Chapter 11
Nonpoint Source
Water Pollution
As rain falls and strikes the ground a complex runoff process begins, carrying with
it dissolved and suspended material from the watershed into adjacent streams, lakes,
and estuaries. Even before people entered the picture, sediments transported from the
watershed would accumulate behind natural dams, along the inside curves of river
bends, and at the mouths of streams.
Now view the world as it has been since the dawn of humankind - a busy place
where human activities continue to influence our environment. For hundreds, and even
thousands of years, these activities have included farming, harvesting trees, construct-
ing buildings and roadways, mining and industrial production, and disposal of liquid
and solid wastes. These activities have led to disruptions in the watershed vegetation
and soils, increases in the amount of impervious surfaces (e.g., pavement and roads),
introduction of agricultural chemicals, fertilizers, and animal wastes into the watershed,
and the deposition of many types of atmospheric pollutants (e.g., hydrocarbons from
automobile exhaust) in the watershed. The combination of these types of pollutants
from diffuse, widespread sources is generally called nonpoint source pollution.
The kinds of nonpoint source pollutants entering a stream vary depending on
the type of human activity in the watershed (Table 11-1). Runoff from agricultural
regions typically contains elevated concentrations of suspended solids, dissolved salts
and nutrients from fertilizers, biodegradable organic matter, pesticides, and pathogens
from animal wastes. Activities that disrupt the vegetation cover or soil surface, such
as construction and silvaculture, will contribute suspended sediments and sediment-
bound nutrients such as phosphorus to surface runoff. Runoff from silvaculture sites
may also contain herbicides that were applied to control the growth of undesirable
plants. Urban runoff, one of the worst sources of nonpoint source pollution, often con-
tains high concentrations of suspended and dissolved solids; nutrients and pesticides
from landscaped areas; toxic metals, oil and grease, and hydrocarbons from roads;
pathogens from pet wastes and leaking septic tanks; and synthetic organics such as
detergents, degreasers, chemical solvents, and other compounds that accumulate on
impervious surfaces or are carelessly poured down storm drains.
Water movement is the prime mode of transport for nonpoint source pollutants,
whether they are dissolved in water or suspended in surface runoff. The concentrations
of soluble pollutants such as road deicing salts, acids from abandoned mine sites,233