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The relationship between saline groundwater within the Arava Rift Valley in Israel and the present and ancient base levels as detected by deep geoelectromagnetic soundings. U. Kafria*, M. Goldmanb,c and E. Levib a

Geological Survey of Israel, 30 Malchei Israel St., Jerusalem 95501, Israel

b

The Geophysical Institute of Israel, P.O.B. 182, Lod 71100, Israel

c

Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA

Abstract

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The time domain electromagnetic (TDEM) geophysical method was employed to detect saline groundwater bodies within and in the close margins of the Arava Rift Valley. The Arava Valley aquifers are known to occupy fresh to saline groundwater. The lateral subsurface

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inflow to the Arava from west and east is characterized by fresh to brackish waters. The results of the present study indicate that salination of groundwater is controlled by both present day and ancient base

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levels, namely by the Dead Sea in the north and by the Gulf of Elat in the south. The configuration obtained by the TDEM survey exhibits interfaces and palaeo-interfaces between fresh to brackish waters and underlying seawater or diluted seawater intruded inland from both base levels as well as brines intruded from the northern base level. The central Arava structural and hydrological divide

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seems to escape seawater or brine encroachment at least to the considerable depth of the TDEM measurements.

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Keywords: Arava Rift Valley, groundwater, salination, base levels, deep TDEM

Introduction General The Arava Rift Valley is located between the Gulf of Elat in the south and the Dead Sea in the north (Fig. 1). From hydrogeological point of view, the main problem here is related to salination processes, which are abundant in this area. Different mechanisms as well as deep seated saline sources or end members were proposed in numerous studies to explain the salination processes. Unfortunately, in most cases

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2 none of the existing boreholes or geophysical soundings in the study area was deep enough to penetrate or directly detect those end members. In order to obtain the necessary information, a deep TDEM survey was carried out along the entire Arava Valley targeted to detect those end members, where present, and to delineate their extension. In addition, the survey was aimed to reveal the genetic relationship between those end members and the different base levels and palaeo-base levels in the south and north. Hydrogeological and hydrochemical background The hydrogeology and groundwater chemistry of the Arava Rift Valley and its western margins were discussed in numerous studies (i.e. Bartov and Bein, 1994;

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Bein et al., 1995,1998; Fleisher et al., 1996, 1997; Guttman et al., 1999; Issar et al., 1972; Kroitoru et al., 1981; Kronfeld et al., 1992; Magal and Yechieli, 2002 and Rosenthal et al., 1992.)

A concise summary based on previous studies as well as on some of the present observations is as follows:

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The regional aquifers are the Palaeozoic to early Cretaceous Kurnub Group (or

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Nubian sandstone) aquifer and the late Cretaceous Judea Group aquifer. Both are considered as the mountain aquifers being exposed to recharge at the margins of the Rift Valley and serving as outlets to ancient and renewable inflow to the Graben fill aquifer.

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The Graben fill aquifer includes the Neogene to Recent Hazeva Formation, the Arava

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Conglomerate and the young graben fill, which occupy mainly the Rift but are also exposed at its margins.

The general groundwater flow direction is from both western and eastern mountain aquifers into the Graben fill aquifer. The existence of a hydrological divide within the Rift Valley (see below) dictates the separation of groundwater flow to the Gulf of Elat base level in the south and to the Dead Sea base level in the north. The drainage basin of the Arava valley is divided into two, namely the southern and northern Arava sub-basins (Fig. 2). Rainfall distribution in the Arava drainage basin, which creates runoff and base flow recharging the Graben fill aquifer, is not even. The southern sub-basin gets low rainfall values from both sides whereas the northern one gets low rainfall from the west but rather higher ones from the Edom Mountains in the northeast. The surface area of the southern sub-basin is 2076 square km, while that of

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3 the northern one is 8658 square km. The latter extends from eastern Sinai and drains by a few long streams with perennial floods that end up recharging the northern Arava Graben fill aquifer. As a result, the northern sub-basin gets considerably higher fresh water contributions from rain and floods than the southern one. The average groundwater flow gradient within the Arava Valley is approximately 1.3 per mil to the southern base level and 3.5 per mil to the northern one. The steepness of gradients in some segments of the northern flow was attributed to passage across some of the existing transversal faults within the Rift Valley (Guttman et al., 1999). An alternative interpretation is given herein based on Graben fill aquifer thickness variations, lateral inflows and the existence of an interface(s) in the north.

