Understanding Water Pollution: Sources and Impacts
Organic Contamination
Organic pollution can cause anoxic conditions, to the extent that only chemotrophs (reducing anaerobic bacteria) can persist. During anaerobic biodegradation, organic acids, alcohols, aldehydes, methane, hydrogen sulfide (H2S), and ammonia are produced. Intermediate compounds include phenols, indoles, and mercaptans. This results in a very high chemical oxygen demand (COD), which must be offset by dissolving oxygen (O2) in water. Organic contamination stems from the lack of O2 and the liberation of ammonium, sulfides, and phenols.
Example: Deposit of iron sulfide (FeS2) under rocks at the bottom of a river heavily contaminated with organic waste from a biodegradable cellulose-producing plant.
Chemical Contamination
Chemical contamination is produced by unnatural compounds introduced into the aquatic environment by human activities. Its effect is unpredictable.
Detergents
Detergents contain surfactants (alkyl sulfates, alkyl sulfonates) that separate fat from the canvas, supported by polyphosphates, carbonates, and silicates, turning them into eutrophicating agents. Surfactants limit the dissolution of O2 at the air-water interface and affect fauna as weak toxins. In the presence of these, insects that alight on the water surface due to surface tension cease to have support and disappear.
Pesticides
This includes fungicides and insecticides. Up to 25 micrograms per liter (?g/L) of organophosphate insecticide in the water shall be deemed as non-toxic for oligochaetes, mollusks, crustaceans, and copepods. However, it kills larvae. The harmful effect of pesticides is rather acute. Their persistence is relatively short (50% degraded in 48 hours in soils). Their persistence in the environment is related to their bioaccumulation in living organisms. Such accumulations result from an incorporation greater than the capacity of biodegradation of the product.
Bioaccumulation
Chemical pollutants that are poorly biodegradable or non-degradable accumulate in the bodies of living beings affected by them.
Pesticides
Especially organochlorines (DDT, lindane, dieldrin, etc.). They dissolve in fat, so they accumulate in adipose tissue. In invertebrates, they are set in the digestive glands. They tend to be concentrated in top predators in the food chains (large carnivores, including humans). Bioaccumulation of DDT in American bald eagles between the 1950s and 1980s nearly extinguished the species. DDT interferes with calcium metabolism and the formation of the eggshell. The fragile eggshell broke easily, and incubation was not completed.
Heavy Metals
Particularly heavy metals like mercury (Hg), copper (Cu), lead (Pb), cadmium (Cd), zinc (Zn), cobalt (Co), and chromium (Cr) in the form of metal salts. They tend to set in organic sediments and clays. They are released under negative redox conditions.
Mercury (Hg)
Mercury forms stable organometallic compounds that inhibit enzymatic activity. Thus, HgCl2 is incorporated into organic sediments and processed into methylmercury (CH3HgCl) under anoxic conditions, which has an enormous capacity to penetrate tissues. A notable case is the pollution of Minamata Bay, Japan, in the late 1970s.
Acidification
Derived from atmospheric acidification by H2S and NO2 from various combustion sources (fossil fuels). These gases combine with rainwater and precipitate as acid rain, with a pH of 4.8 to 3.8 (as seen in Canada). Calcareous soils neutralize the acidity of rain. The same applies to marble sculptures and buildings, which are eroded and eaten away by acids transported by the rain! Ephemeroptera are the continent’s most sensitive aquatic organisms to acid rain, unlike Plecoptera and Trichoptera, which are more resistant. However, the diversity of Trichoptera, Coleoptera, and Diptera decreases from the pollution source.