PUBLICATIONS. Journal of Geophysical Research: Earth Surface. Salt dissolution and sinkhole formation: Results of laboratory experiments

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1 PUBLICATIONS Journal of Geophysical Research: Earth Surface RESEARCH ARTICLE Key Points: Experiment results validate the common mechanism for formation of thousands of sinkholes along the Dead Sea shores In case of stratification in the Dead Sea the rate of sinkhole formation might decrease by 1 order of magnitude Supporting Information: Supporting Information S1 Movie S1 Movie S2 Correspondence to: I. Oz, imri.oz@mail.huji.ac.il Citation: Oz, Imri., S. Eyal, Y. Yoseph, G. Ittai, L. Elad, and G. Haim (2016), Salt dissolution and sinkhole formation: Results of laboratory experiments, J. Geophys. Res. Earth Surf., 121, , doi: / 2016JF Received 28 MAR 2016 Accepted 10 SEP 2016 Accepted article online 13 SEP 2016 Published online 6 OCT 2016 Corrected 7 NOV 2016 This article was corrected on 7 NOV See the end of the full text for details American Geophysical Union. All Rights Reserved. Salt dissolution and sinkhole formation: Results of laboratory experiments Imri Oz 1,2, Shalev Eyal 2, Yechieli Yoseph 2,3, Gavrieli Ittai 2, Levanon Elad 1,2, and Gvirtzman Haim 1 1 The Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel, 2 Geological Survey of Israel, Jerusalem, Israel, 3 Department of Environmental Hydrology & Microbiology (EHM), Ben-Gurion University, Beersheba, Israel Abstract The accepted mechanism for the formation of thousands of sinkholes along the coast of the Dead Sea suggests that their primary cause is dissolution of a salt layer by groundwater undersaturated with respect to halite. This is related to the drop in the Dead Sea level, which caused a corresponding drop of the freshwater-saltwater interface, resulting in fresher groundwater replacing the brines that were in contact with the salt layer. In this study we used physical laboratory experiments to examine the validity of this mechanism by reproducing the full dynamic natural process and to examine the impact of different hydrogeological characteristics on this process. The experimental results show surface subsidence and sinkhole formation. The stratigraphic configurations of the aquifer, together with the mechanical properties of the salt layer, determine the dynamic patterns of the sinkhole formation (instantaneous versus gradual formation). Laboratory experiments were also used to study the potential impact of future stratification in the Dead Sea, if and when the Red Sea-Dead Sea Canal project is carried out, and the Dead Sea level remains stable. The results show that the dissolution rates are slower by 1 order of magnitude in comparison with a nonstratified saltwater body, and therefore, the processes of salt dissolution and sinkhole formation will be relatively restrained under these conditions. 1. Introduction Sinkhole formation is a well-known geologic feature that takes place, where subsurface cavities induce collapse and land failure. Sinkholes are found in many parts of the world including, among others, the United States, Spain, Italy, Thailand, Turkey, and UK. The mechanism responsible for the cavity formation is dissolution of soluble rocks such as limestone and dolomite, whose dissolution rates are relatively slow, as well as evaporites, such as gypsum and halite layers, which are dissolved much faster [Festa et al., 2016; Johnson, 1997; Galloway et al., 1999; Martinez et al., 1998; Gutiérrez and Cooper, 2002]. In the past decades thousands of sinkholes have been developing along the western shores of the Dead Sea and the Lisan Peninsula (Figure 1); some of which reach a depth of 20 m and a diameter of 25 m. These have become a major concern, posing a threat to life and property and hampering local building and development [Arkin and Gilat, 2000; Abelson et al., 2003; Shalev et al., 2006; Yechieli et al., 2006]. The sinkhole formation is associated with the rapid decline of the Dead Sea water level and the consequent migration of the mixing zone between fresh groundwater and the Dead Sea brine [Abelson et al., 2006; Yechieli et al., 2006]. Analyses of groundwater systems are usually performed by using field studies, numerical modeling, or physical models in the laboratory. In some cases it is a complicated or impossible task to attain good field data or reliable parameters for numerical simulation; thus, physical laboratory experiments serve as a good alternative. In addition, the use of physical models provides an ideal setup for measurement, conceptualization, and visualization of groundwater flow and solute transport through the aquifer. Therefore, physical models are used in a variety of groundwater studies, such as density-driven groundwater flow [e.g., Simmons et al., 2002; Illangasekare et al., 2006; Oostrom et al., 1992], saltwater intrusions [e.g., Oz et al., 2014; Chang and Clement, 2012; Goswami and Clement, 2007; Kuan et al., 2012; Morgan et al., 2013], transport of solutes and contaminants [Chang and Clement, 2013; Katz and Gvirtzman, 2000], and effects of heterogeneous porous media [Levy and Berkowitz, 2003; Post and Simmons, 2010; Barth et al., 2001; Illman et al., 2007; Konz et al., 2009]. In this study a physical model is used for conducting laboratory experiments to investigate processes that lead to salt dissolution and sinkhole formation. OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1746

2 Figure 1. Location maps showing the Dead Sea Transform, the distribution of sinkhole sites (black squares), and two selected sinkhole sites (En Gedi date plantations and Samar) along the Dead Sea coast. Figure 2. Schematic cross sections of two subaquifers in an alluvial fan perpendicular to the Dead Sea shore. The level of the Dead Sea and the location of the freshwater-saltwater interface for 1970 and 2015 are shown by black and red dashed lines, respectively Hydrogeology and Limnology of the Dead Sea The Dead Sea is located in the deepest part of the Dead Sea Rift (water level of 429 m above sea level in 2015). The Dead Sea basin developed along a dilatational step between two major strike-slip segments of the Dead Sea OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1747

