Reports on contamination and pollution in the region indicate that the predominant contaminants are trace elements. Other major contaminants include pesticides, hydrocarbons and polychlorinated biphenyls (PCBs). The anthropogenic sources of soil pollution in sub-Saharan Africa include mining and quarrying, agriculture, industrial activities as well as waste disposal and processing. Geogenic sources of soil pollution are, for the largest part, attributed to the geological origin of the soil in areas such as the Central African Copper Belt, South Africa and Santiago island of Cabo Verde. The largest volume of published research on the drivers for soil pollution, originates from Nigeria and South Africa. Other countries for which relevant papers are available include Botswana, Ghana, Kenya, Namibia, Uganda, the United Republic of Tanzania, and Zambia.
Sub-Saharan Africa has significant mineral resources and oil deposits, the extraction and processing of which can stimulate economic activity and provide livelihoods for the population. Oil producing countries in sub-Saharan Africa have shown significant economic growth from the end of the 1990’s (Altenburg and Melia, 2014). Nigeria is currently the region’s leading oil exporter, followed by Angola Chad, Gabon and the Congo (World Bank, 2019b).
Mining and quarrying industry can be subdivided into large-scale operations (Figure 2) or artisanal and small-scale operations based on the approaches and technologies used. The mining and processing of mineral ore bodies in both these sectors, has caused extensive environmental damage in the region, including soil pollution (Fasinu and Orisakwe, 2013).
Large-scale mining is usually characterized by prior planning of the surface and underground infrastructure. The infrastructure may include pollution control dams, tailings storage facilities, ore processing facilities and ancillary operations, all of which can be possible sources of soil pollution. The handling and transportation of the ore by conveyor, road or rail may also lead to spillage and pollution.
The gaseous and particulate emissions from ore smelters can cause soil pollution of surrounding areas. For example, in the Kabwe area in Zambia, dust emissions from the lead and zinc smelter has been identified as the main cause of soil pollution in the area (Kříbek et al., 2019). Other sources of contaminated dust from ore processing include the storage infrastructure for processing wastes such as the flash roasting plant residues, tailings dams, slag dumps and the settling ponds of flotation tailings. The highest concentration of lead in soil was found in the vicinity of the old smelter and the industrial waste areas. The pseudo-total lead concentration in soil (as extracted with aqua regia) is more than 20 000 mg/kg. The mean bioavailable concentrations of trace elements in these areas were: 354 mg/kg lead, 251 mg/kg zinc, and 105 mg/kg iron. Cadmium has the highest bioavailable fraction, representing 56 percent of the pseudo-total concentration, followed by copper at 45 percent and lead at 37 percent.
Tailings particles from the Kombat Mine, in Namibia, was found to be the contamination source of nearby agricultural fields (Mileusnić et al., 2014). Transported by wind and water from the tailings dams, these particles have resulted in copper and lead concentrations of up to 150 mg/kg and 164 mg/kg, respectively, in the cropland soil located west of the tailings areas. The contamination is sufficiently high that the consumption of vegetables prone to lead and copper accumulation is discouraged.
Dust suppression is a practice implemented in the large-scale mining industry that aims to reduce the negative impact of dust emissions from unsurfaced haul roads that are used to transport the ore from the mine. However, some dust suppression techniques potentially introduce contaminants to the soil. In addition to compaction and surface coverage with cement or tar, the unsurfaced haul roads can be treated with dust suppressants that contain tar and bitumen emulsifiers, petroleum resins, lignosulphonates, polymer emulsions and hygroscopic salts (Thompson and Visser, 2007). Another approach includes regular spraying with water that may contain chemical additives that aim to stabilize the soil surface and reduce the dust impacts of regular heavy traffic.
Trace elements at mining sites are often accompanied by other organic contaminants which results in more complex combinations that requires advanced remediation techniques (Rösner, 1999). Acid mine drainage from tailings storage facilities is a major cause of soil acidification in mining areas. The reduced pH levels of both soil and tailings increases the mobility of trace elements and radionuclides (Rösner, 1999).
