Encyclopedia of Environmental Science and Engineering, Volume I and II

MODELING OF ESTUARINE WATER QUALITY 721

While as a general rule the modeling of the hydrody-

namic transport of a constituent in an estuary is much fur-

ther advanced than the modeling of its reaction kinetics,

the most commonly unsatisfactory aspect of present water

quality models is the specification of the source and sink

terms. Many of the physical-chemical processes affecting

the concentration of parameters lack adequate formulation.

These include sedimentation and deposits of particulate

matter, non-linear reaction kinetics, surface exchange of

gaseous constituents, and chemical and biological reactions.

Modeling of the relation of water quality and estuarine biota

is not well advanced. Models of phytoplankton production,

of nitrogen cycling, and of gross ecological parameters have

been attempted with limited success.

Sources

Sources for the materials which are found in the waters of

estuaries include two major sources, river inflow and ocean

water inflow. The concentrations (or ranges) of selected

chemical constituents of fresh and ocean waters are given in

Table 1. Fresh waters may have large ranges of concentra-

tions of the lands which they drain. These ranges are quite

different from those of oceanic waters. In fresh waters, cal-

cium is usually the most abundant cation and sulfate is the

most abundant anion although carbonate may also be quite

high in concentration. In sea water, on the other hand, chlo-

ride is the most abundant constituent and anion followed by

sulfate and bicarbonate. Sodium and magnesium constitute

the majority of the cations. Depending on the relative balance

of river inflow and the incursion of seawater brought in by

tidal action, the quality of the water in the estuary assumes a

composition in proportion to the two sources. However, the

location of constituents from the various sources either later-

ally in the estuary or vertically in the water column is highly

dependent on the circulation patterns existing in the estuary

which were discussed earlier.

Although in relation to river and tidal flows, direct pre-

cipitation is a small hydraulic input to an estuary, its water

quality cannot be ignored. In shallow bays with little river

inflow and a restricted opening to the ocean such as bar-built

estuaries, rainfall directly on the estuary may be an impor-

tant source of fresh water.

Waste discharges may exert a dominant influence on

the water quality of estuaries depending on the amount of

material discharged and its character. Because urbaniza-

tion typically occurs around estuaries, waste discharges

are usually directed to the estuaries since they are the most

convenient waste disposal site. Domestic wastes, wastes

derived from municipalities and ultimately humans, con-

tains large amounts of organic and nutrient (nitrogen,

phosphorus, trace) materials. Some typical concentration

values are given in Table 2. The relative amounts of mate-

rial discharged to estuaries or other bodies of water may be

estimated by knowing the population served by a sewerage

system and mass discharge coefficients. These coefficients

indicate the amount of material discharged per person per

day. Such coefficients are also given in Table 2.

Industrial wastes also reach estuaries either as a direct

discharge to the estuary, as spills from vessels carrying mate-

rials to or from the industries, as the result of dredging activ-

ities, as the discharge of heated effluents from power plants

and heated effluents from nuclear power plants which also

carry radioactive materials, and in other forms. Most indus-

trial activities involve the use of and/or the disposal of water.

Such waters usually contain the by-products of the industrial

process and are characteristic of the process. For example in

manufacturing steel, a certain amount of water is required

for cooling and washing purposes. The amount of water used

to produce a ton of steel by a given process is fairly consis-

tent and the quality of the water resulting from the process is

also fairly consistent. Table 3 lists various types of industrial

activities, the amount of water used in the activity, and the

pounds of oxygen required to oxidize the organic material

in the wastewater as well as the pounds of suspended solids

produced in making some unit amount of product.

Another source of waste material is urban and rural

runoff. Urban runoff may consist of storm water runoff from

the streets and gutters which is routed to the nearest water-

way by storm water pipes, or it may consist of a mixture of

storm water runoff and sanitary sewage in what is called a

combined sewer system. Such systems are typical of older

cities in the United States and other countries which built one

pipe to carry both sanitary wastes and storm water wastes.

TABLE 1

Quality of fresh and ocean water

(Concentration units are mg/L)

Constituent Fresh Ocean

Chloride 1.0–1,000 18,980

Sodium 1.0–1,000 10,560

Sulfate 1.0–1,000 2,560

Magnesium 1.0–1,000 1,272

Calcium 1.0–1,000 400

Potassium 0.01–10.0 —

Bicarbonate 1.0–1,000 142

Carbonate 0.01–10.0 —

Bromide 0.0001–0.1 65

Strontium 0.01–10.0 65

Boron 0.01–10.0 4.6

Fluoride 0.01–10.0 1.4

Aluminum 0.0001–0.1 0.16–1.9

Iodide 0.0001–0.1 0.05

Silicate 1.0–1,000 0.04–8.6

Nitrogen 0.01–10.0 0.03–0.9

Zinc 0.0001–0.10 0.005–0.014

Lead 0.0001–0.1 0.004–0.005

Iron 0.01–10.0 0.002–0.02

Phosphorus 0.0001–0.1 0.001–0.10

Mercury — 0.0003

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