Water Resour Manage (2013) 27:4005–4020 DOI 10.1007/s11269-013-0392-2
Numerical Simulation of Submarine Groundwater Flow in the Coastal Aquifer at the Palmahim Area, the Mediterranean Coast of Israel N. Amir & U. Kafri & B. Herut & E. Shalev
Received: 23 December 2012 / Accepted: 16 June 2013 / Published online: 27 June 2013 # Springer Science+Business Media Dordrecht 2013
Abstract The lower sub-aquifers of the Mediterranean coastal aquifer of Israel, at the Palmahim area, are hypothesized to be laterally blocked to connection with the sea, and thus to seawater intrusion. This is mostly due to the detection of fresh water bodies at these sub-aquifers. This study examine this hypothesis by using two dimensional numerical model simulations of the groundwater flow system at this area, which conducted in order to reveal which hydrogeological setting enables the existence of these fresh water bodies in the lower sub-aquifers and to assess the on-land pumping rates that will prevent their salinization. The hydrogeological settings were examined by steady state simulations followed by simulations of the last 15,000 years sea level changes. These simulations imply that the presence of fresh water in the lower sub-aquifer, whether blocked or connected to the sea, requires offshore separation between the upper and lower sub-aquifers. On-land pumping simulations, with a well located inside the lower sub-aquifers at the shoreline, show a maximum pumping rate of 250 m3/m strip width/year, hereafter m2/year, to prevent the salinization of the lower subaquifers. The various pumping scenarios revealed differences in salinization trends between the scenarios with impermeable separating layers and those with semi permeable layers. Scenarios with extreme pumping rates emphasize these differences, and together with field test, can allow assessing the amount of separation between the sub-aquifers. Keywords Numerical modeling . Submarine groundwater flow . Sea water intrusion . Coastal aquifer
N. Amir (*) : B. Herut Department of Marine Geosciences, Leon H. Charney School for Marine Sciences, Faculty of Science and Science Education, University of Haifa, Mount Carmel, Haifa 31905, Israel e-mail:
[email protected] N. Amir : B. Herut Israel Oceanographic and Limnological Research, National Institute of Oceanography, Haifa 31080, Israel U. Kafri : E. Shalev Geological Survey of Israel, Jerusalem 95501, Israel
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1 Introduction The Mediterranean coastal aquifer of Israel is one of the three main reservoirs of the national water system. The aquifer is replenished by rain and in some parts by the mountainous aquifers, and it drains westward to the Mediterranean Sea. The aquifer suffers from severe salinization during the last decades due to over exploitation that led to deeper inland seawater encroachment mostly into its upper sub-aquifers. The coastal aquifer of Israel was studied and described in several studies (i.e. Issar 1968; Ecker 1999). The southern and central coastal aquifer of Israel extends from the Gaza Strip in the south to the Mount Carmel in the north (Fig. 1). The lateral extent of the aquifer from the sea to the foothills in the east increases from 8 km at the north end to 30 km at the south end. The aquifer thickness decreases from a maximum of 200 m at the southern shoreline to a thickness of only a few meters at the eastern boundary. The aquifer, of the Plio-Pleistocene Kurkar Group overly the impermeable dark shale and clay sequence of the Pliocene Saqiye group. The aquiferous units, that consist mainly of sand and calcareous sandstone (Kurkar) are separated by aquicludal clay and loam layers that thin out eastward, a few kilometers from shoreline. Therefore, as done in previous studies, the aquifer is subdivided herein, into the upper phreatic (namely A and B) sub-aquifers and the lower (namely C and D) confined ones (Fig. 2a). The offshore extension and thickness of these sub-aquifers is unknown and it was assumed in the past (Bear and Kapuler 1981) that all sub-aquifers are open to the sea and thus subjected to seawater intrusion. In fact, the upper sub-aquifers are known to be intruded by seawater along the entire Israeli coastal plain. This proposed setup was challenged by the assumption (Kolton 1988) that in some places the lower sub-aquifers thin out westward by lateral facies change into a continuous aquicludal shale sequence that blocks the lower sub-aquifers to the sea and to seawater intrusion. The sole data, by now, that supports Kolton’s (1988) assumption is the offshore Nordan 4 wildcat, showing a thick continuous shale sequence below the upper sub-aquifers (Kafri and Goldman 