In contrast, artisanal and small-scale mining largely depends on human labour and primitive technologies to extract the mineral from the ore (Figure 3). While the exact number of people dependent on this type of mining in sub-Saharan Africa could not be determined, it contributes significantly to twenty-three of the regional economies. Countries whose rural livelihoods benefit from artisanal and small-scale mining include Burkina Faso, Mali, United Republic of Tanzania, Sierra Leone, and the Democratic Republic of the Congo (Fritz et al., 2018).
The two major contaminants of concern associated with the artisanal and small-scale mining of gold are mercury and cyanide. Elemental mercury is used to remove gold from silt by formation of a mercury-gold amalgam. The washing process leaves mercury behind in the tailings which adds it to soil or nearby water resources (Pure Earth, 2019a). The amalgam is then heated to volatilize the mercury leaving the gold. Arsenic and mercury contamination of soil is also present in and around Obuasi, in Ghana, where gold mining is the main activity (Amonoo-Neizer, Nyamah and Bakiamoh, 1996). The use of cyanide to recover gold is now used by small-scale and artisanal miners in Mozambique, the United Republic of Tanzania and Zimbabwe (Fritz et al., 2018). In Burkina Faso, more than 20 percent of the Zougnazamiline human settlement area is exposed to soil polluted with cyanide (Razanamahandry et al., 2018).
Other minerals such as cobalt and coltan are also extracted through artisanal and small-scale mining. The Democratic Republic of the Congo has the largest cobalt resources of all the countries in the world (Milesi et al., 2006) and between 15 percent and 20 percent of the total production of 2015-2016 were produced by artisanal miners (Barazi et al., 2005). A study was conducted to determine the health exposure risk of people living in the Kasulo neighbourhood of the city of Kolwezi after artisanal cobalt mining activities started following the discovery of cobalt-rich material in a residential plot (Barazi et al., 2005). Analysis of dust in neighbouring house and their plots found the following trace element concentration ranges: 205 to 8 140 mg/kg cobalt, 0.4 to 15 mg/kg uranium, 72 to 458 mg/kg manganese and 3 830 to 12 170 mg/kg iron. The study found that the main causes of soil contamination were the stockpiling and processing of ore and the spillage containers while the ore was hoisted from the pit.
Apart from the mining of minerals, the extraction and processing of fossil fuels are also contributing to soil pollution in the region. The highest occurrence of soil pollution as a result of the petroleum industries has been reported for Nigeria and Angola (Fayiga, Ipinmoroti and Chirenje, 2018). The major contaminants released into the soils from this industry are trace elements and hydrocarbons.
According to Yabe, Ishizuka and Umemura (2010), Nigeria has been subject to more than 4 000 oil spills between 1960 and 2010 with volumes estimated to be more than 2 million barrels (320 000 m3). Sabotage was identified as the main cause of oil spills in four states of Eastern Nigeria (Aprioku, 2003). Akande, Ogunkunle and Ajayi (2018) described accidental spills of crude and refined oil around oil and gas storage facilities in Ibadan. As remediation efforts face several limitations, there are now a large number of historical and recently polluted sites (Ogbonnaya et al., 2017). Ogoko, (2014) undertook a study of soil pollution around the Nigerian National Petroleum Corporation’s depot in Aba city in south-eastern Nigeria. Total petroleum hydrocarbons (TPH) showed a range of concentrations between 5 120 and 24 902 mg/kg while polycyclic aromatic hydrocarbons (PAHs) ranged between 6.3 and 7.4 mg/kg. The main trace elements and their concentration ranges were: lead (16.1 - 32.3 mg/kg), cadmium (1.9 - 11.78 mg/kg) and mercury (0.3 - 1.66 mg/kg).
In Angola, the oil and gas industry is responsible for the oil leaks and spills that have been reported since 2009 in the provinces of Cabinda and Zaire (França, Muteca and Oliveira, 2015). Three studies have confirmed that coastal and riverine sediment were polluted with at least 15 PAHs that include phenanthrene, anthracene, naphthalene, fluorene and chrysene. The mean concentrations were 70.1 mg/kg for phenanthrene and 102.7 mg/kg for anthracene. The average TPH concentration was 21 500 mg/kg, dominated by petroleum hydrocarbons in the diesel range (TPH-DRO).
