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Geofluids
Volume 2019, Article ID 6727681, 21 pages
https://doi.org/10.1155/2019/6727681
Research Article

Applying Rare Earth Elements, Uranium, and 87Sr/86Sr to Disentangle Structurally Forced Confluence of Regional Groundwater Resources: The Case of the Lower Yarmouk Gorge

1Dept. of Catchment Hydrology, Helmholtz Centre for Environmental Research (UFZ), Halle/Saale, Germany
2Helmholtz Centre Potsdam, German Research Centre for Geosciences (GFZ), Section 3.4, Potsdam, Germany
3Dept. of FA 2, Federal Office for the Safety of Nuclear Waste Management (BfE), Berlin, Germany
4Hydrogeology, Freie Universität, Berlin, Germany
5Geological Survey of Israel (GSI), Jerusalem, Israel
6The School of Earth Sciences, Tel Aviv University, Tel Aviv, Israel
7The Inter-Islamic Network on Water Resources Development and Management (NWRDAM), Amman, Jordan
8Dept. of Computational Hydrosystems, Helmholtz Centre for Environmental Research (UFZ), Leipzig, Germany

Correspondence should be addressed to Christian Siebert; ed.zfu@trebeis.naitsirhc

Received 24 May 2019; Revised 23 September 2019; Accepted 3 October 2019; Published 3 December 2019

Guest Editor: Tanguy Robert

Copyright © 2019 Christian Siebert et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The conjoint discussion of tectonic features, correlations of element concentrations, δ18O, δD, and 87Sr/86Sr of groundwater leads to new insight into sources of groundwater, their flow patterns, and salinization in the Yarmouk Basin. The sources of groundwater are precipitation infiltrating into basaltic rock or limestone aquifers. Leaching of relic brines and dissolution of gypsum and calcite from the limestone host rocks generate enhanced salinity in groundwater in different degrees. High U(VI) suggests leaching of U from phosphorite-rich Upper Cretaceous B2 formation. Both very low U(VI) and specific rare earth element including yttrium (REY) distribution patterns indicate interaction with ferric oxyhydroxides formed during weathering of widespread alkali olivine basalts in the catchment area. REY patterns of groundwater generated in basaltic aquifers are modified by interaction with underlying limestones. Repeated sampling over 18 years revealed that the flow paths towards certain wells of groundwater varied as documented by changes in concentrations of dissolved species and REY patterns and U(VI) contents. In the Yarmouk Gorge, groundwater with basaltic REY patterns but high U(VI) and low Sr2+ and intermediate sulfate concentrations mainly ascends in artesian wells tapping a buried flower structure fault system crossing the trend of the gorge.

1. Introduction

Since Roman times, the hot springs of Hamat Gader (HG), Israel, and Ain Himma, Jordan, in the Lower Yarmouk Gorge (LYG) were used for health care (Figure 1). At present, only Ein Balsam at HG is publically in use. Hydrogeological and hydrochemical studies of springs and well waters in the gorge reveal that groundwater of widely different composition discharges at short distances [1]. By major and minor elements and distribution patterns of rare earth elements including yttrium (henceforth termed REY), it was ascertained that thermal groundwater discharging through springs in the LYG is infiltrated in basaltic regions of the Hauran plateau, Syria [1]. Parts of these waters are mixed in various proportions with limestone water from Ajloun. The hot waters of Hamat Gader and Meizar get salinized by either mixing with relic seawater evaporation brines [2, 3] or leaching of evaporites. The recent study is based on 18 years of repeated sampling of wells and springs and reveals significant variations in REY patterns and element concentrations suggesting variation of flow paths and associated interactions with host rocks and leaching residual brines and evaporites.

Figure 1: Overview of the study area, showing geological background (a) and sampling locations (b) including structural features recently introduced by Inbar et al. [15] and Sneh (unpublished).

The chemical and isotopic composition of the large amounts of fresh artesian groundwater produced in the Jordanian Mukheibeh well field contrasts with that of the saline groundwater in the Meizar wells and the springs of Hamat Gader. This gave rise to the conceptual model that the LYG is the surface expression of a fault zone, preventing transboundary flow [1]. 2D and 3D modelling supported that concept of continuous groundwater aquifers with the absence of transboundary groundwater exchange due to a zone of high hydraulic anisotropy underneath the gorge’s centerline [47]. The gorge seemingly acts as a complex conduit-barrier system, along which groundwater from the Golan in the north and Ajloun in the south converges and drains towards the Lower Jordan Valley (Figure 2). Flow paths in the underground of the gorge possibly occur along faults oriented perpendicular to the major axis of the gorge [5, 6, 8].

