WATERSHEDS - Water Quality Engineering in Natural Systems - page 205

reasonable to assume that a part of the dirt origi-
nates from mechanical breakdown of larger litter
particles.
Vegetation. In residential areas, fallen leaves and
vegetation residues, including grass clippings, typi-
cally dominate street refuse composition during
the fall season. During defoliage, a mature tree
can produce 15-25 kg (35-55 lb) of organic leaf
residue, which contains significant amounts of
nutrients. Fallen leaves are about 90% organic and
contain 0.05-0.28% phosphorus. Leaf fallout in
urban areas and its wash-off into storm sewers is
a significant source of biodegradable organics.
Urban Animals and Birds. Fecal matter of urban
animals, including pets and birds, is a significant
source of bacterial contamination in urban runoff.
Fecal coliforms in urban runoff have been found
at levels greater than 10 6 CFU/100 mL (Banner-
man et al., 1993), however, more typical values are
on the order of 20,000 CFU/100 mL; these levels
tend to be higher in the warm months and lower
in the cold months (U.S. Environmental Protec-
tion Agency [USEPA], 1983a). Although these
fecal coliform levels are high, this indicator may
not be useful in identifying health risks in urban
runoff.
Trafic. Motor vehicle traffic is directly responsible
for deposition of substantial amounts of pollut-
ants, including toxic hydrocarbons, metals, asbes-
tos, and oils. In addition to exhaust emissions, tire
wear, solids carried on tires and vehicle bodies,
wear and breakdown of parts, and loss of lubrica-
tion fluids add to the pollutant inputs contributed
by traffic. Vehicular loss of oil on roads and parking
lots has been found to be a major source of
polyaromatic hydrocarbons (PAhs). Regulatory
actions to control exhaust emissions and vehicle
wear, mandatory vehicle emission testing, and
improved fuel additives all contribute to per-
vehicle reduction in traffic-related pollution.
however, increased traffic volume causes increased
pollution, particulary of heavy metals.
Other Sources of Toxic Chemicals. Urban runoff is
the major source of toxic pollutants, such as toxic
metals and organic toxic compounds, such as
PAhs and pesticides. The primary sources of these
compounds are summarized in Table 6.1.
Nutrients from Fertilizers. Excessive use of lawn fer-
tilizers can be a significant source of phosphorus
and nitrogen in runoff from landscaped urban sur-
faces. Runoff from areas with a large percentage
of land cover in lawns can contribute significantly
to phosphorus levels in receiving waters.

Pesticides from Lawns and Golf Courses. Many pes-
ticides applied in urban areas have contaminated
surface and groundwaters, and in many cases, turf
is the major source of these water-quality impacts.
Deicing Chemicals. The application of deicing chem-
icals and abrasives to provide safe driving condi-
tions during winter is practiced in the snowbelt
areas of the United States, all of Canada, and in
many European countries. In the United States,
road deicing salts are applied at rates of 75-330 kg/
km (270-1200 lb/mi) of highway (street) lane.
Typical road salt is 96-98% sodium chloride. Some
applications use calcium chloride (CaCl 2 ) at lower
temperatures, either as a liquid or as a dry mixture
with salt (NaCl). Water containing calcium chlo-
ride has a lower freezing point than water contain-
ing salt and is often applied when temperatures
range from −25°C to 0°C. Mixtures of sand and
other abrasives with road salt in various propor-
tions have been used in many communities and by
some state highway departments. Abrasives can
clog urban storm drains and roadside swales and
generate significant cleanup costs in urban areas.
Airport Deicing. Most aircraft anti-icing and deicing
chemicals are based on formulations of either eth-
ylene or propylene glycol. Glycols themselves are
not acutely toxic; however, deicing and anti-icing
mixtures have been found to have significant
chronic toxicity. Airport runoff containing high
concentrations of glycols can be toxic to animals
if they drink it (pets and animals might like the
taste). Glycols are biodegradable in soils and
aquatic environments and have very high BOD.
Consequently, runoff containing deicing chemicals
poses a great hazard to the oxygen levels in receiv-
ing waters and must generally be treated before
discharge. Five-day BOD in snowmelt water from
airports may be as high as 22,000 mg/L, and at an
application rate for large aircraft of 1000 L
(260 gal) per aircraft, the BOD load from a single
application is equivalent to the sewage BOD daily
load from about 10,000 people.
Erosion. Urban erosion can be divided into surface
erosion of pervious surfaces and channel erosion.
Surface erosion is driven by the energy of rainfall
and overland flow, while channel erosion is related
to the channel flow rates. Construction site erosion
is most severe and can be responsible for the
major part of the sediment load in urban and sub-
urban streams. Although the sediment load from
construction sites is only a fraction of the total
erosion load, it can be most devastating to urban
streams that drain small watersheds that are also