Radon and Salinity Mass Balance Constraints on Groundwater Recharge on a Semi-Arid Island (Catalina, California)

Quantifying the freshwater component of submarine groundwater discharge (SGD) is critical in the analysis of terrestrial influences on marine ecosystems and in assessing the water budget and groundwater recharge of coastal aquifers. In semi-arid to arid settings, this quantification is difficult because low SGD rates translate into low concentrations of groundwater solutes in coastal waters. In this study, fresh SGD (FSGD) was quantified for Toyon Bay on Catalina Island, California, for wet and dry seasons using a combination of radon and salinity mass balance models, and the results were compared to watershed-specific groundwater recharge rates obtained from soil water balance (SWB) modeling. Calculated FSGD rates vary only slightly with season and are remarkably similar to the recharge estimates from the SWB model. While sensitivity analyses revealed FSGD estimates to be significantly influenced by uncertainties in geochemical variability of the groundwater end-member and fluctuations of water depth, the results of this study support the SWB-model-based recharge rates. The findings of this study highlight the utility of the radon-and-salinity-mass-balance-based FSGD estimates as groundwater recharge calibration targets, which may aid in establishing more refined sustainable groundwater yields.

Aquifer Response to Estuarine Stream Dynamics

While seawater intrusions are widely discussed, the salinization of coastal aquifers via narrow rivers is hardly documented. This study investigates groundwater dynamics in an aquifer next to an estuarine stream on the eastern Mediterranean coast. Groundwater levels and salinization patterns were examined as a response to dynamic changes in estuary water, both in low-and high-permeability aquifer units. In the high-permeability unit, the extent of salinization was relatively constant, reaching a distance of at least 80 m from the river, with no long-term changes in fresh-saline interface depth, indicating that the system is in a quasi-steady state. Groundwater salinity in the low-permeability unit showed frequent and large fluctuations (up to 36 and 22 at 5 and 20 m from the river, respectively). We suggest that the river may have a more immediate impact on a low-permeability than on a high-permeability aquifer. This is dependent on the history of seawater encroachments to the river, which are better preserved in the low-permeability unit, and on the hydrogeology of this unit, where sand lenses can serve as high-permeability conduits. However, this unit can efficiently prevent a large extent of salinization of the regional coastal aquifer by the estuary water.

Modeling the Potential of Submarine Groundwater Discharge to Facilitate Growth of Vibrio cholerae Bacteria

Submarine groundwater discharge (SGD), the discharge of terrestrial groundwater to the ocean, can govern the coastal benthic environment. Bacteria such as Vibrio cholerae inhabit coastal waters and sediments, whose growth can be influenced by SGD. In particular, salinity changes introduced by SGD could have a positive effect on the abundance but also virulence of non-halophilic V. cholera bacteria dwelling in coastal waters and shallow marine sediments. Here we assess potential effects of SGD on the environmental properties that favor V. cholerae in a numerical modeling study representing multiple scenarios. Approaching natural systems, simulation results reveal a high sensitivity of non-halophilic Vibrio cholerae growth to SGD and its primary driving factors. This dependency leads to highest growth potential at high groundwater inflow and low hydraulic conductivity of the aquifer as well as for steep sea-side boundary slopes. Besides its minor impact on the extent of SGD in our model, dispersion is a crucial limiting factor for V. cholerae habitat. We conclude that there is a close connection between the driving factors of SGD and low salinity zones along a coastal slope, and recommend taking these into consideration for evaluating local V. cholerae outbreaks.