3 Dead Sea level [BSL] Year Transform [Garfunkel, 1997]. It is one of the largest pull-apart basins on Earth, with an overall length of ~150 km, width of km, and subsidence of more than 10 km, filled with Neogene and Quaternary sediments. The Dead Sea is a hypersaline, extremely dense, terminal lake (total dissolved solids >340 kg/m 3,density>1240 kg/m 3 ), and its brine is saturated with respect to halite [Gavrieli et al., 1989]. The Dead Sea water is Ca-chloridic with a very low Na/Cl ratio compared to normal ocean water (0.20 today versus 0.86, respectively). This unique composition is the result of the evaporation of seawater that intruded the rift valley, followed by halite precipitation and dolomitization [Starinsky, 1974; Katz and Starinsky, 2009]. There are three main aquifers in the Dead Sea region [Arad and Michaeli, 1967; Yechieli et al., 1995]: Lower Cretaceous Kurnub aquifer, composed mostly of sandstone (~300 m thick); Upper Cretaceous Judea Group aquifer, composed of carbonate rocks (~500 m thick); and Quaternary alluvial aquifer (~ m thick) (Figure 2). The alluvial aquifer is bounded on its west by the margin faults of the Dead Sea basin, which set the Judea Group carbonates against the Quaternary sediments. In the east, the alluvial aquifer is in direct contact with the lake along most of its shoreline. The sediments in the alluvial aquifer are clastic deposits in fan deltas (gravel, sand, and clay) and lacustrine sediments in mud plains (clay, aragonite, gypsum, and salt). The salt layers are usually separated from the gravel by impermeable clay layers. These alternations between gravel and clay and/or salt create several subaquifers that differ in their groundwater heads and chemical compositions (Figure 2). The hydraulic connection of the different subaquifers and the impermeable clay and salt layers with the Dead Sea is not entirely clear. In some locations they seem to continue directly to the lake bottom and have a good connection with the lake. In others, there is a clay layer near the lake and in its bottom, blocking the connection between the lower confined subaquifer and the lake and between the salt layer and the lake. The recharge of fresh groundwater to the Quaternary aquifer is through lateral flow across the rift faults from the Judea Group aquifer, which is replenished in the highlands km to the west. Some additional recharge results from flash floods. Direct recharge to the Quaternary aquifer over the alluvial sediments is negligible because of the arid climate and high evaporation in the region. The fresh groundwater hydraulic gradient is m/m and varies along the Dead Sea coast [Yechieli, 2000]. The flow of fresh groundwater to the Dead Sea creates a mixing zone between the freshwater and the Dead Sea brine within its coastal aquifer. Due to the extremely high density of the Dead Sea brine, this freshwater-saltwater interface (FSI) is very shallow, ~10 times shallower than that found near the ocean [Yechieli, 2000]. The Dead Sea level has been dropping since the 1960s (Figure 3) due to diversion by both Jordan and Israel of freshwater that used to flow to the lake through the Jordan River. The rate of decline of the lake level has increased gradually to ~1 m/yr in recent years. This decline is accompanied by changes in the Dead Sea and its surrounding area, and has a major effect on the adjacent groundwater system, exhibited in the migration of the FSI seaward. In order to stop the decline of the Dead Sea level and to stabilize the water level, a project to convey seawater from the Red Sea to the Dead Sea ( Red Sea-Dead Sea Canal ) is being considered [Gavrieli et al., 2005]. The mixing of the Dead Sea brine with the seawater during the operation of this project is expected to lead to the development of stratification in the water column of the Dead Sea [Gavrieli et al., 2006]. Oz et al. [2011, 2014] used numerical simulations and laboratory experiments to model the steady state and the transient configuration of the FSI and the general flow patterns within a coastal aquifer adjacent to a long-term stratified saltwater body. They showed that three interfaces between the three different water types (fresh groundwater and two layers of the saltwater body) develop within the aquifer along with three circulation cells Figure 3. Dead Sea level (solid line; data from the Hydrological Service of Israel) and number of sinkholes (dashed line) along the shores of the Dead Sea. 500 Number of sinkholes 1.2. Dead Sea Sinkholes Sinkholes along the Dead Sea are known since the 1960s, but their development has accelerated significantly since the mid-1990s [Abelson et al., 2003]. The formation of sinkholes in the Dead Sea area is strongly related OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1748