In contrast to developed countries, the rapid urbanization in some sub-Saharan African countries did not follow conventional land-use zoning approaches. Settlements have been established adjacent to industrial areas, mines or agricultural processing facilities in order to be close to areas with employment opportunities (OECD and Sahel and West Africa Club, 2020). A plethora of small industrial operations ranging from dry cleaning to lead battery recycling and auto-mechanical workshops can be found within residential areas. Ettler (2016) and Manhart et al. (2016) documented high levels of trace elements in soil and biota around metal smelters leading to human exposure. Prior to selling second-hand lead-acid batteries, traders often drain the acid into environmental media, including soil, as buyers offer higher prices for empty batteries (Manhart et al., 2016). While each individual operation may be small, cumulatively they represent a significant source of soil pollution.
Vehicle workshops provide a range of services including engine repair and maintenance, welding and paint spraying (Sam et al., 2015). The inappropriate management, use and disposal of chemical products and wastes from vehicle workshops can lead to soil pollution (Figure 4). These materials include paints, paint primers and solvents, old hydraulic liquid, lubricating oil and grease (Ekeocha, Nwoko and Onyeke, 2017), all of which are potential sources of contamination with PAHs (Marr et al., 1999; Wong and Wang, 2001). Even though a wide range of potential contaminants are used, the soils of these areas have only been evaluated for trace element contamination. For example, a study on three such sites in Abuja, Nigeria indicated variability in the concentration and distribution of cadmium, chromium, copper, iron, lead, nickel and zinc (Ekeocha, Nwoko and Onyeke, 2017). In Ethiopia, analysis results of twelve workshop sites in Shashemane City, indicate elevated soil concentrations of five trace elements. Analysis of thirty six composite soil samples collected from twelve vehicle workshops indicated overall mean concentrations of 782 mg/kg lead, 443 mg/kg nickel, 331 mg/kg cobalt, 290 mg/kg chromium and 133 mg/kg cadmium (Demie, 2015). In addition, three samples from control sites were analysed for the calculation of the contamination factor values. The average trace element concentrations from the control sites were 118 mg/kg chromium, 53 mg/kg cobalt, 20 mg/kg nickel, 10.5 mg/kg cadmium and 8.5 mg/kg lead.
Paint has been identified as a source of soil pollution with lead and cadmium in residential areas in Ibadan and Lagos in Nigeria (Adeyi and Babalola, 2017). The levels of these trace elements exceeded the Standards for soils of the Netherlands. Other small or home-based industries such as the preparation of fried foods for sale and personal care services can also result in soil contamination. It was reported that oils containing PCB have been removed from old transformers, sold and subsequently used for frying food and in the preparation of hair- and skin-care products (Alemu, undated). Polychlorinated biphenyl filled electrical capacitors and transformers can be a source of soil pollution through leakage from damage and corrosion while in service, during maintenance operations and when decommissioned. Sites that could be polluted with PCB include electrical substations, transformer maintenance workshops, storage areas, and recycling and disposal facilities for used oils and old equipment (Weber et al., 2018b).
In sub-Saharan Africa, approximately 69 percent of waste ends up on open waste sites where it is highly likely to be burned (Kaza et al., 2018). Waste is often dumped without a prior sorting processes () and can include combinations of medical, agricultural, and domestic waste (Yabe, Ishizuka and Umemura, 2010). Between 2000 and 2018, the waste treatment approaches at 80 percent of the approved municipal solid waste (MSW) dumpsites in the region consisted of uncontrolled dumping and burning. Some sites were staffed and had limited equipment and some containment to control the combustion processes (Idowu et al., 2019). Chicken eggs and milk produced around dump sites in Nigeria were found to be contaminated with polybrominated diphenyl ethers (PBDEs). Elevated levels of PBDE were discovered in chicken eggs produced 5 km away from the dumpsites (Oloruntoba et al., 2019).
