Environmental Engineering Reference
In-Depth Information
2.6 UNDERSTANDING POLLUTANT TRANSPORT IN STORMWATER
Our current understanding of pollutant transport in stormwater is summarized
next.
Stormwater Quantity and Quality
Water resource qualities are inextricably linked and need to be managed together.
Although the most obvious impact of land development is the increased rate
and volume of surface runoff, the pollutants transported with this runoff com-
prise an equally if not more significant impact. In fact, the distinction between
water quality and water quantity is imposed and somewhat artificial. Manage-
ment strategies that address quantity will in most cases address quality , especially
infiltration BMPs.
As land development increases the rate and volume of stormwater runoff,
increased runoff scours the land surfaces, both impervious and pervious. The
kinetic energy of the raindrops and runoff suspends the solid pollutant particles
and transports them with the initial runoff, where the energy is greatest. The
pollutants that can dissolve in the rainfall become solutes and move in the runoff
in a more distributed concentration. The resulting turbid flow of runoff carries a
mix of pollutants, including sediment and organic detritus, nutrients (phosphorus
and nitrate), synthetic organic chemicals, and petroleum hydrocarbons. These
non-point source (NPS) pollutants generally are materials that are deposited on the
land surface. NPS loads are generated in higher quantities from impervious areas
that are often defined as “hot spots,” such as fueling islands, trash dumpsters,
industrial sites, fast-food parking lots, and heavily traveled roadways.
Many so-called “pervious” surfaces, such as chemically maintained lawns and
landscaped areas, also add significantly to the NPS load, especially where these
areas drain to impervious surfaces (such as gutters) with storm sewers that drain
directly to surface waters. The soil compaction process applied to many land
development sites results in a vegetated surface that is close to impervious in
many instances (Table 2-1) and produces far more runoff than does the original
soil mantle. In fact, compaction turns natural soil into concrete. A new lawn
surface has probably also been heavily loaded with fertilizers, which results in
polluted runoff that degrades all downstream ponds and lakes.
Table 2-1 Increase in Soil Bulk Density with Compaction
Land Use
Soil Density (g/cm 3 )
Undisturbed land: forest and woodlands
1.03
Residential neighborhoods
1.69-1.97
Golf courses, parks, athletic fields
1.69-1.97
Concrete
2.2
Source: David B. Friedman, District, Director, Ocean Co. Soil Conservation District.
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