Environmental Engineering Reference
In-Depth Information
9
OCEANS AND ESTUARIES
9.1
INTRODUCTION
deltas of large rivers is minimal and the freshwater
impact extends out into the sea. Whether the conflu-
ence of a river with a sea is called an estuary or a delta
depends on the flow and amount of sediment carried
by the river.
Beaches, coastal waters, estuaries, and oceans are used
for a variety of commercial and recreational activities.
Pollutants frequently found in these areas include bac-
teria, excess nutrients, and algal growth, with the primary
sources of pollution being urban runoff and disposal of
domestic wastewater. Pathogen contamination by
sewage discharges can result in the closure of shellfish-
ing areas and bathing beaches, and high nutrient levels
from sewage discharges can result in algae blooms that
consume oxygen and attract predators, such as sea
urchins and crown-of-thorn sea stars, which can destroy
living coral.
An
estuary
is a semiclosed coastal body of water
having a free connection to the open ocean and con-
taining a measurable quantity of seawater. Estuaries
vary greatly in size and shape and are sometimes called
bays, lagoons, harbors, inlets, or sounds, although not
all water bodies by those names are estuaries. Estuaries
are commonly found at the lower reaches of rivers,
where ocean tides and river flows interact. A typical
estuary is illustrated in Figure 9.1, where the view is
toward the open ocean. Some familiar estuaries in the
United States include San Francisco Bay, Puget Sound,
Chesapeake Bay, and Tampa Bay. Estuaries are typi-
cally the sink for all pollution activities that take place
in their contributing watershed, and runoff from urban
areas is typically the leading cause of impairments to
estuaries. River deltas, such as the Mississippi River
delta and the Nile River delta, are not normally con-
sidered as estuaries since saltwater intrusion in the
9.2 OCEAN OUTFALL DISCHARGES
Ocean outfalls are used by many coastal communities
to discharge treated domestic wastewater into open-
ocean waters. In some cases, treated domestic wastewa-
ter is blended with wastewater from other sources prior
to discharge. The wastewater discharged by an ocean
outfall experiences rapid mixing in the immediate vicin-
ity of the outfall, with dilution resulting from the entrain-
ment of ambient seawater as the buoyant freshwater
plume rises in the denser saltwater environment. As
ambient seawater is entrained, the effluent plume
becomes denser, rising until the plume density equals
the density of the ambient seawater. If the ocean is
(density) stratified, there is the possibility that the
density of the wastewater plume and surrounding sea-
water will equilibrate prior to the plume reaching the
surface, in which case the plume will be “trapped” below
the surface. Conversely, if the ocean is unstratified, the
density of the freshwater plume can never equal the
density of the surrounding ocean water (no matter how
much of the ocean water is entrained), and the plume
reaches the ocean surface, possibly forming a noticeable
boil
. Trapping of wastewater plumes beneath the ocean
surface is desirable because contaminants that make it
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