The leachate from these unmanaged landfill sites is a source of chlorides and sulphates as well as cobalt, nickel and zinc as was found at Koshe in Addis Ababa, Ethiopia (Haile and Abiye, 2012). Soil concentrations of trace elements at and around a 56 ha municipal waste landfill site in the Eastern Cape of South Africa were compared to the soil threshold values of both the World Health Organization (WHO) as well as the South African Department of Environmental Affairs (Nyika et al., 2019). The average soil concentrations measured were 33.4 mg/kg arsenic, 481 mg/kg cobalt, 219 mg/kg copper, 1 335 mg/kg chromium, 8 991 mg/kg manganese, 354 mg/kg nickel, 46 mg/kg lead, 160 mg/kg zinc, 435 mg/kg vanadium and 155 268 mg/kg iron.
The impact of open waste dumpsites on soil pollution in the Nairobi metropolitan area has been described by Kimani (2007) and Njagi et al. (2016). At the Kadhodeki waste site on the outskirts of Nairobi several old quarries have been used for waste dumping since 1986. The area is now also used by subsistence farmers for crop production (Njagi et al., 2016). The researchers analysed soil sampled at depths of 0.15 m directly around the waste dump areas and found the following mean trace element concentrations: 14 419 mg/kg manganese, 11 968 mg/kg nickel, 6 003 mg/kg cobalt, 5 077 mg/kg vanadium, 2 089 mg/kg copper, 525 mg/kg iron, 436 mg/kg mercury, 289 mg/kg zinc and 60 mg/kg lead. An earlier report by Kimani (2007), on the Dandora waste dump area in Nairobi also reported high exposure risk to the trace elements cadmium, copper, lead and zinc (Figure 6). The concentrations in the soil were higher than the soil quality standards of both the Netherlands and Taiwan.
In addition to waste generation within the region, the shipment of waste from other regions adds to the heavy pollution burden associated with waste in sub-Saharan Africa. Nigeria was one of the main receivers of waste electronic and electrical equipment (e-waste) from Asia and Europe (Sthiannopkao and Wong, 2013). Currently, a contentious aspect of this waste shipment is the larger volumes of electronic waste (e-waste) that ends up in the region. The processing of waste, especially e-waste releases PCBs, flame retardants, such as polybrominated diphenyl ethers (PBDEs), oil and polybrominated and polychlorinated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs and PCDD/Fs) (Babayemi et al., 2015; Kone, 2014; Sindiku et al., 2015b, 2015a).
Toxic waste has been delivered from Europe to Mozambique, Nigeria, Togo, Benin, Somalia, Djibouti, Equatorial Guinea and the Congo (Kone, 2014). The waste from these shipments contained cyanides, pesticides, solvents, radioactive material and pharmaceutical waste. A notorious example of this practice, was that of the Probo Koala tanker that dumped 554 tonnes of hazardous waste in Abidjan, Côte d’Ivoire in 2006 after waste treatment in the port of Amsterdam was considered too expensive (Wingerde, 2015). The informal sector of e-waste recycling is predominant in many countries of the region, although landfill and incineration are also common practices. Both management approaches contribute to the release of hazardous compounds into the soil (Bimir, 2020). With the adoption of the global ban on the export of hazardous waste by the Parties of the Basel Convention in December 2019, this problem is expected to be significantly reduced in the coming years, although it effectiveness will depend on the non-acceptance of hazardous waste by importing nations and the control of illegal traffic (Joshi, 2020).
The practice of open burning e-waste, such as at the Agbogbloshie waste site in Ghana, has been shown to be a source of soil pollution (Figure 7). Soil samples from these sites contain high levels of both chlorinated and brominated dioxin-related compounds that have been identified as a human health risk (Tue et al., 2016). The contaminated soil samples result in contamination of livestock such as cattle and chicken and related meat, milk and eggs (Weber et al., 2018a). Chicken eggs at Agbogbloshie had the highest PCDD levels ever measured in an egg (Petrlik et al., 2019).
