Yoseph Yechieli

Sinkhole “swarms” along the Dead Sea coast: Reflection of disturbance of lake and adjacent groundwater systems

More than a thousand sinkholes have developed along the western coast of the Dead Sea since the early 1980s, more than 75% of them since 1997, all occurring within a narrow strip 60 km long and <1 km wide. This highly dynamic sinkhole development has accelerated in recent years to a rate of ∼150–200 sinkholes per year. The sinkholes cluster mostly over specific sites up to 1000 m long and 200 m wide, which spread parallel to the general direction of the fault system associated with the Dead Sea Transform. Research employing borehole and geophysical tools reveals that the sinkhole formation results from the dissolution of an ∼10,000-yr-old salt layer buried at a depth of 20–70 m below the surface. The salt dissolution by groundwater is evidenced by direct observations in test boreholes; these observations include large cavities within the salt layer and groundwater within the confined subaquifer beneath the salt layer that is undersaturated with respect to halite. Moreover, the groundwater brine within the salt layer exhibits geochemical evidence for actual salt dissolution (Na/Cl = 0.5–0.6 compared to Na/Cl = 0.25 in the Dead Sea brine). The groundwater heads below the salt layer have the potential for upward cross-layer flow, and the water is actually invading the salt layer, apparently along cracks and active faults. The abrupt appearance of the sinkholes, and their accelerated expansion thereafter, reflects a change in the groundwater regime around the shrinking lake and the extreme solubility of halite in water. The eastward retreat of the shoreline and the declining sea level cause an eastward migration of the fresh–saline water interface. As a result the salt layer, which originally was saturated with Dead Sea water over its entire spread, is gradually being invaded by fresh groundwater at its western boundary, which mixes and displaces the original Dead Sea brine. Accordingly, the location of the western boundary of the salt layer, which dates back to the shrinkage of the former Lake Lisan and its transition to the current Dead Sea, constrains the sinkhole distribution to a narrow strip along the Dead Sea coast. The entire phenomenon can be described as a hydrological chain reaction; it starts by intensive extraction of fresh water upstream of the Dead Sea, continues with the eastward retreat of the lake shoreline, which in turn modifies the groundwater regime, finally triggering the formation of sinkholes.

Will the Dead Sea die

The level of the Dead Sea (the lowest surface on Earth) is currently declining at a rate of 0.8 m/yr, and has dropped about 20 m since the beginning of the twentieth century; it reached −410 m in 1997. We address the question of whether the level of the Dead Sea will continue to decline. A numerical model, developed in this study to determine the water balance, accounts for the increase in salinity and the concomitant decrease in the rate of evaporation that accompanies reduction in the activity of the water. Simulations based on ranges of water withdrawal scenarios suggest that the Dead Sea will not “die”; rather, a new equilibrium is likely to be reached in about 400 yr after a water-level decrease of 100 to 150 m.

The sulfur system in anoxic subsurface brines and its implication in brine evolutionary pathways: the Ca-chloride brines in the Dead Sea area