4 Figure 4. Schematic diagram showing the full laboratory setup in the experimental sand tank. In experiments 1AGN 6CBN (Table 2), outlets C and D, the lower overflow collector, and the low-salinity reservoir are not included. Unit a1 and a2 are peristaltic pumps, and b1 and b2 are laboratory lifts which control the overflow outlet heights. Numbers and points represent the 168 electrical conductivity electrodes. The highlighted electrodes (#6, #30, and #53) were used in some of the experiments to support the qualitative results of the pictures. to the decline of the Dead Sea level (Figure 3). This continuous decline led to the migration of the FSI eastward, causing the salt layer below the FSI to be exposed to fresh groundwater unsaturated with respect to halite (Figure 2). The exposure of salt layers to such water causes extensive dissolution and creates cavities. These cavities expand rapidly until their ceilings collapse, followed by the collapse of unconsolidated rocks above them and formation of sinkholes at the surface. Direct contact between the chemically unsaturated water and the salt layer is made possible by stratigraphic discontinuity at the bottom of the soft impermeable clay layer that surrounds the salt layer or by faults that cut through this clay layer and channeled fresh groundwater from the deeper aquifer to the shallower one. The sinkholes appear along lineaments, following young fault systems [Abelson et al., 2003]. The mechanism presented above is the conventional, well-accepted mechanism for salt dissolution and formation of thousands of sinkholes along the Dead Sea shores, based on field observations, data from boreholes, and geochemical and geophysical data [Abelson et al., 2006; Yechieli et al., 2006]. However, the full dynamic process that leads to sinkhole formation has never been observed in the field. Furthermore, numerical models are difficult to apply due to the inherent complexity of the natural system and lack of real values for the necessary parameters (such as hydraulic conductivities, geomechanical properties, and dissolution rates). In this study the experimental approach was adopted in order to test and visualize, for the first time, the accepted mechanism that leads to salt dissolution and formation of thousands of sinkholes along the Dead Sea shores. Visualization of the process would help to evaluate its validity. The second objective is to use the experimental approach to examine the effects of different hydrogeological configurations, salt layer properties, and the future impact of stratification in the Dead Sea during the operation of the Red Sea-Dead Sea Canal project, on the rate and patterns of salt dissolution and sinkhole formation. 2. Materials and Methods 2.1. Physical Model Laboratory experiments were conducted in a vertical, thin, rectangular flow tank made of 1.2 cm thick Plexiglas, simulating the coastal aquifer adjacent to the Dead Sea (Figure 4). The flow tank is divided into three distinct chambers: a central flow chamber containing the porous medium (the aquifer) and two side chambers, which maintain constant water heads, which form the boundary conditions. The right chamber represents the saltwater boundary (the open water system) and contains either a stratified or a nonstratified water column. The left chamber represents the inland boundary of the regional fresh groundwater. The side chambers are separated from the main porous medium chamber by a fine net that prevents the passage of granular material. At the backside of the flow tank 168 electrodes were placed for in situ electrical conductivity (salinity) measurements. OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1749

5 Table 1. The Characteristics of the Water Types Used in the Laboratory Experiments High-Salinity Brine (HSB) Low-Salinity Brine (LSB) Fresh Groundwater Measured density (kg m 3 ) (25 C) Salinity (TDS) (g/l) Electric potential (mv) Color Green Red No color The initial conditions and the boundary conditions throughout the experiment were determined by the water levels within the side chambers. These levels were controlled by a system of laboratory lifts, pipes, and pumps (Figure 4). The full experimental setup that was used for maintaining the boundary conditions in the case of stratification is described in Oz et al. [2014]. The setup used in the experiments with a nonstratified saltwater body does not include outlets C and D, the lower overflow collector, and the lowsalinity reservoir (Figure 4). The freshwater was pumped from the supply reservoir with a peristaltic pump (a2) into an overflow bottle, located on an elevator, and connected to pipe A, thus maintaining the water level within the chamber. The freshwater flowed through the porous media and out from the saltwater chamber through pipe B. The salt water was pumped from the supply reservoir with a peristaltic pump (a1), flowed into the saltwater chamber from pipe E, and out into the overflow collector through pipe B. The height of this outflow (pipe B) determined the water level within the saltwater chamber, which represents the level of the open saltwater body. The coastal aquifer adjacent to the Dead Sea is mainly composed of three different rocks or sediment types: gravel, salt, and clay, represented in the experiments by sand, salt grains or consolidated massive salt, and bentonite/clear varnish, respectively. The packing of the experimental tank was carried out under saturated conditions, with the grains being poured through the water to avoid entrapment of air bubbles. The sand is silica grains from Negev Industrial Minerals Ltd., which were sieved to a diameter range of mm and to a range of mm for the coarse sand layers. The sand was first washed with distilled water to remove dust and clay minerals. The oxide coatings of the quartz grains were removed by HCl acid to prevent the sorption of the dye tracers that were added to the salt water [Magal et al., 2008]. The salt layer in the experiments is represented by two salt types: (1) salt grains and (2) salt blocks; their characteristics are described in section In experiments with the salt grains, bentonite, a sodium-rich phyllosilicate consisting mostly of montmorillonite clay of volcanic ash origin, was used to simulate the clay layer that surrounds it. The bentonite swells upon contact with water and thus was used as a low-permeability barrier. In experiments with the massive salt block, the bentonite penetrated between the salt block and the front wall of the Plexiglas tank, occluding the results. Therefore, in these experiments the impermeable bentonite, which simulates the clay layer that surrounds the salt blocks, was replaced by impermeable clear varnish that was sprayed around the salt prior to the experiment. The three water types used in the laboratory experiments were (1) fresh groundwater, (2) low-salinity brine (LSB), and (3) high-salinity brine (HSB) (Table 1). The LSB was used only in the experiments that included stratification of the saltwater body. Both brines were prepared by dissolving halite (NaCl) in tap water. The HSB, which represents the Dead Sea brine, was prepared by dissolving halite until saturation was achieved. The brines were dyed by adding ~10 g to 20 L of the solutions: commercial red food color (AmeriColor Ltd.) to the LSB and pyranine (green color, 8-hydroxypyrene-1,3,6-trisulfonic acid, and trisodium salt; 75%) to the HSB. The adsorption of the food color and the pyranine to the quartz grains in these concentrations is negligible (our own batch experiments and Magal et al. [2008]). The densities were measured by a digital density meter (Kyoto Electronics, DA130N) normalized to a temperature of 25 C. The experimental process, including the distribution of the dye tracers within the flow tank, and the formation of sinkhole and/or cavities, was documented by digital and time-lapse cameras. The documentation was done either at varying time intervals, depending on the rate of changes in the location of the water types, or continuously, at fixed intervals, throughout the entire experiment. In some of experiments the distribution of the dye tracers, as presented in the pictures, were confirmed by OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1750