A study of contaminant concentrations in the blood of 245 people from 16 African countries linked negative human health implications to pollution from artisanal e-waste processing sites (Henríquez-Hernández et al., 2017). The majority of the participants from sub-Saharan Africa were from Western Africa and Central Africa. It was found that children from areas where e-waste processing is a major activity, are particularly susceptible to exposure to trace elements such as aluminium, arsenic, chromium, lead, mercury and vanadium.
On an international level, the incineration of medical waste has been identified as a potential source of PCBs, PAHs, dioxins and other carcinogenic organic contaminants (WHO, 2020). In sub-Saharan Africa, medical waste treatment in the region is mainly limited to incineration and dumping (UNEP, 2018). The ash generated by incineration, is then transported and dumped on landfill sites. Medical waste incineration ash was identified as diffuse source of soil pollution with cadmium, chromium, lead, and zinc within a 60 m radius around a medical waste dump site of a teaching hospital in Ghana (Adama et al., 2016). The elevated levels of chlorine and lead at a waste site of a hospital in South West Nigeria may also pose a risk of pollution to the surrounding agricultural soil (Inyang, Akpan and Obiajunwa, 2013).
Agricultural activities in the region have also been a source of soil pollution (Fayiga, Ipinmoroti and Chirenje, 2018). Pesticide use by the agricultural sector is considered the most significant contributor of soil contaminants. The high levels of endosulfan and DDT, both organochlorine insecticides, were detected in soils of the Awash valley state farms in Ethiopia are attributed to historical agricultural practices in the area (Westbom et al., 2008). In Burkina Faso, endosulfan and profenofos, a organophosphate insecticide, were present in the soil of both old and new agricultural fields used for cotton production (Ondo Zue Abaga et al., 2011). A higher concentration of profenofos is present in the newly developed cotton fields than the old production areas. Another study of leaf and root vegetables cultivated in soils with low levels of PCB congeners and DDT, showed variable levels of DDT, DDD and DDE as well as six PCB congeners in the crops (Olatunji, 2019). Endosulfan was listed under annex A of the Stockholm Convention in 2012 and exemptions for its continued use ceased to be granted in 2017. There are now 16 pesticides listed for elimination with a further two, including DDT, whose use are restricted.
The use of DDT for agricultural purposes is banned. Some countries in the region have been granted exemptions under the Stockholm convention to continue to use DDT for indoor residual spraying (IRS) as a control of malaria vectors. Unfortunately, supplies of DDT can sometimes leak from malaria control to be sold through local markets into the agricultural sector (Sun et al., 2016). In Ethiopia, leaves of the indigenous tree species, Catha edulis, are sold at markets and the raw leaves are chewed by consumers to experience high energy levels and euphoria. However, it was found that DDT is applied as a foliar spray prior to selling the leaves as it makes the leaves shine that makes it more attractive to potential customers (Mekonen et al., 2017).
The past burial of obsolete pesticides is another source of soil pollution that may also be a diffuse source of groundwater pollution through seepage. Results from the soil analysis of five Tanzanian sites in the Arusha and Mbeya regions indicated that, although the buried persistent organic pesticides had aged, the degradation rate is slow (Mahugija, 2013). The range of the total concentrations of DDT and hexachlorocyclohexane (HCH) were respectively: 5.2 mg/kg to 5 410 mg/kg dry weight, and 1.4 mg/kg to 42 200 mg/kg dry weight. Other pesticides detected included the persistent organic pollutants (POPs) aldrin, dieldrin, endrin, endosulfan, chordane and heptachlor, amongst others.
The lower cost of biosolids and manure makes it the preferred source of soil nutrients in the region (Fayiga, Ipinmoroti and Chirenje, 2018). While it may be more affordable, it can also contribute to soil pollution, as already described in Chapter 3. A study conducted to determine the source of trace element contamination in soil and vegetables of the Philippi Horticultural Area of the Western Cape Province of South Africa, showed that the highest contributor of trace elements, were a mixture of pig and cattle manure (referred to as kraal manure) (Malan et al., 2015). The mean concentrations of trace elements measured in the kraal manure, were 559 mg/kg zinc, 87 mg/kg copper, 8.5 mg/kg lead and 0.84 mg/kg cadmium. The range of concentrations of cadmium measured in the kraal manure were 0.1 - 3.8 mg/kg.