Figure 2: Schematic geological cross sections (not to scale). Section (a) starts in the Lower Jordan Valley, continues through the LYG, and branches into the Hauran NE-ward, while section (b) cuts from the Ajloun northward across the LYG and the Golan Heights into Mt. Hermon. The map shows the location of cross sections in red.

Based on stratigraphic data [9, 10], topographic data, deep seismic survey data [11, 12], shallow fault mapping [13], and thickness irregularities of the Turonian and Senonian sequences in the study area [1417] support the occurrence of strike-slip flower structure faults along and across the gorge creating a series of structural fault blocks and numerous buried faults at close proximity to the Dead Sea Transform Fault (DSTF) (Figure 1).

Applying REY distribution patterns, U(VI), 87Sr/86Sr, and water isotopes in a new, complete, and synchronous set of sampled spring and well waters in 2016, we aim for joint discussion of hydrochemical and geological features to improve the knowledge of the sources of groundwater and of their flow paths.

After the introduction (Section 1), we will present the hydrogeological setting of the studied area (Section 2), the sample acquisition and the techniques used to analyze them (Section 3), the results on major and minor element, particularly on REY and U(VI), and Sr isotopes (Section 4), and a detailed discussion (Section 5). Section 6 concludes this study.

2. Hydrogeological Setting

Geographically, the Yarmouk drainage basin comprises (i) the volcanic Hauran plateau and the western flank of the Jebel Druze volcano [18], (ii) the southern and southeastern slopes of Hermon, (iii) the Golan Heights with numerous volcanic cones, (iv) the northern plunges of the Ajloun anticline, and (v) the Azraq-Dhuleil basin ENE of Ajloun (Figure 1(a)).

The Mediterranean climate in the Yarmouk basin causes rainy and cool winters and hot and dry summers. The distinct differences in altitude and the distance from the Mediterranean force strong gradients in annual precipitation. Highest values (up to 1300 mm/a) fall in the Hermon Massif and the highest parts of Jebel Druze; medium elevated regions such as the Hauran and Ajloun plateaus and the Golan Heights receive 600-800 mm/a, while the low-lying LYG and the xeric region SE of the surface drainage basin receive <500 mm/a only (e.g., [1821]). The resulting recharge fractions are calculated to range from 0.06 to 0.1 [20, 2224].

In the south of the Yarmouk River, geological formations dip NW-ward (Figure 2). Here, the oldest hydrogeological relevant formations comprise the highly karstified lime- and dolostones of the Upper Cretaceous A7 aquifer and the overlaying heavily fractured silicified limestones of the Eocene B2 aquifer, altogether forming the 160 m thick regional A7/B2 aquifer system (Figure 3). This system becomes efficiently confined due to its descent and the appearance of the covering B3 aquiclude. On top of the southern flank of the LYG, remnants of the B4 sequence form a local limestone aquifer.

Figure 3: Stratigraphic table of the hydrogeological formation in the Yarmouk Basin.

All formations older than B4 continue in the underground of the Golan Heights syncline before they partly resurface in the foothills of the Hermon anticline [25]. Underneath the Golan Heights, Jurassic limestones form the base of the formations before they become uplifted in the Hermon anticline in the north (Figures 1(a) and 2(b)). Since the drainage basin extends into three nations with different geological terminologies. Figure 3 compiles the relevant parts of the stratigraphic columns for the entire region.

Morphologically, the Golan Heights is restricted southward by the LYG, westward by the Hula Valley and Lake Tiberias, northward by Wadi Sa’ar at the foothills of Mt. Hermon, and eastward by Wadi Raqqad. The entire Golan Heights is unconformably overlain by Plio-Pleistocene cover basalts, which continue E- and SE-ward into the Hauran plateau, Jebel Druze, and Azraq-Dhuleil Basin and form the uppermost supraregional aquifer in the area [26, 27]. Within the Golan Heights, the thickness of the basalts varies with more than 750 m in the central part and less than 50 m along the LYG (Figure 2(b)) and B3 layers form the impervious base of the basaltic aquifer [2830]. However, the basaltic aquifer is connected to underlying aquifers at certain locations [31], either where B3 was already eroded or where structurally prominent features of post-Pliocene age cut the formations [12, 28, 32, 33]. An aeromagnetic survey in N Jordan revealed a SW-NE lineament branching from the DSTF towards Hamat Gader in the LYG [34], which was later proven to be a fault by geological mapping (Sneh et al., unpublished) (Figure 1(b)).

