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The Pliocene Yafo Formation in Israel: Hydrogeologically inert or active? D. Avisar · E. Rosenthal · H. Shulman · M. Zilberbrand · A. Flexer · J. Kronfeld · Z. Ben Avraham · L. Fleischer

Abstract For several decades the “Saqiye beds” (later renamed Yafo Formation) underlying the Coastal Plain aquifer (Kurkar Group) aquifer of Israel, were regarded as an extremely thick, tectonically undisturbed, and absolutely impervious aquiclude. Following intensive groundwater exploitation from the overlying Kurkar Group aquifer, brackish and saline waters were locally encountered in the lower parts of this aquifer and always at the contact with the underlying Yafo Formation aquiclude. The present study revealed that this aquiclude is not a uniform and impervious rock unit, but rather an alternation of pervious and impervious strata within the Yafo Formation containing highly pressured fluids of different — mostly high — salinities. The permeable beds are at an angular unconformity and in direct contact with the overlying Kurkar Group aquifer. The Yafo Formation and the underlying and overlying rock units are dislocated by numerous fault systems, which facilitate accessibility of brines into the Kurkar Group aquifer. The mobilization of the saline fluids and their injection into the Kurkar Group aquifer could be due either to diffusion of saline fluids occurring in the permeable horizons of the Petah Tiqva Member through the clays of the Yafo Formation or to their upconing following intensive pumping in the Coastal Plain aquifer. It could have also been caused by up-dip movement of saline water as the result of overpressure generated by major accumulation of gas in the permeable horizons. Another possible mechanism could be hydraulic Received: 27 November 2002 / Accepted: 15 October 2003 Published online: 16 April 2004  Springer-Verlag 2004 D. Avisar ()) · E. Rosenthal · H. Shulman · A. Flexer · J. Kronfeld · Z. Ben Avraham Dept. of Geophysics and Planetary Sciences, Tel Aviv University, Ramat Aviv, 69978 Tel Aviv, Israel e-mail: [email protected] M. Zilberbrand Hydrological Service of Israel, POB 36118, 91360 Jerusalem, Israel L. Fleischer Geophysical Institute of Israel, POB 182, 71100 Lod, Israel Hydrogeology Journal (2004) 12:291–304

contact with pressurized brines up-flowing along fault zones from deep-seated Jurassic or Cretaceous reservoirs. The squeezing of saline interstitial water from the clays of the Yafo Formation into the overlying Kurkar Group aquifer, is of secondary importance for groundwater salinization (its input is comparable with salt input from rain). R
sum
Depuis longtemps, les «couches de Saqiye», nomm
es maintenant formation de Yafo, constituant le mur de l’aquifre ctier (s
rie de Kurkar) d’Isral, ont
t
consid
r
es comme un ensemble extrÞmement
pais, sans d
formation tectonique et totalement imperm
able.  la suite de l’exploitation intensive de l’eau souterraine de l’aquifre sus-jacent de la s
rie de Kurkar, des eaux sal
es et des saumures sont rencontr
es occasionnellement dans les parties les plus profondes de cet aquifre et toujours au contact de l’imperm
able sous-jacent de la formation de Yafo. Cette
tude a r
v
l
que cet imperm
able n’est pas une unit
g
ologique uniforme et imperm
able, mais qu’il s’agit plutt d’une alternance de couches perm
ables et imperm
ables dans la formation de Yafo contenant des fluides sous des pressions fortes avec des salinit
s diff
rentes (?) et en g
n
ral
lev
es (?). Les niveaux perm
ables sont en discordance angulaire et en contact direct avec l’aquifre sus-jacent de la s
rie de Kurkar. La formation de Yafo et les unit
s g
ologiques situ
es dessous et dessus sont disloqu
es par de nombreuses systmes de failles, qui facilitent le passage des saumures dans l’aquifre de la s
rie de Kurkar. La mobilisation des fluides salins et leur injection dans l’aquifre de Kurkar pourraient Þtre dues soit  la diffusion des fluides salins dans les horizons perm
ables du Petah Tiqva au travers des argiles de la formation de Yafo, soit leur remont
e par upconing sous l’effet de pompages intensifs dans l’aquifre de la plaine ctire. Il peut aussi Þtre caus
par la remont
e des eaux salines selon le pendage sous l’effet d’une surpression provoqu
e par une accumulation consi-d
rable de gaz dans les horizons imperm
ables. Un autre m
canisme possible pourrait Þtre un contact hydraulique avec des saumures sous pression, remontant le long des zones de faille depuis des r
servoirs profonds jurassiques ou cr
tac
s. L’expulsion d’eau saline interstitielle des argiles de la formation de Yafo dans l’aquifre de Kurkar est d’importance secondaire pour la salinisation de l’eau souterraine; cet apport est comparable  celui de sels par les pluies. DOI 10.1007/s10040-004-0322-8

292

Resumen Durante d
cadas, las “Saqiyebeds” (rebautizadas como Formacin Yafo) que se sitfflan bajo el acu fero Costero (Grupo Kurkar) de Israel fueron consideradas como un acuicludo extremadamente potente, tectnicamente inalterado y totalmente impermeable. Como resultado de la explotacin intensiva del Grupo Kurkar, se produjo la mezcla de aguas salobres y salinas en la zona inferior de dicho acu fero y siempre en el contacto con el acuicludo subyaciente de la Formacin Yafo. Este estudio revel que el acuicludo no es una unidad rocosa uniforme e impermeable, sino que consiste en una alternancia de estratos permeables e impermeables dentro de la Formacin Yafo, que contiene fluidos a alta presin de salinidades diferentes pero generalmente elevadas. Las capas permeables se hallan en inconformidad angular y en contacto directo con el acu fero superior del Grupo Kurkar. La Formacin Yafo y las unidades rocosas infra- y suprayacentes estn dislocadas por numerosos sistemas de fallas, lo que facilita el acceso de las salmueras al acu fero del Grupo Kurkar. Los fluidos de los horizontes permeables del Miembro Petah Tiqva a trav
s de las arcillas de la Formacin Yafo o hacia el ascenso de los domos salinos en respuesta a la extraccin intensiva en el acu fero del Llano Costero. Podr a tambi
n haber sido causado por el movimiento a favor del buzamiento de las aguas salinas como resultado de la sobrepresin generada por una acumulacin mayor de gas en los horizontes permeables. Otro posible mecanismo podr a basarse en el contacto hidrulico con salmueras presurizadas, que fluyen hacia arriba a favor de zonas de falla desde los reservorios profundos del Jursico o del Cretcico. La extraccin de agua salina intersticial desde las arcillas de la Formacin Yafo hacia el acu fero superior del Grupo Kurkar es de importancia secundaria para la salinizacin de las aguas subterrneas, ya que su aportacin es comparable a la procedente del agua de lluvia. Keywords Permeable beds · Sources of groundwater salinity · Israel · Leaking aquiclude · Pressurized brines

