Environmental Engineering Reference
In-Depth Information
• Leaching of pollutants from septic systems and other sources, such as landfills/solid
waste disposalsites, onto surfaces and into ground water and subsequently into storm
drainage.
• Application of pesticides and fertilizers in gardens and other cultivated urban lands;
• Illegal discharges of sewage and industrial wastewater designated to be discharged into
sanitary sewers
• Cross connection of sewage and/or industrial wastewater from sanitary sewers, failing
septic tanks andother sources into the storm water system.
• Pollution contained in precipitation (wet atmospheric deposition).
• Non point sources of pollution from agricultural activities within the municipal areas.
Atmospheric depositions of pollutants washed during the first stages of rainfall have been
found to have a significant impact on the quality of surface water runoff. Both industrial
(urban and transportation) and agricultural activities may contribute to the pollution
content of atmospheric deposits. Rain droplets and snowflakes absorb pollutants from the
atmosphere, including acid forming components. In residential areas, the fugitive dust
(dry deposition) mostly originates from surrounding soils, construction sites, and refuse
disposal sites and from biological sources (pollen, spores and other organic residues). It
has been estimated that in larger cities the deposition rate of atmospheric particulates in
wet and dry fallout ranges from 7 tonnes/km
2
per month to more than 30 tonnes/km
2
per
month (Novotny & Olem 1994). Higher deposition rates occur in congested downtown
and industrial zones, and lower rates are typical for residential and other low-density
suburban zones. For other measured pollutants (phosphorus, cadmium, chromium, lead
and zinc) rainfall contributions were insignificant. It has been found (AWRC 1981) that
in terms of pollutant concentrations, surface sources of storm water pollution were much
greater than the atmospheric contribution. Exceptions were the atmospheric deposition of
nitrate and zinc, which were apparently related to stack outputs from upwind industries
The following conclusions were made, based on a precipitation sampling program in
Washington DC:
• Pollutants tend to wash out relatively uniformly over the metropolitan area, even though
the origins ofpollutants may tend to be area specific.
• Atmospheric contamination is washed out during the first stages of a precipitation
event. Thus the resulting ground surface loading of atmospheric pollution is largely
independent on the magnitude or intensity of precipitation.
• Other than atmospheric conditions prior to or during rainfall, the primary factor
affecting pollutant loadsfrom rainfall is the period of time since the previous
precipitation event.
It should be noted that in terms of diffuse pollution in the region, atmospheric depositions
could be considered a significant pollution source only with respect to heavy industrial
sites, which generate a considerable amount of air pollution, and areas of very intensive
traffic in urban areas.
Streets refuse deposition is another factor that influences the runoff quality. Particles
that are larger in size than dust (> 60 µm) are considered as street refuse or street dirt
(Novotny & Olem 1994). In NURP (National Urban Runoff Program) studies in USA,
these deposits were divided into median-sized deposits (street-dirt particles range from 60
µm to 2 mm) and litter (>2 mm). Litter deposits contain items such as cans, broken glass,
