Accumulationwashoff

Most urban watersheds are dominated by accumulation and wash-off processes, depending on impervious areas. The accumulation of solids on impervious urban surface areas is described by Sartor and Boyd (1972), as shown in Figure 9.20.2.

The sources of urban diffused pollution are:

• Litter, including large-sized materials (greater than 3.2 mm) containing items such as cans, broken glass, vegetable residues and pet waste. Pet fecal deposits can reach alarming proportions in urban centers where large numbers of people reside in highly impervious zones.

• Medium size deposits (street dirt) represent the bulk of street surface-accumulated pollution. The sources are numerous and very difficult to identify and control. They may include traffic, road deterioration, vegetation resides, pets and other animal waste and residues, and decomposed litter.

• Traffic emissions are responsible for potentially toxic pollutants found in urban runoff, including lead, chromium, asbestos, copper, hydrocarbons, phosphorous, and zinc. Pollution also comes from particles of rubber abraded from tires.

• Road deicing salts applied in winter cause highly increased concentrations of salts in urban runoff.

Road salts are applied at rates of 100-300 kg/km of highway and contain sodium and calcium chloride.

• Pesticides and fertilizers applied onto grassed urban lands.

In fully developed urban areas, where most land surfaces are impervious because of paving and rooftops, washoff of deposited particles and their transport to the watercourse become the important mechanism. The relationship of imperviousness to the quantity of some pollutants are shown in Table 9.20.2.

Table 9.20.3 shows values and ranges of accumulation of street and surface pollutants estimated by Ellis (1986). A list of specific nonpoint sources is presented in Table 9.20.4. The list is not exhaustive. The importance of the sources varies with local conditions.

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