Groundwater salinization and seawater intrusion tracing based on Lithium concentration in the shallow aquifer of Jerba Island, southeastern Tunisia

Highlights

• Seawater intrusion is the mechanism controlling the interaction between groundwater and aquifer materials.

• Lithium content and groundwater abnormal salinizationAre highly correlated.

• Lithium is a tracer for the processes associated to seawater intrusion.

Abstract

The objective of this study was to identify groundwater salinization origins and develop a new geochemical approach based on Lithium and major element concentrations to understand the processes associated to seawater intrusion into the shallow aquifer of Jerba Island, Southeast of Tunisia. An integrated application of geochemical modeling and statistical approaches (Principal Component Analysis and Hierarchical Cluster Analysis)based on a major and trace ions from the Jerba unconfined aquifer was used. The hydrochemistry and the statistical investigations revealed that the groundwater geochemical behavior is controlled by seawater intrusion, rock-water interaction and anthropogenic impact. Seawater intrusion is the mechanism controlling groundwater abnormal salinization and the interaction between groundwater and aquifer materials. In areas where seawater intrudes a fresh coastal aquifer, cation exchange reactions influence groundwater chemistry and Li⁺ ions released into the solution resulting in a Li⁺ concentration increase. This result proves that seawater intrusion activates Li⁺ leaching to groundwater. Taken together, these results suggest Li⁺ as a groundwater salinization tracer.

Introduction

Studying the groundwater contamination mechanisms is crucial to effective groundwater resource management and protection. Several studies have shown that groundwater chemistry was influenced by both natural and anthropogenic processes which contribute to their quality degradation (Dragon and Gorski, 2014; Machiwal and Jha, 2015). Geochemical methods using the conservative ions (Cl⁻ and Br⁻) as natural tracers are common for determining groundwater mineralization origins (Andreasen and Fleck, 1997, Hsissoua et al., 1999, Cartwright et al., 2006, Alcala and Custodio, 2008, McArthur et al., 2012). Also, the isotopic approach and factor analysis are pertinent approaches to identify groundwater degradation mechanisms (Ben Hamouda et al., 2009, Bonton et al., 2010, Huang et al., 2013). However, few past studieshave involved the use of Lithium in Hydrogeology (Négrel et al., 2010 in Kloppmann et al., 2011). Lithiumhas two isotopes: ⁶Li and ⁷Li (Rodier et al., 2009), with natural abundances of 7.5% and 92.5%, respectively (Négrel et al., 2010). Indeed, Lithium isotopes can be used as a proxy of saline groundwater origin and agricultural diffuse pollution indicator (Négrel et al., 2010 in Kloppmann et al., 2011). High Lithium concentrations which are influenced by the lithology of the aquifer could occurin brackish groundwater (Rodier et al., 2009). In fact, Lithium is a reactive salinity tracer (Pauwels, 1995). It is also successfully used as a conservative salinity tracer (Zellweger, 1994). Using Li⁺ as a groundwater salinization tracer was proposed to identify the origin of saline groundwater (Russak et al., 2016). It can also be used to show hydro-reservoir signatures and water/rock exchanges (Négrel et al., 2010).

It has been demonstrated that the Li⁺ concentration in underground water can be used as an indicator of residence time of seawater in aquifers when seawater intrusion occurs (Santucci et al., 2016). When water coming from a saline source remains in aquifers for a relatively long period due to rock/water exchange, Li⁺ concentration can rise with the increase of residence time of these waters in the aquifer.

The natural and anthropogenic processes that govern groundwater quality may not be easily distinguished from the chemical composition of groundwater alone. For this reason, modern approaches and tools such as multivariate statistical techniques, namely Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) areusedfor the efficient management of groundwater quality (Davis, 2002, Hu et al., 2013, Hynds et al., 2014, Kumar, 2014, Montcoudiol et al., 2014, Montcoudiol et al., 2015, Machiwal and Jha, 2015). The multivariate statistical approaches offer a valuable tool for the interpretation of complex groundwater quality datasets and the detection of geochemical processes which govern groundwater quality. Multivariate statistics help to separate the anthropogenic and geogenic phenomena influencing the groundwater hydrochemistry (Dragon and Gorski, 2014).

In this investigation, statistical analyses (PCA and HCA) were adopted to identify the major processes involved in the groundwater quality degradation and determine the sources of groundwater contamination in Jerba Island.

The purpose of this study was to assess the geochemical quality of the Mio-Plio-Quaternary aquifer in Jerba Island. This paper focuses on the relationship between mineralization and Lithium and the use of this chemical element as an indicator of groundwater salinization. Geochemical and statistical approaches were adopted to achieve aims.