Introduction Throughout long years of exploitation of fresh groundwater from the Coastal Plain aquifer of Israel, brackish and saline groundwater — occasionally exceeding sea water salinity — have been encountered, particularly in the lower parts of the aquifer and always at the contact with the underlying aquiclude (Rosenthal et al. 1992; Ben-Gai 1994; Vengosh and Rosenthal 1994; Avisar 2001). The chemical composition of these waters clearly indicates that they were not related to recent seawater encroachment induced by overpumping, which began in mid 1930s (Zilberbrand et al. 2001). The present study was conducted in the central and southern parts of the Coastal Plain in Israel (Fig. 1). It examined whether the irregular and spotty occurrence of saline groundwater in the deeper parts of the Coastal Plain aquifer could be related to the permeable, brine-bearing Hydrogeology Journal (2004) 12:291–304

beds and to structural disturbances in the upper parts of the Yafo Formation. It also conceptualizes the hydrogeological model that controls the flow of saline groundwaters penetrating the overlying Coastal Plain aquifer.

Stratigraphy and Lithology

Saqiye Group L wengart (1927) was the first to define the marine, blue, sandy clays rich in molluscan shells as the major aquiclude underlying the Coastal Plain aquifer. These impervious beds, encountered in a borehole east of the city of Jaffa (Yafo), were named Saqiye beds of Miocene or Pliocene age (L wengart 1928). Picard (1938) corrected their age to Plaisancian–Pontian (Pliocene). For decades, the beds underlying the Coastal Plain aquifer were identified as ‘Saqiye beds’, a term which became a synonym for a regional rock unit, enormously thick, and assumed to be tectonically undisturbed and absolutely impervious. The stratigraphic position of the beds underlying the Coastal Plain aquifer was amended by Gvirtzman and Reiss (1965), who established the Saqiye Group (Fig. 2) of Late Eocene to Early Pleistocene age (Gvirtzman 1970). In the western parts of the Coastal Plain, the group is extremely developed in the subsurface and its maximum thickness reaches approximately 2,000 m beneath the coastline (Gvirtzman and Buchbinder 1977). In the central and eastern parts of the Coastal Plain and in the foothill area, the Saqiye Group displays its marginal facies and only scattered outcrops occur. There, the thickness varies in the 0–150 m range. The Saqiye Group is subdivided into two major units: the Lower and the Upper Saqiye, separated by the Mavqi’im Formation (Fig. 2). The present paper focuses on the Upper Saqiye unit.

Lower Saqiye The Lower Saqiye rock sequence is composed of the Ziqlag, Ziqim, and Bet Guvrin Formations (Fig. 2). It contains mainly of marly chalks, marls, reefal and bioclastic limestones deposited during a sequence of sedimentary cycles representing transgressions and regressions (Gvirtzman and Buchbinder 1977). During the early Oligocene, a major westward regression occurred, which was accompanied by deep incision of erosional non-conforming surfaces (Gvirtzman1969). According to Druckman et al. (1995), geological evidence indicates a sea-level drop of over 800 m, which resulted in the incision of large submarine channels into the drowned shelf. This phase, known as the Messinian desiccation, is characterized by the massive deposition of evaporites on a strongly dissected topography and particularly within the incised submarine channels. These evaporites comprise part of the Mavqi’im Formation (Gvirtzman 1970), which consists of anhydrite, gypsum, rock-salt, and dark marly shales of Late Miocene age. The maximum thickness of this formation attains in the basinal domain is more than DOI 10.1007/s10040-004-0322-8

293 Fig. 1 Location map displaying: zero line of Yafo Formation indicating the subsurface distribution of Neogene erosive channels, seismic lines, geological cross sections and boreholes

1,000 m whereas in the erosional channels beneath the Coastal Plain the thickness is about 200 m (Garfunkel et al. 1979). In the erosional channels and within the anhydrite sequence, halite beds were encountered attaining thicknesses of several tens of meters. These beds were deposited in a sabkha environment (Hsu et al. 1973; Gvirtzman and Buchbinder 1977). Within the evaporite sequence, up to seven marine intercalations were identified (Druckman et al. 1995). In the subsurface, the Mavqi’im evaporites form a characteristic continuous veneer, plastering the erosional surface of the Ziqlag Formation (Fig. 2).

Upper Saqiye (Yafo Formation) Major erosional channels identified beneath the Coastal Plain are shown in Fig. 1. In these channels, the thick sequence of evaporites is covered by sandstones and conglomerates of the Afiq Formation (Gvirtzman 1970; Fig. 2). This formation also contains variegated shales, siltstones, and marls. It is restricted to the thalwegs of the erosional channels and was deposited in fluvial and lacustrine–marsh environments, in river channels, and on flood plains. The Afiq Formation represents continuous fluvial deposition, which terminated the Ziqlag– Mavqi’im sedimentary cycle (Gvirtzman 1970).

