Water Pollution and Its Control Questions and Answers

Answer: D

Such lakes are hypereutrophic.

Hypereutrophic lakes are the most nutrient-rich class. Dense, frequent algal blooms cut transparency to less than about 3 feet (under 1 m Secchi depth), and when the algae die and decay they use up the dissolved oxygen in deeper water, suffocating fish and other fauna.

Oligotrophic lakes are nutrient-poor and very clear, mesotrophic lakes are moderate, and eutrophic lakes are nutrient-rich with reduced clarity, but the extreme low transparency and oxygen loss described mark the hypereutrophic stage.

Answer: A

The decade is the International Decade for Action on Water for Sustainable Development.

The UN General Assembly proclaimed 2018-2028 as the International Decade for Action 'Water for Sustainable Development', running from World Water Day, 22 March 2018, to 22 March 2028, to speed up progress on water goals including SDG 6.

The 1981-1990 decade was the International Drinking Water Supply and Sanitation Decade, and 2005-2015 was 'Water for Life'. The other titles do not match the 2018-2028 decade.

Answer: C

When BOD becomes very high, the water becomes anaerobic.

Biological oxygen demand is the amount of dissolved oxygen that aerobic microbes consume while breaking down the organic matter in a water sample. A very high BOD means a heavy organic load, and microbial decomposition uses up the dissolved oxygen faster than it is replaced. The water becomes oxygen-depleted, fish die, and anaerobic decomposition takes over.

So dissolved oxygen falls rather than rises, and conditions turn against aerobic microbes. Microbial numbers are high, not low, when BOD is high, which rules out the last option.

Answer: D

Eutrophication leads to the death of fish by decreasing the dissolved oxygen content, option D: the excess nutrients cause algal blooms, and their decay uses up the oxygen.

Increasing oxygen would help fish, and decreasing algae would remove the cause of the problem, so options A and C are wrong.

The sequence in eutrophication is nutrient enrichment, an algal bloom, death and decay of the algae and consumption of dissolved oxygen by decomposers. The shortage of oxygen then kills fish. This is why increasing the oxygen is wrong, and decreasing the algae would stop the problem rather than cause it.

Answer: A

The bird affected by the oil spill at Kosasthalaiyar in December 2023 was the spot-billed pelican, option A.

Flamingo, macaw and pigeon are not the species named in the key.

After Cyclone Michaung in December 2023 an oil spill from a Chennai refinery spread in the Kosasthalaiyar river and the Ennore creek, and it affected birds of the wetland, such as the spot-billed pelican. The key names the pelican, and the other three birds are not associated with the incident.

Answer: A

The abundant growth of algae in water reservoirs is due to eutrophication, the enrichment of water by nutrients, option A.

Thermal pollution, bio magnification and radioactive pollution are different forms of harm.

Eutrophication is the enrichment of lakes and reservoirs by nutrients such as nitrates and phosphates from fertilisers and sewage, which causes rapid growth of algae. Thermal pollution is the heating of water, bio magnification is the build-up of toxins along the food chain, and radioactive pollution is by radiation.

Answer: A

Biological oxygen demand is the amount of dissolved oxygen needed by microbes (aerobic bacteria and other micro-organisms) to break down the organic matter present in water. A high BOD therefore signals heavy organic pollution.

Plants, animals and seaweeds do use dissolved oxygen, but BOD is defined by microbial decomposition of the waste, so they are not the answer.

BOD is measured in milligrams of oxygen per litre over five days at 20 degrees Celsius, and clean river water has a BOD of only a few mg/L, while untreated sewage runs to several hundred.

Answer: B

Eutrophication is caused by excess nutrients.

When a lake, pond or coastal water receives too much nitrogen and phosphorus, mainly from fertiliser run-off, sewage and detergents, algae and water plants grow explosively in an algal bloom. When they die, bacteria decomposing them use up the dissolved oxygen, killing fish and other life.

Excess oxygen is the opposite of what happens: eutrophic water ends up starved of oxygen. Salinity and temperature affect aquatic life, but they are not what drives eutrophication.

Answer: C

Algal bloom results in eutrophication.

When lakes and ponds receive excess nutrients, especially nitrates and phosphates from fertilisers and sewage, algae multiply rapidly into a bloom. When the algae die, their decomposition by bacteria uses up the dissolved oxygen, so fish and other aquatic life suffocate. This nutrient-driven deterioration of a water body is called eutrophication.

Global warming is caused by greenhouse gases, salinization is the build-up of salts in soil, and biomagnification is the increase of toxic substances along a food chain.

Answer: B

Eutrophication is caused by Nutrient enrichment.

When nitrates and phosphates from fertiliser run-off, sewage and detergents enter a lake or pond, they feed dense blooms of algae. When the algae die, their decomposition by bacteria uses up the dissolved oxygen, and fish and other animals suffocate.

Sand and silt cause siltation, heavy metals cause toxicity and biomagnification, and radioactive pollution causes radiation damage. None of them over-fertilises the water.