INTRODUCTION - Water Quality Engineering in Natural Systems - page 5

much as possible, since the direct pollution of streams
by pathogens is a likely and undesirable consequence.

capping the landfill with a low permeability cover and
installing a leachate collection system underneath the
landfill. Many older landfills do not have leachate col-
lection systems.

1.2.2.3 Urban Runoff. urban runoff contains con-
taminants that are washed from pavement surfaces and
carried to surface water bodies. Contaminants contained
in urban runoff include petroleum products, heavy
metals such as cadmium and lead from automobiles, salt
and other deicing compounds, and silt and sediment
from land erosion and wear on road and sidewalk sur-
faces. Bacterial contamination from human and animal
sources is also often present. The initial “flushing” of
contaminants during storm events typically creates an
initial peak in contaminant concentration in the surface
runoff, with diminishing concentration as pollutants are
washed away.
A major factor associated with the impairment of
receiving waters is the amount of impervious area that
is directly connected to urban runoff systems. An
example of directly connected impervious area is shown
in Figure 1.5, where the impervious area in the fore-
ground also surrounds the stormwater inlet, and so
runoff from the impervious area flows directly into the
inlet, without flowing over any pervious area. The pervi-
ous (vegetated) area contributing to the inlet is shown
in the background. Stormwater inlets, such as the one
shown in Figure 1.5, typically discharge collected storm-
water directly into a receiving stream or other drainage
pathway. A typical rule of thumb is that receiving-stream
degradation can occur when the contributing watershed
is more than 10% impervious, and degradation is
unavoidable when the contributing watershed is more
than 30% impervious.

1.2.2.5 Recreational Activities. recreational activi-
ties, such as swimming, boating, and camping, can have
a significant impact on water quality. The impact of
human activities has typically been reported in terms of
increased levels of pathogenic microorganisms.

1.3 CONTROL OF WATER POLLUTION

Polluted water is defined as water that does not meet the
water-quality criteria or standards associated with its
use. Control of water pollution ultimately requires that
the level of pollutants introduced from point and non-
point sources be controlled such that the receiving
waters meet their applicable water-quality criteria or
standards. Pollutants of concern vary depending on the
type of water body, its designated use, and local circum-
stances. For rivers and streams, the most common water-
quality problems are high pathogen concentrations,
siltation, habitat alteration, oxygen depletion caused by
excessive levels of biodegradable organics or nutrients,
and heavy metals that have the potential to bioaccumu-
late in fish and other aquatic life. In lakes and reservoirs,
low oxygen levels exacerbated by high nutrient levels is
the most common water-quality problem. In groundwa-
ter, contamination by carcinogenic organic substances
originating from above-ground spills and poor handling
practices of hazardous substances, as well as pathogenic
viruses originating from septic tanks, are common water-
quality problems.
The types of water-quality concerns expected in any
particular situation usually depends on the type of water
body, since the dominant fate and transport processes
can vary significantly between types of water bodies. For
example, rivers are fast moving and most commonly the
recipients of uncontrolled surface runoff and wastewater
discharges; lakes are slow moving, deep, and prone to
retaining nutrients and other anthropogenic contami-
nants; and groundwater is typically a pristine slow-moving
and direct source of drinking water that is prone to con-
tamination from surface spills of hazardous substances
that interact with the subsurface solid matrix in unique
ways. Given these fundamental differences between the
fate and transport of pollutants in different types of water
bodies, the approach to pollution control is significantly
influenced by the type of water body. As a consequence,
the dominant fate and transport processes in rivers,
groundwater, lakes and reservoirs, and coastal waters are
covered separately in different chapters of this topic.

1.2.2.4 Landfills. leachate from landfills can be a
source of contamination, particularly for groundwater.
Water percolating through a landfill (leachate) contains
many toxic constituents and is typically controlled by

Figure 1.5. Directly connected impervious area.