Displacement of springs and changes in groundwater flow regime due to the extreme drop in adjacent lake levels: The Dead Sea rift

Highlights

• Extreme lake-level fluctuations at the Dead Sea and its precursor, Lake Lisan.

• Groundwater flow modeling within aquifers discharging into fluctuated lakes.

• Large and small-scale spring displacement due to lake level fluctuations.

Abstract

Lake-level fluctuations brought on by climatic changes and anthropogenic factors may affect the flow regime in adjacent aquifers that discharge toward those lakes. Such fluctuations may also cause displacement of springs that discharge these aquifers. Using a calibrated numerical model, an extreme example of such phenomenon is observed in the Dead Sea rift valley and the adjacent Eastern Mountain Aquifer of the Judea and Samaria Mountains. Lake levels along the Dead Sea rift have changed dramatically and rapidly by hundreds of meters, followed by changes in the lake areas by hundreds of square kilometers. Simultaneously, the aquifer exhibited spring displacements, both on large and small scales. Currently, 50% of the aquifer water discharge in the Zuqim zone, and 10% north of the Dead Sea. But in the past, only 30% discharged at Zuqim and 40% north of the Dead Sea. There is evidence for such an occurrence in the past, and it is likely to recur in the future, based on the predicted progressive decline of the current Dead Sea level. This may have an impact on wetland habitats along the coast.

Introduction

The levels of terminal lakes fluctuate due to climate changes and/or anthropogenic factors. Most terminal lakes serve as drainage basins for adjacent aquifers and, as a result, their groundwater flow regime is also affected (Kafri and Yechieli, 2010). The area of Lake Chad, for example, one of the biggest lakes in Africa, was much larger than its current area during the early Holocene due to a wetter climate (Gasse, 2000, Leblanc et al., 2007, Quade et al., 2018); in recent years, it has been progressively shrinking, in part due to water overexploitation (Birkett, 2000, Leblanc et al., 2007). The highest level of the Great Salt Lake in Utah, USA, during the last glacial period, some 15,000 years ago, was 1550 m above sea level (masl). Over the last century the level of this lake, which is replenished mainly by groundwater (Manning and Solomon, 2003), has fluctuated between 1277 and 1284 masl, due to annual and seasonal variations of evapotranspiration and precipitation (Hart et al., 2004, Pedone and Folk, 1996). The Aral Sea level varied during the Holocene by more than 40 m as a result of climate changes (Boomer et al., 2000, Micklin, 1988). The area of this lake was the fourth-largest inland water body in the world in 1960. However, due to massive anthropogenic water withdrawal for irrigation and the deflection of the rivers that fed it, it had shriveled by the early 21st century to one-tenth of its 1960 size (Johansson et al., 2009, Micklin, 2007). The heightened effect of climate change on inner lakes can be similarly seen in Lake Eyre, Australia (Magee et al., 2004); Mono Lake, CA, USA (Stine, 1991, Stine, 1990) and Asal Lake, Afar Depression, Djibouti (Gasse and Fontes, 1989).

The Dead Sea in the southern Levant, its precursor- Lake Lisan, and the adjacent Judea and Samaria Eastern Mountain Aquifer (EMA), all described in this study, together comprise an additional relevant case study for examining the impact of climate change and anthropogenic activities on groundwater systems.

The Dead Sea (Fig. 1B) is the lowest terrestrial lake on Earth; its present level (2019) is −433 m above sea level (masl) (Israel Water Authority). It is a unique hyper-saline and deep terminal lake with a maximum depth of about 300 m. Its margin slopes are relatively steep, reflected in extreme lake-level fluctuations that follow climate changes or anthropogenic activity. The Dead Sea level currently falls at an average rate of >1 m per year, mainly due to the damming of its feeding rivers and exploitation of upstream aquifers (Abelson et al., 2017, Lensky et al., 2005). This drop is predicted to continue in the next decades until a new limnological equilibrium will be achieved, probably at −500 to −550 masl (Gavrieli et al., 2005, Krumgalz et al., 2000, Yechieli et al., 1998). In geological history, lakes in the Dead Sea rift valley have often spread or shrunk in the wake of climate changes and variation in rainfall amounts and evapotranspiration. During the last glacial period (70–16 kBP), the 220-km long Lake Lisan extended in the valley, and its level varied between −250 and −160 masl (Fig. 1B), i.e., up to 270 m above the present Dead Sea level (Bartov et al., 2002, Stein et al., 2010, Torfstein et al., 2013).

The Dead Sea and its precursor lakes serve as a discharge basin to the adjacent aquifers. On the west side of the valley, the EMA is discharged through springs mostly located along the Dead Sea shoreline. The extreme changes of the lake level are hypothesized to affect the EMA flow regime.

The EMA is part of the Judea Group of the Albian to Turonian age. The rocks are mostly karstic limestone and dolomite interbedded with some marls and chalks (Arkin and Braun, 1965, Arkin and Hamaoui, 1967). The Judea Group overlies shales and marls of the Kurnub Group (Aptian-Albian) (Fleischer et al., 1993), and itself is overlain by mostly impermeable chalks and marls of the Mt. Scopus Group (Santonian-Paleocene). Judea Group rocks are exposed in the elevated western part of the area (Fig. 1, Fig. 2). Their thickness increases gradually from about 500 m at the southern part of the EMA to about 800 m in the aquifer’s central and northern parts.