The groundwater in the phreatic and shallow basaltic aquifer mainly follows the morphology. Within the Golan Heights, it flows W- and SW-ward towards the Hula Valley, the Lake Tiberias, and the LYG [28, 33]. In the east, a subterranean meridional ridge forms a water divide against the Hauran [19] (Figure 2(a)). The thin lava flows east of the water divide, hosts only modest amounts of groundwater, and discharges locally into incised wadis, e.g., the Raqqad. The basaltic cover of the Hauran plateau is mainly recharged at the elevated southeastern flanks of the Hermon Massif and western piedmont of Jebel Druze, from where the groundwater flows SE- and W-ward, respectively. The groundwater most probably converges in the central part of the Hauran and flows from there SW-ward towards the LYG. There, the observed groundwater of this study discharges either naturally at the valley floor through springs in Hamat Gader, Suraya, and Himma or artificially through the (mostly) artesian wells of Mukheibeh and Meizar, located in the flanks of the gorge, either north (Meizar wells) or south (Mukheibeh wells) (Figure 1(b)).

3. Analytical Procedures

The elements Ca2+, Mg2+, U(VI), and REY are determined by ICP-MS (Elan DRC-e). K+ and Na+ were analyzed by ICP-AES (Spectro Arcos) using matrix-adjusted standard solution for calibration. Cl-, Br-, and SO42- are determined with Dionex ICS (AS18 column). The alkalinity is titrated to pH 4.3 with H2SO4 and given as HCO3-.

To determine REY and U(VI), preconcentration is required. Therefore, about 4 l of sample is filtered in the field by using a peristaltic pump coupled to 0.2 μm filters (Sartorius, Germany). The samples are acidified by subboiled (index sbb) HCl, and 1 ml of Tm spike solution is added. At the same day, the samples are adjusted to using HClsbb and subsequently passed through preconditioned C18 Sep-Pak cartridges (Waters, USA), loaded with an ethylhexyl phosphate (Merck, Germany) liquid ion exchanger, at a rate of 1 l/h. Thereafter, each cartridge is washed with 50 ml of 0.01 M HClsbb and subsequently eluted with 40 ml of 6 M HClsbb at a rate of 3 ml/min. The eluates are evaporated to incipient dryness, and the residues are dissolved in 1 ml of 5 M HNO3 sbb (Merck, Germany) and transferred into 10 ml volumetric flasks. 1 ml of spike solution is added which is used, if necessary, for drift corrections of the response factors during the ICP-MS measurements.

Stable isotopes of oxygen and hydrogen are determined in separate filtered samples (0.2 μm) using laser cavity ring-down spectroscopy (Picarro L2120-i, USA) without further treatment of the water samples. The respective analytical precision is ±0.1‰ and ±0.8‰ for δ18O and δD, respectively. The results are reported relative to Vienna Standard Mean Ocean Water (VSMOW).

Analyses of 87Sr/86Sr in water samples were performed at TUBAF, Freiberg, Germany. Samples were prepared and analyzed after Tichomirova et al. [35] by applying TIMS (Finnigan MAT 262) with an acceptable relative error of ±0.005% for 87Sr/86Sr. Sr-isotope ratios are given in respect to NBS-987. To analyze Sr2+ in basaltic rock samples, rocks have been powdered to <150 μm, pressed to pellets, and analyzed applying energy-dispersive X-ray fluorescence (EDXRF) (Spectro XEPOS HE 2000). Chemical and isotopic analyses are given in Tables 13.

Table 1: Compilation of groundwater analyses from the Lower Yarmouk Gorge and surrounding areas.
Table 2: REY composition of the discussed groundwater from the Lower Yarmouk Gorge.
Table 3: Sr2+ concentration and 87Sr/86Sr isotope signatures of groundwater and rocks from the Lower Yarmouk Gorge and surrounding areas.

4. Results

Depending on the sampling location, the results are classified in the following way: Mukheibeh well field (M1-M13), Ain Himma (AH), Hamat Gader springs (HG), Meizar wells (Me1-Me3), and the Yarmouk River (YR). Sampling locations, which have been repeatedly sampled, are indicated by the year of sampling given in parentheses. The Hebrew and Arabic term of springs is transliterated as Ein and Ain, respectively.

4.1. Major and Minor Element Correlations with Cl-

From the low-salinity Mukheibeh clusters, two (Figures 4(a)4(f)) or one (Figures 4(g)4(j)) mixing lines evolve with high-salinity end members. They indicate that two end member brines occur in the study area: one is salinizing the Meizar wells and Ain Himma and the other the springs of Hamat Gader (Ein Maqla, Ein Reach, Ein Balsam, and Ain Sarayah). The Ca2+ concentration in Ain Himma switches between the two trends, probably because the access point to sample the spring water within the increasingly ruined Himma resort varied over the years. Hamat Gader brines are lower in SO42- but higher in Cl- and Br- than Meizar brines. Waters from wells such as Meizar 1 and Mukheibeh 8, 9, and 11 sometimes deviate from the indicated trend lines. The Yarmouk River water is mostly comparable to Mukheibeh water, but not in diagramms with Na+, Cl- and SO42-.