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The northern sub-basin exhibits gradual thickening of the Graben fill aquifer from south to north (Fig. 3). In its southern portion, the combination of relatively thin Graben fill aquifer with considerable lateral floods inflow from the large streams (Hiyon, Paran) results in higher groundwater levels and steeper gradients (Fig. 3). Farther north, the Graben fill aquifer thickens and the gradients as expected are

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milder. Towards the Dead Sea base level, despite the considerably thick Graben fill aquifer, the flow cross section above the inclined interface is gradually reduced and

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the flow gradient as expected, steepens again.

Since electric resistivity measured by TDEM cannot distinguish between different genetic water types, only total salinity or chlorinity of groundwater is discussed in the

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present study. The distribution of chlorinities at different regions and aquifers according to the existing borehole data is given in Table 1. The results can be summarized as follows:

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The Kurnub and Judea Group aquifers contain only fresh to brackish waters along the entire Arava Valley and its margins.

In the Graben fill aquifer, fresh water is found close to the divide and the outlets of the large streams from the west to the northern sub-basin. In all other regions, fresh to brackish waters are abundant. Seawater concentrations are encountered in the immediate vicinity of the Gulf of Elat and concentrated brines, up to 202,000 mg/l Cl, were detected in the Arava 1 well close to the Dead Sea. Superficial to shallow brines were encountered in the so called Sabkha sediments at Yotvata (up to 100,000 mg/l Cl) and Evrona. These were formed due to the emergence of brackish waters from depth and evaporation close to the surface (Michaeli and Galai, 1963; Michaeli, 1971; Kafri, 1977).

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4 The different brackish waters found in the study area were defined in several studies as mixtures of fresh waters and emerging deep seated brines despite the fact that seawater or brine end members were encountered only close to the southern and northern base levels. The internal structure of the Rift Valley The internal structure of the Rift Valley has an important bearing on its hydrology and its subdivision to sub-basins. The internal structure is deduced from geological as well as geophysical data (Fleischer and Gafsou, 1998; Frieslander, 2000). The Rift Valley is a segment of the Transform system. Due to the movement along the

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Transform and to the resultant existence of diagonal transversal faults, the Rift Valley is subdivided into sub-basins (Frieslander, 2000). A topographic and structural divide occurs in the center of the Arava Valley, where the Rift is narrow and closed. From here towards the base levels in the south and north, the Rift deepens gradually and the thickness of the Graben fill aquifer is dictated by the internal structure of the Rift (Fig.

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3). The thickness of the Graben fill aquifer at the divide is between 0-200 m. Further to north, its thickness exceeds 3500 m in the Dead Sea sub-basin and towards south it

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attains a thickness of up to 3000 m in the southern Arava close to the Gulf of Elat. The TDEM method

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Depending on the required depth to the target, there are two principal modifications

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of the TDEM also known as TEM (transient electromagnetic) method. The first modification, termed the near zone or short offset TEM (SHOTEM) uses transmitter (Tx) – receiver (Rx) separations that could be significantly less than the depth to the target. The most common array used in the SHOTEM modification is the central loop (CL) array (Fig. 4, left). By using this array, one can detect a target at depths two to three times of the Tx size. The SHOTEM modification provides the best lateral and excellent vertical resolution with regard to electrically conductive targets. It is practically insensitive, however, to resistive (particularly thin) structures and suffers from extremely low signal-to-noise characteristics. Another modification named the far zone or long offset TEM (LOTEM) uses Tx-Rx separations, which are several times greater than the depth to the target. In most cases, the LOTEM array consists of a long grounded line as Tx and both grounded and

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5 inductive antennae as receivers (Fig. 4, right). The LOTEM modification suffers from low spatial resolution, but is sensitive to both conductive and resistive targets and provides still reasonably high signal-to-noise characteristics. The choice of either modification depends on many, sometimes contradictory factors, but, if the target is highly conductive and the survey is carried out in a desert area, the SHOTEM modification is superior compared to LOTEM and any other geoelectric or geoelectromagnetic technique. While LOTEM is, by definition, the depth oriented technique, the SHOTEM method is further subdivided into three different modifications according to the required penetration depth. These are: shallow SHOTEM up to approximately 100 m depth

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(e.g. Geonics EM-47, Zonge Nanotem, FASTTEM, etc. systems), medium depth systems up to approximately 500 m (Geonics EM-37 or EM-67, Sirotem Mark-IV, etc.) and deep SHOTEM up to approximately 2 km depth (e.g. Geonics EM-42 and Cycle-5M). The Cycle-5M deep TDEM system by Elta Ltd., Novosibirsk, Russia, has been used to detect deep aquifers, as well as to solve various deep