The rift-filling sediments that compose the Dead Sea Group (Fig. 1C and 2) are relatively young, namely Pliocene-Holocene. These sediments, exposed along the Dead Sea shore, are roughly divided into two hydrogeological facies: permeable (conglomerate, gravel and sand) and impermeable (marl, silt–clay and salt). The two youngest formations of the Dead Sea Group are the Lisan and Ze’elim Formations.

There are two dominant geological structures in the EMA: folds and fractures, each related to a different stress field (Eyal and Reches, 1983). Folds are the product of the Syrian Arc stress field, and are expressed as a series of asymmetric anticlines and synclines with axes plunging to the northeast. The two noticeable folds are the Hebron and the Ramallah Anticlines, located in the western part of the EMA (Fig. 1B). As for the fractures, most are related to the Dead Sea stress field, with a dominating horizontal extension trending west-east. The Dead Sea Basin is the largest product of this stress field in the region. It is a rhomb-shaped pull-apart basin, and the deepest structure along the Dead Sea rift valley (Garfunkel and Ben-Avraham, 1996). Its western fault (the escarpment fault) is the primary morphological feature in the study area, creating a steep cliff above the Dead Sea shore. This fault places the Judea Group carbonates, which comprise the cliff, in contact with the Dead Sea Group in the east (Fig. 2).

The EMA is one of three aquifers that drain the Judea and Samaria Mountains, the others being the western and the northern aquifers. The EMA drains toward the Dead Sea rift (Gvirtzman, 2019); its boundaries are based on the structural configuration of the Judea and Samaria Mountains (Levy et al., 2019). Its northern and western boundaries are the regional drainage divides, while groundwater flows from the south through the southern boundary (Fink, 1994) and the eastern boundary is the drainage base, namely, the Dead Sea rift valley. The EMA is divided vertically into two sub-aquifers, upper and lower, with an aquitard layer in between (Fig. 1, Fig. 2). The recharge occurs on the Judea Group outcrops located along the Judea and Samaria Mountains (Fig. 1, Fig. 2) and the aquifer discharges mainly in springs along the Dead Sea shore: Zuqim, and Qane-Samar (Avrahamov et al., 2018, Gräbe et al., 2013, Guttman and Zuckerman, 1995, Laronne Ben-Itzhak and Gvirtzman, 2005). Practically, the groundwater of the EMA crosses the rift’s faults into the rift filling units (Dead Sea Group) west of the Dead Sea. In recent decades, dozens of wells have been drilled into the EMA and the exploitation has intensified (Gvirtzman, 2019). The EMA is characterized by a steep hydraulic head gradient. A difference of more than 800 m was measured over a distance of ~20 km, from ~400 (masl) in the west to ~−400 (masl) near the Dead Sea. In accordance with Darcy’s law, such a steep hydraulic gradient is usually found in media with relatively low hydraulic conductivity. However, this is not the case at the EMA; in fact, the opposite is true. The steep gradient is due to the structure of alternate anticlines and synclines that characterize the region. As a result, the aquifer includes strips of steep-inclined layers, whose horizontal hydraulic conductivity component is smaller, due to the inherent anisotropy of the layers and the folding compression. These flexures thus serve as hydraulic barriers to the groundwater flow and divert the flow sideways (Guttman and Zuckerman, 1995, Levy et al., 2019).

Despite the increased pumping from the aquifer in recent decades, the discharge quantities from the main outlets along the Dead Sea coast (Zuqim, Qane, Samar springs) have not yet diminished (Burg et al., 2016). Due to the rapid decrease of the Dead Sea level, the location of these springs has shifted over time (Burg et al., 2016). Zuqim springs, the largest outlet of the EMA, discharged in the mid-20th century at an elevation of ~−380 masl next to the escarpment fault. At present, part of the discharge appears at ~−410 masl. Moreover, when the Dead Sea drops, the impermeable clay of the Ze’elim Formation is exposed in between the active alluvial fans as a “mud” plain, preventing further displacement of the springs eastward. However, hydrological connectivity of the EMA to the Dead Sea still occurs through the permeable Kidron, Darga and Arugot alluvial fans (Fig. 1C). These lithological-derived relationships are important in terms of the hydrological system's response to future level changes.

The current extreme fluctuations in the Dead Sea level offer a special opportunity to monitor the mutual response of the hydrogeological system in a nearby aquifer. This includes the hydraulic head changes as well as the discharge amounts, the flow regime, location of the discharging springs and the depth of the fresh-saline water interface. Accordingly, the main purpose of this study is to examine the effects of level fluctuations of terminal lakes like the Dead Sea on an adjacent aquifer. Based on today’s observations, potential effects of future anthropogenic processes will be examined. The Dead Sea rift valley example may serve as a special case study due to the very rapid and extreme changes in its lake levels. Such changes happened in the past due to climate changes and are expected to recur in the future due to anthropogenic processes.