The assessment of processes controlling the spatial distribution of hydrogeochemical groundwater types in Mali using multivariate statistics

Highlights

• The analysis enabled the definition of seven groundwater types.

• HCO3 is the dominant anion.

• Mali groundwater is comparatively high in Na, Mg and HCO₃.

• Geographical location and environments have a greater influence on main ion proportions.

• The regional chemical tendencies are reliable with geology and climate.

Abstract

Sustainable management of groundwater resources is a major issue for developing countries, especially in Mali. The multiple uses of groundwater led countries to promote sound management policies for sustainable use of the groundwater resources. For this reason, each country needs data enabling it to monitor and predict the changes of the resources. Also given the importance of groundwater quality changes often marked by the recurrence of droughts; the potential impacts of regional and geological setting of groundwater resources requires careful study. Unfortunately, recent decades have seen a considerable reduction of national capacities to ensure the hydrogeological monitoring and production of qualit data for decision making. The purpose of this work is to use the groundwater data and translate into useful information that can improve water resources management capacity in Mali.

In this paper, we used groundwater analytical data from accredited, laboratories in Mali to carry out a national scale assessment of the groundwater types and their distribution. We, adapted multivariate statistical methods to classify 2035 groundwater samples into seven main groundwater types and built a national scale map from the results. We used a two-level K-mean clustering technique to examine the hydro-geochemical records as percentages of the total concentrations of major ions, namely sodium (Na), magnesium (Mg), calcium (Ca), chloride (Cl), bicarbonate (HCO₃), and sulphate (SO₄). The first step of clustering formed 20 groups, and these groups were then re-clustered to produce the final seven groundwater types. The results were verified and confirmed using Principal Component Analysis (PCA) and RockWare (Aq.QA) software. We found that HCO₃ was the most dominant anion throughout the country and that Cl and SO₄ were only important in some local zones. The dominant cations were Na and Mg. Also, major ion ratios changed with geographical location and geological, and climatic conditions.

Introduction

Worldwide, groundwater is the main source of freshwater for daily consumption for about 2 billion people in urban and rural communities (Nyagwambo and Nyasha, 2006, Anayah et al., 2009). It is a particularly important water resource in semi-arid and arid areas where rainfall precipitation and intensity is relatively low (Oboubie, 2008). Groundwater resource management is, therefore, important as this valuable resource must be managed sustainably. This, however, is a major issue for developing countries, such as Mali. The fact that groundwater has multiple uses in developing countries including Mali means that robust management strategies must be implemented to avoid over-exploitation. Therefore, each country needs to carry out extensive monitoring to derive the necessary data that are needed to support the prediction of the ongoing quality changes of these water resources. Also, given the importance of groundwater quality in an environment where drought recurs frequently, this information should be suitable for determining the potential impacts of the regional and geological settings on groundwater resources. Unfortunately, in recent decades there has been a considerable reduction in the national capacity in Mali to maintain on-going hydrogeological monitoring and to produce good quality information that is suitable for end users, for example, policy makers and regulatory bodies. The purpose of this study, therefore, was to determine the main groundwater types and their spatial distribution in Mali.

Statistical analysis to understand freshwater resources (groundwater) in terms of …. .play a crucial role in all development sectors, including agriculture (Dandan, 2011), irrigation (Ali El, 2004), industry (Pradeep and Naik, 2007, Kanagaraj and Elango, 2016), domestic activities (Packialakshmi et al., 2015) and energy (Ali El, 2004) as well as in regional scale (Sarra Bel Hadj Salem, 2011, Guggenmos, 2011). Therefore, the spatial variations in the types and quality of these groundwater resources are very important as shownby Zhu et al., 2016, Nosrat Aghazadeh, 2011, Abdel Wahed et al., 2014 and Jingsheng (2002) demonstrated the importance of groundwater types spatial variation in water management. Many studies have demonstrated that the geographical and geological settings of groundwater resources are closely related to the groundwater type (Green Timothy et al., 2011) and (Mariko et al., 2012) also applied multivariate statistics to assess hydrogeochemical groundwater types in different parts of Mali. In China (Guo et al., 2015) applied the same technics to identify groundwater evolution and recharge sources in the arid Shule River Basin. (Bates et al., 2008). and (Cloutier et al., 2006, Cloutier et al., 2008) also revealed a link between groundwater hydrogeochemical conditions and climatic, geological and geographical conditions. Vadillo (2005) and Edet et al. (2011) similarly established that the origin of groundwater types may be influenced by four factors, namely (1) increases in air temperature, (2) increases in ambient CO₂ concentrations, (3) extreme precipitation events and the way in which precipitation is transported to aquifers, and (4) human activities, such as irrigation and other types of farming activities. Our goal is to verify if geological conditions, geographical and climatic conditions are related to groundwater types and to confirm that by using the multivariate statistical method.