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geological/hydrogeological problems, which could not be solved by the existing SHOTEM equipment due to its limited penetration depth. The estimated penetration

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depth of the Cycle-5M system varies between 800 to roughly 2000 meters, depending on local geoelectric conditions and the level of EM noise in the area. The deep Cycle-5M system

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Similarly to standard SHOTEM systems such as Geonics EM-67, etc., CYCLE-5M

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consists of a receiver (ZEI-5M) and a transmitter (GTE-45). The transmitter is connected to a transmitter loop (Tx), the size of which may vary from a few hundred by few hundred meters to as much as 1 km by 1km. In the present survey, the Tx loop size varied between 500 by 500 m and 600 by 600 m at all thirty measured stations (besides a few points, where, due to local topographical limitations, the Tx loop was a rectangle of a similar moment). The receiver is connected to a receiver coil (Rx) normally located at the transmitter loop center. In order to perform depth sounding, the current in Tx is abruptly terminated thus generating induction currents within the subsurface. These induction currents move downwards and outwards with increasing time. The response from these currents is measured at Rx site as a time varying signal or transient. In its amplitude and shape, this transient contains information about the subsurface resistivity, while earlier times

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6 provide the information regarding shallow depths and later times provide the information about deeper depths. More detailed description of the physical background of the TDEM method can be found for example in Fitterman and Stewart (1986). In order to achieve a maximum possible signal-to-noise ratio (SNR), Cycle-5M uses a specially designed multi-turn Rx with low noise preamplifier providing an effective area of 250,000 m2. Such an area is equivalent to a loop of 500 by 500 m size, while the actual dimensions of the Rx are only 4 by 4 m. It is clear that such a dramatically increased Rx moment is mostly essential in sparsely inhabited areas with relatively low external EM noise level, such as the Judea Desert, Negev, the Arava Valley, etc.

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In areas with strong ambient noise, a significant increase in SNR is achieved by using very powerful current transmitting system (Fig. 5) as well as by using various physical and digital filtering techniques. The current transmitting system consists of an 80 to 100 KVA motor generator, a high power ballast resistors, a current rectifier, a current transmitter and a control unit

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including GPS for the synchronization between the transmitter and the receiver (Fig. 5).

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In the course of the present survey, most of the measurements were carried out using the deep Cycle-5M TDEM system. At some points, the Cycle-5M measurements were accompanied by shallow and middle depth range measurements (Geonics EM 67) to

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provide better resolution of relatively shallow structures and to control the deep measurements.

Previous TDEM surveys in the Arava Valley

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Several shallow TDEM surveys, up to a depth of a few hundred meters, were carried out in the past along the Arava Valley. The results of these surveys are described by Bartov et al. (1998), Goldman and Izersky (1999), Kafri et al. (2001) and are summarized in the database of the Geophysical Institute of Israel (Gendler and Goldman, 2003). Recent geoelectromagnetic surveys conducted in the framework of the Dead-Sea-Rift-Transec (DESERT) project across the Arava fault provide additional local geoelectric information regarding both shallow (TDEM) and deep (magnetotelluric) structures in the eastern margins of the central Arava valley (Ritter et al. 2003, Koch et al., 2004 and Maercklin et al., 2005).

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7 In general, the shallow measurements revealed in both the southern and northern Arava rather shallow, low resistivity structures interpreted as saline groundwater bodies of limited extension. In the north, close to the Dead Sea those measurements detected a very low resistivity unit (below 1 ohm-m) at a relatively shallow depth of several hundred meters. The magnetotelluric measurements in the eastern Arava valley revealed a considerable drop of resistivity to roughly 1 to 2 ohm-m at a depth of approximately 1.2 km. The working hypothesis The relationship between TDEM resistivity and groundwater salinity.

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The relationship between TDEM resistivities and groundwater salinity for different aquifers in Israel was discussed among others by Kafri and Goldman (2005). Based on the experience gained in Israel, the conclusions that were obtained and employed in the present study are as follows:

In coastal granular aquifers, resistivities below 1 ohm-m represent hosted brines

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(sometimes diluted); resistivities between 1 and 3 ohm-m are typical of normal seawater concentrations (also diluted to some extent), resistivities between 3 and 10

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ohm-m are associated with hosted brackish to fresh waters and resistivities exceeding 10 ohm-m represent hosted fresh water.