Figure 4: Crossplots of elements in the groundwater in the Lower Yarmouk Gorge. For details, refer to text.
4.2. Uranium

U(VI) is correlated neither with any other element mentioned before nor with Eh varying between -200 and +200 mV (Table 1). The Mukheibeh field splits into three subgroups (Figure 5). U(VI) with 80-105 nmol/l has the highest values in Mukheibeh groundwater. Groundwater from wells Mukheibeh 5 and 11, Ain Himma, Hamat Gader shows values between 3 and 10 nmol/l. The groundwater with <0.1 nmol/l and that with U(VI) below the detection limit comprise all well waters from Meizar 2 and 3 and Mukheibeh 1, 8, and 9. The lowest U(VI) values are either in the lowest or in the highest sulfate groundwater (Figure 5(b)).

Figure 5: Crossplots of U(VI) and Na/Cl values in the groundwater of the Lower Yarmouk Gorge.
4.3. Rare Earths and Yttrium

Weathering of omnipresent alkali olivine basalts in the Yarmouk basin releases Fe(II) which precipitates as colloidal ferric oxyhydroxides (HFO) under oxidizing conditions. These colloids later aggregate to gels on all solid surfaces along the pathways within and below the basaltic layer. In aqueous systems, however, HREE and Y are slightly fractionated. The REY patterns of samples in this study are subdivided into 6 types. The first group (t1) typifies groundwater derived from weathered alkali olivine basalts. The patterns t2 and t3 show the results of increasing mixing with limestone water (t4) (Figures 6(a)–6(d)). In Figure 6(e), three REY patterns of type t2 are compiled which show very high LREE contents but low HREE and Y. Otherwise, they resemble type t2. Another different feature of t2 is that positive Gd anomalies exceed those of Y.

Figure 6: REY distribution patterns of groundwater in the Lower Yarmouk Gorge. The visual grouping of patterns shows their high variability.

All of the above patterns show positive Y anomalies. The dissolution of REY-enriched HFO yields convex patterns of type t5 with enhanced abundances of medium REE compared to light and heavy REE and negative Y anomalies (Figure 6(f)). These Y anomalies develop because Y prefers to stay in the aqueous phase during the stage of REY adsorption by HFO [36].

The water from Ain Himma in 2001 and 2007 and well Mukheibeh 4(16) shows REY patterns, typical of water from limestone aquifers such as those of Ein Dan and Ein Banyas in the Mt. Hermon Massif but without the negative Ce anomalies typical of spring waters from karstic limestones (Figure 6(g)) or from Cretaceous limestones along the rift valley [37]. Note that the REY abundance in waters from Mukheibeh 4(16) and Ain Himma from years 2000 and 2007 is lower than that in the spring waters of Dan and Banyas, which may be a result of interaction with HFO.

4.4. 87Sr/86Sr

Although the waters show a wide spread in Sr2+, their 87Sr/86Sr isotope ratios vary only between 0.7070 and 0.7077 (Figure 7). This corresponds with the range of 87Sr/86Sr in Cretaceous limestones of Israel, which is about 0.7070-07086 (Wilske et al., unpublished data). Only the Yarmouk River with 0.70710 points to mixing with basaltic rock drainage water which shows an 87Sr/86Sr value of 0.70455, slightly above Phanerozoic upper mantel alkali olivine basalts from Israel with 87Sr/86Sr of 0.7033-07035 (Table 3).

Figure 7: Crossplots of Sr isotope ratios and Sr2+ in the groundwater from the Lower Yarmouk Gorge.

In the plot of Sr2+ vs. 87Sr/86Sr, Mukheibeh field groundwater clusters at low Sr2+, whereas the samples from Hamat Gader, Meizar 2, and Himma show a wide spread. Mukheibeh 8 water fits into the Hamat Gader-Meizar 2-Himma trend, whereas Meizar 3 approaches the Mukheibeh field.

