8 Harmful Effects of Water Pollutants – Explained!

(I) Effect Of Sewage And BOD Problem:

The sewage and other organic matter discharged to a water body are degraded by oxygen-requiring micro-organisms.

The amount of oxygen consumed by the microbes is the Biochemical Oxygen Demand (BOD). BOD is an indication of the presence of sewage and other organic wastes.

After the addition of organic wastes, several zones may be distinguished in a flowing stream.

These are zone of degradation, zone of active decomposition, zone of recovery and unaffected zone. If the volume of incompletely digested sewage is small in relation to the volume of water into which it is released, the oxygen demand may be easily satisfied. But when the volume of sewage is greater than the carrying capacity of the water, the oxygen demand may remain unsatisfied for a long period of time creating low-oxygen (hypoxic) conditions. Thus high levels of BOD can deplete the oxygen in water.

Near the point of organic discharge, the bacterial population rapidly increases with active growth of sewage fungus (Fig. 6.9). Protozoa species that feed on bacteria increase in numbers, but the oxygen depletion causes a decrease of algae and clean water fauna. Fish being sensitive to DO concentration are often eliminated through mass fish deaths and only a few macro-invertebrates such as sludgeworms (Tubifex) and bloodworms (Chironomus) can exist in hypoxic water.

Effects of organic pollution in a lentic water body, as opposed to the flowing water environment, are modified by the morphometric features of the receiving water. Zonal changes that have been described for flowing water do exist, but may become compressed in great measure either laterally or vertically when the discharge is to a lake or estuary.

Such compression may tend to decrease the severity of pollution that is often observed in lotic water body and on the other hand, may increase substantially the development of algal blooms or rooted aquatic plants that may develop from the nutrients released with the introduced organic waste and decomposed flora.

(ii) Effects of Pesticides, Detergents and Synthetic Fertilizers:

Pesticides are synthetic chemicals used for pest control. A major source of pesticides in water bodies is the runoff from agricultural fields. Some pesticides also enter inland waters from industries, which use pesticides in their manufacturing processes, or from the manufacturing of pesticides themselves. Pesticides adversely affect a wide range of organisms including insects and fish. Pesticides accumulate in the tissues of aquatic organisms through bio-concentration and biological magnification and adversely affect their metabolic processes including reproduction.

Some pesticides kill not only the target organisms but they also adversely affect many non-target organisms. Hence the term ‘biocide’ is often used for them. In the environment, a biocide may be detoxified or its toxicity is changed in some way. A good example of the second possibility is DDT, which is quite toxic to many insects and relatively nontoxic to birds. DDT is metabolized by detritivores along two pathways, under un-aerobic condition to TDE (also called DDD) and under aerobic condition to DDE, as shown below (Fig. 6.10)These metabolites, DDD and DDE both are toxic. However, the biocidal activity of the metabolites is different from DDT. DDE is relatively nontoxic to insects, but it seriously affects female birds. It disrupts their calcium metabolism. As a result, the eggs laid by affected birds may have very thin shells (Peakall, 1974).

The birds most affected by DDT poisoning are the raptors, the carnivorous birds, since they are at the end of the food chain. Pesticides show the phenomenon of ‘food chain magnification’, by which species at the end of the food chain ingest more amount of the chemical than those at the beginning. Many other insecticides show similar patterns of biodegradation. By bacterial action, Aldrin is changed to Dieldrin, which is more toxic than the parent molecule.

Pesticides also affect non-target organisms. These effects may be mainly physiological and behavioural. Physiological effects may be manifested by death, sterility or some other dysfunction. The worst harm is done when the non-target organism is a beneficial insect. Pesticides enter the human body by a number of routes. Some insecticides remain in the body; they are stored in certain tissues (especially in adipose tissue) and cause pathological effects. Behavioural changes caused by pesticides may be too many.

For example, sub-lethal doses of DDT cause trout to forget most of their learned avoidance responses. Salmon exposed to sub-lethal dose of DDT become increasingly sensitive to cold water and may even be led to lay their eggs in abnormally warm water (Ogilive and Anderson, 1965). Mosquito fish exposed to relatively low concentrations of DDT tend to prefer waters that are more saline than usual for the species. Such behavioural changes might have quite serious impacts on the populations of desirable species.

Detergents are washing powders and materials. They are composed of complex phosphates which eventually breakdown into phosphates usable by aquatic plants. The use of detergents has been responsible for increase in the phosphorus in sewage effluents. Phosphate pollution of rivers and lakes causes extensive growth of algae, which depletes the DO content of water and disrupts the natural food chains.