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In the case of carbonate aquifers, due to their generally lower porosities, the appropriate resistivities are higher than those of granular aquifers, namely, resistivities between 3 and 5 ohm-m represent hosted seawater; resistivities between 5 and 20

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ohm-m are typical of brackish waters and those exceeding 20 ohm-m represent fresh water.

In the present study, the Judea Group aquifer was regarded as a carbonate aquifer whereas the Kurnub Group and the Graben fill aquifers as granular aquifers. The appropriate groundwater salinities within the aquifers were interpreted accordingly. Regional base levels as sources of salination. The existence of the different (both in time and space) current and ancient base levels in the north and south related to the Rift Valley has an important bearing on its hydrology. It is most important to reveal salination as well as flushing mechanisms within the time “memory” of the system. Hypothetically, salination by seawater or

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8 brines can also be contributed to the Rift Valley laterally from the east and west but those potential aquifers (Kurnub and Judea Groups) are known to occupy only brackish to fresh waters. Parts of the Rift Valley in the north acted as a base level already in the Pliocene following the marine transgression from the north (Schulman, 1962). The elevation of that base level, several tens of meters above sea level (hereafter asl), is documented by the uppermost exposures of the Bira Formation in the north as well as of the Mazar Formation south of the Dead Sea (Eyal, 1984; Yechieli, 1987; Yechieli et al., 1994). In addition, subsurface information reveals the existence of the same formation in the southern Arava related to the transgression from the Gulf of Elat (Bartov and Bein,

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1994), apparently disconnected from the northern transgression by the structural divide in between. One can assume that at this stage seawater (or partly diluted seawater) could penetrate from these base levels and fill both the underlying, already existing Hazeva Formation and, laterally, the mountain aquifers. This assumingly formed a configuration of convergent fresh to brackish flow to the Rift Valley above a fresh/seawater interface.

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Following the detachment of the above northern transgression from the

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Mediterranean, an inland terminal lake or base level was formed, which was gradually depleted accompanied by formation of brines and evaporites deposition. In parallel, the subsurface intruded Pliocene saline waters were continuously flushed back to the

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receding base level. On the other hand, there is no clue to assume that the southern transgressed system was disconnected from the main seawater body in the south.

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The younger Lisan lake, which occupied a large segment of the Dead Sea Transform system since some 70,000 years ago, attained an ultimate level of 180 m below sea level (hereafter, bsl) exhibiting salinity approximately 5 times greater than normal seawater salinity (Begin et al., 2004). The internal Lisan lake base level occupied the Rift Valley south of the Dead Sea reaching as south as the Idan area. At this time span, the Lisan lake brine assumingly managed to penetrate and fill the underlying Graben fill aquifer and laterally the mountain aquifers. Since then, the consequent continuous (although fluctuating) drop of the base level ended up with the present, still depleting Dead Sea base level. This is assumingly accompanied by continuous flushing of the pre-existing intruded saline waters to the dropping base level. The present Dead Sea level of 420 m bsl and its contained brine salinity, which is roughly 10 times greater than that of normal seawater salinity,

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9 contributes to the salinity of the regional aquifers due to the downward and lateral penetration of the brine into the aquifers thus forming an interface with the overlying cyclic fresher water. In the southern Arava, the base level dropped since the Pliocene and at the peak of the last glacial, some 18,000 years ago, it reached the level of 120 m bsl and consequently had risen to the present level. This process is supposed to be accompanied by flushing and intrusion of only normal seawater, different from the case of the northern Arava. Local base levels as salination sources. Local areas of closed drainage are known in the southern Arava as sabkhas (e.g. the

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sabkhas of Yotvata and Evrona). As described before, these are typical of shallow to superficial brines that are limited in thickness and lateral extent. Such appearances or ancient sabkhas are expected to be encountered in the subsurface as having a limited lenticular geometry in different levels, not related to the regional base levels. The conceptual model

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Based on the above working hypothesis, a conceptual model exhibited on Fig. 6 as a

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longitudinal cross-section along the Arava Valley between the Gulf of Elat and the Dead Sea.

The inputs to the model were as follows:

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The present base levels are at 0 m and at 420 m bsl in the Gulf of Elat and the Dead Sea, respectively. The Pliocene marine base level in the northern and southern Arava

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is assumed at 60 to 70 m asl, and the Lisan lake base level in the northern Arava is at 180 m bsl. The marine base level at the peak of the last glacial in the Gulf of Elat is at 120 m bsl.