4.5. δ18O vs. δD

The stable water isotopes in the LYG range from low values of Meizar 2 in the southern Golan Heights and springs and wells on the eastern plunge of the Mt. Hermon Massif towards the Hauran plateau to high values in water of the Yarmouk River (Figure 8). All data from the LYG are plotted between the Syrian and Mt. Hermon meteoric water lines (MWL). The Mukheibeh waters like the groundwater from the Hauran plateau nearly cover the whole array, whereas the samples of Meizar, Hamat Gader, and Himma cluster. Ein Sahina, located uphill of the Hamat Gader group, is plotted among the Mukheibeh field. Ain Sarayah, located close to Ein Reach of the Hamat Gader cluster, is plotted among the heaviest Mukheibeh waters. Meizar 2 and Meizar 3(08) show the lowest isotope values.

Figure 8: δD vs. δ18O in groundwater of the Lower Yarmouk Gorge ([1]; this study) and the Hauran plateau [18]. MWL for Mt. Hermon and SW Syria are taken from Brielmann [52] and Al Charideh and Zakhem [53], respectively.

5. Discussion

5.1. Sources of Groundwater

The stable isotopes of water and the element correlations reveal different origins of fresh and saline contributions to the groundwater in the LYG. Distinct groups of stable isotopes suggest regional infiltration areas at different elevations. The Meizar 2 groundwater from 2001 to 2016 with (i) light δ2H and δ18O signatures and (ii) REY patterns of nearly limestone water shape and least affected by HFO (t4 in Figure 6(d)) suggests infiltration of precipitation on the outcropping Triassic to Cretaceous limestones of the foothills of the Mt. Hermon Massif. The increase in Cl- is higher than that in Na+ probably pointing to mobilization of highly evaporated seawater brines and admix of these brines to the limestone water.

The water of Meizar 2(08) and Meizar 3(08) shows similar chemical and isotopic composition and the same type of REY patterns (t5). Although showing similar U(VI) concentrations, Meizar 2(01) and Meizar 2(16) are dissimilar in REY patterns (t4 and t5). This suggests that these types of groundwater discharge from the same reservoir but the flow path of recharging water differs over the years.

The groundwater with δ18O and δD of about -6‰ and -30‰, respectively, typifies the groundwater from Hamat Gader, Himma, Meizar 3 in 2001 and 2016, and the Mukheibeh field. Most of the Mukheibeh and Hauran groundwater shows a trend of increasingly heavy stable isotopes of water, suggesting evaporation of recharge prior to infiltration (Figure 8). The effect of evaporation on stable isotope enrichment is shown by heaviest δD and δ18O signatures in the Yarmouk River.

High molar values of Na+/Cl- and Ca2+/SO42- but low Br-/Cl- and low concentrations of Na+, Cl-, Ca2+, Mg2+, K+, Sr2+, and Br- typify the basaltic waters [31]. Pure basaltic water is characterized by (Table 1) and typical REY patterns of type t1 (Figure 6(a)). With increasing leaching of halite from sedimentary rocks, the basaltic waters approach the lowest Na+/Cl- value of about 1, whereas mixing with evaporated seawater brines yields (Figure 9). Comparison of the Mukheibeh waters with those of basaltic composition reveals that the former waters are enriched in all elements (Table 1). The dissolution of anhydrite/gypsum by thermal waters of Meizar and Himma leads to enhanced concentrations of Ca2+ and SO42- (Figure 4(b)). Ca2+ may also increase by dissolution of calcite at enhanced temperatures and albitization of plagioclases in basalts. Mg2+, Rb+, Br-, and K+ may be gained by ion exchange against Ca2+ in marly layers in the aquifers (Figures 3 and 4). A Br- increase may be gained by contact with the bituminous-rich B2 formation. The correlations of Cl- with SO42-, Ca2+, Mg2+, Sr2+, Rb+, and Br- reveal that, with few exceptions, waters from Hamat Gader, Himma, and Meizar are mixtures of basaltic water and remnants of brines from the Triassic-Cretaceous Arabian carbonate platform. The strong correlation of Rb+ and Sr2+ indicates a common source but not necessarily the same mineral (Figure 4(h)). The two trends in the correlation of Rb+ and Br- verify the different sources of both elements (Figure 4(j)).

Figure 9: Crossplots of 1000Br/Cl and Na/Cl of the groundwater from the Lower Yarmouk Gorge.

The molar 1000Br-/Cl- vs. Na+/Cl- values show several trends for groundwater in the Yarmouk basin and the trend of evaporated seawater in salt pans (Figure 9). In this plot, the springs of Hamat Gader, Himma, and well Meizar 2 define vertical trends which are only explainable by leaching of Br- from the organic-rich B2 formation (Figure 3). Meizar 3 in 2001 and 2016 and all the groundwater with the lowest Br-/Cl- values in the vertical groups suggest mixing between Mukheibeh groundwater and seawater brine characterized by Na+/Cl- and 1000Br-/Cl- of about 0.5 and 5.3, respectively. Such ratios resemble those of the Ha’On type of brine, emerging at SE shoreline of the Lake Tiberias [2, 38, 39]. A second mixing line is indicated by Ein Sahina and Mukheibeh wells 1 and 6; both lines only differ in the Mukheibeh end member.