Fertilizers containing nitrates and phosphates are used in modem agriculture. Some of these are washed off the agricultural fields through irrigation, rainfall and drainage into nearby rivers and ponds where they seriously disturb the aquatic ecosystem. Excessive use of fertilizers often leads to accumulation of nitrates in water. When such waters are used by animals, these nitrates are reduced to toxic nitrates by intestinal bacteria. Nitrates can cause a serious disease called methaeglobinemia, which can damage respiratory and vascular systems and even cause suffocation.

(iii) Effects of Hazardous Pollutants (Toxic Metals and Other Chemicals):

As is the case with toxic air pollutants, there are many potentially toxic water pollutants. Of these, inorganic pollutants include arsenic and the metals cadmium, mercury, lead, nickel, zinc and copper. These can kill or sicken fish and other aquatic animals. Organic pollutants under this category are benzene, toluene, chloroform, formaldehyde and many pesticides such as Parathion.

Benzene can irritate the skin and eyes, cause headache and dizziness and has been associated with leukemia and aplastic anaemia. Formaldehyde, which is released from furniture factories or pressed wood products, can irritate the eyes and lungs and at high doses is an animal carcinogen.

Exposure to chloroform can damage liver and kidneys and at high concentrations it is an animal carcinogen. Chloroform is released from sewage treatment plants and from pulp-bleaching facilities that use a chlorine-containing chemical. Cadmium, which is emitted by metal refining operations and incinerators, is a highly toxic metal that concentrates in shellfish, animal kidneys and liver and plants. Mercury, which is a volatile liquid metal, is especially toxic after conversion to methylmercury by bacteria. Methylmercury concentrates in animal tissues.

(iv) Effects of Pathogenic Micro-organisms:

Micro-organisms (bacteria, viruses and protozoa) are naturally found m water and elsewhere in the environment and can cause infections. They reach water bodies through point and nonpoint sources. The latter include runoff from livestock operations and storm water runoff. Poorly performing municipal sewage treatment plants are point sources of pathogenic micro-organisms.

Examples of communicable disease indicators and organisms are given in Table 6.6. Infectious micro- organisms are a tremendous threat when they occur in drinking water and cause waterborne diseases. In third world countries, up to 80% of infectious diseases are attributed to poor drinking water and according to WHO, more than 35 per cent of all deaths in these countries are directly related to contaminated drinking water.

(v) Effects of oil spills (Marine Pollution):

Oil and grease are also considered serious pollutants of water. Oil spills are a major problem in many coastal waters and can kill or adversely affect fish, phytoplankton and zooplankton, and birds and mammals. Oil spills have been reported from various parts of the world. Steiner (1994) has recently described the dreadful effects of the Alaskan oil spill, the Exxon Valdez that affected the entire biotic community including river otters, bald eagles, seals, sea lions, whales, and diving birds.

According to Hill (1997), oil spills can kill or reduce populations of organisms living in coastal sands and rocks, and may kill the insects and worms that series as food to many birds and other animals. When oil moves into coastal wetlands, it kills fish, shrimps, birds and other animals. An oil spill may also foul beaches used for swimming. However, spills are not the only source of oil in water. Oil leaking from vehicles or released during accidents is washed off roads with rain water. Improper disposal of used oil from cars is another source. This oil adversely affects soil and water organisms and green plants.

(vi) Effects of Acids Deposition:

Some water bodies may be naturally acidic, but others are made so by acidic deposition or acid runoff from mines. The sulphur in coal produces acids in the water, which is sometimes responsible for lowering the pH of streams. The lowered pH usually kills all the organisms of streams.

Acid rain or acid deposition, which lowers the pH of lakes and streams to 4 or 5, adversely affects fish, algae and other sensitive organisms. Acid rain also affects green plants; there is reduced rate of photosynthesis and growth and increased sensitivity to drought and disease. Acid deposition weakens frees, so they could be easily killed by drought and pathogens. Nutrients like nitrates may be leached from the soil by acid runoff waters. The activity of nitrogen-fixing bacteria is inhibited, thus reducing soil fertility.

(vii) Effects of Thermal Pollution:

Thermal pollution may be defined as “the warming up of an aquatic ecosystem to the point where desirable organisms are adversely affected” (Owen, 1985). A large number of industrial plants use cold water from the rivers and discharge it hot. Effect of thermal pollution depends on the temperature of the hot water released at a particular place (or point). In extreme cases it may cause direct mortality of fish and other desirable organisms. The lethal temperature for trout is above 25°C and for yellow perch 31°C.