Current elevation of the water table is based on existing data, whereas palaeo-water tables are reconstructed according to the location of the ancient base levels and assumed groundwater flow gradients, similar to the present ones. The calculated saline water densities that occupy or occupied the different base levels are: 1.029 for normal seawater, 1.115 for the Lisan lake brine and 1.23 for the present Dead Sea brine. Based on the above data, the configurations of the interfaces for each stage were calculated using the Ghyben-Herzberg approximation.

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10

Fig. 6 exhibits the obtained configuration for the different stages: 1

The central Arava region, between latitude coordinates 420 and 480, which

coincides with the structural and hydrological divides, is supposed to contain only fresh to brackish waters to a depth of 1200 m bsl in its center. 2 In the southern Arava, only remains of unflushed or partly flushed Pliocene seawater are expected in most of the area as well as younger to modern seawater intrusions closer to the Gulf of Elat. 3 The northern Arava can be subdivided into two sub-regions. The southern one, up

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to latitude coordinate 520 in the north is expected to occupy only remains of unflushed or partly flushed Pliocene seawater. Farther north towards the Dead Sea remains of Pliocene seawater, Lisan lake brine as well as current Dead Sea brine are expected to be found in a zoned configuration. It should be noted that some differences between the “ideal” model and the actual

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situation might stem from the existence of a transition zone between fresh and saline water bodies rather than the suggested sharp interface. Additional factors that may be

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responsible for such differences are partial flushing and dilution processes as well as tectonic instability, which prevent the system from reaching a steady state and retaining the memory of older stages.

Results and discussion of the TDEM survey

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Goldman et al. (2006) described in details the results of the different TDEM measurements carried out in the course of the present survey. Based on the above, a longitudinal cross-section between the Gulf of Elat and the Dead Sea (Fig. 7) and two transversal cross sections (Figs. 8 and 9) were constructed. The cross-sections exhibit resistivity intervals indicative of specific groundwater salinities within different types of aquifers as described above. In addition, top Basement, top Judea Group and base Graben fill aquifers are shown enabling by extrapolation to decide whether a specific resistivity interval is related to a granular or to a carbonate aquifer. Correlation is supported by available subsurface borehole data.

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11 Based on structural hydrological and resistivity characteristics, the longitudinal cross section (Fig. 7) can be subdivided into four segments or basins from north to south. Resistivities discussed below are already translated to salinities:

Segment A (The Hazeva-Dead Sea basin) The entire segment consists of a granular Graben fill aquifer sequence bottomed at absolute depths between 2500 to 4000 m bsl. An interface between fresh and brackish waters declining from north to south underlain by very low resistivity (below 1 ohmm) brines is encountered at measurement points (mp) 2, 1, 3, 20. The top of the very

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low resistivity unit reaches a depth of at least 1400 m bsl. Such brines were detected in the Arava 1 well, adjacent to mp 1. In mp 3, the brine is overlain by a 2 ohm-m resistive body representing a transition zone or unflushed earlier seawater. Moreover, the very low resistivity brine exhibits a gradual increase of resistivity southward indicating an increasing dilution with distance from the base level. More to the south

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(mp 11, 19) a southward declining interface between fresh to brackish waters underlain by waters of approximately seawater concentration is encountered.

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Segment B (The En Yahav-Zofar basin)

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This segment is aligned at the margins of the Rift and the soundings, thus, penetrate only the mountain formations and aquifers. The Judea Group carbonate aquifer contains fresh to brackish waters in its upper part, underlain in places (mp 17 and 8)

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by waters of seawater concentration. The underlying Kurnub Group granular aquifer contains waters of seawater concentration.

Both the W-E directed TDEM transversal cross-section through mp 8 and 9 (the Zofar 20 well) and the magnetotelluric cross-section (Ritter et al., 2003) in the center of the Arava (Fig. 8) exhibit the following: An interface between fresh water and underlying waters of seawater concentration within the Kurnub Group aquifer is encountered at an elevation of 1200 to 1300 m bsl. According to the Zofar 20 well, artesian water of 3500 mg/l Cl emerge from the bottom of the borehole at around 1000 m bsl, still above the interface as obtained by the TDEM measurements.