6 Table 2. The Experimental Setups Based on the Hydrogeological Characteristics and the Limnological Conditions of the Dead Sea Experiment Name a Cross-Section Type Salt Type Limnological Conditions Experiment 1AGN A Grains Nonstratified Experiment 2BGN B Grains Experiment 3BBN B Block Experiment 4BBN B Block Experiment 5CGN C Grains Experiment 6CBN C Block Experiment 7BBS B Block Stratified Experiment 8CBS C Block a Experiment names include experiment number (1 8) and the initials of the cross-section type (A, B, or C; defined in section 2.2.1), salt type (G grain and B block), and limnological conditions (N nonstratified and S stratified; defined in section 2.2.2). semiquantitative data from the electrodes located in the back of the experimental tank. These electrodes measure real-time electric potential, which was calibrated against solutions with known salinity (Table 1) Experimental Setups In addition to verification of the mechanism for sinkhole formation, the experiments were used to examine the impact of two different hydrogeological configurations of the coastal aquifer: (1) its stratigraphic configuration and (2) the properties of the salt layer. In addition, we examined what the impact would be of different limnological configurations, such as stratification in the Dead Sea during the operation of the Red Sea-Dead Sea Canal project [Gavrieli et al., 2006]. These experimental setups were tested in eight different experiments, and their characteristics are presented in Table Hydrogeological Configurations The impacts of different hydrogeological configurations of the aquifer were tested using three different stratigraphic cross sections (Figure 5), which generally represent the coastal aquifer of the Dead Sea. These cross sections were made based on geological data from boreholes in different sinkhole sites in fan deltas and mud plains along the Dead Sea shores. The first cross section simulates a stratigraphic case in which the outer part of the salt layer is in direct contact with gravel sediments, with no separation of the clay layer between them (Figure 5a). This scenario was used in experiment 1AGN. The second (Figure 5b) was used in experiments 2BGN, 3BBN, 4BBN, and 7BBS, simulating a case in which most of the outer part of the salt layer is surrounded by a clay layer, but direct contact between the salt layer and the gravel sediments is possible through gaps below the salt layer, where the clay layer is missing. The third cross section (Figure 5c) simulates a stratigraphic case in which the outer part of the salt layer is fully surrounded by a clay layer. Vertical displacement of the salt and clay layers enables direct contact between the salt layer and the sand and creates upper and lower gaps along the fault plane locally exposing the salt layer to the gravel. This scenario was used in experiments 5CGN, 6CBN, and 8CBS. The stratigraphic configurations in cross sections A and C are found in the En Gedi site (Figure 1), where there is a clay layer above and below the salt layer in the western side (C), while in the east, near the Dead Sea, there is no clay (A) [Shalev and Yechieli, 2008]. Data from boreholes in the Mineral site (Figure 1) show the presence of a clay layer above the salt layer, and the absence of this clay layer below [Shalev and Yechieli, 2008], as shown in cross section B. In addition to the impact of different stratigraphic configurations, we also tested the impact of the two different properties of the salt layer using two types of the salt: (1) salt grains in experiments 1AGN, 2BGN, and 5CGN and (2) consolidated salt blocks in experiments 3BBN, 4BBN, 6CBN, 7BBS, and 8CBS. These two types are end-members that represent salts found in sediment cores from boreholes drilled in sinkhole sites. The salt grains that were used in the experiments are commercial halite grains (99.5%), with a diameter range of mm. This type of nonconsolidated salt was found in sediment cores that were taken from boreholes (i.e., Mineral site in Figure 1). The consolidated salt blocks are salt rocks that were taken from recent salt deposits that precipitated along the shore of the Dead Sea (~100% halite) and from Mount Sedom, which is a salt diaper located at the southern part of the Dead Sea (Figure 1). This salt consists predominantly of salt OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1751

7 Figure 5. Three cross sections perpendicular to the Dead Sea shore, which portray different stratigraphic configurations for the aquifer. rock (halite), cyclically interbedded with gypsum; anhydrite; dolomite; and minor amounts of silt, marl, and clay [Zak, 1967]. The salt blocks were cut and shaped to fit the flow tank and the desirable dimensions Stratification in the Dead Sea During the operation of the Red Sea-Dead Sea Canal project, three interfaces and three circulation cells are expected to develop within the coastal aquifer adjacent to the Dead Sea [Oz et al., 2011, 2014]. The impact of stratification in the Dead Sea and the corresponding unique groundwater configuration on salt dissolution and sinkhole formation was not studied before. Therefore, the impact of stratification was tested in experiments 7BBS and 7CBS (Table 2). Stratigraphic cross sections B and C were used in these experiments. 3. Results 3.1. Sinkhole Formation Along the Dead Sea Shores Experiments 1AGN to 6CBN (Table 2) simulate the natural processes of salt dissolution and sinkhole formation along the shores of the Dead Sea. The experimental procedure in all of the experiments was conducted according to the following steps: (see section 1.2): (1) an initial steady state of the FSI is created between the HSB and fresh groundwater. The salt layer was located below the FSI at the onset of the experiments and is thus initially exposed only to the saturated HSB; (2) the water level in the saltwater chamber is lowered, demonstrating lake-level decline; and (3) consequently, the FSI withdraws back toward the saltwater boundary, and the salt layer is exposed to fresh groundwater flow through zones, where it is connected directly to the sand layers. In experiment 1AGN (Figure 6), the dissolution of the salt layer is followed by a gradual subsidence of the surface and the formation of cavities in the sand above the salt layer. The dissolution of the salt layer under this configuration starts from its top left side, shortly after the FSI reaches that area. Thus, subsidence of the surface begins at the inner parts of the aquifer, far from the saltwater body, and its progress depends on how far the FSI has receded. The final geometry of the salt layer after the dissolution is shaped by the parabolic geometry of the FSI (Figure 6b). The qualitative data from the pictures are confirmed by semiquantitative data from an electrode located at the top left side of the salt layer (Figure 6). The electric potential value of this electrode decreased rapidly (<8 min) after the FSI crossed this point, from values of HSB (0.803 mv), OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1752

