Sinkhole hazard above salt, Dead Sea shore

The hazard of sinkholes (collapse dolines) is commonly associated with karst (landscapes dominated by dissolution and subsurface drainage), where subsurface cavities undermine the overlying strata, causing subsidence and collapse. In limestone karst terrains clusters of sinkholes usually develop over long periods (on the order of thousands of years or more). Much faster dynamics, on the order of human lifespans, are observed at sinkholes formed in evaporitic areas, where dissolution rates are one to three orders of magnitude faster than that in limestone.

Evaporites vary largely in their solubility and dissolution rates. Salt (consisting of halite - NaCl), a common rock (and mineral) in subsurface sediments, is about 100 times more soluble than gypsum. Salt is soluble when in contact with the vast majority of environmental water. Due to the high solubility it is often completely dissolved and eliminated by groundwater close to the surface. Therefore salt outcrops are rare but can produce typical surface karst characterized by localized sinkholes, blind valleys and underground streams. Partial dissolution of subsurface salt and collapse of overlying material are hazardous. The resulting sinkholes are surface manifestations of subsurface dissolution and internal erosion and deformation, commonly hidden from direct observation and from most subaerial geomorphic study methods. Sinkhole development can be extremely rapid, particularly in areas where human activities alter groundwater circulation.

During the last thirty years hundreds of sinkholes have occurred along the Dead Sea shores in both Israel and Jordan (Fig. 1). The process began in the southern part of the Dead Sea coast and spread northward along the western coast. The steeper eastern coast has been less affected, and most of its sinkholes are concentrated in the flat-lying region close to the Lisan Peninsula and along the north-eastern shore. The sinkholes have already caused considerable damage to infrastructure, and at least four people have fallen into sinkholes which collapsed under their feet. There is an obvious potential for further collapse beneath the main highways and other man-made structures. Thus, a high sinkhole hazard threatens human lives, property (Fig. 2), and future economic development in the Dead Sea basin.

The Shalem-2 site (also called 'Mineral'), covering ~1 km², is located at the western coast of the central part of the northern Dead Sea basin, between the Dead Sea shoreline and Route #90 (the main road along the western Dead Sea shore). The area comprises the Mineral Spa, where natural black mud and sulfur-rich hydrothermal water with a temperature of 39 °C attract tourists from Israel and abroad. Around Mineral Spa, sinkholes develop in both mud flat (south) and alluvial fan (north) areas. Sinkhole distribution is mainly linear, along a salt layer edge (Fig. 3). A salt layer with a cavity was intersected by a borehole (Mn-2). Groundwater sampled from the cavity had Na/Cl ratio of 0.55–0.60, indicating mixture of Dead Sea brines (Na/Cl ratio of 0.30) and dissolved salt rock (Na/Cl ratio of 1). Soon after drilling, the borehole structure collapsed into a newly developed sinkhole which was then east of the salt layer edge.

The western edge of the salt is located in alluvium composed of gravel, sand, and clay. These sediments permit direct contact of the salt layer with highly mineralized (200 g/l chloride) rising water at a temperature of 29–40 °C. Sinkholes are located close to the salt edge, above a dissolution front which gradually consumes the underlying salt lyer. Further to the west, a borehole did not cross the salt layer. This borehole, located west of the salt edge, revealed water with relatively low chloride concentration — 120 g/l. Such water is 'aggressive', i.e. has a high dissolution power towards salt rock, and is believed to create voids and eventually sinkholes. The large cluster of sinkholes south of Mineral Spa started to develop in 1992. During recent years sinkhole development progressed northward, damaging the infrastructure (Fig. 4).

The hazardous salt layer was deposited during the latest Pleistocene. The salt layer wedges out toward its western margins where aggressive aquifers create the dissolution front. Shallow burial (~10s m) does not significantly alter the primary porosity and permeability of the salt. After the salt is covered by insoluble layers, it is protected from direct rainfall, so dissolution is primarily controlled by groundwater dynamics. Within the vadose zone (above the watertable) the salt is mainly attacked by meteoric water (rainfall or runoff infiltration), either diffuse or concentrated. Overlying unconsolidated sediments often collapse or subside into voids soon after they are formed by salt dissolution. The salt can also be dissolved within the shallow phreatic or confined zone by aggressive groundwater.

Confined artesian flow of hypogenic (deep) water plays an important role dissolving the salt layer along the Dead Sea shore. Flowing through salt-poor aquifers, this water is aggressive with respect to salt and often hydrothermal. The water can rise from depth through a permeable layer or along faults or fractures, forming dissolution voids in the salt body. The effect of wide-scale dissolution is transferred upward by successive failures, uplifting voids towards the surface (Fig. 5). So far, sinkholes rarely developed east of the edge of the salt layer, because of the increasingly thicker clay aquicludes bounding the salt layer.

To conclude, the rapid dynamics of the system renders the Dead Sea coastal environment a hazardous area because of sinkhole formation. Salt distribution, Dead Sea level, and aggressive water are the main factors determining the salt sinkhole hazard. Geophysical methods (such as radar) are applicable to predict imminent sinkholes and mitigate the hazard. The possibility of importing water into the basin and raising the Dead Sea to its mid 20th century level is being evaluated today, but the costs seem to be high.

• Frumkin, A. and Raz, E., 2001, Collapse and subsidence associated with salt karstification along the Dead sea: Carbonates and Evaporites, v. 16, 2, p. 117-130.
• Frumkin, A., Ezersky, M., Al-Zoubi, A., Abueladas, A.-R. (2011). The deadly hazard of the Dead Sea: geophysical assessment of salt sinkholes. Geomorphology 134, 102–117.
• Gutiérrez, F., Cooper, A., Johnson, K., 2008. Identification, prediction, and mitigation of sinkhole hazards in evaporite karst areas. Environmental Geology 53, 1007–1022.
• Yechieli, Y., Abelson, M., Bein, A., Crouvi, O., 2006. Sinkhole "swarms" along the Dead Sea coast: reflection of disturbance of lake and adjacent groundwater systems. Geological Society of America Bulletin 118, 1075–1087.