Hydrogeology Journal (2004) 12:291–304

The Yafo Formation, which represents the upper Saqiye (Gvirtzman and Reiss 1965), lies on an erosional topography cut into the Mavqi’im and Afiq beds. The age of this formation is Pliocene to Early Pleistocene. It varies in thickness from a few meters in the eastern Coastal Plain to 1,500 m in the western, offshore area and represents a large-scale marine transgression that penetrated deeply inland to the foothills of the Judea Mountains (Issar 1961). The predominant lithology of this formation consists of impermeable, plastic, blueish-gray shales. Mineralogically, it is distinguished by the prevalence of montmorillonite from the Nile River basin (Gvirtzman 1970) and is thus in clear contrast to the illite-kaolinite assemblage, which is dominant in the clayey beds in the lower parts of the Saqiye Group. Since the early 1920s, the beds of the Yafo Formation, underlying the Kurkar Group sequence, were regarded as a regionally flat, continuous, uniform, and impermeable hydrological boundary. However, many wells drilled in the Coastal Plain of Israel since the early 1930s encountered hydrocarbon gas and saline water in porous horizons within the Saqiye beds (Blake and Goldschmidt1947; A. Issar, personal communication, 1960). In numerous places, the upper boundary of the Yafo Formation contains lumachelle beds (such as the Petah-Tiqva Member; Fig. 2; Gill 1965), or sands and conglomerates occurring in incised erosional channels DOI 10.1007/s10040-004-0322-8

294

locally, the aquifer is of uniform sandy lithology and is not subdivided by these shale barriers. Cumulative evidence (Rosenthal et al. 1992; Schlein et al. 1992; Ben-Gai 1994; Vengosh and Rosenthal 1994) has proved that the Kurkar Group unconformably covers an incised and highly irregular surface of the Yafo Formation.

Geological Evolution and Structures in the Yafo Formation The Yafo Formation was deposited during the Pliocene by sedimentation during a period of apparent tectonic quiescence. However, the Pliocene was not only a period of successive marine transgressions, which deposited mainly clay, but was also characterized by active tectonism and volcanism. The Pliocene transgression was cyclical with short periods of shallow-sea deposition of beds of sand and lumachelle. Along the Gaza area and the Israeli coastline, the Yafo Formation forms a huge sedimentary prism (Fig. 4) 200 km long and over 100 km wide extending offshore (Neev et al. 1976). Along its western parts, this sedimentary prism subsides due to two processes:

Fig. 2 Hydrostratigraphic column with ages given in millions of years (modified after Rosenthal et al. 1999)

containing saline water. Brines and gas traps were encountered in these lenses and in underlying thick shales interbedded with thin, shelly, sand beds (Blake and Goldschmidt 1947: Schlein et al. 1991; Ben-Gai 1994).

Kurkar Group The dominantly sandy sequence overlying the Yafo Formation is known as the Kurkar Group of Pleistocene age, forming the Coastal Plain aquifer of Israel. The two terms — Kurkar Group aquifer and Coastal Plain aquifer — are synonymous. It consists of marine calcareous sandstones deposited in near-shore environments, indurated eolianites, sandy loam, fossil soils, swamp mud, and alluvial and colluvial deposits (Gvirtzman and Buchbinder 1977). The Kurkar Group reaches an average thickness of 200 m in the west and wedges out towards the east. In the western and central part of the Coastal Plain, the Kurkar Group aquifer is irregularly subdivided by intercalated shale beds into four subunits: A–D (Fig. 3). In the east where bodies of brackish water occur Hydrogeology Journal (2004) 12:291–304

1. Deposition of an intensive supply of mostly silt and clay detritus carried from the Nile River delta by longshore currents and exerting a heavy load on the underlying beds (Nachmias 1969; Gvirtzman 1970). 2. Tectonic subsidence due to the subduction zone of the Cyprian Arc north of the Eratosthenes seamount that is approximately located 200 km from the Israeli shoreline (Robertson 1974; Mart1984; Ben-Avraham et al. 1995), accompanied by uplifting of the mountainous backbone of Israel (Negev to Galilee), as indicated by Picard (1938) and Widowinski and Zilberman (1996). The westward and northwestward tilt of the Yafo prism would create an extensional regime and the development of slumping and listric faulting (Fig. 5). Moreover, during the Pliocene, intensive tectonic and volcanic processes took place in the Dead Sea–Jordan Rift Valley (Picard 1938; Ben-Avraham et al. 1995). It is not plausible to assume that this geodynamic period of tectonic subsidence, major lateral displacements, and volcanism in the Rift Valley, together with conspicuous uplift of its neighboring regions (i.e., the Negev, Judea, Samaria, and the Galilee), would not affect the area of Yafo Formation sedimentation. On the contrary, one would expect tectonic activity and rejuvenation of older, deep-seated fault-systems. Figure 6 demonstrates the spatial distribution of various fault systems. Moreover, the deep oil wildcat well (National Park 1, see Fig. 1) drilled near Tel Aviv encountered a basaltic body that erupted during the Pliocene (The National Park Volcanic, Grader and Gvirtzman 1961) and many small volcanic extrusions and dikes occur on the west flanks of Samaria Mountains (Picard 1965; Livnat 1974; Sneh and Buchbinder 1984). These Pliocene volcanic bodies

DOI 10.1007/s10040-004-0322-8

295 Fig. 3 Schematic cross section indicating subdivision of the western part of the Kurkar Group by argillaceous beds and location of brines trapped in the lower part of the Kurkar Group aquifer (in red). Unlike the west-central parts of the Coastal Plain, the eastern part lacks impermeable beds that could block and trap the brines at its base. Consequently, the brines could migrate by upconing to the upper parts of the aquifer and degrading the fresh water

(Ginzburg et al. 1976; Mart 1984). Garfunkel et al. (1979), and Folkman and Shoresh (1979) suggested that these were rootless gravitational slumps on top of the lubricant evaporitic section of the Mavqi’im Formation, and are not direct expressions of deeply rooted faults. By analyzing high-resolution seismic sections, Ben-Gai (1996) and Ben-Gai et al. (1996) showed the occurrence of faults cutting through the Yafo Formation. The accumulation of gas deposits, discovered in permeable beds within the Yafo Formation under the Coastal Plain in the Ashdod gas wells, is controlled by a system of (listric) faults that border the reservoir to the east. Ben-Gai (1996) and Ben-Gai et al. (1996) suggested too, that these disturbances were mainly related to gravitational slumps.