8 Figure 6. The setup and the results of experiment 1AGN. Cross section A, salt grains, and nonstratified limnological conditions were used. The red and black dashed lines in box B represent the surface of the sand at the beginning and the end of the experiment, respectively. The red and black circles represent the spatial location of electrode #30 (Figure 4). saturated with respect to halite, to a value of unsaturated mixed water (0.234 mv), less saline than the LSB (Table 1), which made the dissolution of the salt layer possible. In experiment 2BGN (Figure 7 and Movie S1 in the supporting information for the full dynamic process of the experiment), at first, the impermeable clay layer blocks the direct access between the sand and the salt layers. The dissolution of the salt layer under this configuration starts from the bottom of the salt layer, through the gap in the impermeable clay layer, shortly after the FSI reaches that area (Figure 7). Subsequently, the fresh groundwater dissolves the salt layer from below and carries the dissolved salts seaward. The dissolution progresses from the area above that gap to the margins of the salt layer. Accordingly, a sinkhole develops at the surface above the gap and progresses gradually to both sides above the dissolving salt layer margins. During the salt dissolution, the incompetent clay and sand layers sink and replace the dissolved salt layer (Figure 7b). At the end of the experiment (Figure 7c) the salt layer is completely dissolved (100%), and the size of the sinkhole at the surface increases. As in experiment 1AGN, the qualitative data from the pictures are confirmed by semiquantitative data from an electrode located below and to the left from the gap in the salt layer (Figure 7). The electric potential value of this electrode decreased rapidly (<10 min) after the FSI crossed this point, from values of HSB (0.75 mv), which are saturated with respect to halite, to a value of unsaturated mixed water (0.6 mv), which made the dissolution of the salt layer possible (Table 1). Figure 7. The setup and the results of experiment 2BGN. Cross section B, salt grains, and nonstratified limnological conditions were used. The red and black dashed lines in boxes B and C represent the surface location at the beginning and the end of the experiment, respectively. The red and black circles represent the spatial location of electrode #53 (Figure 4). In experiment 3BBN (Figure 8 and Movie S2 for the full dynamic process of the experiment), as in experiment 2BGN, dissolution of the salt layer begins when fresh groundwater reaches the gap in the bottom of the salt layer, shortly after the FSI reaches that area. The dissolved salt is carried seaward by the fresh groundwater that dissolved them. This was inferred qualitatively from the pictures. However, in this experiment, due to the mechanical properties of the consolidated salt block, the dissolution initially forms a cavity in the salt layer that does not affect the sand layers and the surface above (Figure 8b). Continuous dissolution of the salt increases the size of the cavity almost to the original size of the salt layer (Figure 8c). During this phase, the impact on the sand layers and the surface above the salt layer is minor OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1753

9 Figure 8. The setup and the results of experiment 3BBN. Cross section B, consolidated salt blocks, and nonstratified limnological conditions were used. The red and black dashed lines in box D represent the surface location at the beginning and the end of the experiment, respectively. and mainly concentrated on the left side. Shortly afterward, an additional dissolution pushes the system beyond its mechanical failure threshold, and instantaneous collapse of the cavity takes place. The overlying ceiling layers collapse into the empty space and fill the cavity, and a collapse sinkhole is created at the surface (Figure 8d). Experiment 4BBN is similar to experiment 3BBN, but the volume of the salt layer is smaller and its shape is not rectangular (Figure 9a). As in experiment 3BBN, the cavity that develops in the beginning of the dissolution Figure 9. The setup and the results of experiment 4BBN, which shows the dynamic formation of the cavity in the unsaturated zone. Cross section B, consolidated salt block, and nonstratified limnological conditions were used. OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1754

10 Figure 10. The setup and the results of experiment 5CGN. Cross section C, salt grains, and nonstratified limnological conditions were used. The red and black dashed lines in boxes B and C represent the surface location at the beginning and the end of the experiment, respectively. The red and black circles represent the spatial location of electrode #6 (Figure 4). does not affect the sand layers and the surface above it (Figure 9b). However, as the salt layer continues to be dissolved, the overlying ceiling collapses instantaneously into the empty space. However, in this case the collapse of the sand layers above the salt layer doesnotreachthesurfacebutremains in the unsaturated zone, without the formation of a sinkhole (Figure 9c), due to the high mechanical strength in that zone (discussed in section 4.2). The dissolution of the salt layer in experiment 5CGN starts shortly after the FSI reaches the left salt block from the upper gap of the fault plane (Figure 10). Although this gap faces counter to the direction of the fresh groundwater flow, it is located higher than the gap at the lower part on the faultplane.therefore,thefsicrosses the upper gap first, exposing the salt layeratthisparttothefreshgroundwater flow, which enables its dissolution. The fresh groundwater that dissolves the salt becomes heavier, forming a vertical downward flow along the fault plane and through the left block. Consequently, the clay layer and the surface above the left part of the salt block subsided gradually, and a cavity was formed next to the surface (Figure 10b). The experiment terminated after 4 h, 2 h after the salt layer on the left side of the fault plane had completely dissolved. Toward the end of the experiment the cavity volume above the left block expanded and the surface continued to subside (Figure 10c). The dissolution of the salt layer to the right of the fault plane was very limited throughout the experiment, if at all, and the layers above it remained stable. That happened because the clay layer that surrounds the left block subsided and blocked the direct contact between the fresh groundwater and the salt of the right block. The qualitative data from the pictures are also confirmed in this experiment by semiquantitative data from an electrode located within the right salt block (Figure 10). The electric potential value of this electrode remained constant on HSB value (~0.8 mv) throughout the whole experiment. This value represents saturation with respect to halite (Table 1), so it confirms that dissolution of the salt layer was impossible. In experiment 6CBN (Figure 11), the dissolution of the salt layer also starts shortly after the FSI reaches the salt block from the upper gap on the fault plane and continues downward. Similar to experiment 5CGN, the dissolution process starts from the upper gap and the subsidence of the layers above the salt, and the formation of the sinkhole is also focused on the left block and the fault plane. However, in this experiment, since the bentonite was replaced by varnish, the direct contact between the fresh groundwater and the right blocked was not blocked, and most of it dissolved as well (Figure 11c). As in experiment 5CGN, the surface subsidence and sinkhole formation patterns were gradual even though the salt layer was composed of massive, competent salt blocks. The explanation for this difference, in relative to the other experiments with salt blocks, is discussed in section 4.2. The results of this experiment are based on pictures from the time-lapse cameras Sinkhole Formation Adjacent to a Stratified Dead Sea Two experiments were conducted to examine the impact of possible future stratification in the Dead Sea on salt dissolution and sinkhole formation along the shores of the Dead Sea (experiment 7BBS-8CBS in Table 2). In experiment 7BBS the salt layer is made of a massive salt block, surrounded by clear varnish with a gap in the bottom. The initial conditions of the experiment are a single FSI between the HSB and fresh groundwater, OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1755

