Groundwater quality in Africa
Groundwater quality can be influenced by both natural and anthropogenic processes. A detailed review of groundwater quality issues in Africa can be found in:
Xu Y and Usher B (eds). 2006. Groundwater pollution in Africa. Taylor & Francis/Balkema, Netherlands.
Some of the key issues and studies related to groundwater quality in Africa are described below.
Geogenic contamination refers to naturally occurring elements that are generally present in groundwater due to dissolution of the aquifer material. Geogenic contaminants in groundwater, such as arsenic and fluoride, have a negative effect on human health, particularly when consumed over prolonged periods of time.
The Swiss Federal Institute of Aquatic Science and Technology (Eawag) are currently developing global probability maps of fluoride and arsenic. The first versions of these maps use measured concentrations of arsenic and fluoride from around 20,000 and 60,000 globally distributed observations points, respectively. They combine this data with available environmental information on soil, geology, climate, and topography, and use statistical methods to determine the probability of the occurrence of groundwater fluoride and arsenic concentrations above the WHO guidelines.
The global probability maps can be downloaded from the Eawag website:
Further information on the methodology can be found in the following publications:
Amini M, Mueller K, Abbaspour KC, Rosenberg T, Afyuni M, Moller KN, Sarr M, Johnson A. 2008. Statistical Modeling of Global Geogenic Fluoride Contamination in Groundwaters. Environmental Science and Technology, 42, 3662-3668.
Amini M, Abbaspour KC, Berg M, Winkel L, Hug SJ, Hoehn E, Yang H, Johnson A. 2008. Statistical Modeling of Global Geogenic Arsenic Contamination in Groundwater. Environmental Science and Technology, 42, 3669-3675.
Salinity is another important groundwater quality issue that can be driven by both natural and anthropogenic processes. Processes such as sea level rise and intense evaporation can lead to naturally high salinity in groundwater, while over-abstraction, irrigation and waste disposal can exacerbate groundwater salinity issues. Salinity has important consequences for human health and agricultural productivity.
IGRAC have compiled a global map of groundwater salinity occurrence by extrapolating documented cases into larger areas of high probability of saline occurrence. The global salinity map can be downloaded from the IGRAC website:
Nitrogen occurs naturally in the environment and is essential for plant growth. Nitrogen-based fertilisers are therefore often applied to increase crop yields. Leaching from agricultural land can lead to high concentrations of nitrogen in groundwater, which can have a negative impact on both the environment and human health.
IGRAC are currently carrying out a global assessment of nitrate contamination which will result in global scale maps of nitrate in groundwater. Further information can be found on the IGRAC website:
Urban and peri-urban areas are expanding in many parts of Africa, particularly across sub-Saharan Africa. Groundwater is often a very important source of improved drinking water in urban and peri-urban environments however high population densities put pressure on these resources in terms of both quantity and quality.
Groundwater quality can be influenced by a large number of contaminants in the urban environment, from microbiological pathogens and heavy metals to macronutrients, herbicides and pesticides.
Some of the key sources of urban pollution include:
- Pit latrines, which are often located close to abstraction points, particularly in densely populated peri-urban or unplanned urban settlements
- Sewer leakage and sewage effluent
- Uncontrolled disposal of household and industrial waste
- Peri-urban agriculture, which includes pesticides/fertilisers and livestock waste
- Storm water runoff
- Vehicle emissions, power stations and mine waste
There are few studies looking at urban groundwater issues in Africa and those that have been carried out mainly focus on large cities and include only basic chemical and microbiological parameters. A detailed review of urban groundwater quality issues can be found in:
Lapworth DJ, Stuart ME, Pedley S, Nkhuwa DCW & Tijani MN. 2014. A review of urban and peri-urban groundwater quality studies in sub-Saharan Africa. British Geological Survey (Report Number)