Impact of evaporation on groundwater salinity in the arid coastal aquifer, Western Saudi Arabia

Highlights

• Groundwater chemistry is predominantly regulated by the evaporation process.

• RIE, mineral dissolution and wastewater infiltration also governed water chemistry.

• High NO₃⁻ in this aquifer resulted from the agriculture and residential activities.

• Non-saline sources are differentiated from saline sources by major and minor ions.

Abstract

Understanding groundwater salinization and pollution in the arid coastal aquifer is crucial due to complex geochemical processes and sources. This study intends to evaluate the impact of evaporation on groundwater salinity in the arid coastal aquifer, Al Lusub basin, Saudi Arabia using geochemical and multivariate statistical tools. Groundwater samples were collected (n = 52) and analysed for major and minor ions. Groundwater is brackish and shallow wells have higher salinity compared to deeper ones. Hierarchical cluster analysis (HCA) classified the wells into three groups (CG1, CG2, CG3). In these cluster groups, salinity is in the order of CG1(1448 mg/l) < CG2 (3704 mg/l) < CG3(11018 mg/l). PHREEQC modelling reveals that groundwater in this basin is saturated with carbonate, sulphate (CG2, CG3), fluorite (CG3) and silicate minerals, and under-saturated with chloride and hydroxide minerals. Thermodynamic stability diagrams confirm the silicate weathering and kaolinite equilibrium. Precipitation of carbonate minerals at high salinity due to evaporation reduces Ca activity in the groundwater, which triggered the solubility of gypsum and fluorite through common ion effect. Pearson correlation analysis, principal component analysis, ionic ratios (Na/Cl, Cl/Br and F/Cl) and ionic deltas (+ΔBr, +ΔF, +ΔCa, +ΔSO₄, +ΔNO₃, −ΔNa and −ΔK) justified that non-saline sources and processes, namely evaporation, reverse ion exchange, mineral dissolution and wastewater infiltration predominantly affected the water chemistry in this aquifer. Repeated irrigation practice with high salinity groundwater leads to salt accumulation in the vadose zone due to evaporation, which flushed by the subsequent irrigation events and resulted in higher salinity and NO₃⁻ in the groundwater. Hence, sustainable groundwater management and smart irrigation should be implemented.

Introduction

In the arid and semi-arid environments, groundwater management is a challenging task due to lack of water resources, low and erratic rainfall, high evaporation rate, low recharge and climate change (DeNicola et al., 2015, Shrestha et al., 2016, Hu et al., 2019). Water quality degradation often leads to stress in water management and distribution system. Groundwater and soil salinization are largely intensified by evaporation in arid and semi-arid regions (Wu et al., 2014, Re and Sacchi, 2017). In the coastal aquifer, salinization is a global issue that endangers groundwater utilization and management in the arid and semi-arid environments (Vallejos et al., 2020). Evaluating the salinization processes in the coastal aquifer is crucial due to multiple contamination sources and complex processes (Re and Zuppi, 2011, Kloppmann et al., 2013, Nogueira et al., 2019, Ziadi et al., 2019, Vallejos et al., 2020). Further, coastal aquifers often have a shallow water table and are highly vulnerable to surface contamination sources (Vrba and Romijn, 1986, Rosenthal et al., 1992, Zhang et al., 2014, Jaunat et al., 2019).