11 Figure 11. The setup and the results of experiment 6CBN. Cross section C, consolidated salt blocks, and nonstratified limnological conditions were used. The red and black dashed lines in boxes B and C represent the surface location at the beginning and the end of the experiment, respectively. as in the case of a nonstratified lake (Figure 12a). The salt layer is located below the FSI, exposed only to the saturated HSB. The experiment starts when the LSB is pumped into the upper part of the saltwater chamber, demonstrating stratification in the lake. Once present, the LSB penetrates from the saltwater boundary into the aquifer, forming three interfaces between the three water bodies, and a wedge of the LSB surrounds the salt layer (Figure 12b). Consequently, the salt layer, which was now exposed to the halite-undersaturated LSB, begins dissolving. However, the dissolution rate in this case is relatively slow and the experiment was ended after 6 h, during which only 14% of the salt block dissolved (Figure 12c), most of it during the dynamic intrusion of the LSB from the saltwater boundary into the aquifer. In experiment 8CBS the two massive salt blocks were displaced to represent a fault, and were bounded by clear varnish, excluding the faces that turn to the fault plain (Figure 13). The procedure of experiment 8CBS is similar to that in experiment 7BBS. The penetration of the LSB creates the wedge and the three interfaces (Figure 13b) and exposes the two salt blocks to the unsaturated LSB that dissolves them. Similar to experiment 7BBS, the dissolution process is also relatively slow, and at the end of the experiment, after 12 h, only 25% of the salt blocks dissolved (Figure 13c). As in experiment 5CGN and experiment 6CBN, the temporal pattern of the dissolution process of the two blocks is different; the left block started to dissolve first, and thus, at the end of the experiment it weighed less than the right one, even though their weight at the beginning of the experiment was almost the same (Table 3). 4. Discussion 4.1. Mechanism for Sinkhole Formation at the Dead Sea Six experiments (1AGN 6AGN; Table 2) were used to visualize the full dynamics of the accepted mechanism for salt dissolution and formation of thousands of sinkholes along the shores of the Dead Sea. Overall, the experimental results support the accepted mechanism and plausibly explain the process of salt dissolution and sinkhole formation adjacent to the Dead Sea. The results show that in all the examined hydrogeological configurations, when the FSI moves across the area in which the salt layer is in direct contact with the sand layers, the fresh groundwater dissolves the salt layer. The dissolution of the salt layer causes subsidence of incompetent clay and sand layers above and leads to the formation of cavities below the surface and/or sinkholes at the surface. The dynamics of the subsidence and the formation of sinkholes are either gradual or instantaneous (see section 4.2 hereafter). The location and width of the cavities and sinkholes in all the experiments are limited to the area above the salt layer, enabling to conclude that there is a relationship between the dissolution of the salt layer and the formation and subsidence of the incompetent layers above. Atzori et al. [2015] show that the induced stress field favors generation of sinkholes at the perimeters of the subsiding areas rather than at their centers and that is in agreement with field observations. We do not see clear evidence for that in our experiments, probably due to the different scale of the experimental system in comparison to the natural system. OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1756

12 Figure 12. The setup and the results of the experiment 7BBS. Cross section B, salt grains, and stratified limnological conditions were used. The experiment shows the salt dissolution dynamics in the case of stratification in the Dead Sea. The accepted mechanism presented in this work for the formation of thousands of sinkholes ties between the presence of a soluble salt layer and a major change in the regional groundwater system. The experimental results presented in this work support this mechanism, yet they take into account only the special configuration adjacent to the Dead Sea shores and the dynamics of the Dead Sea and the surrounding groundwaters. However, dissolution-induced subsidence above evaporates, derived by changes in the groundwater system, has been observed in many other places in the world. For example, Fidelibus et al. [2011] describe gypsum dissolution and sinkhole formation due to changes in the groundwater circulation. These changes are related to precursor changes in the hydrogeological functioning after the Acquarotta canal in Lesina Marina area, Italy, was established, and gypsum rocks were exposed to unsaturated groundwater. Johnson [1997] and Martinez et al. [1998] described karst processes and dissolution of salt and gypsum rocks throughout the United States and Northern America, where these rocks came in contact with unsaturated groundwater. Thus, our experimental method and results are not only relevant to the Dead Sea area but also can be applied to study the mechanisms of sinkhole formation under different geological configurations and groundwater systems in other geographic locations Surface Subsidence and Sinkhole Formation Patterns The experimental results clearly distinguish between two different patterns of surface subsidence and sinkhole formation: (1) gradual subsidence and sinkhole formation and (2) instantaneous subsidence and collapse sinkhole formation. The difference between the two, as demonstrated by the results, depends on (1) the stratigraphic configurations of the aquifer, which dictates the location from which the dissolution starts, and the dissolution direction, and (2) the existence or absence of a competent layer with sufficient mechanical strength above the developing dissolution cavity. In the Dead Sea, the instantaneous subsidence, and the formation of collapse sinkhole, is found mainly at the alluvial fans. This type of sinkhole are related to the more competent gravel sediments there, relative to the weaker, muddy sediments built of silt and clay which form the mudflats and are characterized by more gradual subsidence and sinkhole formation [Abelson et al., 2006; Avni et al., 2016; Shalev and Lyakhovsky, 2012]. OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1757

