APPLICATION OF THE TIME DOMAIN ELECTROMAGNETIC (TDEM) METHOD FOR STUDYING GROUNDWATER SALINITY IN DIFFERENT COASTAL AQUIFERS OF ISRAEL Mark Goldman PhD, Head of Geoelectric Department, Geophysical Institute of Israel
[email protected]
Uri Kafri PhD, Senior Hydrogeologist, Geological Survey of Israel
[email protected]
Yossi Yechieli PhD, Head of Hydrogeological Department, Geological Survey of Israel
[email protected]
ABSTRACT The Mediterranean coastal aquifer of Israel is represented by calcareous sandstone divided by impermeable clays into generally four sub-aquifers. The upper two sub-aquifers are heavily overexploited and, as a result, are subjected to salinization by seawater intrusion. The ground water salinity in lower sub-aquifers is rarely known due to the lack of borehole information from the appropriate depths. The application of surface geoelectric measurements using time domain electromagnetic (TDEM) method allowed to detect relatively fresh ground water with in the lower sub-aquifers below seawater intrusion into the upper sub-aquifers (hydrological reversals). Thick clay layers within the lower sub-aquifers could be misinterpreted as saline groundwater. Similar problem exists in other coastal aquifers of Israel as well.
Key words Mediterranean coastal aquifer of Israel, time domain electromagnetic method (TDEM), salinity.
RESUMEN El acuífero costero mediterráneo de Israel está constituido por areniscas calcáreas divididas en cuatro sub-acuíferos por capas impermeables de arcilla. Los dos sub-acuíferos superiores se encuentran intensamente sobreexplotados, y como resultado, están afectados por la intru sión de agua de mar. La salinidad de los sub-acuíferos inferiores se conoce muy poco, debi do a la ausencia información procedente de sondeos que tengan la profundidad adecuada. La aplicación de medidas geoeléctricas superficiales usando el método electromagnético del dominio temporal (TDEM) permitó detectar agua relativamente dulce en los sub-acuíferos inferiores por debajo del área afectada por la intrusión en los sub-acuíferos superiores (inversión hidrológica).Gruesas capas de arcilla intercaladas en los sub-acuíferos inferiores pueden ser erróneamente interpretadas como agua subterránea salina. Problemas similares existen en otros acuíferos costeros de Israel.
Palabras clave Acuífero costero mediterráneo de Israel, método electromagnético del dominio temporal, agua subterránea salina. 45
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INTRODUCTION All geoelectric and particularly electromagnetic (EM) methods are generally suitable for studying salinity of groundwater owing to the close relation, which exists between the salinity and electrical resistivity (conductivity) measured by these methods. It is well known that the more saline the groundwater is to be detected, the more accurate and unique are the geoelectric results. From a hydrological viewpoint, this is due to the fact that, starting from certain salinities (e.g. normal sea water or greater), the lithology of the aquifer plays a much less significant role so that the resistivity of the aquifer is almost solely determined by the water salinity (Goldman, et al, 1988). It is important to emphasize, however, that the latter observation is mostly applicable to porous clastic aquifers rather than to fractured carbonate aquifers, the resistivity of which may vary over a much wider range, even when they are saturated with seawater. Moreover, the resistivity of seawater saturated carbonate aquifers can be similar to or even greater than the resistivity of some dry or freshwater saturated clastic formations (e.g. clays, marls, etc.). From a geophysical viewpoint, the geoelectric parameters of seawater saturated lithologies, characterized by extremely low resistivities, may be uniquely and accurately determined, using a properly selected geoelectric (EM) technique. Both theoretical investigations and an extensive practical studies (Kaufman and Keller, 1983; Fitterman and Stewart, 1986; Nabighian and Macnae, 1991; Goldman, et al., 1991) show that the superior method in detecting salt water saturated layers in the subsurface, is the time domain electromagnetic (TDEM) method. The method possesses the highest sensitivity to electrically conductive targets, provides excellent vertical and lateral resolution and can be easily employed in terrains with electrically difficult surface conditions such as dry sands, hard rocks, etc. The method suffers, however, from an extremely low sensitivity to thin resistive horizons and possesses relatively low noise protection capabilities.
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The TDEM method has been extensively applied for detecting saline groundwater in practically all clastic aquifers of Israel. A total of approximately 800 soundings have been carried out here during last 15 years. Most of the soundings were conducted in the Mediterranean coastal plain (nearly 400) and in the Sea of Galilee and its close vicinity (roughly 300). The rest of about 100 TDEM measurements were performed in the Dead Sea coastal area, the Arava desert and the Gulf of Eilat coastal aquifer. A number of deep TDEM soundings have been recently carried out to detect saline groundwater within the regional carbonate aquifer. The most recent results of the TDEM surveys in all coastal aquifers of Israel are discussed below.
