Environmental Engineering Reference
In-Depth Information
Because of these problems, informal settlement residents are rarely provided with
proper water supply, sanitation or waste disposal services. As a result, residents are
frequently forced to rely on shallow groundwater for domestic water supply (Butcher
2003, Dahiya 2003' Tsvere et al
.
2004). In fact, urban groundwater is thought to supply
up to half of the world's urban population (Foster, 2001). This exposes settlement
residents to health risks from the contaminated groundwater. Informal settlement
communities have become highly prone to water-borne diseases (Chidavaenzi et al
.
2000)
- although respiratory diseases are also common in overcrowded slums (Gulis et al
.
,
2004). In cases where supply is erratic, the situation is worsened as residents store water
in containers or sinks for more than a day (Tsvere et. al
.
2004).
Inadequate water supply and sanitation are largely responsible for the high levels of
such diseases in Southern Africa (Makoni 2001). Human excreta may contain eggs of
helminthes (worms), protozoa, bacteria and viruses. These may be excreted in vast
numbers depending on the age and state of the individual. Fecal matter contains on
average 10
9
bacteria per gram (not necessarily pathogenic) and the excreta of infected
individuals as high as 10
6
viruses per gram (National Academy of Sciences 1987). When
feces are deposited in shallow water an easy pathway is provided for pathogens to enter
surface and groundwater, and hence to the local population via contaminated drinking
water.
1.2
Impact on groundwater of semi-formal settlements
In developing countries, the subsurface environment frequently acts as a major sink for
domestic waste, leading to significant degradation of groundwater quality (Forster 1999).
Rapid urbanization is associated with rapid deterioration in water quality (Ren et al
.
2003). Contamination of shallow groundwater from peri-urban semi-formal settlements is
especially problematic in cities where the settlement is in a water source area for
municipal reservoirs. The semi-formal settlements of Old Naledi, Gaborone, Botswana
(Gwebu 2003), and Epworth, Harare (the settlement studied in this chapter) are cases of
this kind.
Nitrate, nitrite, phosphate and organic compounds are the main contaminants that
frequently percolate to groundwater from informal settlements (Wright 1999).
Microbiological contamination is also frequent. The combination of a lack of well-
developed service infrastructure and a lack of proper settlement planning means that a
variety of activities in informal and semi-formal settlements contribute to groundwater
contamination, see Table 6.1.
Probably the most important of these problems are those caused by on-site sanitation -
since the latrines are abundant, and spread throughout an informal settlement. On-site
sanitation can be of various
Table 6.1. Summary of possible groundwater
contamination problems associated with informal
settlements.
Activity
Associated Contaminants
On-site sanitation (various methods)
Nitrate, phosphate, coliform bacteria, other pathogens