5.2. The Impact of HFO Precipitation on U and REY

U(VI) is highly adsorbed onto the high surface area of HFO [40]. The U content of alkali olivine basalts is in the range of 1 ppb [41]. The infiltrating basaltic groundwater with low U(VI) content passes the growing HFO “filter” within and below the basaltic cover of the Hauran plateau and elsewhere. During the alteration of HFO to goethite, lepidocrocite, or hematite, the adsorbed U(VI) is reduced to U(V) which is more resilient to oxidation than uraninite (UO2) or adsorbed U(IV) [42, 43]. Adsorption of U(VI) in the pH range of 6.6-7.3 (Table 1) is not affected by additional adsorption of phosphate [44].

The high U(VI) contents of 80 to 105 nmol/l in the groundwater of Mukheibeh artesian wells 1, 2, 4, 6, and 7 are most probably supplied later from the phosphorite-rich B2 aquifer. The phosphorites from the B2 formation in Syria, Jordan, and Israel contain about 100 ppm U [45]. Assuming that U(VI) is mobilized by phosphate as UO2(HPO4)22+ [46], the phosphate concentration should be in the range of 0.2 μmol/l or 6 μg/l which was much below our routine detection limit of phosphate of 1 mg/l.

Meizar 2 water has its source in the flanks of the Mt. Hermon Massif and in the western elevations of the Hauran plateau, which agrees with light stable isotopes of water. Although limestone waters contain 2-20 nmol/l U(IV) from elsewhere in Israel (Siebert unpublished), Meizar 2 and Mukheibeh 8, 9, and 11 waters show less than 0.1 nmol/l U(VI) suggesting that these waters must have had contact with HFO but did not interact with the B2 formation. Though having similar low U, considerably heavier stable isotope signatures in Mukheibeh 8 and 9, the most northeastern samples in the LYG, refer to a recharge area differing from Meizar 2.

HFO scavenges not only U(IV) but also REY and HPO42-. There may be some synergetic interaction between phosphate and REY resulting in type t1 patterns. This seems to be indicated in type t2, which is possibly due to Y-phosphate precipitation (possibly churchite, Y, and HREEPO4) due to which the light REE are released [47].

All groundwater in the gorge is produced from limestone aquifers. When the REY poor basaltic water passes the limestones at enhanced temperatures, some calcite dissolves and thereby its aliquot of REY is released and mixed with REY load of the groundwater. More than 99% of the REY is immediately adsorbed onto calcite surfaces [48]. This way, the REY patterns of groundwater change from type t1 to t2, t3, and finally t4 (Figures 6 and 10). At enhanced temperatures, release of LREE is faster than that of HREE and Y because their Coulomb binding forces are less for the former bigger than the latter smaller ions. This may qualitatively explain the change in REY patterns of groundwater in the Yarmouk basin.

Figure 10: Mixing of basaltic and limestone groundwater showing the change in rare earth distribution patterns.

Although the groundwater of the Yarmouk Gorge is produced from limestone aquifers, their REY patterns still indicate that the groundwater originates from basaltic catchment regions or, more precisely, has passed HFO layers. Although the patterns are similar in shape, the spring waters of Dan and Banyas from limestones of the Mt. Hermon Massif without contact with HFO show higher abundances than the limestone-like waters from the Yarmouk Gorge such as in Himma spring and Mukheibeh well 4(16) (Figure 6(g)). Types t1 to t4 in Figure 5 represent the continuous change of REY patterns due to the interaction of basaltic groundwater after passing the HFO filter t1 and limestones resulting in changes according to t2-t4. These types of patterns result from mixing limestone and basaltic rock waters. It could well be that not the whole volume of water changes due to the interaction but only parts of it and mixing of various types yields the final patterns as shown in Figure 10.

Type t5 (Figure 6(e)) is not showing dissolution of phosphate minerals such as apatite but leachates of altering HFO that loses REY at high levels. The difference between the latter two is that the former should show a positive Eu anomaly [36], whereas the latter is characterized by a negative one.

How does it come that these types of groundwater still show REY patterns typical after infiltration in basaltic catchment areas? The reason is that the REY in calcite surfaces along the pathways in limestones equilibrate with the low REY abundance from the basaltic catchment. Under steady-state conditions, the groundwater from limestones shows patterns achieved by the interaction with groundwater that has passed HFO layers [31].