Methods

Fig. 4 Block diagram of the structural setting of the Coastal Plain. The Yafo Formation creates a huge regional sedimentary prism extending into the offshore (after Rosenthal et al. 1999)

bear witness to tectonic activity during the time interval of Yafo Formation deposition. These two previously mentioned geological processes created the necessary conditions for the formation of dislocations and structural disturbances within the Yafo Formation. These phenomena were documented and their origin has been debated since the late 1970s. One approach related them to sedimentary slumps caused by neo-tectonic triggering within the pre-Pliocene section Hydrogeology Journal (2004) 12:291–304

The conclusions presented in this paper are based on lithological evidence from the zone of transition between the Yafo Formation (upper part of the Saqiye Group) and the overlying Kurkar Group. They are also based on highresolution seismic profiles from the southern part of the Coastal Plain of Israel (Schlein et al. 1991; Ben-Gai 1996; Fig. 1). The almost random distribution both of the lithological and of the seismic evidence, reflects the interests of gas prospectors rather than a properly planned survey project. In order to outline the subsurface geological model of the study area, several selected seismic profiles were related to the lithological evidence from deep boreholes drilled for oil and gas exploration or for water supply. By using velocity surveys and synthetic seismograms, it was possible to identify the major rock formations and to trace them on the seismograms. Within the selected horizons, displacements and discontinuities were marked as disturbances — or as faults. The interpreted time sections were then converted to depths using velocity information derived from various sources such as DOI 10.1007/s10040-004-0322-8

296

Fig. 5 Conventional migrated two-way-time seismic section (B– B0 : see location in Fig. 1) in the Shiqma region showing permeable beds within the Yafo Formation that are in direct contact and at angular unconformity with the base of the overlying sandy, waterbearing sequence of the Kurkar Group. These beds are cut by faultlike disturbances that are in direct contact with halite beds in the

Mavqi’im Formation. Some of these disturbances reach and affect the base of the Kurkar Group Aquifer. Note that easternmost deepseated faults that extend up to the Mavqi’im Formation terminate close to the base of the Kurkar Group. Note that the total depth of well Nir-Am 8 (2,117 m) is below the bottom of the section

velocity surveys and Root Mean Stacking velocities (RMS; Sheriff 1991). Based on this depth conversion, several geological cross sections were constructed and will be discussed below. Estimation of a possible diffusion flux of chloride from the clays of the Yafo Formation to the Kurkar Group aquifer was made by mathematical modeling using MODFLOW software (McDonald and Harbaugh 1984) with an ARGUS ONE pre-processor (Shapiro et al. 1997), and by considering formal similarity (Freeze and Cherry 1979) of one-dimensional equations of molecular diffusion and water flow in a confined aquifer: dC=dt ¼ D
d2 C=dz2 dH=dt ¼ ðk=sÞ
d2 H=dz2 ;

Fig. 6 Structural map showing location of wells that contain brackish water in the Kurkar Group aquifer. These wells are situated above faults at the top of the Judea Group, and close to the eastern margins of the Neogene channels. These faults, marked in green, reached the Mavqi’im Formation (modified after Fleischer et al. 1993; Gelberman 1994)

ð1Þ

in which C is chloride concentration, D is the moleculardiffusion coefficient of Cl in porous media, z is the vertical coordinate, t is time, H is hydraulic head, k is hydraulic conductivity, and s is the specific storage. Hydrogeochemical modeling for explanation of changes in chemical composition of groundwater was performed using the NETPATH program (Plummer et al. 1994).

Findings

Occurrence of Permeable Beds In the upper parts of the Yafo Formation in wells Ashdod Gas 1, Rishon LeZion 2a, Palmahim 1, Saqiye 2, and

Hydrogeology Journal (2004) 12:291–304

DOI 10.1007/s10040-004-0322-8

297

Hadassim 2 (Fig. 1), the typically argillaceous beds are interlayered with permeable strata containing sands, conglomerates, and lumachelle beds of the Petah Tiqva Member (Figs. 5, 7 and 8). The permeable beds within the Yafo Formation are in direct contact and at angular unconformity with the base of the overlying sandy, waterbearing sequence of the Kurkar Group. The thickness of these permeable beds varies between several meters to several tens of meters and they occur within an interval ranging between a few meters to several hundreds of meters beneath the base of the Kurkar Group.

Disturbances and Dislocations within the Yafo Formation

Fig. 7 High resolution two-way-time seismic section (A–A0 : see location in Fig. 1) in the Afiq Channel showing the structural relationships of the uppermost seismic horizons of Yafo Formation and the base of Kurkar Group. These horizons are permeable beds in direct contact and at angular unconformity with the base of the overlying sandy, aquiferous sequence of the Kurkar Group. Moreover, the permeable beds are cut by faults that reach and effect base Kurkar Goup Aquifer. After Schlein et al. (1992)

Conventional and high-resolution (HR) seismic sections from different parts of the Coastal Plain (Ashdod, Afiq, and Shiqma areas: Figs. 5, 7, and 8) show fault-like disturbances and dislocations crossing the whole sequence of the Saqiye Group (including the Yafo Formation at its top). These disturbances reach not only to the base of the Kurkar Group, but also affect it structurally. According to Ben-Gai (1996), in the Ashdod gas field (Fig. 8), faultlike disturbances running through the whole sequence of the Saqiye Group (including the clastic beds of Yafo Formation) affect the overlying Kurkar Group and, at their lower extremity, reach close to the warped, evaporite-bearing Mavqi’im Formation. Of particular interest is the evidence from seismic sections measured along the erosional channel of Afiq (Fig. 7; Schlein et al. 1991) and in the Shiqma area (Fig. 5). In both cases, listric disturbances develop from the Mavqi’im beds, crossing the whole sequence of the Saqiye Group, dislocating the

Fig. 8 High resolution twoway-time seismic section (C– C0 : see location in Fig. 1) in the well field of Ashdod

Hydrogeology Journal (2004) 12:291–304

DOI 10.1007/s10040-004-0322-8

298

clastic beds at the top of the Yafo Formation, and reaching to the base of the Kurkar Group.