The trout eggs will not develop in water having temperature above 14°C.The thermal pollution causes interference in spawning and reproduction of fish since all fish require an optimum temperature for breeding. It is apparent that because thermal pollution may raise stream temperature to about 32°C during summer, any Coldwater fish will have to leave the area to survive. It is well known that most freshwater fauna populations decline with rising temperatures and few species can exist in a water temperature of over 40°C. Above 30°C green algae and diatoms are reduced in numbers, but there is increased growth of blue-green algae and sewage fungus.

Thermal pollution causes reduction in DO content of water. For example, at a temperature of 0°C, water has DO content of about 14.5 ppm, but at 18°C it contains only 6.5 ppm. It is apparent that Coldwater fish, which require about 6 ppm to survive, could not tolerate the high water temperatures resulting from thermal pollution. If they remained in the area, they would die from oxygen starvation. Besides these effects thermal pollution causes increased vulnerability to disease and permits invasion of organisms that are tolerant to warm water and disturb ecological balance.

(viii) Effect of Eutrophication:

Eutrophication means the excessive growth of aquatic plants, both rooted and planktonic, to levels that are considered to be an interference with desirable water uses. Claphem Jr (1981) defines eutrophication as a natural phenomenon whereby the productivity of a lake increases from initial oligotrophic stage, through a moderately productive or mesotrophic stage, to a very fertile or eutrophic stage. This phenomenon is called natural eutrophication and leads to lake – aging in due course of time. When the process of eutrophication is speeded up by anthropogenic activity, it is called cultural eutrophication.

The increased production of aquatic plants has the following consequences regarding water uses:

(a) Large diurnal variations in DO can result in low levels of DO at night, which in turn can result in the death of desirable fish species.

(b) Phytoplankton and weeds may settle to the bottom of the water body and create a sediment oxygen demand (SOD), which results in low levels of DO in the hypolimnion of lakes and reservoirs as well as in the bottom waters of deeper estuaries.

(c) Lager diatoms and filamentous algae can clog water treatment plant filters.

(d) Toxic algae have sometimes been associated with eutrophication in coastal regions and have been a key factor in the occurrence of “red tide”, which may result in paralytic shellfish poisoning.

(e) Excessive growths of rooted aquatic macrophytes interfere with navigation, aeration and channel-carrying capacity.

(f) The algal blooms impair water quality by giving it a bad taste and odour.

The eutrophication of a given water body may vary depending on the degree of penetration of the solar radiation to different depths, the amount and type of nutrient inputs (Phosphorus, Nitrogen, Silica), the particulars of the water movement through flow transport and dispersion, and the geographical location of the water body. The principal external sources of nutrient inputs are municipal sewage and wastes, industrial wastes, agricultural runoff, forest runoff, and urban and suburban runoff.

The nutrients are present in several forms. However, all the forms are not readily available for uptake of the phytoplankton. For example, total phosphorus is present in two forms – dissolved and particulate. The dissolved form, in turn, is composed of several forms, one of which is the dissolved reactive phosphorus.

This form of phosphorus is used for phytoplankton growth. Total nitrogen is composed of four major components – organic, ammonia, nitrite and nitrate forms. The latter three forms make up the total inorganic nitrogen, which is utilized for phytoplankton growth. The organic form of nitrogen represents both the dissolved and the particulate component. The particulate form, in turn, is composed of organic detritus particles and the phytoplankton. The direct input of nutrients may be reduced by several means.

In many cases the process of eutrophication is speeded up by human activity; this is called cultural eutrophication and affects recreation, fisheries, water supply and public health. It is well known that about 80% of the nitrogen and 75% of the phosphorus added to the water bodies has its source in human activities. This human-generated nutrient input is derived from several sources, including domestic sewage, agricultural fertilizers, detergents, industrial wastes and livestock wastes.

When the average concentration of soluble inorganic nitrogen exceeds 0.30 ppm and the soluble inorganic phosphorus exceeds 0.01 ppm, algal blooms may appear. During die summer, the algal bloom problem usually becomes more acute with adverse effects on the whole biota of the lake.

Such cultural eutrophication has caused lake Erie to age 15000 years in only 25 years between 1950 and 1975 (Owen, 1985). Lake Mendota and lake Washington have also undergone rapid eutrophication as a result of human activities. In India, the ever going problem of cultural eutrophication has already reduced the recreational value of the Kashmir lakes. The Nainital lake is also undergoing rapid eutrophication, especially due to inputs of sewage and domestic wastes and detergent discharges (Nagdali and Gupta, 2002). Several studies have indicated that cultural eutrophication can be controlled if phosphate inputs are reduced.