2006). Other studies can also support this assumption by showing that there is a significant hydraulic separation by clay layers between the described on-land sub-aquifers, resulting in different chemical composition, salinity and piezometric heads between these sub-aquifers (Nativ and Weisbrod 1994). The old ages of the groundwater in the lower sub-aquifers which exceed 10,000 years opposed to the young ages (approximately tens of years) in the upper sub-aquifers (Yechieli et al. 2001, 2009; Sivan et al. 2005), imply to a restricted flow or stagnation in the lower sub-aquifers, possibly due to their blockage to the sea. The time domain electromagnetic (TDEM) is a geophysical method that is used to monitor seawater intrusion mostly into the upper sub-aquifers (Goldman et al. 1991). Onland TDEM measurements along the shoreline reveal high resistivities in the lower subaquifers, interpreted as fresh water bodies beneath the seawater-intruded upper sub-aquifers (Kafri and Goldman 2006; Goldman et al. 2008). Direct borehole water chemical measurements confirmed these findings (Yechieli et al. 1997). This phenomenon is most prominent in the Palmahim area, exhibiting an approximately 20 km strip between Ashdod and Bat Yam (Fig. 1). Offshore TDEM measurements, which were carried out later enabled to detect this phenomenon to at least 2.5 km seaward, extending some 30 km parallel to the shoreline (Levi et al. 2010). However the fully offshore extend of these bodies is still not clear. These freshwater bodies raise again the question about the connection of the sub-aquifers to the sea (Kafri and Goldman 2006). The connection of the sub-aquifers to the sea whether connected or blocked, was not examined by past simulations of the Israeli coastal aquifer (i.e. Prieto and Destouni 2005; Yechieli et al. 2010; Yakirevich et al. 1998) which ignored the subdivision to sub-aquifers or
Numerical Simulation of Submarine Groundwater Flow
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HAIFA
10
20 Kilometers
32.5° N
ME DIT ERR AN E AN
SEA
5
L ME AR TC UN MO
0
HADERA
NETANYA
HERZELIYA
TEL AVIV _ YAFO
A
BAT YAM
PA LM AH
IM A
RE A
32° N
ASHDOD
RISHON LEZIYON
A'
Array of pumping wells represented in the pumping simulations
ASHQELON NORDAN 4
GAZA 31.5° N
34.5° E
35° E
Fig. 1 Location map of the coastal aquifer of Israel (gray area) showing the Palmahim area, A-A′ cross section and the location of Nordan 4 wildcat
focused only on the upper sub-aquifers and did not considered the offshore extent of the lower sub-aquifers. The objective of the present study is to understand by using numerical simulations, the groundwater flow system in the Palmahim area regarding to the hydrogeological configuration and the conductivity of separating layers which enables the existence of fresh groundwater in these lower sub-aquifers, and to assess the production potential of fresh groundwater from the lower sub-aquifers while causing no inland salinization.
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Fig. 2 a Schematic hydrogeological cross section of the coastal aquifer based on the topography of cross section A-A′ b The model cross section of the “Open” scenario and boundary conditions c The model cross section of the “Closed” scenario. The four sub domains are: 1) The upper A and B sub-aquifers 2) The lower C and D sub-aquifers 3) The undivided aquifer 4) The separating clay layers
2 Model Setup The groundwater flow system is simulated using FeFlow (Diersch 2005), a finite-element simulator that solves the coupled variable density ground water flow and solute transport. The two known transport mechanisms are advection and diffusion/dispersion (Kolditz et al. 1998). Three series of simulations were conducted as described below: 1) Steady state simulations. 2) Transient sea level rise simulations. 3) Pumping simulations. Two scenarios are simulated in each simulations series in order to examine the effect of the hydrogeological configuration, an “Open” scenario whereby both the upper and the lower sub-aquifers are open to and connected
Numerical Simulation of Submarine Groundwater Flow