The application of fertilizer to enhance crop production, has also been shown to be a source of soil pollution in the region. The pineapple industry of the East London area, in the Eastern Cape of South Africa suffered significant economic losses when cadmium concentrations above 0.05 mg/kg were detected in canned pineapples in Switzerland and the whole consignment was rejected (Hill, Fraser and Baiyegunhi, 2012a; Morris, 2007). The source of the elevated cadmium levels was found to be imported zinc-phosphate fertilizer contaminated with cadmium, affecting 2 000 hectares of planted pineapples (Hill, Fraser and Baiyegunhi, 2012b; Morris, 2007).
Manure as a by-product of livestock production is also a source of waste that can result in soil pollution of the areas when the manure is disposed of in landfill areas or other bare soil surfaces that are used to stockpile the manure on. A study conducted at the Animal and Livestock Science experimental farm of the Agricultural University of Abeokuta in Nigeria, evaluated the levels of trace element accumulation in soil resulting from manure stockpiles of different types of animals (Azeez et al., 2009). The results indicated that soil pollution was the most significant where poultry manure was disposed, followed by that of swine and cattle. The highest accumulation of cadmium, copper, iron, lead, manganese and nickel occurs at soil depths of 0.8 - 1.2 m. The mean concentrations of trace elements at these depths measured were 54 385 mg/kg iron, 285 mg/kg manganese, 24.3 mg/kg zinc, 16.5 mg/kg copper, 1.54 mg/kg nickel, 15.4 mg/kg lead, 1.27 mg/kg cadmium, and 0.44 mg/kg chromium.
Historically, one of the major sources of soil pollution associated with transport was the contamination of roadside soil with lead through the use of leaded fuel. Although the phasing out of leaded fuels started almost forty years ago, progress in sub-Saharan Africa was slow in comparison to that made in Europe and the United States of America and even other developing countries (Makokha, 2011). In June 2001, a workshop held in Dakar was attended by representatives from 25 countries in the region in an effort to eliminate the use of leaded fuel by 2005 (Pure Earth, 2019c). In 2002, it was estimated that 80 percent to 85 percent of all the vehicles in sub-Saharan Africa were still old vehicles with soft valve seats that presented a technical difficulty to the use of unleaded fuel (Cox and Doll, 2002).
The use of leaded fuels has been banned in Kenya, Uganda and the United Republic of Tanzania since 2006 (Makokha, 2011) and the concentration of lead in fuel reduced significantly in Senegal between September 2006 and March 2007 (Pure Earth, 2019c). Both of these reports indicated a positive correlation between restricting or banning the use of leaded fuel and the reduction of lead pollution from environmental matrices such as soil, water, air and vegetation. While the Pure Earth report focusses on the improvement of air quality (Pure Earth, 2019c), samples taken of soils along roads in Kisumu, Kampala and Mwanza showed a definite reduction in lead concentrations between 2007 and 2009 (Makokha, 2011). Although the lead concentrations that were present in the initial samples of 2007 as measured by Makokha (2011) do not degrade, the reduced concentrations measured in 2009 may indicate that the lead particles are possibly transported away in dust. It is likely that as time progressed, the absence of lead emissions from the traffic on the roads resulted in significantly reduced addition of lead to roadside soil.
While none of the countries in sub-Saharan Africa use leaded fuels anymore (WorldAtlas, 2020), transport is still a source soil contamination in the region. There has been significant growth in the number of motorcycles, especially since their use as a mode of public transport provides a source of income (Kumar, 2011).
Vehicular emissions from areas with high traffic density has been linked to elevated trace metal concentrations in urban soils of the city of Pretoria, South Africa (Olowoyo, van Heerden and Fischer, 2012). It was found that the antimony levels in areas with high traffic volume were seven times higher than that of areas with low volumes with the highest mean concentration of antimony measured at 3.49 mg/kg. The brake linings used in vehicles were considered the main source of the antimony contamination. Elevated cadmium concentrations in soil in high traffic areas were attributed to emissions from vehicle engines. The highest mean concentration of cadmium measured in street dust and soil were 1.12 mg/kg and 0.56 mg/kg respectively. Although unleaded fuel has been used in Pretoria since 2005, Olowoyo and co-workers stated that soil and road dust are still contaminated with lead as a result of the past use of lead fuels.