THE MEDITERRANEAN COASTAL AQUIFER The aquifer is approximately 110 km long and extends from the Gaza Strip in the south to the Mount Carmel in the north. For convenience of hydrogeological studies, the aquifer is divided into strips perpendicular to the coast, each of these being 2 km wide (figure 1). The lateral extension of the aquifer varies from approximately 5 km in the north to about 30 km in the south (figure 1). Figure 2 shows a typical hydrogeological cross-section of the aquifer including the geometry of lithological structures, the water table and seawater intrusions into different sub-aquifers. The average thickness of the aquifer is approximately 150 m and it decreases in both eastern and northern directions. The aquifer consists of a Quaternary sequence of marine and continental deposits composed predominantly of calcareous sandstone resting on impermeable black shales and clays of the Saqiye group of Pliocene Miocene age. The western part of the aquifer extending from the shoreline to about 4 km inland, consists of impermeable (mostly clay) horizons which subdivide the aquifer into generally four sub-aquifers (figure 2) herein denoted A
APPLICATION OF THE TIME DOMAIN ELECTROMAGNETIC (TDEM) level, and this results in further invasion of seawater into the aquifer (up to approximately 2 km inland). Due to the dense population, the upper sub-aquifers are also exposed to heavy agricultural and urban/industrial pollution. To date, the exploitation of the aquifer is being carried out solely from the upper subaquifers (A and B). The existence of fresh or brackish water in lower sub-aquifers (C and D), often below seawater intrusion (hydrological inversion), was generally known from a few deep observation wells, in which the salinity measurements were conducted at appropriate depths. However, this water has never been exploited despite the permanently growing deficit of fresh water within the upper sub-aquifers and despite the fact that the water in C and D is almost free of any agricultural or industrial pollution. There are two main reasons for such discrimination, which result from extremely sparse data regarding water salinity within the lower sub-aquifers: • The true dimensions of fresh/brackish water bodies within C and D are practically unknown. • The hydrological connection between the sea and the lower sub-aquifers is not clear. Both problems are crucial for rational management of the lower sub-aquifers, the natural recharge of which is far slower than that in the upper ones. It is clear that the exploitation of deep sub-aquifers becomes efficient only at those parts, where the amount of fresh/brackish water is significant and where, in addition, the sub-aquifers are hydrologically disconnected from the sea, e.g. as a result of the known thickening of clay layers towards the sea. (Kolton, 1988). In order to resolve both of the above-mentioned problems, an extensive TDEM survey including almost 100 soundings have been recently carried out along the Mediterranean coastal plain. Contrary to all previous surveys, most of the measurements were conducted as close as possible to the shoreline using 100 by 100 m transmitter (Tx) loops. In the previous surveys,
Figure 1. Schematic map of the Mediterranean coastal aquifer of Israel.
(phreatic), B (phreatic or confined), C and D (primarily confined). The coastal aquifer is of paramount importance to Israel’s water system owing to its geographic location in close proximity to the most densely populated region of the country. As a result, the aquifer is heavily overexploited despite the existence of a sophisticated water management system based on both a large number of the observation wells and an extensive geophysical (TDEM) monitoring. The overpumpage, in turn, leads to the formation of hydrological depressions, in which the water table sinks below sea
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Figure 2. Typical hydrogeological cross-section of the Mediterranean coastal aquifer.
the Tx size was dictated mainly by the estimated depth to fresh/sea water interface in the upper sub-aquifers. Therefore those few measurements, which have been carried out very close to the seashore in the previous surveys, usually did not provide reliable information about the deep sub-aquifers because of their limited penetration depth. Although the required information has been obtained in numerous measurements remote from the seashore, it was insufficient to prove whether C and D are connected to the sea or not. Indeed, if the water in lower sub-aquifers is fresh far away from the sea, it does not necessarily mean that there is no seawater intrusion closer to the seashore. However, if this is the case near the sea, it is reasonable to assume that C and D are blocked towards the sea in the vicinity of the measurement point. Of course, in any case, the geophysical data must be supported by relevant hydrogeological/geochemical information (Merkado, 1989, Yechieli et al., 1997). In order to evaluate the amount of fresh/brackish water in lower sub-aquifers, a
number of TDEM soundings have been carried out along selected lines perpendicular to the seashore. The results of these measurements are presented below in the form of quasi-2D resistivity cross-sections.