Chemistry of Waters and Brines in the Yafo Formation and in Overlying Beds Chemistry of Water in the Permeable Beds of the Yafo Formation As previously mentioned, the Yafo Formation was always regarded as an aquiclude and was of no particular interest to hydrologists or water managers. Therefore, the chemical composition of the saline fluids encountered in the permeable beds in the upper parts of the Yafo Formation was never systematically investigated and no overall regional picture could be established. In well Saqiye 2 (Fig. 9), salinities increase with depth, from 780 mg/l Cl immediately beneath the Kurkar Group to 2,200 mg/l Cl at base of the Saqiye Group, i.e., 350 m deeper. In well Rishon Le’zion 2a, the salinity is 8,000 mg/l Cl (Fig. 9). In the gas field of Ashdod, salinities were found to be irregularly distributed and are in the 8,620–46,311 mg/l Cl range. The chemical composition of the waters collected in the gas well field of Ashdod was partly distorted as the result of massive addition of K- and Ca-rich compounds during drilling and borehole cementation operations. Ta-

Fig. 9 Strata in water wells Rishon Le’zion and Saqiye 2. Permeable beds marked in yellow in the Upper Saqiye, Yafo Formation containing saline water and brines (see location in Fig. 1). In Saqiye 2 well, the Saqiye Group is characterized by an increase with depth of salinities ranging from 780 mg/l Cl immediately beneath the base of the Kurkar Group to 2,200 mg/l Cl at the base of the Upper Saqiye. These salinities occur in permeable as well as in impermeable beds throughout the entire section of the Yafo Formation. In the Rishon Le’zion 2a well, salinities in permeable beds reached 8,000 mg/l Cl . Modified after Muravinsky et al. (1985) Hydrogeology Journal (2004) 12:291–304

ble 1 contains three analyses that were found to be reliable, i.e., from wells Ashdod Gas 2, 8, and 10. Use has been made of the Cl/Br ratio from well Ashdod Gas 1 to interpret the features of the water chemistry because these parameters were not contaminated in this well by drilling additives. These waters (in the 8,620–46,311 mg Cl/l range) have several common chemical features characteristic of groundwater that is in contact with hydrocarbon gas (Matthess 1982; Appelo and Postma 1994). The waters of Ashdod-Gas 8 and 10 stand out with low concentrations of SO4 (11 and 82 mg/l respectively) and of HCO3 (72 and 127 mg/l). The waters are also characterized by high excess of Ca [as indicated by the Q=Ca/ (SO4+HCO3) ratio of up to 43.67] and by the very low Mg/Ca ratio values (0.05–0.28). The SO4/Cl ratio values are also very low (0.0009–0.01). Such ratios could be due to reduction of sulfate and precipitation of carbonates. These waters are oversaturated with aragonite, calcite, and dolomite, and are undersaturated with sulfates and chlorides. Results of calculating the salt norms (Jones and Bodine 1987) indicate that the chemical composition of these waters is dominated by NaCl (75–87%) and by Cachloride compounds. The wide range of Na/Cl ratios (0.28–0.84) and the occasional low values could be indicative of ion exchange with montmorillonite clays, which are prevalent in the surrounding rocks of the Yafo Formation. The relatively low Cl/Br weight-ratio (132) would indicate seawater evaporation, whereas the higher ratio (253) from borehole Ashdod Gas 1 indicates a marine environment (Starinsky 1974). The chemical variability of the Ashdod gas-field waters may reflect the paleogeographic setting of their genesis. The clays of the Yafo Formation represent the eastward transgression, whereas the irregularly interbedded permeable beds of lumachelle, sand, and gravel indicate local and discontinuous shallow littoral to lagoonal conditions. The prevalent water mass was marine (with Cl/Br=253) whereas, in some lagoons, which were detached from the main body of the seawater, seawater was evaporated (as indicated by the low Cl/Br ratio value of 132). Similar conditions were reported by Levy (1972) for the Bardawill lagoon in northern Sinai. The resultant brines formed by these processes, i.e., partly of the marine type and partly generated by seawater evaporation, accumulated together with biogenic gas in these clastic beds and were further differentially diluted by fresh water percolating from the overlying Kurkar Group aquifer. The saline waters and the brines found in the Yafo Formation could have also evolved by other processes. As previously indicated, beneath the Coastal Plain and offshore, the Saqiye Group (with the evaporites of the Mavqi’im Formation) overlies unconformably the carbonates of the Upper Cretaceous Yagur and Negba Formations. Deep-seated faults cut through the whole stratigraphic sequence, i.e., through Cretaceous beds and the overlying Saqiye Group (including the Mavqi’im beds) to reach and affect the base of the overlying Kurkar Group. The saline fluids originating in the deep-seated Cretaceous beds could migrate along faults and penetrate into DOI 10.1007/s10040-004-0322-8

299 0.01 0.004 0.0009

SO4Cl

132.62 43.67 10.7 32.9

0.13 0.05 0.28

Mg/Ca Cl/Br Qa

0.28 0.84 0.73

Na/Cl

293 127 72

HCO3

46,311 12,624 8,620

658 82 11

SO4

412 116 101.5

CT

8,500 6,900 4,100

K

1,347 26.75 160

Na

16,184 812 929

Mg

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Ratio Ca/(SO4+HCO3)

