Encyclopedia of Environmental Science and Engineering, Volume I and II

WATER: PROPERTIES, STRUCTURE, AND OCCURRENCE IN NATURE 1303

about 10% of the total organic carbon. In much of the ocean

there is a “soluble” organic carbon concentration of around 500

micrograms C per liter, with the higher values occurring in the

surface concentration of some 50 m g C per liter, all ultimately

derived from a surface phytoplankton population equivalent to

a concentration of about 5 m g C per liter. Below a depth of 400

to 600 meters the concentration variations in organic carbon

become very slight. This suggests that materials present in

deep waters are resistant to further metabolic decomposition.

WATER QUALITY CRITERIA AND WATER

POLLUTION CONTROL

In a broad sense, pollution has been defined as an alteration of

man’s surroundings in such a way that they become unfavor-

able to him. This implies that pollution is not solely caused

by contaminants or pollutants added to the environment but

can also result from other direct or indirect consequences of

man’s action. Anything that produces a maladjustment of the

ecological balance constitutes pollution.

Every ecosystem has a rather delicate balance of inputs

and outputs. Ecological unbalance results, for example, when

the rate of extraction of an element (e.g., by mining) by man

exceeds the natural rates of cycling or when the output of a

product (e.g., crops or forests) is maximized.

Disturbance of the Balance between

Photosynthesis and Respiration

Pollution of surface waters frequently results from a disturbance

in the balance between photosynthesis ( P ) and respiration ( R ).

(Reactions (c) and (d)). As Figure 13 illustrates, this balance is

upset by adding either an excess of organic wastes or an excess

of inorganic algal nutrients (e.g., phosphorus and nitrogen

compounds). In the first case ( P! R ), heterotrophic processes

tend to dominate and dissolved oxygen may become exhausted

due to the biochemical oxygen demand and ultimately NO 3  ,

SO 4 2 , and CO 2 may become reduced to N 2 (g), NH 4 , HS^ ^ , and

CH 4 (g). In the second case, the incipient result is progressive

accumulation of autotrophic biomass (e.g., algae and aquatic

plants). Eventually this biomass decomposes enhancing the

rate of respiration, which again may reduce dissolved oxygen

levels. In either case the initial perturbation is followed by a

readjustment, such as a shift in biological species, that finally

leads to a new balance.

The P−R balance may also be distributed by a localized,

physical separation of the P and R functions. For example, in a

stratified lake an excessive production of algae and oxygen in

the upper layers may be paralleled by anaerobic conditions (i.e.,

lack of dissolved oxygen) at the bottom because much of the

photosynthetic oxygen escapes to the atmosphere and does not

become available to heterotrophs in the deeper water layers.

When P–R, the organic material is decomposed as fast as

it is produced photosynthetically. Such a balance,

Import org.material  P ; Export org.material  R,

is necessary to maintain a water in an aesthetically pleasing

condition.

The Pollutants

Much of our concern in the future should be with the fact that

most of the energy dissipation by our industrial society for agri-

culture, landscaping, urban construction and other interventions

of civilization ultimates causes a simplification of the ecosys-

tem: specifically, a reduction of the food web and a shorten-

ing of the food chains. Generally the less complex a natural

Lake River

R P P

R

Waste

Input

Distance

Depth

(a) (b)

FIGURE 13 Separation of photosynthesis and respiration in polluted lakes and rivers. The separation of photosynthesis and respiration

causes pollution by impeding the circulation of chemical constituents with consequent localized increases in photosynthetic or heterotro-

phic microbial populations. The dashed curves represent the net disturbance of the P−R balance and are indicative of the net rate of oxygen

production or consumption.

a) Vertical separation of the P−R functions in lakes can be induced by inorganic nutrients, chiefly nitrogen and phosphorus, which fertilize

surface algal growth. Upon settling, the algae stimulate bacterially mediated oxygen utilization and other redox reactions of lower pe. The

resultant anaerobiosis stifles higher trophic levels.

b) Longitudinal separation of the P−R functions in rivers can be caused by localized injection of organic waste, which stimulates increased

bacterial activity and concomitant oxygen consumption. The resultant mineralized nutrients enhance algal growth downstream which

upon decay exerts an oxygen demand still further along the river.

C023_004_r03.indd 1303C023_004_r03.indd 1303 11/18/2005 11:12:34 AM11/18/2005 11:12:34 AM