5.3. Tracing Mixing by Sr2+ and 87Sr/86Sr

The above discussed findings, which trace back the genesis of the groundwater in the LYG by variable interactions of basaltic water with late Tertiary brines of Ha’On type and with calcite and limestone of the discharging Cretaceous/Paleogene aquifers, can be fortified by model calculations, which try to resemble the measured 87Sr/86Sr values in the groundwater of the LYG by at least interaction of basaltic water and brine (Figure 7).

Using the fraction of brine in the mixture of brine and basaltic water, the mix of Sr2+ (Equation (1)) and the mix of the Sr2+ isotope ratios (Equation (2)) are estimated. where index BW is the basaltic water.

Considering the analytical data on Sr2+ concentration of groundwater in Table 3, brine, basaltic water, and dissolved calcite and gypsum and their corresponding 87Sr/86Sr values and the Sr2+ concentration of basaltic water must be below 0.5 mg/l, the lowest value in Mukheibeh water. Indeed, pure basaltic water sampled from 2 springs in the cover basalt of the Golan Heights shows . The Sr2+ concentration of the brine may be between 79 mg/l as analyzed in Ha’On brine [2] and 300 mg/l, depending on the amount of dissolution of calcite from limestone with assumed average Sr2+ concentrations of 100 mg/mol calcite and about 25 mg/mol gypsum from evaporites [49]. The 87Sr/86Sr value of basaltic water is 0.70455 to 0.70457, and that of the brine is assumed to be 0.7078, matching the spread of data in Figure 7. The 87Sr/86Sr value of 0.7078 may result from mixing of Late Tertiary Tethys seawater of 0.7089 [50] and dissolved average Upper Cretaceous limestone in Israel ranging between 0.7076 and 0.7078 (Wilske et al., unpublished data).

The model curves in Figure 6 are fitted by varying Sr2+ in basaltic water and in brine as well as the 87Sr/86Sr value of the brine. Several information can be derived by the following procedure. (1)The observed groundwater cannot be fitted by one curve, and the results are sensitive to assumed values of 87Sr/86Sr and the resulting (2)To fit most Mukheibeh groundwater and that of Me3(16), the requested Sr2+ concentrations must be 0.05 mg/l, much lower than the observed 0.21 mg/l (Figure 7(a)). Hence, the positive shift of these types of groundwater along the ordinate is assumed to result from the interaction of the proposed fluid mix with calcite and gypsum in the discharging limestone aquifers, which show 87Sr/86Sr values as high as 0.7078 (Wilske et al., unpublished data)(3)The fitting curves are invariant in respect to variations of Sr2+ in the brine (compare red curve in Figure 6(a) and blue curve in Figure 7(b))(4)If 87Sr/86Sr values of brine are larger than 0.7078, neither the group of groundwater from Mukheibeh wells and Meizar 3 nor the group of Hamat Gader, Meizar 2, and Ain Himma can be represented (Figure 7(b))

In summary, the 87Sr/86Sr of the groundwater in the LYG is the result of relic brine, which is diluted by basaltic water and subsequently dissolves calcite and gypsum and experiences some exchange of Ca2+ against Mg2+, Na+, and K+ in marly layers of the aquifers (Figure 3). Only Meizar 3 is mainly limestone water.

5.4. Regional Distribution of Dissolved Species

The regional distribution of U(VI), Sr2+, and REY shows comparable structures, whereas SO42- behaves differently. High and low U(VI) concentrations are present in the NE of the Lower Yarmouk Gorge (Figure 11(a)). The high values of 80-105 nmol/l U mark the area in which Mukheibeh wells 1, 2, 4, 5, 6, and 7 produce artesian water from the phosphorite-rich B2 aquifer. These high U(VI) concentrations decrease to 20 nmol/l SW-ward, downstream the Yarmouk River and to both sides of the gorge. North of and NE-ward in the gorge groundwater contain U(VI) below 1 nmol/l. Such low values can only be established by adsorption of U(VI). In the case of Mukheibeh 8 and 9, this could be HFO in the Hauran plateau; in the case of Meizar, saline groundwater contact with dissolving HFO is documented in Figure 7(f) in the year 2008. According to Shimron [51], basaltic intrusions are present in the Mt. Hermon anticline, being probably responsible for the low U(VI). Additionally, the long pathway through the limestone aquifers from Mt. Hermon to the LYG altered the REY patterns in groundwater to type t4. In 2016, Meizar 3 shows the REY pattern of type t1. However, in 2001, it resembled type t4 of Meizar 2 in 2001 and 2016.