Numerous articles discussed the groundwater quality and chemistry in the coastal aquifers and arid/semi-arid areas (Rajmohan et al., 2009, Ahmed et al., 2017, Re and Sacchi, 2017, Gamboa et al., 2019, Naeem et al., 2019, Nogueira et al., 2019, Ziadi et al., 2019, Vallejos et al., 2020). Huang et al. (2013) reported that groundwater chemistry in Dongguan, south China, is governed by the seawater intrusion, lateral flow of river water, water-rock interaction, sewage, industrialization and agricultural pollution. Kloppmann et al. (2013) studied the groundwater salinity in French aquifer and stated that seawater intrusion is a predominant process in the coastal aquifer while inland salinity is caused by the evaporites. Comte et al. (2016) studied the groundwater resources management in the coastal aquifer of East Africa and reported that poor hydrological knowledge, lack of longterm monitoring plan and inefficient interaction between stakeholders and local communities are major challenges in these regions. In addition, they stated that shallow wells are less saline compared to deeper one due to saltwater invasion. Ahmed et al. (2017) studied the groundwater chemistry in the coastal volcano-sedimentary aquifer, Djibouti, Horn of Africa and reported that saline water intrusion and geogenic processes affected the groundwater quality. Gamboa et al. (2019) evaluated the origin of groundwater solutes in a hyper-arid environment in Atacama Desert in Chile and reported that atmospheric dust, evaporation and water-rock interaction in various geological formations are contributed well in groundwater chemistry. Earlier studies justified that groundwater chemistry is determined by natural processes and anthropogenic factors (Rajmohan et al., 2017, Li et al., 2019, Rajmohan, 2020). Natural processes such as mineral weathering, evaporation, ion exchange, saline water invasion and associated geochemical processes are prevalent in the coastal aquifer (Rajmohan et al., 2009, Ahmed et al., 2013, Huang et al., 2013, Abdalla, 2015, Gamboa et al., 2019, Nogueira et al., 2019, Ziadi et al., 2019). Likewise, anthropogenic factors namely agricultural, industrial, domestic activities and urban development altered the chemistry of coastal groundwater (Huang et al., 2013, Bamousa and El Maghraby, 2016, Ziadi et al., 2019).

In Saudi Arabia (KSA), rapid developments in agriculture and industrial sectors resulted in tremendous pressure on shallow aquifer. Agriculture (81–83.5%) followed by the industries are the major consumer of groundwater in this country (MoWE, 2014, Chowdhury and Al-Zahrani, 2015). Rainfall and floods are the major recharge sources of shallow aquifers (Zahrani et al., 2011, Masoud et al., 2018). On the other hand, high pumping rates degraded the groundwater resources, both quantity and quality (FAO, 2009, Ziadi et al., 2019). Groundwater quality degradation by various sources is documented well in KSA. The impact of the landfill and dumping sites (Hejazi, 1989, Al-Arifi et al., 2013, Rehman and Cheema, 2016), cesspools and sewage systems (Al-Shaibani, 2007, Alyamani, 2007, Alabdula’aly et al., 2010, Rehman and Cheema, 2017) and municipal and industrial waste (Al-Oud, 2008) on groundwater quality are reported in the literature and these studies mostly documented only the contamination status. Moreover, intensive agricultural activities led to groundwater salinization and nitrate accumulation in the shallow aquifers worldwide (Stigter et al., 1998, Oren et al., 2004, Re and Sacchi, 2017, Gutiérrez et al., 2018, Ziadi et al., 2019). In KSA, the relation between groundwater contamination and agricultural activities is not well documented yet except in few studies (Subyani, 2005, Bamousa and El Maghraby, 2016). Further, source of groundwater salinity and related processes are not explored well in this country. Hence, a detailed knowledge and understanding about the groundwater chemistry and geochemical processes will aid for sustainable management of groundwater resources in this country.

The present study was performed to evaluate the impact of evaporation on groundwater salinity in the arid coastal aquifer of Al Lusub basin, Western Saudi Arabia. Rajmohan et al (2019) reported that groundwater usage is restricted in this basin due to high salinity and contamination. Therefore, a clear understanding of groundwater chemistry is a key factor to manage available groundwater resources in the shallow coastal aquifer. The objectives of this study are to identify the origin of solutes in the groundwater, to evaluate the geochemical processes governing water chemistry and to differentiate the role of evaporation and other factors from seawater intrusion on groundwater chemistry using geochemical and multivariate statistical approaches. In the Al Lusub basin, few studies have been performed to evaluate aquifer sustainability, the impact of pumping rate, groundwater potential and groundwater quality (El-Didy, 1999, El-Hames and Al-Ahmed, 2005, El-Didy, 2006, Ewea, 2011, Rajmohan et al., 2019). The groundwater chemistry in this basin is governed by distinguished sources and processes, which are not well defined in the earlier studies. This study can help to protect and manage this shallow coastal aquifer in an efficient way. Further, the outcomes of this study will contribute to improve the overall knowledge of geochemical characteristics of this shallow aquifer in the arid environment and also will serve as a valuable base for groundwater salinization and pollution assessment in similar arid and semi-arid regions in the world.