Although the focus of a trial in the Jinja Municipality in Uganda was to measure the effect of a steel rolling mill on soil contamination, it concluded that nearby road traffic (Figure 8) is most likely the cause of elevated cadmium levels in the area (Namuhani and Cyrus, 2015).
In sub-Saharan Africa, backup generator sets provide a solution to interruptions in electricity supply but are a source of pollution through their significant consumption of fossil fuels. It is estimated that of the total regional fuel consumption, twenty two percent of diesel and fifteen percent of gasoline are used by backup generator sets (IFC, 2019). Although not directly linked to soil pollution, a study conducted in the Abeokuta metropolis of Nigeria showed that these generators contribute to air pollution (Oguntoke and Adeyemi, 2016). The range of mean concentrations of contaminants measured with a portable air quality sampler within a ten metre radius around these generators are 3.5 ppm to 65.6 ppm sulphur oxides, 4.0 ppm to 85.7 ppm nitrogen oxides and 141 ppm to 4 167 ppm carbon monoxide.
The transmission of energy is also a source of PCBs, especially through the presence of PCBs in the oil of old and damaged transformers and capacitors. The leakage of obsolete transformer oil has been credited as the main source of pentachlorobiphenyl and hexachlorobiphenyl detected in Kenyan soil samples (Sun et al., 2016).
Countries that are parties to the Stockholm Convention have prepared inventories of PCBs as part of their National Implementation Plans. These inventories identified old transformers and capacitors in the countries that most likely used PCB-containing oil (Stockholm Convention, 2019). These inventories provide the most reliable information of potential sources of soil pollution by PCBs in the region. Currently a PCB management project is ongoing in the SADC region executed by UNEP and implemented by the Africa Institute (Africa Institute, 2020).
In South Africa, the majority of electricity is generated by coal-fired power stations. The electricity generated is transported over the entire country through an extensive transmission network. Electricity is also exported to the following seven of the sub-regional countries: Botswana, Eswatini, Lesotho, Mozambique, Namibia, Zambia and Zimbabwe. Soil around three power plants in South Africa were found to be contaminated with PAHs. The total PAH concentrations measured in soil ranged between 9.73 mg/kg and 61.24 mg/kg (Okedeyi et al., 2013). The majority of the PAHs were higher molecular weight (five to six ring) hydrocarbons, which indicates their source was emissions from the combustion of coal at these power stations.
In sub-Saharan Africa, South Africa is the only country with a nuclear power plant. The waste of this plant is stored at a dedicated waste facility in the least-densely populated Northern Cape Province of the country.
The use of military equipment has been reported as a point source of soil pollution through the dispersal of contaminants such as trace elements, dioxins, cyanide and organic solvents (Certini, Scalenghe and Woods, 2013). Even long after military conflicts have ended, the detonation of landmines can contaminate soil with trace elements such as nickel, chrome, manganese, cobalt and copper (Hamad, Balzter and Kolo, 2019). In some of the countries the displacement of people to areas of safety such as refugee camps has caused diffuse pollution due to large volumes of waste are then generated and generally improperly managed (Biswas and Tortajada-Quiroz, 1996).
The contribution of military training to soil pollution has been the focus of two studies in Botswana (Dinake et al., 2018; Sehube et al., 2017). The first study focussed on total concentrations as well as the fractionation of lead at eight shooting ranges (Sehube et al., 2017) and the second study evaluated the pollution risk of cadmium, copper, manganese, nickel and zinc in the soil of the berm (the bank of soil behind the targets that stops the projectiles) of five shooting ranges (Dinake et al., 2018). Together, these shooting ranges are representative of data across almost the entire eastern half of Botswana and the soil therefore derived from a variety of geological strata.