Correlation resistivity/salinity measurements A total of 14 TDEM measurements have been carried out in close proximity to deep observation wells, in which the salinity (borehole conductivity) measurements were performed in all four sub-aquifers. In all cases, the interface within the upper sub-aquifers was accurately detected by TDEM (see figure 3 as a typical example). The resistivity of the seawater-bearing part of the aquifer varied in a fairly narrow range between 1.5 to 1.9 ohm-m. The average resistivity value of 1.7 ohm-m is thus identical to that obtained in the first feasibility study survey, in which the TDEM results where correlated with 39 mostly shallow (within sub-aquifers A and B only) observation wells (Goldman et al., 1991). Figure 3 shows also an increase in the resistivity below seawater intrusion and this
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APPLICATION OF THE TIME DOMAIN ELECTROMAGNETIC (TDEM) observation completely fits the distribution of salinities measured in the observation well. Indeed, the salinities within C and D decrease roughly 2 to 3 times and the TDEM resistivities increase approximately 2 times at appropriate depths (below 86 m). Unfortunately, unlike the upper sub-aquifers, this was not the case in all correlation measurements. Figure 4 shows an example of the correlation measurements, in which the borehole salinities/conductivities within the deep sub-aquifers do not fit the appropriate TDEM resistivities (note that the correlation in the upper sub-aquifers is still good). The water in upper sub-aquifers is fresh up to the depth of about 70 m. Then the salinity somewhat increases in the bottom of B and afterwards decreases again within deep sub-aquifers C and D. The TDEM resistivity accurately char-
acterizes the salinity distribution within Aand B, but then dramatically drops somewhere in the middle of C,D to the values typical of seawater saturated lithologies. A similar picture was observed in approximately 20% of the correlation measurements. It is important to emphasize that in all these cases the TDEM resistivities within C and D consistently testified to much higher salinities than those actually measured in the observation wells. Such a consistency implies an assumption that the disagreement is caused by a real hydrogeological phenomenon rather than by measurement errors or an ambiguous geophysical interpretation. The most reasonable explanation of the phenomenon may be related to the fact that the total thickness of clays within the lower subaquifers is usually much greater than that with-
Figure 3. Correlation of TDEM resistivities and borehole conductivity measurements. Dashed line roughly represents the threshold resistivity value for seawater.
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Figure 4. Correlation of TDEM resistivities and borehole conductivity measurements. Dashed line roughly represents the threshold resistivity value for seawater.
in the upper ones. Due to the very low hydraulic conductivity of clays, they can be saturated with fossil saline water, even, if the water within the aquiferous parts of the subaquifer is fresh. Experience shows that resistivities of aquicludes and aquitards saturated with seawater are similar, if not identical, to those of the seawater saturated aquifers and both are completely different from any freshwater saturated or dry lithologies. Taking also into account an extremely low sensitivity of the TDEM method to the existence of thin resistive layers, it is reasonable to assume that, in the case under consideration, the TDEM resistivies in fact characterize clay layers within deep subaquifers rather than freshwater saturated aquiferous horizons.
The ability of TDEM to accurately characterize the water salinity within C and D thus strongly depends on the aquifer/aquiclude thickness ratio as well as on the depth to the lower subaquifers and on the salinity distribution within the upper sub-aquifers. All these parameters are rarely known a priori and could only be partially determined by TDEM. Therefore the application of TDEM for detecting groundwater salinity within the lower sub-aquifers at separate locations seems questionable in this environment. However, taking into account the small probability of misinterpretation and the fact that TDEM consistently overestimates the water salinity in C and D, one might expect that the method can be successfully applied for the regional mapping of the whole coastal plain or of its significant parts.
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APPLICATION OF THE TIME DOMAIN ELECTROMAGNETIC (TDEM) ble with that in the overexploited upper sub-aquifers Aand B (see figure 5 as an example).
Mapping groundwater salinities in lower sub-aquifers An attempt to characterize groundwater salinity within the lower sub-aquifers at the regional scale, has been recently made along approximately 70 km long line between the city of Ashdod in the south and the city of Caesarea in the north (figure 1). The survey included 60 TDEM measurements, most of which were carried out fairly close to the shoreline. The measurements clearly discriminated between two large areas: the southern area extending approximately between latitudes 140 and 187 and the northern area between latitudes 190 and 206 (figure 1). The overwhelming majority of the TDEM measurements at the southern area clearly delineated fresh to brackish groundwater within the lower sub-aquifers while all the measurements in the northern area showed significantly greater salinity (lower resistivity) within the lower sub-aquifers. Despite the presence of a few outliers in the southern area, which can be easily explained by the above mentioned limitation of the TDEM method, the area is recommended for exploitation of the lower sub-aquifers both for a direct use (fresh water) and/or for a further desalination (brackish water). In order to evaluate the expected amount of fresh/brackish groundwater within the lower subaquifers in the area, it was planned to carry out a number of TDEM traverses perpendicular to the shoreline Three such traverses, which have been measured to date, showed a significant amount of water, compara-
THE DEAD SEA COASTALAQUIFER Dead Sea (hereafter denoted DS) is the terminal lake of the Jordan River system (figure 6), situated in the DS Rift, which is part of the Syrian-African Rift.