Chemistry of Water in the Overlying Kurkar Group The Coastal Plain aquifer (Plio-Pleistocene Kurkar Group), which overlies the Yafo Formation, contains groundwater of diverse salinities. Generally, the water is fresh, with Cl concentrations of 65–290 mg/l. The water pumped from well Ashdod 24 (Fig. 1; 72 mg Cl/l) represents the fresh end member of this aquifer. This is a Cabicarbonate type water with high Na/Cl ratio values (0.9– 1.23), Q=0.44–0.78, Mg/Ca ratio values of 0.43–0.74, and Cl/Br ratio values of 280–320. The waters are oversaturated with carbonates and undersaturated with sulfates and chlorides. The salt norms indicate a composition defined by approximately 35% NaCl, 33% carbonates, and the rest as sulfates. According to Zilberbrand et al. (2001), these ionic ratios are typical for the fresh waters of the Coastal Plain (Kurkar Group) aquifer. In several locations in the eastern parts of the Coastal Plain (Beer Tuviya, Kfar Menahem, and Hazor, Fig. 6) brackish groundwater (in the 300–1,850 mgCl/l range) was encountered in the Kurkar Group aquifer. The waters are of the Na-chloride type and their calculated salt norms indicate up to 62% Na-chloride, 22% Ca-chloride compounds, and the rest as Ca-and Mg-carbonates. The ionic ratios of these waters are low Na/Cl (0.58–0.7), Q>1 (1.09–1.55), Mg/Ca 0.88–1.6, and Cl/Br (308–424). As shown by Avisar (2001), the brackish waters are characterized by an inverse relationship between salinity and Na/Cl and between Na/Cl and Q ratios, indicating a salinity-dependent increase of Ca and impoverishment in Na. Similar ionic ratios and chemical relationships were encountered in the Heletz brines occurring in deep-seated beds beneath the Saqiye group (Rosenthal et al. 1999). As shown in Fig. 6, all wells, which encountered brackish groundwaters in the Kurkar Group aquifer, are located in an area overlying the Neogene erosional channels, close to the Judea Group truncated by the Saqiye Group. They are also located above traces of deep faults, which displace Cretaceous and older rock sequences beneath the Saqiye Group. Some of these deep faults extend directly through the Yafo Formation to the base of the Kurkar Group.

a

Ashdod-Gas 2 Ashdod-Gas 10 Ashdod-Gas 8

(m)

327–329 73–268 438–448

7 8.36

Ca pH Depth Name

Table 1 Representative chemical analyses from boreholes penetrating into the Yafo Formation

the permeable horizons in the Saqiye Group. Such a process, which was initially suggested by Rosenthal et al. (1999), was tested by inverse hydrochemical modeling using the NETPATH program (Plummer et al. 1994). Avisar (2001) showed that in well Ashdod Gas 2, the saline waters (those in which the Cl content exceeds that of sea water) could have evolved from the Heletz brines occurring in deep seated (pre-Tertiary) beds (Rosenthal et al. 1999). The hydrochemical evolution from such brines involved the loss of sulfates by reduction, formation of pyrite, massive precipitation of carbonates, and Ca–Na exchange. The brackish and saline waters encountered in borehole Ashdod Gas 10 seem to be derived from trapped sea water mainly altered by sulfate reduction.

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Interaction between various Hydrogeological Systems No clear conclusions were hitherto drawn as to the origin of the saline water occurring in certain areas at the base of the Kurkar Group in the eastern part of the Coastal Plain. Rosenthal et al. (1992), Vengosh (1992), and Vengosh and Rosenthal (1994) suggested that the saline waters originate from strata underlying the Kurkar Group (i.e., from the Yafo Formation). Artzi et al.(1996) raised the possibility that the source of salinity could be related to anthropogenic, surface-pollution processes. An exploration well (Beer Tuviya T/3) revealed a Cl concentration of 1,060 mg/l in the upper part of the Kurkar Group at a depth of 40 m. However, drilling operations were stopped at the contact with the top of the Yafo Formation and the well did not penetrate into the permeable beds of the Yafo Formation; therefore, its findings are inconclusive regarding this. The contact between the Yafo Formation, including permeable interlayers containing saline waters, and the overlying Coastal Plain (Kurkar Group) aquifer, is characterized by two important structural factors: – The angular unconformity between the permeable beds in the Yafo Formation and the base of the Kurnub Group, – The occurrence of faults that cross the permeable layers from the base of the Yafo Formation, up to the base of the Kurkar Group. The permeable beds and the faults may act as routes to conduct saline waters under high pressure and create local, point sources of saline water at the base of the Kurkar Group aquifer. In order to test whether the chemical composition of the saline waters, occasionally encountered at the base of the Kurkar Group aquifer in boreholes Beer Tuviya 6 and Kefar Menahem 2, could evolve by mixing of the hydrochemical end members (Yafo Formation, Heletz brines, and the Kurkar Group waters) known to exist in the subsurface of the study area, inverse modeling by the NETPATH program (Plummer et al. 1994) was attempted. The results are presented in Table 2. Model 1 explains the formation of brackish groundwater at the base of the Kurkar Group (as in well Beer Tuviya 6) by mixing of fresh water flowing in the Kurkar Group (represented by well Ashdod 24) with 2% of upflowing pressurized saline waters from the permeable beds in the Yafo Formation (represented by water from Ashdod Gas 2). The mixing process also involves Ca–Na exchange and various reactions with clays. Model 2 supports the possibility that the brackish water of well Kfar Menahem 2 (which represents a phenomenon similar to that of Beer Tuviya 6) could evolve by mixing between the fresh-water component (Ashdod 24) and 10% of Heletz brines (represented by the water of well Negba 1), which up-flow along the faults dissecting the whole subsurface stratigraphic sequence in the study area. This process involves dissolution of halite

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Table 2 NETPATH modeling of the formation of brackish groundwater.
Dissolution; + precipitation;
exchange enrichment of Ca in solution; + exchange enrichment of Na in solution

Initial source 1 Initial source 2 Resultant Calcite Dolomite Gypsun Halite Ca–Na Exchange Smectite Mg-montmorillonite

Model 1

Model 12

Ashdod Gas 2 =2% Ashdod 24 =98% Beer Tuviya 6 0.04 (mmol/kg)

Ncoa 1 =10% Ashdod24 =90% Kfar Menahem 2

5.1 (mmol/kg) 2.87 (mmol/kg)

0.42 (mmol/kg)

2.3 (mmol/kg)

0.39 (mmol/kg) 4.8 (mmol/kg)

4.7 (mmol/kg) 4.28 (mmol/kg) 1.2 (mmol/kg)

and of dolomite, precipitation of calcite, and various reactions involving clays (Table 2).