The shallow aquifer of Jerba is located in the Southeastern part of Tunisia. This aquifer is bounded by the Mediterranean Sea in the west, the north and the east and the Boughrara lagoon in the south (between latitude 33°38′ and 33°56′N and longitude 10°43′ and 11°04′E). Jerba Island is characterized by a semiarid climate with an average annual rainfall of 250 mm (Kharroubi et al., 2012). Mean monthly ambient temperature ranges from 16.38 °C in December to 29.48 °C in August (period 2010–2014). Monthly evaporation varies from 107.85 mm in December to 154.58 mm in August (period 2010–2013). The average humidity is 67% (INM, 2014).

The study area is dominated by Quaternary sediments (Fig. 1). The northeastern part is mostly covered by recent Quaternary dunes. The Holocene age is presented by bioclastic sands covering the sand spits in the northern and southeastern parts of the island. Upper Pleistocene sediments are governed by conchoolitic limestones, quartzic sand and sands with shell debris surrounding the south and northwest. The red silts attributed to Middle Pleistocene-Holocene cover the total area of the island. Salmon and nodule crusts attributed to Villafranchien overly the upland area in the south, southeast and the east. The Mio-Pliocene sediments present the basement rock of the study area. The topography of the island is flat with a maximum altitude of 54 m above sea level in the south. The island is affected by a Northwest–Southeast faults system strike (Jedoui, 2000, Bouaziz et al., 2003, Ghedhoui et al., 2015) (Fig. 4).

The main aquifer unit is composed of the Mio-Plio-Quaternary deposits intercalated with clay lenses as well as the marine limestone of the Tyrrhenian formation (Kharroubi et al., 2012). It is a multi-layered lenticular aquifer system with several compartments (Khalili, 1986, Yahyaoui, 2012). It is separated from the confined aquifer by a clayey formation (150 m in thickness) (Teissier, 1967). The schematic cross section oriented Southwest-Northeast was established by Khalili. (1986) (Fig. 2). This lateral section confirms the lenticular form of the aquifer and the existence of several compartments. The heterogeneity of this aquifer is due to the heterogeneity of lithology and the fractured morphology of the island. The aquifer is separated from the Miocene deep aquifer by clayey and marl formations (100–150 m in thickness) (Yahyaoui, 2012) (Fig. 3). The static level and the isopiestic maps of 2012 (Fig. 3a and b) were generated from the hydrogeological database of the Tunisian Water Resources Direction of Mednine. The static level of the aquifer varies between 6 and 38 m (DRE, 2014) (Fig. 3, a). The groundwater flow paths through the bedrock aquifer can be identified from the isopiestic map (Fig. 3, b). The map shows a multidirectional groundwater flow with no specific direction. Piezometric depressions related to the overexploitation of the aquifer were observed. The piezometric head of the aquifer varies from an average of 4 m below sea level to 2 m above sea level. The hydraulic gradient decreases towards coastal regions. The piezometric surface indicates local recharge areas (the east: Mahboubine region; the west: some local areas in the Sidi Jmour region), the dominance of the quaternary sandy sediments in this areas facilitate the rainwater infiltration to the aquifer.

Depth to water table reaches 2 m in coastal regions and exceeds 50 m in some central localities of the island. The Jerba unconfined aquifer is mainly recharged by means of vertical infiltration from precipitation. The fault system also contributes to the recharge of the shallow aquifer through the direct infiltration of rainwater. The transmissivity value reaches 10⁻³ m²/s. Pumping rates higher than the present recharge may exacerbate the pressure on aquifer system and expose it to overexploitation. Indeed, groundwater resources of this aquifer are estimated at 3.47 Mm³, while the extracted ones are about 3.61 Mm³ (ODS, 2014).

The shallow aquifer system of the study area was investigated by Kharroubi et al. (2012). The study identifies the geochemical processes which governed the groundwater salinization and confirms that seawater intrusion and rock/water interaction (especially evaporites dissolution) were the major processes controlling groundwater evolution in the island. Classical geochemical and statistical methods were adopted as the methodology.

Some studies such as Souid et al., 2017a, Souid et al., 2017b have demonstrated that the infiltration of wastewater, the discharge leachates, and the human and animal activities in the behavior of the water wells are the principal sources of groundwater quality degradation. Furthermore, the nature and texture of the soil contribute to the transfer of chemical and bacteriological pollutants from superficial layers to groundwater. Intensive pumping also contributes to the migration of pollutants into the aquifer.