Figure 11: Regional distribution of U(VI) (a), Sr2+ (b), rare earth distribution patterns (c), and sulfate (d) in the Lower Yarmouk Gorge. Legends given in (a) and (c) are representative for the entire Figure 11.

In the central part of the LYG, Sr2+ is about 0.55 mg/l (Figure 11(b)), while it increases to 1 mg/l NE-ward, to 3 mg/l in Himma, to 4 mg/l in Hamat Gader, and to 5 mg/l in both Meizar wells 2 and 3.

A similar shell-like behavior is observable in the REY patterns with t1 patterns in the center followed by t2 SW-ward and t3 type SE-ward and patterns of t4 to t6 in the NW (Figure 11(c)).

The high-uranium water shows SO42- concentrations of 30-50 mg/l (Figure 11(d)). Outside that central part, the groundwater shows either much higher SO42- concentrations, such as in Hamat Gader (150 mg/l) and Meizar (300 mg/l), or almost no dissolved sulfate as in the NE (0.12 mg/l). The increasing SO42- outside the marked center may prove depletion of gypsum in the central region of ascending groundwater. Comparing spatial concentration distribution patterns of Sr2+ and SO42- results in similar patterns, though the concentration levels differ significantly.

Leaching of brines and/or evaporites alters the chemical composition of the initial basaltic water. The light signatures of water isotopes of Meizar 2 support a catchment area at the Mt. Hermon foothills or at elevated places in the Hauran. Meizar 3 water isotopes correspond with those of Hamat Gader and Himma, which may be taken as an indirect proof for its basaltic water. Their variable REY patterns of types t1, t4, and t5 suggest various flow paths of the groundwater including differing contacts with HFO. The shortest pathway of groundwater flow is indicated by REY patterns of type t1 (Figure 6), while patterns of type t2 and t3 suggest a longer pathway with more intense REY exchange with calcite in limestones. The longest pathways are typified by REY pattern type t4. The REY types and the concentrations of U(VI), Sr2+, and SO42- characterize complex flow patterns of groundwater towards the gorge.

The most distinct basaltic water is produced from the B1/B2 limestone aquifers fractured by a complex fault system crossing the LYG [15] (Figure 1(b)). This marks the most important flow path of drainage water from the Hauran into the LYG. The springs of Hamat Gader (including Ein Sahina) and Himma are positioned on an uptilted block, whereby both spring fields are separated from the Meizar field. The deep aquifer which is tapped by Meizar 2 also produced water in the shallow well Meizar 3 in 2008.

Although producing from the same aquifer, the hydrochemical differences in groundwater from Ein Himma and Meizar 3 disprove any transboundary flow below the Yarmouk River. The confined water from basaltic infiltration areas in Syria, however, is present on both sides of the gorge.

6. Conclusions

The conjoint study of major, minor, and trace elements, δ18O, δD, and 87Sr/86Sr in the groundwater of the LYG reveals the following: (i)Mixtures of water from basaltic rocks and limestones are almost omnipresent in the LYG. A clear exception is Meizar 2 that produces groundwater that was infiltrated at the flanks of Mt. Hermon Massif. The mixtures vary from nearly pure basaltic water to nearly pure limestone water. In addition, leaching of residual brines and evaporites enhances the salinity of the various types of groundwater(ii)The sources of salinization in limestone aquifers are given by relic brines, leaching of gypsum, and dissolution of calcite. The origin of high sulfate concentrations could be either the Late Triassic gypsum beds occurring at approximate depths of 2000 m or evaporites of the Late Tertiary rift brine of the inland sea. For instance, groundwater in Meizar 2 and Hamat Gader has leached different amounts of gypsum/anhydrite and calcite. Ion exchange of Ca2+ against Mg2+, Na+, and K+ enhanced the concentrations of the latter. Meizar 3 in 2008 resembles Meizar 2 in the same year. Their REY patterns show that this groundwater had dissolved HFO on its altered flow path. The regional distribution of U(VI), Sr2+, and SO42- and REY distribution patterns reveal that there is a zone with strongly confined groundwater and the hydrochemical composition changes systematically sideward and downstream along the gorge(iii)The regional variation of their chemical composition of groundwater is related to a complex flower-structured fault system crossing the gorge. Groundwater flow in the gorge and the mixing between the different water bodies are controlled by these structural features

Data Availability

All underlying data of the research study are included in the manuscript in the form of Tables 13.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The research was partly funded by the German Science Foundation (DFG) (grant MA4450/2) and the DESERVE Virtual Institute (VH-VI-527) funded by the Helmholtz-Association of German Research Centers. The authors thank the Mekorot Co. Ltd. and the IDF and JAF for providing access and security during sampling along the international borders.

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