Environmental Engineering Reference
In-Depth Information
metals, causing most concern in terms of their levels and toxic effects on aquatic life. The
metals are derived mainly from vehicle exhaust emissions, roof and vehicle corrosion,
tyre wear and atmospheric fallout. Deposition rates on street surfaces vary with the use of
the developed area-industrial zones having greater deposition rates than commercial and
residential areas.
The following points should be kept in mind when evaluating the risks of toxic metals
in urban storm water. First, a large fraction will be attached to suspended particulates,
which effectively reduces the level immediately available for biological uptake and
bioaccumulation. Second, urban runoff events typically occur over shorter durations (1-6
hours) than the exposure intervals used in conventional bioassay toxicity tests (24-169
hours for chronic toxicity criteria). Also, urban storm water runoff will be subject to
substantial dilution following mixing in the receiving water. Despite of these
considerations, it is likely that trace metals in runoff would be toxic to the receiving water
habitat, particularly for the more soluble species of Zn and Cu, if long retention times are
involved (Ellis 1989).
The form and concentration of polluting materials in runoff will vary both with the
density of population and with the type of activity associated with a particular area. In
many cases, the surface runoff characteristics from industrial or commercial areas will
differ from the runoff generated by purely residential areas. In general, the following
classification of different types of land use practice in urban areas could be done,
although it is not applicable to many cities, which have mixed patterns:
• Residential (low, medium and high density)
• Commercial
• Industrial (light and heavy industry)
• Other developed land (parks, sport complexes, large parking areas, cemeteries)
• Transportation (roads, streets, parking slots, highways, airports)
3.2.2
Other sources in urban areas
Urban drainage is considered by far, the most important source of diffuse pollution in
urban areas and at present more efforts have been focused on abatement measures with
respect to this source. However, other sources, as listed below, are also important,
especially for developing countries.
• Solid waste disposal sites - unprotected solid waste disposal sites could be a major
source of diffuse pollution to surface and ground water, characterized by high organic
loads, toxic pollutants and pathogens. Protected disposal sites would require a special
treatment of surface runoff and leachate, and could be classified as a point source of
pollution.
• Illegal discharges and informal settlements - this category includes any point or non-
point discharges, which are not authorized and accounted for. They are classified as
diffuse pollution because of their possible large numbers and difficulty in location.
Major sources of diffuse pollution are informal settlements, which are typical for
developing countries. These are squatter camps at the outskirts of towns and cities,
which most often have a very limited water supply and lack sanitation facilities. Other
form of illegal discharges due to informal settlements are unauthorized buildings in
