Pacific Coastal & Marine Science Center
Submarine Ground-water Discharge
In northern Israel, two aquifers discharge into the sea — the Pleistocene coastal granular aquifer and the Cretaceous carbonate one, which is partly confined. This area contains many young, recently active, transversal faults, some of which are already known for being major pathways for seawater/groundwater exchange. The project, focusing mainly on the role of transverse faults in SGD and on the proportions of recycled seawater in this discharge, involves various methodologies, including geochemical tracing and continuous in-situ measurements of flow rates and solute fluxes, as well as a detailed ecological study of the coastal water.
The geochemical tracing includes the study of Rn and Ra isotopes (including the short-lived 223Ra and 224Ra), as well as salinity and nutrients. Flow rates and solute fluxes and their temporal patterns will be determined using newly developed methodology, which apply the osmotic pumping principle to the in-situ continuous sampling of seawater and pore water. Tracing the profiles of a dye tracer previously injected into the sediment, flow rates as low as a few cm/yr can be determined. Together with chemistry of pore fluid this will allow the determination of fluxes of solutes, and especially of nutrients, into the coastal seawater.
In the ecological part of the project, we will focus on the variability in microbial autototrophic and heterotrophic communities in relation with SGD-induced differences in the input of nutrients (nitrate, nitrite, phosphate, ammonium, silica) and other dissolved anthropogenic constituents (for example, radium, polonium and heavy metals). Initial characterization of the microbial autotrophic and heterotrophic communities will be done using total turbidity, chlorophyll a extraction, HPLC, flow cytometry, microscopy, photosynthetic, and bacterial activity. Areas with distinct, significant SGD will be studied in more detail using molecular and isotopic δ13C, δ15N) methods to characterize the dominant microbial populations in these areas.
The triangle on the map below shows the location of the Dor Beach study area. Click the map to view the study area in more detail (212 kb JPEG image).
A high-resolution, stationary geophysical and geochemical survey was conducted at Dor Beach, Israel, to examine the shallow coastal hydrogeology and its control on the exchange of submarine groundwater with the shallow Mediterranean Sea.
Time-series resistivity profiles using a new 56 electrode (112 m long) marine cable produced detailed profiles of the fresh water/salt water interface and the subtle response of this interface to tidal excursions and other forcing factors. Such information, when ground-truthed with representative pore water salinities and formation resistivity values, can provide unique information of the extent and rates of submarine groundwater discharge (SGD).
Time-series 222Rn measurements of the adjacent coastal water column complemented these geophysical techniques and were modeled to yield integrated advective flow rates across the sediment/water interface, which at Dor Beach ranged from about 0 to 30 cm/day (mean = 7.1 cm/day), depending on the tidal range. Such results suggest that the underlying hydrogeologic framework at Dor is favorable for substantial SGD. Extrapolating these SGD estimates across a 100 m wide coastal zone implies that the Rn derived SGD rate would equal ~7.1 m3/day per m of shoreline, and that the source of this discharging groundwater is a complex mixture of fresh groundwater derived from the upland Kurkar deposits, as well as locally recycled seawater.