Sehube et al. (2017) found that the total lead concentrations from seven of the eight shooting ranges exceeded the USEPA critical level of 400 mg/kg. The highest mean lead concentration was measured in soil samples collected from the Thebephatshwa shooting range at 38 386 10 197 mg/kg, a site regularly used by the nearby air base. The lower concentrations of lead at the eighth range are attributed to its less intensive use for pistol shooting, the bullets of which contained approximately 44 percent less lead than the rifle bullets. It was found that the lead fractions present at all eight of the sites are largely carbonate-bound as cerussite and hydrocerussite (Sehube et al., 2017). Of the trace elements investigated by Dinake et al. (2018), cadmium was found to present the most significant pollution risk, although its total concentration was low in the berm soils of the shooting ranges. Through the use of ecological risk calculations, it was shown that copper, manganese, nickel and zinc also contribute to high levels of soil pollution of the military ranges at Selibe Phikwe, Thebephatshwa and Shoshong (Dinake et al., 2018).
Waste dumping around Rwandan refugee camps in the Democratic Republic of the Congo provides an example of the far-reaching contribution of war to soil pollution (Figure 9). Although the evaluation by Biswas and Tortajada-Quiroz (1996) did not include specific mention of soil pollution, it reported on the extent of waste dumping at these camps and the lack of appropriate waste treatment capacity. Every month, an estimated 150 tonnes of human excreta were transported to other areas such the Virunga National Park. This waste was only treated with a cover layer of lime at the designated defecation areas before it was removed. No other treatment options were available to reduce the levels of pathogens and other contaminants associated with human waste (Biswas and Tortajada-Quiroz, 1996).
While the major sources of soil pollution in sub-Saharan Africa are associated with mineral and petroleum extraction and processing, industrialization, waste mismanagement and energy generation, other minor sources are also present even though information about them is limited.
According to the World Malaria Report of 2019, sub-Saharan Africa has the highest concentration of malaria incidents in the world (WHO, 2019). Even though the Stockholm Convention prohibits the use of Persistent Organic Pollutants, WHO concluded in 2007 that DDT remains an important tool in the fight against vector-borne diseases such as malaria (WHO, 2011). The main reason is that DDT still has higher residual efficacy than pyrethroids, organophosphates and carbamates. However, very few publications directly report this practice as a source of soil contamination in sub-Saharan Africa.
The practice of Indoor Residual Spraying (IRS) with DDT in the fight against malaria, originates from South Africa and has been implemented there since the end of the 1930s. In villages in Vhembe District, IRS has been practiced since 1945 (Van Dyk et al., 2010). DDT and its metabolites have been detected in soil around the houses. It has also been detected in chicken tissue and vegetables in the same areas where the soil samples have been taken.
A Kenyan study conducted on the presence of organohalogenated contaminants in soil, indicated that DDT and endosulfan were the main contaminants (Sun et al., 2016). The presence of DDT in soil was attributed to its recent use for malaria control.
Contaminants that may be naturally present in elevated levels in sub-Saharan Africa include trace elements and radionuclides. The relation between the geological cartography and the concentrations of trace element in soil was illustrated for Santiago island (part of the Cabo Verde archipelago) (Cabral Pinto et al., 2015). This study warns that the natural background values of chromium, cobalt, copper, nickel and vanadium are present at elevated concentrations over large parts of the island and pose a pollution risk in agricultural soil.
Although a large portion of the trace element pollution in the Katanga Copperbelt in the Democratic Republic of the Congo is caused by mining and smelter activities, elevated levels of cobalt and copper are present on hills that are naturally metalliferous (Pourret et al., 2016). These trace elements occur mostly in heterogenite and malachite formations that are considered fairly stable and are not subject to anthropogenic interference that could lower the pH and result in higher mobility.
Radiation in the Vinaninkarena area of Madagascar is mainly caused by the uranium series that migrates in soil while thorium is immobile (Rabesiranana, 2001). An analysis of 85 topsoil samples from the same area in 2004 indicated that the activities of both the thorium and uranium series are higher than the world average values published by UNSCEAR in 2000 (Rabesiranana et al., 2008). The results indicated that people in the area are exposed to higher doses of radioactivity.