13 Figure 13. The setup and the results of experiment 8CBS. Cross section C, consolidated salt blocks, and nonstratified limnological conditions were used. The experiment shows the salt dissolution dynamics in the case of stratification in the Dead Sea. Gradual subsidence and formation of sinkholes are observed in experiments in which the salt layer is composed of salt grains (i.e., experiments 1AGN, 2BGN, and 5CGN). The gradual subsidence in these experiments is due to the absence of a competent layer with sufficient mechanical strength to maintain an open cavity. Thus, the dissolution process is accompanied by continuous subsidence of the incompetent sand and clay layers above the salt layer. In experiment 6CBN, although the salt layer is composed of a massive competent salt block, the subsidence pattern is also gradual. That is, because in this stratigraphic configuration (cross section C in Figure 5) the dissolution of the salt layer starts from the upper gap on the fault plane and the flow direction is vertically downward. As a result, the cavities that formed in the upper part of the left massive salt block are rapidly filled up by the adjacent incompetent sand. Notably, this downward flow is inconsistent with field results, which report a higher groundwater head in the lower subaquifer than in the upper one [Yechieli et al., 2006], indicating an upward flow through the faults. Possible explanations resolving this difference are (1) the laboratory setup cannot produce enough pressure in the lower subaquifer to overcome this buoyancy effect and/or (2) there may be a vertical convective flow downward, without it having been observed in the field. In experiments 3BBN and 4BBN, in which the salt layer is composed of a massive salt block, the subsidence is instantaneous, and collapse sinkholes are formed. In these cases the combination of the competent massive salt block and the stratigraphic configurations (cross section B in Figure 5) enables the development of cavities, which are protected by the remnants of the competent salt layer itself. Therefore, these cavities can continue to develop until the system reaches its mechanical failure threshold, and the overlying ceiling Table 3. Comparison Between the Weights of the Salt Blocks in Experiment 8CBS Before and After the Experiment Original Weight (g) Final Weight (g) After 12 h Dissolved Weight (g) Left block Right block Total OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1758

14 Table 4. Comparison Between the Weights and the Dissolution Rates of the Salt Blocks in Two Experiments with Nonstratified Saltwater Bodies (#3 and #6) and Two Experiments with Stratified Saltwater Bodies (7BBS and 8) Original Weight (g) Final Weight (g) Dissolved Weight (g) Percent Dissolution Time (h) Dissolution Rate (g/h) Experiment 3BBN ~1000 ~0 ~ Experiment 6CBN ~1000 ~0 ~ Experiment 7BBS Experiment 8CBS collapses into the empty space. Experiment 4BBN, which shows the dynamic formation of the cavity in the unsaturated zone, exemplifies a situation in which the instantaneous subsidence is restrained and does not reach the surface, and a collapse sinkhole does not form. Instead, a subsurface cavity develops in the relatively thick unsaturated zone, which is a region of high mechanical strength itself as a result of adhesion forces between the sand grains. Such a cavity in the subsurface forms a potential collapse sinkhole, whose surface expression is negligible [Shalev and Lyakhovsky, 2012]. Based on the above it can be concluded that instantaneous failure of the surface and the formation of collapse sinkhole require the existence of a competent layer. In nature this layer can be the compacted gravel layers in fan deltas or the salt layer itself [Shalev and Lyakhovsky, 2012; Shalev et al., 2006]. For stable cavities to develop below incompetent sediments (e.g., the mud plains adjacent to the Dead Sea), the salt layer itself must be competent, and its dissolution has to start from its lower parts Impact of Stratification in the Dead Sea The impacts of stratification in the Dead Sea on salt dissolution and sinkhole formations, which were not modeled before, are studied in experiments 7BBS and 8CBS. In these experiments the water from the upper layer in the stratified lake (the LSB) intrudes into the aquifer and forms a wedge above the Dead Sea brine (the HSB) and below the fresh groundwater (Figures 12 and 13). The exposure of the salt layer to the LSB results in five major factors, as reflected in the experimental results, which affect the rates and patterns of salt dissolution and sinkhole formation: (1) the LSB is closer to saturation with respect to halite than fresh groundwater (Table 1); (2) the groundwater velocity flow in the wedge is significantly slower than that of the fresh groundwater adjacent to a nonstratified lake; (3) the flow direction of the LSB in the aquifer is landward, opposite the direction of the fresh groundwater, which flows seaward; (4) the salt layer is exposed to the above conditions at the early stages of the process, as the LSB intrudes into the aquifer, creating the composite structure of the wedge and the three interfaces, which develops following the change in the lake s water column structure; and (5) the velocities of the brines are significantly higher during the early stages of their intrusion than at the later stages, when the system approaches steady state conditions. The experimental observations on flow patterns and spatial location of the LSB in the aquifer are consistent with the results of numerical and experimental results conducted by Oz et al. [2011, 2014]. The combined effect of saturation degree and flow rate was examined in laboratory experiments using halite from Mount Sedom [Stilleretal., 2007]. They showed that the dissolution rate decreases significantly with the increase in the salinity of the solutions and that the rate of dissolution is doubled in experiments with stirring compared to those with no stirring. The results of our experiments also show the importance of water salinity and flow rate on the process of salt dissolution (Table 4). Whereas in experiments 3BBN and 6CBN (a nonstratified water column), the salt layer dissolved completely (100%), in experiments 7BBS and 8CBS (a stratified water column), only 14% and 25% of the salt layer were dissolved, respectively. Moreover, the limited dissolution of the salt layer in the latter took more time than the complete dissolution of halite in the former, and most of the dissolution occurred during the transient intrusion of the LSB wedge. Integration of these two experimental results shows that the dissolution rates in the stratified system are slower by 1 order of magnitude than that in the nonstratified system (Table 4), and therefore, the process of salt dissolution and sinkhole formation adjacent to a stratified saltwater body is expected to be restrained. As mentioned above, additional impacts of stratification of the Dead Sea are related to the flow direction of the solvent, the timing of the intrusion, and the velocity rates. In the current system the flow direction of the fresh groundwater is seaward, from west to east, and the dissolution process and the development of sinkhole sites progress in the same direction [Yechieli et al., 2006; Abelson et al., 2006]. The flow direction of the LSB in the stratified system, however, is expected to be landward [Oz et al., 2011, 2014], in the opposite direction. Therefore, OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1759

