the bottom of the lithosphere as a result of shear heating and damage mechanisms. Upon further deformation, melt zones intersect creep damage zones, thus forming channels that may be used for the melt to migrate upward. If a preexisting structure resides only in the brittle crust, it does not control the path of melt migration to the surface, and melt-filled channels propagate from the bottom upwards, independently of upper crustal structures. In contrast, a preexisting weak structure that reaches a critical depth of 20km allows fast (similar to 2Ma) propagation of melt-filled channels that link melt damage from the bottom of the lithosphere to near-surface structures. Our model results may explain the short time scale, volume, and magma extraction from the asthenosphere through a low surface heat flow lithosphere, such as observed, for example, in the Harrat Ash-Shaam volcanic field (northwestern Arabia), which developed in the Arabian Plate and is spatially linked to the Azraq-Sirhan Graben