Figure 6. Location maps: a general map; b location map of TDEM traverses (dashed lines).
Figure 5. Quasi-2D resistivity cross-section along the TDEM traverse at the Ziqim site.
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TÉCNICAS GEOFÍSICAS The major structural feature in the study area is the western border of the DS Rift, where several large normal faults exist. The coastal plain between the faults and the DS consists mainly of Quaternary continental sediments (figure 7). The main source of fresh-water recharge into the Quaternary aquifer is lateral flow across these faults from the Judea Group aquifer (figure 7), which is replenished in the mountain area some 10-30 km to the west. The DS is hydraulically connected to the adjacent groundwater system; the drop of the DS level, therefore, is accompanied by a drop of the groundwater level.
TDEM measurements A number of TDEM soundings have been made along generally W-E traverses between the DS shore and the fault scarp (figure 6b). Some of the traverses show irregular configurations, the hydrogeological reasons for which is discussed below for the Nahal Temarim traverse as a typical example.
The Nahal Temarim TDEM traverse According to the TDEM resistivities, the configuration of the fresh-saline water interface represent a westward dipping boundaries between the brine (5 ohm-m), and a transition (mixing) zone (1-5 ohm-m) between them (figure 8). The following two features seen in figure 8 commonly appear in other traverses: • An unflushed brine body (sounding 23/7), below and above the present water table, and • A steepening of the interface in the western portion of the traverse close to the fault scarp. The hydrogeological analysis of the both phenomena is as follows. • The DS coastal aquifer is, in most places, known to be subdivided into sub-units such as detected in the Tureibe and DSIF boreholes (Yechieli, 1993), which are not continuous and thus defined as sub-aquifers. In such a case, it is possible that the lower fresher water is related to a different sub-
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aquifer, separated from the upper saline sub-aquifer. The irregularity may also be the result of the fact that the DS system is not in steady-state conditions due to the continuous and rapid drop of the DS levels and subsequently of the groundwater levels. This non-steady-state situation is the reason that parts of the sequence have not yet been completely flushed, although they are presently located above the freshsaline water interface. The same situation also prevails in the unsaturated zone in some locations. Hydraulic head in the study area between the fault scarp and the DS base level exhibits a changing gradient. The steep gradient of water levels across the border faults caused by anisotropic permeability
Figure 8. TDEM traverse at Nahal Temarim. Location of interface was calculated using Ghyben-Herzberg approximation modified for the Dead Sea conditions.
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TÉCNICAS GEOFÍSICAS in the fault zone and/or by different permeabilities in both sides of the fault (Yechieli et al., 2001) would demand a parallel steepening of the freshwater/brine interface.
The most interesting results were obtained in the northern coast and they are discussed below in some details.
TDEM measurements in the northern coastal strip The northern strip is characterized by the following main elements influencing groundwater salinity: • Existence of an additional gulf created by two artificial fjords entering several hundred meters inland to the north of the shoreline (figure 9). • Existence of a deep saline end member. • Evaporation ponds filled up with sea water (figure 9). The TDEM measurements have been carried out along two long perpendicular lines FF' and G-G' and along a short line E-E' running from the seashore inland close to the state border (figure 9). The locations of the TDEM stations were selected as much as possible in
THE GULF OF EILAT COASTAL AQUIFER The Gulf of Eilat is served as a terminal base level to the groundwater flow from both western and eastern mountain aquifers of the Cretaceous age to the Quaternary Rift Valley fill aquifer. The latter is characterized by the intercalations of aquiferous coarse clastic rocks and thick impermeable layers of marls and clays dividing the aquifer into a number of subaquifers. Several TDEM traverses and isolated soundings have been carried out in the western and northern coastal strips of the Gulf of Eilat.
Figure 9. Location map of the TDEM measurements at the northern coast of the Gulf of Eilat.
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APPLICATION OF THE TIME DOMAIN ELECTROMAGNETIC (TDEM) accordance with the above mentioned hydrogeological features. For example, line F-F' runs as close as possible to the fjords and evaporation ponds while line E-E' begins very close to the seashore and passes the evaporation ponds. Figure 10 shows an example of a quasi2D resistivity cross-section drawn from the interpreted 1-D TDEM measurements along line F-F'. In order to define layering within the cross-section, the following resistivity and appropriate hydrogeological units have been introduced: Resistivity range (ohm-m) > 10 2-10 1-2
