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Eutrophication is an increasing problem in sub-Saharan Africa (SSA), and, as a result, the ecological integrity
of surface waters becomes compromised, fish populations become extinct, toxic cyanobacteria blooms are
abundant, and oxygen levels reduce. In this review we establish the relationship between eutrophication of
fresh inland surface waters in SSA and the release of nutrients in their mega-cities. Monitoring reports
indicate that the population of mega-cities in SSA is rapidly increasing, and so is the total amount of
wastewater produced. Of the total amounts produced, at present, less than 30% is treated in sewage
treatment plants, while the remainder is disposed of via onsite sanitation systems, eventually discharging
their wastewater into groundwater. When related to the urban water balance of a number of SSA cities, the
total amount of wastewater produced may be as high as 10–50% of the total precipitation entering these
urban areas, which is considerable, especially since in most cases, precipitation is the most important, if not
only the ‘wastewater diluting agent’ present. The most important knowledge gaps include: (1) the fate and
transport mechanisms of nutrients (N and P) in soils and aquifers, or, conversely, the soil aquifer treatment
characteristics of the regoliths, which cover a large part of SSA, (2) the effect of the episodic and largely
uncontrolled removal of nutrients stored at urban surfaces by runoff from precipitation on nutrient budgets
in adjacent lakes and rivers draining the urban areas, and (3) the hydrology and hydrogeology within the
urban area, including surface water and groundwater flow patterns, transport velocities, dynamics of
nutrient transport, and the presence of recharge and discharge areas. In order to make a start with managing
this urban population-related eutrophication, many actions are required. As a first step, we suggest to start
systematically researching the key areas identified above. |