Possible Mechanisms for Penetration of Saline Water from the Yafo Formation into the Kurkar Group Sediments There are four possible mechanisms for penetration of saline groundwater from permeable horizons in the Yafo Formation (hence the Petah Tiqva member) into the overlying Coastal Plain (Kurkar Group) aquifer: – Squeezing of saline interstitial water from the clays of the Yafo Formation as a result of sediment compaction. – Diffusion of saline interstitial water from the clays of the Yafo Formation, upwards into the Kurkar Group aquifer. – Vertical upward leakage of saline groundwater, both through the clays of the Yafo Formation and through fault zones, under a vertical gradient between the permeable beds of the Petah Tiqva Member and the overlying Kurkar Group (Coastal Plain) aquifer. – Salinization could also be caused by lateral flow of saline groundwater from the permeable beds of the Petah Tiqva Member to the base of the Kurkar Group aquifer (see Fig. 5), driven by a horizontal gradient of hydraulic head caused mostly by gas pressure in the Petah Tiqva Member. Squeezing of pore water from the clays as a result of compaction could take place throughout the whole history of the clayey sediments. It is logical to assume that, initially, this pore water had the salinity of ocean water (about 19,000 mg/l). The porosity of fresh clay mud is usually 80–85% of the total volume (Larsen and Chilingar 1967; Gealy 1971) whereas the porosity of the clays in the Yafo Formation ranges from 40–50% (Gill 1965). Therefore, in a 1,000-m-thick clay column with a 1-m2 cross section, there are 500-600 m3 of solid-rock material. This solid material includes 15–20% of fresh mud with a porosity of 80–85%. The initial volume of the mud may thus be estimated at 2,500–4,000 m3. Considering a porosity of 80–85%, it would contain 2,000–3,400 m3 of DOI 10.1007/s10040-004-0322-8

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ocean water. At present, a 1,000-m-thick column of Yafo Formation clay with a porosity of 40–50% contains 400–500 m3 of pore water. Therefore, 1,500–3,000 m3 of ocean water could have been squeezed both into the upper (Kurkar Group) aquifer as well as downwards. This water contained 28.5–57 ton Cl . The distribution of upward and downward fluxes of the squeezed saline water was estimated to be similar. The maximum chloride flux, caused by squeezing of interstitial clay water into the Kurkar Group aquifer, could be estimated by assuming that squeezing started towards the end of the deposition of the Yafo Formation, i.e., at least 1.6 million years ago (Horowitz 1979). The flux of Cl– is estimated at 17.8– 35.6 g m 2 year 1. For comparison, rain introduces 6.8– 16.2 g Cl m 2 year 1 annually to the Coastal Plain aquifer. Hence, the squeezing of interstitial water from the clays of the Yafo Formation into the Coastal Plain (Kurkar Group) aquifer does not contribute substantially to the salinization of its groundwater The assessment of the roles played by the other salinization mechanisms (as previously indicated) was made for an area around drill hole Ashdod-Gas 1 (Fig. 8), for which the most complete data set was available. An estimation of the possible diffusion flux of chloride from the clays of the Yafo Formation to the Kurkar Group aquifer was made possible by mathematical modeling. The conceptual model was outlined as follows, in the vicinity of drill hole, Ashdod-Gas 1, the uppermost 30-mthick sandy horizon within the Yafo Formation, was found at about 180 m below the base of the Kurkar Group. Assuming symmetry of both upward and downward diffusion, the lower model boundary (impermeable) was set at the half-thickness level. The upper model boundary, also impermeable or with a constant chloride concentration, was set at 70 m above the base of the Kurkar Group and corresponding to the fresh groundwater (with minimum chloride concentration) sampled there. The initial profile of Cl– concentrations was set at 50,000 mg/l in the permeable horizon within the Yafo Formation, 19,000 mg/l (as in ocean water) in the clays of the Yafo Formation, and 40 mg/l in the Kurkar Group. Lateral flushing due to horizontal water movement at the base of the Coastal Plain aquifer was ignored in order to attain maximal possible concentrations at the base of the (Kurkar Group) aquifer. The molecular-diffusion coefficient was taken from the literature (Freeze and Cherry 1979; de Marsily 1981), is constant for each lithological species, and attains, respectively, 510 5–510 6 m2/day and 110 6 m2/day for sandy sediments and for clays. Practically, it was simulated by setting D=k/s [see Eq. (1)] with the hydraulic conductivity (k) at 110 8 m/day and the specific storage (s) at 210 3–210 4 and 110 2. Modeling was performed for a column of 1-m2 cross section. The vertical grid was characterized by cells of 0.05–1 m height, condensing near external and internal (lithological) boundaries. Simulations were performed with a time step of 1 year for the total period of 400,000– 500,000 years, which corresponds to reaching the steadystate Cl profile. Hydrogeology Journal (2004) 12:291–304

Fig. 10 Graph of diffusive Cl flux from the Yafo Formation to the Kurkar Group aquifer versus time, calculated by results of mathematical modeling by setting Cl concentrations at the upper and lower boundaries as constant in time. The steady-state flux was attained in about 500,000 years. Large fluxes during the first 10,000 years should be regarded as unreal, caused by idealized initial conditions (sudden concentration increase at the base of the Kurkar Group). Line 1 corresponds to diffusion coefficient values of clayey and sandy sediments of 110 6 and 510 6 m2/day, respectively, whereas line 2 corresponds to values of 510 6 and 510 5 m2/day

