724 MODELING OF ESTUARINE WATER QUALITY
area, and for some systems on the stage of the tide when the
overflow occurs. The quality of such wastewater is given in
Table 4 as well as some mass discharge coefficients. A more
detailed discussion of this type of wastewater is given in
another part of this Encyclopedia (see Urban Runoff ).
Rural runoff, though less innocuous than urban runoff,
cannot be disregarded as a wastewater source. Mass dis-
charge coefficients relating to quality of the runoff water
to the land use and amount of rainfall are given in Table 5.
Miscellaneous waste discharges occur into estuaries which
cannot be quantified in the way done for other types of
waste discharges. These include such waste as oil spills,
spills of toxic or hazardous materials waste from houseboats
or larger vessels with quarters for crew for living purposes,
dredging spoils, heat from power or nuclear plants, and other
sources.
Another source of material in estuaries is biological recy-
cling. Although biological recycling may also be considered
as a sink and will be discussed as such later, recycling of
material is extremely important in transforming waste mate-
rials from man-made waste discharges (or waste discharges
from other lower animals or even dead organisms) to a chem-
ical form in which it may be used again by the biological
system. For example, organic material in a domestic waste
discharge is oxidized at least partially by bacteria to carbon
dioxide and water. Carbon dioxide is a necessary constitu-
ent for the growth of plants in conjunction with light. The
process of photosynthesis using light reduces carbon dioxide
in an organic material which is incorporated in the tissues of
the plant. Since plants comprise the basic food stuff for all
organisms, the recycling of carbon from the organic form to
the inorganic form as carbon dioxide is vitally important.
Similarly, nitrogen, which may be in an organic form in a
waste discharge, is oxidized by plants for growth. In both
cases the organic form acts as a sink while concurrently the
inorganic form becomes the source.SinksThere are several types of water quality sinks in estuaries.
Water withdrawal from an estuary is one type of sink; with-
drawals may be made for industrial uses such as salt produc-
tion or for cooling purposes. In the latter case, some water may
be returned to the estuary in a heated condition, but the water
withdrawn which is lost in the cooling process as evaporation
is lost to the estuary. Sedimentation becomes another sink as
molecules of material which were in the water column sorb
into the particulate matter which settles out and becomes part
of the bottom sediment. Unless the bottom sediment is dis-
turbed, eroded or dredged, this material is essentially lost to
the water column. Precipitation may also occur to tie up sev-
eral minerals which then settle out on the bottom and as with
sedimentation are lost from the water column. Precipitation
quite commonly occurs in estuaries which have little fresh
water inflow but a high evaporation rate. These estuaries are
known as hypersaline estuaries because the salinity content
rises to levels above that of normal sea water. In the Laguna
Madre Bay of Texas, salinity levels reach two or three timesthat of normal sea water. In this bay, crystals of gypsum are
found on the shores.
Another type of sink which is tied intimately to the bio-
logical system of the estuary is the degradation of materials,
that is, the change from one chemical form to another by
biological action. Above, the process of oxidation of organic
material was mentioned. This is one form of oxidation in
which organic material is oxidized to smaller molecular mate-
rial, which is further oxidized to carbon dioxide and water as
the ultimate inorganic products. Oxygen is consumed in the
process and is lost to the water. Another oxidation process is
nitrification. In this process, nitrogen in the organic form is
oxidized to ammonia which is then further oxidized by the
bacterium, Nitrosomonus to nitrite and further to nitrate by
the bacterium, Nitrobacter. Under low oxygen or anaerobic
conditions, the nitrate or nitrite may be reduced by bacteria
to elemental nitrogen gas which may then be removed from
the system according to the solubility of the nitrogen gas.QUALITY NEEDED FOR ESTUARY USE
SUSTENANCENow that some of the hydraulic, biological, and man-made
mechanisms which influence water quality in estuaries have
been described, the use concept for estuaries and the water
quality needed to sustain uses should be considered.
As an example, the San Francisco Bay-Delta Study
Report (1969) listed some of the uses which can be made of
estuaries, and these are given in Table 6. These uses are quite
varied and in some cases would be competing uses if applied
to the same part of an estuary. For combinations of uses for a
given area in an estuary, usually one use will require a water
quality higher than the other uses, and this use will dictate
the water quality needed in that particular area. Some of the
uses listed are not really dependent on water quality such as
shipping, unless the quality is particularly adverse (very acid
water or large floating material).
The general philosophy of estuary use in particular or
resource use in general is that a range of uses may exist
bounded by two extreme views. These views are: (1) a
resource may be used indiscriminately without regard to
the consequences (e.g., total consumption, contamination,
etc.) of that use; and (2) a resource should be preserved with
no interaction with man. Both views, despite being held
by some are recognized as incompatible, and the concept
of sustained use, the designation of some use or uses for
a resource area and management of the system to support
that use best, is favored. As mentioned at the beginning of
this article, the designation of uses for estuarine areas in
all states through public hearings was a recognition of the
sustained use concept. However, subsequent attempts to
manage waste inputs to estuarine systems and the estuarine
systems themselves have shown how difficult management
is. The difficulties lie in the technical aspects of manage-
ment (the interrelatedness of water quality with estuarine
circulation and sources and sinks), the economic aspects
(the cost of management systems directly and indirectly),C013_006_r03.indd 724C013_006_r03.indd 724 11/18/2005 12:49:02 PM11/18/2005 12:49:02 PM