MODELING OF ESTUARINE WATER QUALITY 715
This use context has been supported in subsequent leg-
islation, particularly the 1972 Water Pollution Control Act,
which required that water quality criteria be updated periodi-
cally by the U.S. Environmental Protection Agency as well
as by the states.EstuariesThe estuary is one of those bodies of water which is the
focus of intense competing uses. Estuaries comprise one of
the most important resources of any country for the support
of such uses as navigation, recreation, nursery and resting
grounds for waterfowl and wildlife, nursery and spawning
areas for fish and shellfish, and particularly sites for urban
growth and the consequences or byproducts of urban growth.
It is estimated that about 75% of the entire population of the
U.S. lives within 50 miles of the nation’s coasts (USEPA
1987), and such a large urban population presents heavy use
pressures on coastal areas, particularly estuaries.
Estuaries are semi-enclosed coastal bodies of water having
a free connection with the open sea and within which the sea
water is measurably diluted with fresh water derived from land
drainage (Pritchard, 1967). Along the coasts of the United
States alone some 45,832 square miles of estuarine waters
exist. Of this total, 17,058 square miles are found along the
Atlantic Coast, while along the Pacific Coast south of Alaska
but including the Pacific Islands some 14,353 square miles
exist, 2760 square miles are found along the Coast of Alaska,
and 11,661 square miles are found in the Gulf of Mexico and
Caribbean Islands (National Estuarine Study, 1971). Of the
total, less than 30% is water less than six feet deep, vulnerable
to filling, as well as especially productive of fish, shellfish
and wildlife. At least 6.8% of the latter have been obliterated
through filling, most in the last 50 years (Stroud, 1971).
Development of estuary shorelines indicate some of the
uses of the estuaries. Of the 89,571 statute miles of tidal
shoreline in the United States estuaries, some 17,853 miles
can be described as recreation shoreline, that is, accessible
and useful for recreational pursuits. Of this shoreline, 16,559
miles are privately owned and 1,294 miles are publicly
owned; however, only 770 miles may be considered rec-
reation areas. Marine transportation terminal facilities are
users of a portion of the shoreline estuaries. In 1966, there
were 1,626 marine terminals providing deep water berths in
132 ports on the Atlantic, Gulf, and Pacific Coasts. Industries
use estuarine waters for cooling and return a heated effluent.
Industries and cities use estuaries as disposal sites for their
wastes. With a third of the United States population located
in the estuarine zone, the impact of man on estuaries must
necessarily be quite high (National Estuarine Study, 1971).
Biological uses of estuaries are also quite high. It has
been estimated that nearly 63% of the commercial catch on
the Atlantic Coast is made up of species of fish believed to be
estuarine dependent. Assuming that this applies equally to the
combined catches by foreign nationals as to the US domestic
catch, the fisheries yield to the US Atlantic continental shelf
and present levels of development of the fishery is equivalent
to about 535 pounds per acre of estuaries (McHugh, 1966).Similar but somewhat smaller estimates have been made for
the Gulf of Mexico estuaries based on commercial catches
in the Gulf of Mexico and for the Chesapeake Bay estuary
based on catches within the estuary itself (McHugh, 1967).Factors Influencing Estuarine Water QualityWhat are the factors that control the quality of waters in estu-
aries? The predominant factors are the hydraulic (transport)
characteristics of the estuary, the inputs or sources of materi-
als which make up elements of the quality of the water, and
the sinks present in estuaries—those physical, chemical and
biological phenomena which cause materials in the water to
change in concentration or to be altered chemically to a dif-
ferent form than when originally introduced.
The hydraulic regime of an estuary is dependent upon
three particular factors: the physiography of the bay—its
size, area in relation to volume, depth, and shoreline devel-
opment; the amount and seasonability of river inflow to the
estuary; and the wind and tidal mixing which takes place in
the estuary on each tidal excursion. The latter factor is depen-
dent upon the tidal range, the configuration of the entrance
to the estuary, the volume of the river inflow and the peri-
odicity of the tides. The impact of sources of material to an
estuary are dependent upon the character and amount of the
material and the location in the estuary where the material
enters. Materials which enter with the river inflow will very
likely reach broad areas in the estuary due to mixing within
the estuary and the fact that the material will pass through the
estuary on its way to the ocean. On the other hand, material
discharged near the mouth of the estuary will travel only a
short distance into the estuary and most likely be transported
out of the estuary rather quickly. This generalization does not
apply to all estuaries, particularly those which are strongly
stratified. This type of estuary will be discussed in more detail
later on. The size of the sinks for materials in estuaries is
dependent on the conservative or non-conservative nature of
the material, that is, whether the material can be broken down
into by-products or whether it remains in essentially the same
form throughout its history within the estuary. Conservative
and nonconservative materials may both be removed from
the water column due to flocculation or sedimentation within
the estuary, in which case materials may become part of the
bottom sediments and lost from the water column unless the
sediments are disturbed.
Because of the intimate tie between water quality and
estuarine hydraulics, they will be examined below as well
as the sources and sinks for materials within estuaries, both
natural and man-made materials, before discussing water
quality-estuarine use interactions.ESTUARINE HYDRAULICSA spectrum of hydraulic types may occur or exist in an estu-
ary. These may range from the situation in an estuary in
which the river flow dominates to the estuary in which the
river flow is negligible and the hydraulic regime is dependentC013_006_r03.indd 715C013_006_r03.indd 715 11/18/2005 12:49:00 PM11/18/2005 12:49:00 PM