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12 More to the south, at mp 14, 16, 30 and at the more western mp 10 and 15 (Fig. 9) the same interface, declining to 2000 m bsl towards south and east, is encountered. At the bottom of the Paran 20 borehole, brackish waters were found, assumingly representing diluted saline waters detected by TDEM at a greater depth. Segment C (The Menuha-Qetura basin) This segment partly coincides with the structural divide of the Arava Valley, where the Rift is “closed” and the Graben fill aquifer, overlying the Judea Group aquifer, is very thin. The top of the crystalline basement here (mp 22, 4, 9 and 23) is relatively shallow, between 600 and 1200 m bsl. The depth to the basement is estimated by

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downward extrapolation from higher stratigraphic levels as well as by means of geoelectromagnetic measurements due to the exceptionally high resistivities of the basement (Kafri et al., 2001; Ritter et al., 2003). In its northern part, the segment is characterized by the absence of low resistivities (saline waters) along the entire penetrated sequence (mp 4, 22, 12, 21 and 13). This

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observation is supported by the occurrence of only fresh water encountered in the Yaalon 7 borehole adjacent to mp 4. More to the south (mp 23, 29) high resistivities

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(fresh waters) are encountered also in complete agreement with adjacent boreholes both within the Judea Group aquifer (e.g. the Qetura 19 borehole) and within the

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Kurnub Group aquifer (e.g. the Qetura 9 borehole). However, contrary to the northern part of the segment, those are underlain by lower resistivities (brackish water) within the Kurnub Group aquifer (e.g. the Gerofit 5 borehole). Segment D (The Yotvata-Elat basin)

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This segment is aligned in the Graben fill aquifer that thickens southward. There is no information regarding the formations underlying the Graben fill aquifer here, except for mp 27, which exhibits very high resistivity at 1000 m bsl, thus testifying to the possible occurrence of the crystalline basement at this depth. The segment is characterized by the following main features: Thin low resistivity bodies are encountered in the shallow sequence (up to the depth of 100 m). These are interpreted herein as saline waters related to existing or ancient sabkhas or to unflushed clay layers (Kafri et al.,2001).

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13 The upper high resistivity (fresh water) body is not thick. It is bottomed by an interface declining from 100 m to 600 m bsl in a south-north direction. The underlying water body exhibits brackish to seawater concentrations. The interface is shallower than the calculated one using the current groundwater table (Fig 6). Moreover, the current seawater intrusion is limited to the close vicinity of the Gulf of Elat and only to the uppermost Graben fill aquifer (Bein et al., 1998). It is, therefore, deduced that the discussed saline waters are remains of partly flushed or unflushed saline waters related to the Pliocene transgression. The underlying high resistivities are interpreted as the crystalline basement formations since no fresh waters are expected at such depths below the saline ones.

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No deep, very low resistive brines have been encountered in the entire segment. Saline waters in Timna were interpreted through geochemical parameters as originating from deep seated brines. The heavy stable isotope composition of some of these waters is indeed indicative of evaporation. This can provide an alternative interpretation of density driven brines from the nearby existing or ancient shallow sabkhas.

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The overall salinity distribution configuration along the Arava, as deduced from the

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longitudinal TDEM cross section (Fig. 7) basically resembles the one shown in the conceptual model (Fig. 6).

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Conclusions

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The TDEM survey managed to detect fresh, brackish and seawater to brine concentrations and the interfaces between the appropriate water bodies. The obtained longitudinal resistivity cross-section generally resembles the suggested conceptual hydrogeological model. The convergent groundwater flow into the Arava from the mountain aquifers is fresh to brackish. The salination of groundwater is presently, as in the past, controlled by seawater intrusions from the southern (Gulf of Elat) and northern (Dead Sea) base levels as well as by the ancient Lisan lake and the present Dead Sea brines from the northern base level. As a result, a salinity zonation is observed along the Arava from north to south:

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14 In the Hazeva-Dead Sea basin in the north, fresh to brackish groundwater overlies concentrated brines in its northern portion and seawater in its southern part. In the En Yahav-Zofar basin, the fresh to brackish waters overlie seawater concentrations. In the central Arava structural and hydrological divide, fresh waters are encountered in the entire measured sequence. In the southern Yotveta basin, a fresh to brackish water body is encountered with some residual Pliocene seawater and shallow sabkha type local brines. The TDEM results, shown on the longitudinal cross section generally support the configuration suggested in the conceptual model.

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The obtained configuration might assist in developing an optimal groundwater exploitation scheme, which would minimize salination of groundwater in the whole Arava Valley.

Acknowledgements

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The study was funded by the Hydrological Service of Israel. Our thanks are due to Vladimir Fridman and Yakov Livshitz for providing the necessary hydrological data

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and for valuable and constructive remarks in the course of the study they made along with Avi Burg and Gabi Weinberger.

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The successful field survey would not be possible without an extraordinary assistance of Alex Zakharkin .