Abstract Important elements in the evolutionary history of saline groundwater might be overlooked when they involve both sulfate removal through reduction and input of sulfate via dissolution. These two simultaneous and apparently contrasting processes can result in a negligible net effect on the sulfate concentration. Isotopic composition of sulfur in sulfate and sulfide can be applied to identify the bacterial sulfate reduction (BSR) though the extent of the process is difficult to quantify. Saturation with respect to gypsum may suggest that gypsum dissolution also occurs. However, a more definite identification of these processes and their quantification can be achieved through the use of ammonium concentration in the anoxic brines. This approach assumes that the ammonium is derived only from the oxidation of organic matter through BSR and it requires that the C:N ratio in the oxidized organic matter be known. A minimum estimate for the sulfate reduction can be obtained when the Redfield C:N ratio (106:16) is assumed. Several calculation methods are presented to identify the extent of sulfate reduction prior to, concomitant with, or following gypsum dissolution that are based on combining sulfur isotopic compositions, Rayleigh distillation equation, and calculated gypsum saturation indices. The required assumptions are presented and their validation is discussed. The subsurface hypersaline Ca-chloride brines in the vicinity of the Dead Sea are taken as a case study. Here sulfur isotope compositions of sulfate and sulfide, and high ammonium concentrations indicate BSR occurs in the subsurface. The sulfur isotopic composition of the sulfate makes it possible to distinguish between two major groups of brine and their recent evolutionary histories: (1) the Qedem–Shalem thermal brines (δ 34 S SO4 =21–24‰) which emerge as springs along the shores and are slightly undersaturated with respect to gypsum; (2) DSIF–Tappuah brines (δ 34 S SO4 =30–60‰) which are found in shallow boreholes and are saturated to oversaturated with respect to gypsum. Calculations based on their ammonium content suggest that both groups of brine require apparent unreasonably high oversaturations with respect to gypsum prior to the onset of the reduction. This implies that the groundwater systems were open with respect to sulfate, and that the sulfate reservoir was replenished continuously or intermittently during the BSR. The DSIF–Tappuah brines continue to dissolve gypsum during their BSR. The dissolving sulfate is derived from relatively isotopically enriched gypsums (δ 34 S SO4 >20‰), such as found in the Lisan Formation. These brines approach the steady-state isotopic composition (δ 34 S ss ) dictated by the combination of the δ 34 S of the dissolving gypsum and the fractionation factor accompanying BSR. The sulfur isotopic composition of the Qedem–Shalem brines implies that most of their ammonium content is derived from an earlier phase of BSR and that the last phase of BSR takes place during the brines’ rapid ascent to the surface. Prior to this stage they evolved through either: (1) dissolution of gypsum with δ 34 S SO4 ≤20‰ which occurred after the main BSR in the subsurface; (2) a previous phase in which the brines were part of a lake and later percolated to the subsurface. As such, their isotopic composition and ammonium content were determined by the combined effect of freshwater sulfate input to the lake and BSR in the stratified lake.

What Is the Role of Fresh Groundwater and Recirculated Seawater in Conveying Nutrients to the Coastal Ocean

Submarine groundwater discharge (SGD) is a major process operating at the land-sea interface. Quantifying the SGD nutrient loads and the marine/terrestrial controls of this transport is of high importance, especially in oligotrophic seas such as the eastern Mediterranean. The fluxes of nutrients in groundwater discharging from the seafloor at Dor Bay (southeastern Mediterranean) were studied in detail using seepage meters. Our main finding is that the terrestrial, fresh groundwater is the main conveyor of DIN and silica to the coastal water, with loads of 500 and 560 mol/yr, respectively, per 1 m shoreline. Conversely, recirculated seawater is nutrient-poor, and its role is mainly as a dilution agent. The nutrient loads regenerated in the subterranean estuary (sub-bay sediment) are relatively small, consisting mostly of ammonium (24 mol/yr). On the other hand, the subterranean estuary at Dor Bay sequesters as much as 100 mol N/yr per 1 m shoreline, mainly via denitrification processes. These, and observations from other SGD sites, imply that the subterranean estuary at some coastal systems may function more as a sink for nitrogen than a source. This further questions the extent of nutrient contributions to the coastal water by some subterranean estuaries and warrants systematic evaluation of this process in various hydrological and marine trophic conditions.

Time response of the water table and saltwater transition zone to a base level drop

[1] This paper investigates the effect of a drainage base level drop on the groundwater system in its vicinity, using theoretical analysis, simulations, and field data. We present a simple and novel method for analyzing the effect of a base level drop by defining two characteristic times that describe the response of the water table and the transition zone between the fresh and saline water. The Dead Sea was chosen as a case study for this process because of the lake’s rapid level drop rate. During a continuous lake level drop, the discharge attains a constant value and the hydraulic gradient remains constant. We describe this new dynamic equilibrium and support it by theoretical analysis, simulation, and field data. Using theoretical analysis and sensitivity tests, we demonstrate how different hydrological parameters control the response rate of the transition zone to the base level drop. In some cases, the response of the transition zone may be very rapid and in equilibrium with the water table or, alternatively, it can be much slower than the water table response, as is the case in the study area.

