**1. Introduction**

Water is a valuable commodity, not only for the human population, but also for all living as well as non-living biota. The availability of freshwater resources coupled with the quality of the available resource determines not only where life settles, but the quality of that life too [1].

Although nearly 70% of the surface of the earth is covered with water, over 90% of this is ocean water; an additional 2.2% is contained in ice sheets, glaciers and ice caps [2]. Therefore, according to Davies and Day [2], the fraction of water suitable for domestic use (potable water) is less than 1% of global water supplies. Furthermore, this proportion of freshwater is made up of atmospheric water, surface as well as groundwater resources [2]. With such limited availability, freshwater resources are not only

reserved for domestic use and the maintenance of aquatic ecosystems, benefits derived from available freshwater resources also include agricultural use, industrial production, mining as well as navigation, thus enabling and facilitating global socio-economic development. For this reason alone, global water security is of paramount concern.

Freshwater resources are, however, disproportionately distributed, where some areas have sufficient supplies of the resource, whereas the resource is gravely limited in other areas, thus threatening not only local or regional water security, but socioeconomic advancement too. Moreover, freshwater resources are not only threatened by their natural distribution and availability, population growth coupled with economic expansion also increases the demand for freshwater resources worldwide. In addition, climate variability also threatens the supply and renewal of freshwater resources, more so in water-scarce areas, where extreme effects of climate change, such as droughts, will further exacerbate water scarcity. Although finite, freshwater resources are renewed through precipitation, which, similar to the distribution of freshwater supplies, is also disproportionately distributed. Thus, in order to protect the resource, it is important to safeguard the use of both surface and groundwater resources.

Groundwater resources are often utilised where surface water resources are limited, augmenting surface water supplies. Not only does groundwater supplement surface waters in arid and semi-arid areas, groundwater resources also sustain associated or hydraulically linked ecosystems through river base flows [3, 4], acting as a significant buffer in times of droughts [3, 5]. Thus, groundwater resources are fundamental to the Earth's supply of freshwater. These subsurface resources, which act as a natural reservoir for freshwater, are important for socio-economic development as well as the alleviation of poverty [6–8].

Groundwater resources provide the most dependable and most easily accessible source of freshwater in most arid and semi-arid areas with limited surface water supplies. Many socio-economic activities are reliant on groundwater resources, and over half the global population also relies on groundwater for their domestic supplies, where over 60% is used for agricultural irrigation, 25% is allocated for domestic use, and 10% is used by industry [9–11]. This allocation, however, differs from region to region, for example, groundwater augments drinking water in many developed countries, whereas, in semi-arid regions of the world, it serves as the main source of water [10]. According to Foster and Chilton [9], groundwater systems represent the world's most predominant freshwater reservoirs. However, in many parts of the world, groundwater resources are excluded in the management of water resources, resulting in the overexploitation of the resource with no regard to its sustainability. Although out of sight, and often not easily quantifiable, groundwater resources should be monitored, conserved and efficiently managed to ensure their equitable and sustainable utilisation on national, regional and global scales; however, their subterranean nature makes this difficult to implement, especially where these resources traverse political borders.

#### **1.1 Groundwater and aquifers**

Groundwater is found below the surface of the earth, collecting in spaces in the soil and sediment and also filling up interstices between rocks [12]. These rock formations, together with the water stored within the crevices of the rock, are known as aquifers. Therefore, the shape and size of the spaces within the aquifer (rock formation) affect the total volume of water that can be stored in the spaces, a term referred to as porosity [13]. Moreover, in addition to an aquifer's storage capacity, an

*Groundwater Dynamics in Transboundary Aquifers of Southern Africa DOI: http://dx.doi.org/10.5772/intechopen.109906*

**Figure 1.** *A representation of the movement of water on land as well as below the surface of the earth [16].*

aquifer's ability to diffuse water through the formation affects its yield [12, 14], a term referred to as permeability. Thus, according to Sophocleous [15], an aquifer's permeability measures the connectivity of spaces within the aquifer, which allows for water to move through the aquifer. Aquifers can be superimposed by layers of soil, or other layers of rocks, and they may also sit on similar or different layers of rocks. These top and bottom layers may be permeable (allowing water to flow through the aquifer), thus known as aquitards or impermeable [allowing no movement of water through (into or out of) the aquifer] and, therefore, known as aquicludes (**Figure 1**).

Aquifers are, therefore, classified into three categories (unconfined, semi-confined and confined) based on their level of porosity and permeability. Unconfined aquifers are those aquifers where there is usually a hydraulic connection between surface and groundwater resources, or where there is connection between groundwater and water in the vadose (unsaturated) zone. A semi-confined aquifer (aquifer 2 in **Figure 1**) is an aquifer superimposed by a semi-permeable aquitard (thus allowing limited movement of water between the aquifer and the above environment) and underlain by an impermeable aquiclude, thus restricting the movement of water from below. A confined aquifer, often referred to as a "*fossil aquifer*," is an aquifer that is completely closed-off, covered by an aquiclude on all the sides of the aquifer, thus completely closing off the water that saturated the aquifer during its formation [15]. As mentioned earlier, aquifers and their associated groundwater resources offer a vital relief in water-scarce areas, and similar to surface waters, these resources also cross political borders.

## **1.2 Transboundary aquifers**

Transboundary aquifers (TBAs) are aquifers that are located beneath the surface of more than one country and are, thus, shared by those countries. Transboundary groundwater resources, therefore, sustain ecosystem services and ecological functions

**Figure 2.** *Transboundary aquifers of the world [17].*

of ecosystems reliant on groundwater in more than one country. Similar to national aquifers, transboundary aquifers are vulnerable to over abstraction and contamination; however, their exposure to pollution and risk of over abstraction increases with each border that the resource crosses. According to the International Groundwater Resources Assessment Centre (IGRAC), approximately 468 transboundary aquifer systems have been identified worldwide, as depicted in **Figure 2**. Of these, 135 transboundary aquifers were identified in the Americas, 130 in Asia, 106 in Africa, and 97 in Europe [17].

Of the 106 transboundary aquifers found in Africa, 24 aquifers are shared by the 12 states representing the Southern African Development Community (SADC): Angola, Botswana, Democratic Republic of Congo, Eswatini, Lesotho, Malawi, Mozambique, Namibia, Tanzania, South Africa, Zambia and Zimbabwe (**Figure 3**).

Of the 24 transboundary aquifers found within the SADC region, nine are shared by South Africa and its neighbouring states. This study, however, focuses only on two of the biggest transboundary aquifers shared by South Africa and its neighbouring countries. The study focuses on the Karoo Sedimentary Transboundary Aquifer (KSTA) shared by South Africa and Lesotho, as well as the Stampriet Transboundary Aquifer System (STAS), shared by Botswana, Namibia and South Africa (AF1 and AF5 in **Figure 3**).