References

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Bartov, Y. and Bein, A., 1994. The geology and hydrogeology of the central Arava between Yotveta and Paran. Geol. Surv. Israel, Rep. GSI/5/94 (In Hebrew). Bartov, Y., Goldman, M., Kalbo, R. and Ronen, A., 1998. A TDEM survey in northern Arava. Interim report. Geol. Surv. Israel, Rep. TR-GSI/2/98 (In Hebrew). Begin, Z. B., Stein, M., Katz, A., Machlus, M., Rosenfeld, A., Buchbinder, B., Bartov, Y., 2004. Southward migration of rain tracks during the last glacial, revealed by salinity gradient in Lake Lisan (Dead Sea rift), Quaternary Science Reviews, 23: 1627-1636.

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15 Bein, A., Yechieli, Y. and Halicz, L., 1995. The geochemistry of groundwater in the southern Arava. Geol. Surv. Israel. Rep. GSI/43/95 (In Hebrew). Bein, A., Yechieli, Y. and Ben Shabat, J., 1998. Hydrogeological regional model of the southern Arava. Geol. Surv. Israel, Rep. GSI/15/98 (In Hebrew). Eyal ,A., 1984. The geology of the northern Arava and its western margins in the Hazeva-En Yahav region. M. Sc. thesis, Hebrew University, Jerusalem (In Hebrew). Fitterman, D.V. and Stewart, M.T., 1986. Transient electromagnetic soundings for groundwater. Geophysics, 53: 118-128. Fleisher, E., Fleischer, L. and Frieslander, U., 1996. Groundwater in the Arava,

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Yaalon-Yotveta region. Geophys. Inst. Israel, Rep .820/56/96 (In Hebrew). Fleisher, E., Fleischer, L. and Frieslander, U., 1997. The hydrogeology of the Idan-En Yahav region. Geophys. Inst. Israel, Rep. 820/36/96 (In Hebrew). Fleischer, L. and Gafsou, R., 1998. Top Judea Group – digital structural map of Israel, Phase 1 (Arava Valley and Galilee). Geophys. Inst. Israel, Rep. No. 873/90/97.

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Frieslander, U., 2000. The structure of the Dead Sea,emphasizing the Arava in the light of new geophysical data. Ph. D. thesis, Hebrew University, Jerusalem (In Hebrew).

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Gendler, M., Goldman, M., 2001. Development of the database and calibration of electromagnetic (TDEM) results by hydrological well data throughout Israel.

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GII Report 822/121/01.

Goldman, M., Gilad, D., Ronen, A. and Melloul, A. 1991. Mapping of seawater

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intrusion into the coastal aquifer of Israel by the time domain electromagnetic method. Geoexploration, 28: 153-174.

Goldman,M. and Izersky, M., 1999. A TDEM survey in the region of water reservoirs in the northern Arava. Geophys. Inst. Israel, Rep. 822/90/98(3) (In Hebrew). Goldman, M., Kafri, U. and Levi, E., 2006. Detection of saline water bodies along the Arava Rift between the Dead Sea and the Gulf of Elat using geoelectric methods. Geophys. Inst. Israel, Rep. 932/211/06 (In Hebrew). Guttman, J., Burg, A., Lifshitz, A., Lumelsky, S. and Zukerman, H., 1999. Flow model for the evaluation of the water potential of the Arava aquifer. Tahal. Water Planning for Israel (In Hebrew).

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16 Issar, A., Bein, A. and Michaeli, A., 1972. On the ancient water of the Upper Nubian Sandstone aquifer in central Sinai and southern Israel. J. Hydrology, Vol. 17: 353-374. Kafri, U., 1977. Disposal of waste effluents formed during the proposed production of titamium dioxide in Timna. Geol. Surv. Israel. Rep. Hydro/2/77. Kafri, U. and Goldman, M., 2005. The use of the time domain electromagnetic method to delineate saline groundwater in granular and carbonate aquifers and to evaluate their porosity, J. Appl. Geophys., 57: 167-178. Kafri, U., Goldman M and Gendler, M., 2001. A TDEM survey to characterize groundwater salinity in the southern Arava and the Gulf of Elat. Surv.Israel,

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Rep. TR-GSI/3/2001 and Geophys. Inst. Israel, Rep. 906/94/00 (In Geol. Hebrew).