15 the sinkhole formation would start closer to the lake and migrate westward with time. The fact that the intrusion of the LSB into the aquifer occurs from the very early stages of the process, and that the flow velocity is significantly higher at these stages than later on, implies that most of the salt layer dissolution would occur during these early stages Laboratory Experiment Versus Field Conditions While the experimental approach reproduces and validates the full dynamics that lead to salt dissolution and sinkhole formation, there are some specific limitations, beyond those encountered in any model, which relate to the comparison between our laboratory experiments and the field conditions. The main issue is that the analysis is restricted to the laboratory scale. Obviously, upscaling the presented results to field-scale conditions requires special caution. Nevertheless, if one assumes that the same phenomena observed in the laboratory occur in the field, some comparison of the time scales over which these sinkholes are formed in the field can be made. For example, field observations imply that 10 years is a typical time for an average sinkhole of 1000 m 3 to be developed in the coastal aquifer adjacent to the Dead Sea by the fresh groundwater flow. Accordingly, the dissolution rates that form a cavity in these dimensions in the field are ~20,000 g/h (taking the density of salt to be 2.1 g/cm 3 ). This value is 2 orders of magnitude faster than the dissolution rate of ~200 g/h that is observed in the lab (Table 4). If we also take under consideration that the typical flow rates of the fresh groundwater close to the shoreline (where these cavities are formed, and the flow velocities are the highest due to the narrowing of the flow cross section above the FSI) range between ~100 m/d [Magal et al., 2010] and more moderated values of ~1 m/d [Shalev et al., 2006], and that the flow rates in the experiments are ~100 m/d, then the apparent dissolution rates in the field are between 2 to 4 orders of magnitude higher than those in the lab. However, we need to normalize these rates in relation to the effective surface area for dissolution, which also affects dissolution rates [Stiller et al., 2007]. The average effective surface area of dissolution in the experiments during the dissolution process is ~0.01 m 2 (range between m 2 and ~0.015 m 2 at the beginning and at the end of the dissolution, respectively). Since the effective area of dissolution in the field is much larger, i.e., ~1 100 m 2, the actual dissolution rates per specific area are quite similar in the laboratory and the field. Lastly, the hydrogeology in the experiments is greatly simplified as the layers are different (massive salt block versus porous salt, sand versus gravel, etc.), there are no facies changes, and there is a direct hydraulic connection between the salt layer and the lake. A more complex hydrogeological setup, or the possibility of no hydraulic connection of the gravel and the salt layers to the lake (probably common in mud plains), was not examined in the present study. 5. Summary and Conclusions In this study we examined the commonly accepted mechanism for salt dissolution and sinkhole formation along the shores of the Dead Sea over the past 30 years. Using various laboratory experiments in a sand tank we modeled, for the first time, the full dynamic process and tested the impact of different hydrogeological characteristics on sinkhole formation. The results of the experiments strongly support the accepted mechanism, as they show that salt dissolution due to water-level recession and relocation of the freshwater-saltwater interface is a plausible mechanism explaining the formation of thousands of sinkholes along the Dead Sea shores. Moreover, it is the first time that the full dynamics of these subsurface processes, which were exposed only in their outer expression on the surface, could be visualized from the beginning of the process to its end. The specific location from which the dissolution starts and its direction, together with the salt layer properties (unconsolidated salt grains versus massive salt), determine if the dynamic subsidence of the surface and formation of sinkholes will be gradual or instantaneous. The results show that instantaneous failure of the surface and the formation of collapse sinkhole depend on dissolution of a competent layer such as a massive salt block due to its relatively high mechanical strength. However, when dissolution of the massive salt layer starts at its upper part (i.e., above the competent layer), the subsidence of the surface and the formation of sinkholes is gradual, despite the existence of this competent layer. Gradual subsidence of the surface and formation of sinkholes also developed in experiments in which the salt layer is built of salt grains, which do not have enough mechanical strength to hold a cavity. OZ ET AL. SALT DISSOLUTION AND SINKHOLE FORMATION 1760