[1] The present study examines the response of groundwater systems to expected changes in the Mediterranean Sea (rise of <1cm/yr) and Dead Sea levels (decline of ∼1 m/yr). A fast response is observed in the Dead Sea coastal aquifer, exhibited both in the drop of the water levels and in the location of the fresh-saline water interface. No such effect is yet observed in the Mediterranean coastal aquifer, as expected. Numerical simulations, using the FeFlow software, show that the effect of global sea level rise depends on the coastal topography next to the shoreline. A slope of 2.5‰ is expected to yield a shift of the interface by 400 m, after a rise of 1m (∼100 years), whereas a vertical slope will yield no shift. Reduced recharge due to climate change or overexploitation of groundwater also enhances the inland shift of the interface.
In coastal aquifers, significant vertical hydraulic gradients are formed where fresh water and underlying salt water discharge together upward to the seafloor. Monitoring boreholes may act as "short circuits" along these vertical gradients, connecting between the higher and the lower hydraulic head zones. When a sea tide is introduced, the fluctuations of both the water table and the depth of the mixing zone are also biased due to this effect. This problem is intensified in places of long-screen monitoring boreholes, which are common in many places in the world. For example, all approximately 500 boreholes of the fresh water-salt water mixing zone in the coastal aquifer of Israel are installed with 10 to 50 m long screens. We present field measurements of these fluctuations, along with a three-dimensional numerical model. We find that the in-well fluctuation magnitude of the mixing zone is an order of magnitude larger than that in the porous media of the actual aquifer. The primary parameters that affect the magnitude of this bias are the anisotropy of the aquifer conductivity and the borehole hydraulic parameters. With no sea tide, borehole interference is higher for the anisotropic case because the vertical hydraulic gradients are high. When tides are introduced, the amplitude of the mixing zone fluctuation is higher for the isotropic case because the overall effective hydraulic conductivity is greater than the conductivity in the anisotropic case. In the aquifer, the fresh water-salt water mixing zone fluctuations are dampened, and tens of meters inland from the shoreline, the fluctuations are on the order of few centimeters.
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. Total Organic Carbon (TOC) and manganese, and characterized by strong reducing conditions [12].Seawater intrusion (SI) is also a global concern, exacerbated by increasing demands for freshwater in coastal zones and predisposed to the influences of rising sea levels and changing climates. Open questions still exist, in particular about the transient SI processes and timeframes, and the characterization and prediction of freshwater-saltwater interfaces over regional scales and in highly heterogeneous and dynamic settings [13].Numerical solutions have become standard tools for analyzing aquatic systems, which involve a bi-directional flow [14]. Two different modeling approaches have been used in the literature to represent the freshwater-saltwater relationship, one of which is sharp interface approximation [15][16][17], while the other includes a transition zone between freshwater and seawater [18][19][20][21]. Numerical solution with the FEFLOW cod is a common tool to study the salt water interface movement [22]. Yet, extensive measurement campaigns are necessary to accurately delineate interfaces and their displacement in response to real-world coastal aquifer stresses, encompassing a range of geological and hydrological settings [13]. Analytical studies of tide-induced groundwater fluctuations in confined coastal aquifers that extend under the sea have been studied in References [23-25] and others. Other studies [24,26] suggested a general analytical sol...
A Time Domain ElectroMagnetic (TDEM) survey coupled with hydrogeological information was carried out along the Be'er Sheva valley between the Mediterranean Sea and the Dead Sea base levels, aiming to obtain the subsurface distribution of the different fresh and saline water bodies. Hydrochemical considerations and previous TDEM results hinted that the previously accepted model of an upper fresh water body underlain by solely Dead Sea (R-type) brines in the east and C-type brines in the west does not match the actual observations. Thus, the suggested working hypothesis of additional intruding seawater from the Mediterranean Sea to the Dead Sea base level was checked by the TDEM method. The results, indeed, exhibit an upper high resistivity fresh water body, an underlying low resistivity brine body and a moderate resistivity body in between. The origin of this body is not uniquely determined based on the geophysical measurements alone. Analysis of borehole data testifies that hydrochemical parameters of the body cannot be solely interpreted as a mixture of the above two end members, but rather calls for an additional contribution of intruding seawater. The suggested configuration consists of an upper fresh water body flowing to both base levels in the west and the east. Underneath there is a saline body resulting from seawater intrusion reaching from the Mediterranean to the Dead Sea base level. At the bottom there is a third body of the westward density driven Dead Sea brine. The entire configuration is supported by results of subsurface flow modeling.
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