Koch, O., Helwig, S.L., Meqbel, N., 2004. Vertical near surface conductivity anomaly detected at the Dead-Sea-Transform. Proceedings of the 17th Workshop on Electromagnetic Induction in the Earth, Hyderabad, India. Available at

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http://www.emindia2004.org: 1/6-6/6. Kroitoru, L., Kronfeld, J. and Ginzburg, A., 1981. The geohydrology of the Gerofit-

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Ya'alon area. Isr. J. Ear. Sci., Vol 30: 24-30.

Kronfeld, J., Weinberger, G., Yaniv, A., Zafrir, J., Vulcan, U., Agami, M. and Rosenthal, E., 1992. Uranium isotope disequilibrium studies and the

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geohydrology of the Arava rift valley. Israel. Nucl. Geophys., 6: 535-545. Maercklin, N., Bedrosian, P.A., Haberland, C., Ritter, O., Ryberg, T., Weber, M. and

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Weckmann, U., 2005. Characterizing a large shear-zone with seismic and magneotelluric methods: The case of the Dead Sea Transform. Geophysical Research Letters, Vol. 32.

Magal, E. and Yechieli, Y., 2002. A literature survey of the Yotveta sabkha. Geol. Surv. Israel, Rep. GSI/9/2002 (In Hebrew). Michaeli, A., 1971. A survey of brackish water salinity. Southern Arava. Tahal. Water Planning for Israel (In Hebrew). Michaeli, A. and Galai, A., 1963. A hydrological study in the Yotveta region. Tahal. Water Planning for Israel. P. M. 359 (In Hebrew). Ritter, O., Ryberg, T., Weckmann, U., Hoffmann-Rothe, A., Abueladas, A., Garfunkel, Z., 2003. Geophysical images of the Dead Sea Tranform in Jordan

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17 reveal an impermeable barrier for fluid flow. Geophysical Research Letters, Vol. 30. Rosenthal, E., Adar, E., Issar, A. and Batelaan, O., 1992. Definition of groundwater from patterns by environmental traces in the multiple aquifer system of the southern Arava valley, Israel. J. Hydrology, Vol. 117: 339-368. Schulman, N., 1962. The geology of the central Jordan Valley.Ph.D.thesis,Hebrew University,Jerusalem (In Hebrew). Yechieli,Y., 1987. The geology of the northern Arava Rift and the Mahmal anticline, Hazeva region. Geol. Surv. Istael, Rep. GSI/30/87 (In Hebrew). Yechieli, Y., Elron, E. and Sneh, A., 1994. Geological map of Israel, 1:50,000, Neot

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Hakikar Sheet. Geol. Surv. Israel. (In Hebrew). Figure Captions

Figure. 1. Location map of the study area including TDEM mesurement points, cross-

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sections and selective correlation boreholes. Figure 2. The Arava Valley drainage basins.

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Figure 3. A map showing the thickness of the Neogene to Recent Graben fill within the Arava basins (After Frieslander, 2000).

Figure 4. Two major modifications of the TDEM/TEM method: SHOTEM (left sketch) and LOTEM (right sketch).

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Figure 5. Deep SHOTEM system Cycle-5M.

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Figure 6. A cross-section exhibiting the conceptual model.

Figure 7. A longitudinal columnar TDEM resistivity cross-section between the Gulf of Elat and the Dead Sea. tB =top Basement tJ =top Judea Group B.G.F. =Base Graben Fill Figure 8. A transversal columnar TDEM resistivity cross-section in the Zofar region correlated with the magnetotelluric 2-D resistivity cross-section and borehole data. Figure 9. A transversal columnar TDEM resistivity cross-section in the Paran region correlated with borehole data.

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18

Table Caption Table 1. Groundwater chlorinity (mg/l) in the different aquifers and regions of the Arava Valley. The latitude coordinates as in Fig. 1.

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Aquifer Region

Kurnub Group

Judea Group

Hazeva Group & Graben fill

Neot Hakikar

(Arava 1) 202,000

Hazeva Idan

715

Zofar

520

600 - 700 3,500

200 - 790 500

480 - 900

200 - 480 480

(Zofar 20) 580 - 750

200 - 300

400 - 1,350

Qetura Yotvata

350 - 3,500

Samar Timna Be'er Ora

3,000 - 3,500

450

150 - 850

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Yahel Yaalon

460

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The Arava divide

440

500 – 2,400

shallow brines

200 - 800

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Eilat

400 - 1,700

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Paran Menuha Hiyon

540

200 - 1,800

(Ein Ofarim)

En Yahav

Latitude coordinates

500 -12,000

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420

650 – 7,300

shallow brines 400

1,000 – 5,300 seawater near the coastline

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380

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