Hydrodynamic groundwater modeling and hydrochemical conceptualization of the mining area of Moulares Redeyef (southwestern of Tunisia): New local insights

Highlights

• Calibration, validation and simulation of a Modflow model under severe climate conditions and anthropogenic activities.

• Characterisation of the geochemical composition of groundwater.

• Assessment of the contamination of the Plio-Quaternary aquifer with the Phosphates wash rejects.

• Proposition a novel risk index for the MMR-b aquifer system.

Abstract

The Moulares Redeyef Mining basin (MMR-B) is located on Southwestern of Tunisia. The problem of groundwater mineralization in this basin has long been attributed to the significant decline in groundwater reserves. In order to quantify the aquifers reserves and identify processes and factors governing the groundwater quality, a detailed multidisciplinary study was proposed in this study. The approach used various GIS databases, including hydrodynamic characteristics, geochemistry, geology and climatic parameters. A Modflow hydrodynamic model was developed over 67-year modeling period (1953–2020) and many simulations were performed. Water samples were collected from 39 boreholes from the deep aquifer and 13 wells from the superficial aquifer in Tabeddit area. The hydrogeological model shows a flow convergence zone around the Tabeddit area. The reserve of the aquifer system was simulated to 10 Mm³/year for 2020, while it was 26 Mm³/year in 2000. Yet, 61% of exploitable reserves was consumed in 20 years. The quality of groundwater showed that MMR-b is characterized by 4 main trends: Ca–Mg–HCO₃, Ca–Mg–SO₄, Na–Cl–NO₃ and Na–Mg–HCO₃. For the heavy metals, the 13 alluvial groundwater samples contain high concentration of Cadmium that exceed the Tunisian Standards. Finally, a Risk Index of Aquifer is elaborated based on the Modflow model and the geochemical quality. The index gives that the PQ layer is under very high risk and the M aquifer is in a moderate situation. Accordingly, it is recommended that the mining deciders revise their exploitation strategy urgently in order to preserve water resources for future use.

Introduction

Under climate change and anthropogenic stress, the future of groundwater resources is becoming a major concern around the world. Today, the growing awareness of the groundwater importance as a vital natural resource leads to several fundamental research questions such as: How much exploitable groundwater do we have? And what is the impact of human activities on groundwater quality? Recent hydrogeological research confirms that depletion of groundwater reserves is a common problem in many arid to semi-arid basins of the world (e.g. Afonso et al., 2019; Aliyari et al., 2019; Farid et al., 2013a; Jung et al., 2019; Malekzadeh et al., 2019; Yassin et al., 2019). Climate change is causing water scarcity in many regions due to reduced rainfall and prolonged droughts (Leduc et al., 1996; Ram, 2013; Wang et al., 2015a; Zhu et al., 2020). The situation is exacerbated in those regions, which are characterized by the scarcity and irregularity of surface flows. Groundwater is the main water resource; but it characterized by very low renewal rates and is very sensitive to climate.

The depletion of groundwater has been the subject of several hydrogeological studies (Hamdi et al., 2018a; Wang et al., 2015b). They indicate that the state of the resource depends mainly on the internal architecture of aquifers, precipitation and exploitation. Therefore, it seems important to assess the processes and phenomena controlling the reaction of aquifer systems that are exposed to the global changes. Many hydrogeological approaches have been performed to understand the interactions and predict groundwater dynamics (Leduc et al., 1996; Hassan and Jin, 2016; Hamdi et al., 2018b; Jung et al., 2019). However, the geometry of the aquifer systems, recharge and exploitation are rarely highlighted despite their importance (Farid et al., 2013a; Raiber et al., 2012; Trabelsi et al., 2013; Zhu et al., 2020).

In this context, numerical modeling provides useful tools to facilitate the construction of hydrogeological models and estimation of many parameters of the water budgets (Moya et al., 2014; Raiber et al., 2012; R. Zhu et al., 2020). In fact, hydrogeological conceptualization is to simplify the reality of the environment and helps to improve the functioning comprehension of a hydrosystem, and to estimate of its storativity and resource spatial distribution.

These new visualization, processing and modeling technologies have been widely applied in hydrogeological studies (Gogu et al., 2001; Jerbi et al., 2019; Ross et al., 2005; Yousif et al., 2018; R. Zhu et al., 2020). Hydrogeological modeling is an increasingly used tool as a means of verifying consistency of available data, for better understanding and more efficient analysis of a complex hydrogeological context. It uses different approaches, and its applications are widely discussed by several authors (e.g. Alkhatib et al., 2019; Dehghanipour et al., 2019; Hendriks et al., 2016; Park et al., 2019). The analysis of these works shows the difficulties of acquiring reliable data to better assess the complexity of the systems. The importance of new data acquisition and the operation of novel technologies to better elucidate hydrological processes in complex systems is discussed in this work.

For the implementation and analysis of these technologies, the case of the Redeyef Moulares Mining Basin (MMR-b), located in southwestern Tunisia, is considered in this work. Indeed, many factors make it a complex system such as its large extent, the complexity of its geology and the difficulties of estimating exchanges with the surrounding environment, including recharging and pumping. This is all the truer as access to water becomes increasingly difficult to control in the agricultural sector as well as in the supply of drinking water (Hamed et al, 2010, 2014; Karaouli et al., 2009; Mokadem et al., 2018).

The current research will essentially contribute to the choice of a compromise solution for the various problems involved and incorporate predictive approaches to the state of the basin's groundwater resources. The model developed in this work is defined as computer tools that can simultaneously integrate physical, hydrological and even water resource management decisions and scenarios. Indeed, the main of this work is to evaluate the impact of natural conditions, climate scenarios and projected planning, in general, and to create a refined and well-detailed conceptual risk model for the MMR-b aquifer system. Various types of data were analyzed using several methods, such as geological and geochemical methods, spatialization and statistical processing.