Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 147

4.2 ORGANIC CHEMICALS

On the earth, there are almost 4 million organic chemicals with names given and
catalogued by the International Union of Pure and Applied Chemistry (IUPAC).
Each year approximately 1000 new organic chemicals are synthesized and used
commercially. The vast majority of these compounds break down in the environment,
and only a fraction prove to be toxic and/or carcinogenic; some are known to be
persistent and bioaccumulating. In toxicology, any toxic effect is related to two
factors: the exposure dose and the susceptibility of the organism to the specific
chemical being considered. The dose received by an organism is influenced by the
available concentration of the contaminant in the environment and by the length of
exposure. Therefore, for some compounds, a long-term exposure to a low available
concentration may cause a toxic effect. 104 Uptake of pollutants by biota within the
water column can result in bioaccumulation and transfer of the pollutants into the
food chain.
To determine the exposure concentrations and doses of chemical compounds
to living organisms, it is often helpful to use mathematical models to estimate the
fate of the chemicals and their transport in the aquatic environment. Models can
also be used to perform waste load allocations to meet water quality standards.
According to the Organization for Economic Cooperation and Development
(OECD), there are about 70,000 synthetic chemicals (mostly organic) in daily use.
Numerous compounds are continuously introduced into the environment in large
quantities, e.g., solvents, components of detergents, dyes and varnishes, additives
in plastics and textiles, chemicals used for construction, antifouling agents, and
pesticides. 105
In order to assess the risk and rate of pollution of coastal waters, e.g., lagoons,
attention must be given to the input of pollutants, their transport and transformations,
the transfer of pollutants between the water and sediment, and the effects of the
different pollutants on the aquatic organisms.

4.2.1 S OURCES OF O RGANIC C HEMICALS

There is widespread environmental contamination by organic chemicals from both
natural sources and human activities. The main sources of organic chemicals in water
are the degradation of naturally found organic substances, reactions occurring during
drinking water and wastewater treatment and transmission, and land-based sources
(point and nonpoint) of pollutants of domestic, industrial, and agricultural origin
through atmospheric deposition, silviculture, leaching, etc. Coastal lagoons can
receive some or all of these sources.
The most abundant organic chemicals are humic substances, which are biodegraded
products of natural organic matter, humus. Humus is rich in nutrients and remains an
important food supply for microorganisms for long periods.
Organic compounds formed during water treatment consist of disinfection
by-products, such as trihalomethane and chloramines. Other chemicals, e.g., acryl-
amide, are coagulants used in water treatment. In addition, compounds, such as