Environmental Engineering Reference
In-Depth Information
bottles, pull tabs, papers, building materials, plastic, garbage, parts of vegetation, dead
animals and insects, animal excreta, and the like. The sources of street dirt are numerous
and often very hard to control. A comprehensive survey of a number of U.S. cities with
respect to the accumulation of pollutant on street surfaces (AWRC 1981) points out that
runoff from street surfaces is generally highly contaminated. It was estimated that, based
upon a complete cleansing of street surfaces by a moderate to heavy storm of one hour
duration, the storm runoff water produced during this storm would contribute
considerably more pollution load than would the same city's raw sewage during the same
period of time. This type of urban diffuse pollution is common for many African
countries and contributes a substantial pollution load.
Pollution input from vegetation is generated by leaf fallout and other residues as grass
clipping. During characteristic seasons, it dominates street refuse composition and is a
source of large urban loads of biodegradable organics. Only a portion of this vegetation
residue that accumulates on impervious surfaces is a pollution threat to surface waters.
Vegetation fallout on soils becomes an integral part of the soil composition, and in most
cases may even improve soil permeability and erosion resistance. During defoliage in the
fall a mature tree can produce 15 to 25 kg of organic leaf residue (dry weight) that
contains 90% organics and 0.04%-0.28% phosphorus (Novotny 2003). With respect to
the countries in the region, grassed open ditches are a preferred option for conveyance of
road/street runoff. Grass clippings and other vegetation material accumulate in the ditches
and could be a considerable source of pollution if not cleaned regularly.
Motor vehicular traffic is directly responsible for the deposition of substantial amounts
of pollutants, including toxic hydrocarbons, metals (from exhaust emissions, worn off
tyres, clutch and brake linings, lubricants, coolants, rust and decomposing coatings
dropped from the underside of mudguards and undercarriages) asbestos and oils. The
particulates contributed by traffic are primarily inorganic. Most of the traffic exhaust pipe
emissions are dust size (< 60 µm). Only a small portion (< 5%) of the traffic related
pollution could be directly traced to vehicle emissions. However, the pollutants that
motor vehicles emit are among the most important because of their potential toxicity. In
addition to traffic density and vehicle pollutant emissions, pavement conditions and
compactions are significant in determining the traffic impact on pollutant loads. Streets,
whose conditions are rated as fair to poor, were found to have total solid loadings 2.5
times greater than those rated as good to excellent. Summarized data on the quality of
runoff from highways in the USA show average concentrations of lead and zinc of 0.53
mg/l and 0.37 mg/l respectively (Novotny & Olem 1994).
Toxic chemicals could become a diffuse source of pollution from industrial sites,
where no provision for safe storage of fuels, raw materials or auxiliary chemicals is
made, and spillages or leakages could be washed out by the runoff and reach the drainage
system.
The major pollutants associated with agriculture include sediment, nutrients, pesticides
and other toxins, bacteria and salts or increased salinity levels. Different types of
agricultural land use are more likely to contribute certain pollutants than others from
runoff and subsurface water. Typical types of agricultural land use patterns are dry land
and cropland, irrigated cropland, pastureland, rangeland, forestland, confined animal
feeding operations, orchards and wildlife land. It has been mentioned (Chapter 3) that
informal small-scale agricultural activities become an often-met part of the urban