The source and age of groundwater brines in the Dead Sea area, as deduced from 36Cl and 14C

36Cl and 14C analyses of saline groundwater in the Dead Sea area were performed in order to study the chloride source of the brines and their age. Similarity among the 36Cl/Cl ratio in saline groundwater (7−15 × 10−15), Dead Sea water (12.8 × 10−15), and in an halite layer (11 × 10−15) indicates that they all have a similar chloride source. The 36Cl/Cl ratios also suggest a significant contribution of chloride from rain water. It is postulated that groundwater brines are the result of direct infiltration of brines from a precursor Dead Sea lake, such as Lake Lisan, which covered the area in the past. This lake underwent several evaporation stages precipitating halite and infiltrating brines of varying chemical composition. 14C data (3.8–25.8 PMC) indicate that the percolation of brines into the sediments took place more than 9,000 years ago.

Radiocarbon in seawater intruding into the Israeli Mediterranean coastal aquifer.

Saline groundwaters from the Israeli coastal aquifer were analyzed for their radiocarbon and tritium content to assess the rate of seawater penetration. The low (super 14) C values (28-88 pMC versus 100-117 pMC in seawater) imply an apparent non-recent seawater source, or water-rock interactions along the penetration route. The latter process is supported by measurable tritium values at some locations, which imply a relatively rapid rate of seawater intrusion. In other locations, low tritium values (

Anaerobic oxidation of methane by sulfate in hypersaline groundwater of the Dead Sea aquifer

lt;2 T.U.) indicate that recent seawater (

CHARACTERIZATION AND DATING OF SALINE GROUNDWATER IN THE DEAD SEA AREA

lt;50 yr) did not penetrate inland. The low delta (super 13) C values in saline groundwater (average of -5.3 per mil versus 0 per mil in seawater) indicate that the dissolved carbon pool is comprised of a significant fraction of organic carbon. A linear negative correlation between delta (super 13) C and (super 14) C implies that this organic source is old (low (super 14) C values).

Buoyancy‐Induced Flow of a Tracer in Vertical Conduits

Geochemical and microbial evidence points to anaerobic oxidation of methane (AOM) likely coupled with bacterial sulfate reduction in the hypersaline groundwater of the Dead Sea (DS) alluvial aquifer. Groundwater was sampled from nine boreholes drilled along the Arugot alluvial fan next to the DS. The groundwater samples were highly saline (up to 6300 mm chlorine), anoxic, and contained methane. A mass balance calculation demonstrates that the very low δ13CDIC in this groundwater is due to anaerobic methane oxidation. Sulfate depletion coincident with isotope enrichment of sulfur and oxygen isotopes in the sulfate suggests that sulfate reduction is associated with this AOM. DNA extraction and 16S amplicon sequencing were used to explore the microbial community present and were found to be microbial composition indicative of bacterial sulfate reducers associated with anaerobic methanotrophic archaea (ANME) driving AOM. The net sulfate reduction seems to be primarily controlled by the salinity and the available methane and is substantially lower as salinity increases (2.5 mm sulfate removal at 3000 mm chlorine but only 0.5 mm sulfate removal at 6300 mm chlorine). Low overall sulfur isotope fractionation observed (34ε = 17 ± 3.5‰) hints at high rates of sulfate reduction, as has been previously suggested for sulfate reduction coupled with methane oxidation. The new results demonstrate the presence of sulfate-driven AOM in terrestrial hypersaline systems and expand our understanding of how microbial life is sustained under the challenging conditions of an extremely hypersaline environment.