The received vertical gradient of Cl concentration at the base of the Coastal Plain aquifer decreased exponentially in time. Concurrently, the steady-state upward diffusive chloride flux at the base of the Kurkar Group (calculated using the modeling results as DdC/dz) also drops rapidly to steady-state values of 0.37–5.11 g Cl m 2 year 1 (Fig. 10). This calculated flux is also relatively small and similar to the flux from squeezing of interstitial water, and can be disregarded for short-time periods in its effect on the groundwater salinity in the Kurkar Group aquifer. Nevertheless, over the geological time scale and under conditions of reduced lateral groundwater flow in the Kurkar Group aquifer, both fluxes could lead to salinization of water in the lower parts of this aquifer, above and close to the upper surface of the Yafo Formation. Vertical upward leakage of saline groundwater, both through clay layers and fault zones, may occur only if the hydraulic head in the permeable horizons in the Yafo Formation (Petah Tiqva member) exceeds the hydraulic head in the Kurkar Group aquifer. In drill hole Ashdod Gas 1, the maximal water pressure (P) measured in a permeable horizon located at depth of 521 m (497.62 m below MSL) was 731 psi (Muravinski et al. 1985). Brine with a weight density (g) of about 1.05 g/cm3 was found at this depth. The hydraulic head (H) of this brine at the absolute height of the sampling point (Z) was assessed by Eq. (2) (Freeze and Cherry 1979): H ¼ Z þ P=g

ð2Þ

For the given data, the hydraulic head of the brine is 497.62 m+731 psi 6894.757 (newton/m2)/psi/ (1.05 103Kg/m39.80665 newton/kg)= 497.62+489.47=

8.15 m. The hydraulic head of phreatic groundwater in the Kurkar Group aquifer, as measured during 1959–2002 in water wells Nir Galim 15/2 and USOM 1S, which are, DOI 10.1007/s10040-004-0322-8

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respectively, about 1.5 km NE and SW of well Ashdod Gas 1 (Fig. 1), varied between 0–5.3 m. Therefore, at present, upward and updip flux of saline water from the permeable beds of the Yafo Formation to the overlying Kurkar aquifer could be only possible as a result of excessive pumping, which would lower the water levels in the Kurkar Group aquifer by more than 8.2–13.5 m. However, such a flux could have been possible during the geological past, at the beginning of the deposition of the Kurkar Group sediments.

Discussion The present study has revealed that alternating pervious strata within the Yafo Formation contain pressurized fluids of different — mostly high — salinities. These permeable beds and the overlying Kurkar Group (Coastal Plain aquifer) are in direct contact by angular unconformity. The Yafo Formation and other underlying and overlying rock units, are crossed by several systems of faults and/or by fault-like disturbances. This evidence indicates that the Saqiye beds (later renamed Yafo Formation), are far from being what they had been considered to be — a hydrogeologically and chemically inert aquiclude, but are actually a leaky rock unit that contributes saline waters to the overlying Kurkar Group aquifer. Lithological features and structural disturbances facilitate access of brines into the base of the Kurkar Group, and originate in deep-seated reservoirs such as the Mavqi’im Formation or even in deeper Cretaceous and Jurassic beds (Starinsky 1974; Fleischer et al. 1977; Fleischer 1987; Vengosh and Rosenthal 1994; Rosenthal et al. 1999). This interpretation has gained strong support from hydrochemical evidence. This new evidence not only provides an explanation for the point sources of saline waters in the southern Coastal Plain (such as at Beer Tuviya, Hazor, and Migdal), but bears considerably on broader issues such as the general hydrological model and the salt balance of the entire Coastal Plain aquifer. The base of this huge regional aquifer cannot anymore be regarded as a flat, impervious, and chemically inert surface, but in contrast, it is a morphologically irregular surface that is locally in contact with updipping permeable beds containing saline fluids. As a result of the traditional interpretation by which the Yafo Formation has been regarded as an ideal aquiclude, no direct hydrological observations were initiated and, therefore, there is no systematic information on hydraulic heads in its permeable beds. Occasional data of fluid and gas pressures in wildcat wells have been collected from geological completion reports (Grossowicz1976; Teleman 1977; Ben-Zaqen1980; Spivak et al. 1983; Shomrony et al. 1984). The occurrence of point sources of saline waters at the base of the Kurkar Group aquifer, the discovery of pressurized saline waters in the Petah Tiqva Member, and the direct contact via an angular unconformity between these permeable beds and the Kurkar Group aquifer have proHydrogeology Journal (2004) 12:291–304

voked the investigation of various possible models for the penetration of these saline waters into the Coastal Plain aquifer. Modeling has indicated that, at present, upward flux of saline water from the permeable beds of the Petah Tiqva Member to the Kurkar Group aquifer could be only possible under strong overexploitation of the fresh water, which would lower water levels in the Coastal aquifer by more than 8.2–13.5 m. It should, however, be kept in mind that the hydraulic head of the deep brine depends on its weight density (g). Therefore, less saline water would have, under similar conditions, a higher hydraulic head and, thus, could penetrate into the base of the Kurkar. In the vicinity of the gas well Ashdod 1, several permeable horizons were identified in the Yafo Formation. In these horizons, the salinity of groundwater varies considerably, from 780 to over 50,000 mgCl/l. Unfortunately, water pressures were measured only in the uppermost horizon and this was the only figure considered in the previously presented model calculations. Usually, anoxity increases with depth, and gas-producing processes might be of higher intensity in deeper permeable horizons within the Yafo Formation. This would generate higher water pressures and hydraulic heads [Eq. (2)], increasing the probability of lateral updip flow towards the base of the Kurkar Group. All these factors would contribute — either separately or in concerted ways — to the accumulation of saline waters in the lower parts of the Kurkar Group aquifer. Moreover, intensive exploitation of fresh groundwater from this aquifer could lead to the mobilization of the saline fluids within the Yafo Formation and to their upconing into the fresh groundwater body of the overlying Kurkar Group.

Conclusions 1. The Yafo Formation is a leaky aquiclude and not a perfect one as hitherto postulated. 2. Within the Yafo Formation, the permeable horizons containing saline waters are tilted beds of sand that meet the base of the Kurkar Group at an angular unconformity. Listric disturbances may also act as conduits of saline waters. 3. The pressurized brines and saline waters within the Yafo Formation constitute a potential risk factor for the fresh waters of the overlying Coastal Plain (Kurkar Group) aquifer. The saline waters could penetrate massively into the Kurkar Group aquifer as a result of excessive pumping of fresh waters to result in lowering of its water levels. 4. The conclusion that the Yafo Formation is hydrogeologically active, is important for the identification of subsurface “salinization foci”. This information is important for the optimization of water management and for the prevention of salinization of the overlying fresh water.

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