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400 Chapter 12 Urban Runoff and Combined Sewer Overflow Management

Figure 12.3 2007 Midwest Flooding
(Source: Wikipedia, http://en.wikipedia.org/wiki/Image:Ottawa_OH_2007_
Blanchard_River_ Flood_Coast_Guard.jpg)

which can in turn change the biological community structure. With increased sunlight,
algae in nutrient-rich receiving waters grow faster and the dominant species changes,
which affects the composition of higher organisms. Increased imperviousness and loss of
groundwater resupply can lead to more frequent low-flow conditions in perennial streams.
The effects of hydrologic changes due to urbanization therefore should be prevented or
mitigated to minimize urban runoff pollution.

12.2 URBAN RUNOFF POLLUTION
Prevention and control of urban runoff pollution requires an understanding of pollutant
categories, of the major urban sources of these pollutants, and of the pollutants’ effects.
When stormwater runoff and municipal wastewaters are carried in the same collector sys-
tem, the spills of untreated discharges are termed combined sewer overflow (CSO).
Significantly, 56% of the populations in U.S. cities with 100,000 or more inhabitants are
served by such combined or partially combined systems. Also, the separately collected or
free-discharging stormwaters alone can produce mass releases of contaminants harmful to
receiving waters.
Table 12.1 lists the primary categories of urban runoff pollutants, pollutants associated
with each category, typical urban runoff pollutant sources, and potential effects. Table 12.2
summarizes the relative contribution of predominant pollution sources to the degradation
of U.S. rivers, lakes, and estuaries. For municipalities, urban storm-generated runoff and
construction are the most prevalent sources; outlying agricultural activities also can play a
significant role in many urban areas.
The effects of urban runoff pollutants vary for different water resource types. A given
municipality’s pollutants of concern, therefore, depend on the types of water resources in
and downstream of the community and their desired uses.