Quantification of Submarine Groundwater Discharge in the Gaza Strip

Gaza Strip has suffered from seawater intrusion during the past three decades due to low rainfall and high abstraction from the groundwater resource. On a yearly basis, more than 170 million m3 of groundwater is abstracted, while the long-term average recharge from rainfall is 24.4 million m3/year. Submarine groundwater discharge (SGD) has never been studied in the Gaza Strip, due to lack of experience in this field, next to the ignorance of this subject due to the seawater intrusion process taking place. Continuous radon measurements were carried out in six sites along the Gaza Strip to quantify the SGD rate. The final result shows SGD to occur in all sampled sites. The range of SGD rates varies from 0.9 to 5.9 cm·day−1. High values of SGD are found in the south (Rafah and Khan Younis governorates). The high values are probably related to the shallow unconfined aquifer, while the lowest values of SGD are found in the middle of Gaza Strip, and they are probably related to the Sabkha formation. In the north of Gaza Strip, SGD values are in the range of 1.0 to 2.0 cm·day−1. Considering that SGD would occur with the measured rates in a strip of 100 m wide along the whole coast line, the results in a quantity of 38 million m3 of groundwater being discharged yearly to the Mediterranean Sea along Gaza coast. Nutrient samples were taken along Gaza Strip coastline, and they were compared to the onshore wells, 600 m away from the Mediterranean Sea. The results show that SGD has higher NO3− + NO2− than nutrient-poor seawater, and that it is close to the onshore results from the wells. This confirms that the source of SGD is groundwater, and not shallow seawater circulation. In a coastal strip of 100 m wide along the Gaza coast, a yearly discharge of over 400 tons of nitrate and 250 tons of ammonium occurs from groundwater to the Mediterranean Sea.

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.

Temporal evolution and variability of dissolved inorganic nitrogen in beach pore water revealed using radon residence times

We coupled measurements of beach pore water residence time, determined using the radioisotopic tracer (222)Rn, with dissolved carbon and nitrogen chemistry to identify the temporal evolution and variability of dissolved inorganic nitrogen (DIN) concentrations in beach pore water along the Santa Barbara, California coastline. Pore water dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) ratios (DOC:TDN) were negative exponentially correlated with residence time. Mean pore water residence times were positively correlated with tidal amplitudes, and ranged from 4.4 to 6.4 days. We used this range in mean residence times to model radon residence time distributions (RTDs), and integrated them with modeled DIN vs residence time relationships (DIN-temporal evolution, or DIN-te curves) to derive volume-weighted mean (VWM) DIN concentrations. We observed 1.2-fold and 5.2-fold differences (20% and 420% increases) in VWM DIN concentrations over the range in modeled RTDs and DIN-te curves, respectively, and a maximum 6.4-fold difference (540% increase) in VWM DIN concentrations for an interactive shift in the RTD and the DIN-te curve. Our study suggests that accounting for temporal variability in the RTD and DIN concentration of pore water is necessary to obtain more accurate estimates of DIN delivery to coastal oceans.

What is the role of fresh groundwater and recirculated seawater in conveying nutrients to the coastal ocean?

Submarine groundwater discharge (SGD) is a major process operating at the land-sea interface. Quantifying the SGD nutrient loads and the marine/terrestrial controls of this transport is of high importance, especially in oligotrophic seas such as the eastern Mediterranean. The fluxes of nutrients in groundwater discharging from the seafloor at Dor Bay (southeastern Mediterranean) were studied in detail using seepage meters. Our main finding is that the terrestrial, fresh groundwater is the main conveyor of DIN and silica to the coastal water, with loads of 500 and 560 mol/yr, respectively, per 1 m shoreline. Conversely, recirculated seawater is nutrient-poor, and its role is mainly as a dilution agent. The nutrient loads regenerated in the subterranean estuary (sub-bay sediment) are relatively small, consisting mostly of ammonium (24 mol/yr). On the other hand, the subterranean estuary at Dor Bay sequesters as much as 100 mol N/yr per 1 m shoreline, mainly via denitrification processes. These, and observations from other SGD sites, imply that the subterranean estuary at some coastal systems may function more as a sink for nitrogen than a source. This further questions the extent of nutrient contributions to the coastal water by some subterranean estuaries and warrants systematic evaluation of this process in various hydrological and marine trophic conditions.