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to the sea and a “Closed” one where the lower sub-aquifers are blocked to the sea by confining clay layers. The simulations are done along cross-section A-A′, south of Bat Yam and perpendicular to the shoreline (Fig. 1), based on the topography and bathymetry along it, extracted from a 25 m digital terrain model (Hall 2008). The 42166 by 474 m cross section represents the coastal aquifer from its eastern boundary, some 17 km east of the shoreline, to a depth of 400 m below sea level in the west, some 25 km west of the shoreline (Fig. 2). Based on the on-land known subsurface information at the shoreline and in the absence of offshore information, the sequence was extended in the “Open” scenario westward at the same slope and thickness until it intersected the sea floor (Fig. 2b). The thicknesses used were 120 m, 50 m for the upper sub-aquifers and the lower sub-aquifers respectively. For the separating clay layers a thickness of 30 m was set at the shoreline, which thin out 4 Km eastward and increases to around 50 m westward of the shoreline in order to conserve the thickness of the sub-aquifers. For the case of the “closed” scenario, the only difference was the arbitrarily blockage of the lower sub-aquifers some 10 km west of the shoreline (Fig. 2c). Based on the known configuration of the coastal aquifer of Israel, the aquifer is simulated assuming a heterogeneous aquifer. Four sub domains in the cross section with different hydraulic properties exist, which represent the different anisotropic sub-aquifers and the clay layers that separate them (Fig. 2). The grid mesh consists of around 25,000 triangular elements (13.000 nodes). Two boundaries, namely the upper and the lower boundary delimit the model. The lower boundary is set as no flow boundary. The upper boundary is divided into two segments, namely east and west of the shoreline (Fig. 2b). Boundary condition at the eastern (land side) segment is set for all the simulations as the recharge influx of 0.17 m/year for the flow equation and a salinity value of 0 g/l for the transport equation. In the present study only the simple possibility of no change of the recharge was considered although recharge could have changed in the past. This is done due to the absence of detailed relevant data and in order to obtain only the effect of the hydrogeological configuration and the conductivity of the separating clay layers on the seawater intrusion. Conditions at the western (sea side) segment are set differently for each series of simulations. Initial head is set as the height above −400 m level as described below along with initial mass concentration for each series of simulations. The parameters used herein are detailed in Table 1. The different hydraulic parameters are based on information obtained from Lutzky and Shalev (2010). Parameters values are valid for the entire cross section except for the hydraulic conductivity which has a unique value for each domain. A range of hydraulic conductivities was chosen for the separating layers from an extreme impermeable value of 0.000001 m/d to a permeable value of 0.1 m/d. This was done due to the possibility that the offshore separation might not be continuous and in order to explore the role of heterogeneity of the separating layers. Salinity in the simulations is given as concentration of total dissolved solids (TDS). Time steps for the transient simulations were arranged automatically using the FeFlow -Forward Euler/Backward Euler time integration scheme. The FeFlow algorithm for the time step size is controlled by the conversion of the solution of the previous time step. The maximum step length was restricted to 400 days.
3 Results and Discussions 3.1 Steady State Simulations These simulations were executed in order to reach a steady state condition in both the “Open” and the “Closed” scenarios using different hydraulic conductivities for the separating (K4) layers, ranging from 0.1 to 0.000001 m/d. The purpose of these simulations was to
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Table 1 Parameters used in the simulations
N. Amir et al.
Parameters
Value
Units
Hydraulic conductivity Upper sub-aquifers (K1) Lower sub-aquifers (K2)
20 10
m/d m/d
Undivided aquifer (K3)
15
m/d
Separating clay layers (K4)
0.1–0.000001
Anisotropy (Kv/Kh)
0.1
Porosity
0.3
Specific storage
0.0001
Specific yield
0.031
Longitudinal dispersivity Transverse dispersivity
10 1
m/d
1/m m m
Freshwater density
1000
Kg/m3
Seawater density
1031
Kg/m3
Freshwater salinity Seawater salinity
0
g/l
39
g/l
detect which cases enable the existence of fresh groundwater in the lower sub-aquifers close to shoreline (which is the actual situation) and to rule out the cases where sea water already intruded the lower sub-aquifers to this area. The boundary condition at the upper western sea side segment is set as freshwater head equivalent to the sea water head. Salinity value at this side is 39 g/l (Fig. 2b). Initial mass concentration of 39 g/l and initial head of 400 m are set for the entire model meaning that the entire cross section is saturated with seawater. The steady state simulations of the “Open” scenario (Fig. 3) show that in the case of extremely impermeable separating clay layers (K4
