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

Abstract

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...

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

“…The CHIRP results indicate that seawater intrusion can take place, as observed in unit B (not in the underlying unit C and the overlaying unit A) at the southern part 75 m from sea, and not at the northern part. This was supported by numerical simulations, which showed that the difference in the offshore extension of the clay may delay SWI by years [18]. The hydraulic conductivity determined by field tests for units A, B and C were 30, 68 and 109 m/day, respectively, and the storativity in units B and C were 0.0001 and 0.0016, respectively [18].…”

“…The study area includes two observation wells screened at unit B (T72 and T71-2), located 50 and 75 m from shore, respectively, and one pumping well (P211) opened both to unit B and to unit C, which is located 125 m from the sea (Figure 2). Due to pumping, water level in unit B (which is the subject of this study) is on the average 1.1 and 1.3 m lower than in the overlaying unit A and the underlying unit C, respectively [18]. Profiles in units A and C, 75 m from shore, show low and uniform EC values (5-6 mS/cm, like the natural conductivity in this area), while in unit B an interface was observed at a depth of 22 m, and salinity of bottom water reached 40 mS/cm [18].…”

“…Due to pumping, water level in unit B (which is the subject of this study) is on the average 1.1 and 1.3 m lower than in the overlaying unit A and the underlying unit C, respectively [18]. Profiles in units A and C, 75 m from shore, show low and uniform EC values (5-6 mS/cm, like the natural conductivity in this area), while in unit B an interface was observed at a depth of 22 m, and salinity of bottom water reached 40 mS/cm [18]. In this study, electrical conductivity was measured by several methods: SMD (subsurface monitoring device) is a new automatic, remote-controlled multi sensor geophysical tool, which allows the measurement of water EC profiles and other parameters, such as water level and temperature profiles [19][20][21][22][23].…”

“…Although exploitation is very high, the water level in this area is quite stable, and seawater intrusion is limited. This is explained by the increase of inflow from the underlying Yarkon Taninim aquifer [18]. Offshore geophysical surveys by CHIRP (Compressed High Intensity Radar Pulse) in the shallow sea showed that the offshore extension of the shallow clay layer, which confines unit B, reaches more than 700 m in the northern part of the study area, while at the southern part of the area is missing already, at 100 m offshore.…”

Recent Research Results on Groundwater Resources and Saltwater Intrusion in a Changing Environment

“…Tal et al [7] investigated the interrelationship between a multi-layered coastal aquifer at the southern Carmel plain in Israel, fish-ponds, and the sea using off-shore seismic surveying, on-land time-domain electromagnetic (TDEM) surveying, electrical conductivity (EC) profiles, hydrological field experiments, and groundwater levels. Using groundwater modelling, they showed that the exact location of the hydraulic connection between the confined aquifer unit and the sea (variable continuity of confining clay) played a significant role in the sensitivity of the aquifer unit to seawater intrusion.…”