Water Pollutants and Effects Questions and Answers

Answer: D

Eutrophication is caused by agricultural run off and domestic sewage.

Eutrophication is the enrichment of lakes and slow rivers with nutrients, chiefly nitrogen and phosphorus. Fertiliser washed off farms and sewage carrying detergents and human waste supply these nutrients, causing algal blooms. When the algae die and decay, bacteria use up the dissolved oxygen, and fish and bottom life suffer.

Transport, industrial and biomass burning emissions are air pollutants. They can add some nitrogen through deposition, but the main and direct causes of eutrophication are the nutrient-rich runoff and sewage reaching the water.

Answer: A

Fluoride in water mainly affects the teeth.

Small amounts of fluoride (around 1 mg/L) protect teeth against decay, but above about 1.5 mg/L, the BIS limit for drinking water, it causes dental fluorosis: mottled, chalky or brown-stained enamel. Long exposure to higher levels causes skeletal fluorosis, with stiff joints and bone deformities, a problem in parts of Rajasthan, Andhra Pradesh and other states.

The brain, muscles and kidneys are not the main targets. Kidney damage is more typical of cadmium, and nervous system damage of lead and mercury.

Answer: A

Blue baby syndrome is caused by nitrate.

Infants given water or formula made with water high in nitrate convert it to nitrite in the gut. Nitrite oxidises the iron of haemoglobin to form methaemoglobin, which cannot carry oxygen, so the baby's skin turns bluish: methaemoglobinaemia. Infants under six months are most at risk. Nitrate from fertilisers and sewage in well water is the usual source, and the drinking water limit (45 mg/L as nitrate in India, 50 mg/L by WHO) is set to prevent it.

Sulphate in excess has a laxative effect, while calcium and potassium are essential minerals that do not cause this condition.

Answer: D

Sewage-polluted water shows a high concentration of nutrients and excess growth of algae.

Sewage carries large amounts of nitrogen and phosphorus. These nutrients fertilise the water and cause algal blooms (eutrophication). When the algae die, bacteria decompose them and use up dissolved oxygen, so oxygen falls and fish may die.

The options pairing a low nutrient level with excess algae, or high nutrients with low algal growth, break the link between nutrients and algae. Low algal growth with less dissolved oxygen gets the oxygen right but not the algae, since sewage pollution stimulates algal growth rather than suppressing it.

Answer: B

Contaminated water spreads typhoid.

Typhoid is caused by Salmonella Typhi, which is shed in the faeces of infected people and carriers and is spread when sewage contaminates drinking water or food: the faecal-oral route. Safe water supply and sanitation are the main means of controlling it.

Tuberculosis and diphtheria are bacterial infections spread through the air by coughing and sneezing. Tetanus is caused by spores of Clostridium tetani from soil entering a wound; it is not spread by drinking water.

Answer: D

The answer is All of the above.

All three are turbidity units. The JTU (Jackson Turbidity Unit) comes from the old Jackson candle turbidimeter. The FTU (Formazin Turbidity Unit) is based on a formazin polymer standard. The NTU (Nephelometric Turbidity Unit) is measured with a nephelometer that reads scattered light at 90°, and is the unit used by the BIS and WHO drinking-water standards today.

For formazin standards NTU and FTU give the same numbers, so choosing any single one would leave out units that are equally valid for turbidity.

Answer: B

Excess nitrate causes blue baby syndrome in infants.

In an infant's gut, nitrate is reduced to nitrite, which oxidises haemoglobin to methaemoglobin. Methaemoglobin cannot carry oxygen, so the baby turns bluish (methaemoglobinaemia). This is why the BIS limit for nitrate in drinking water is 45 mg/L.

Itai-itai disease was cadmium poisoning in Japan, fluorosis comes from excess fluoride, and nerve damage is typical of lead or mercury exposure, not of nitrate.

Answer: C

Organically polluted water shows low D.O. and high BOD.

Organic waste, such as sewage, is food for bacteria. They consume dissolved oxygen as they decompose it, so the biochemical oxygen demand (the oxygen needed to break down the organic load) is high, and the dissolved oxygen left in the water falls.

Low D.O. with low BOD would point to some other oxygen sink, not organic pollution. High D.O. with high BOD cannot last, because the high demand uses up the oxygen. D.O. and BOD being equal has no diagnostic meaning.

Answer: A

Such a water body is called hypoxic.

Hypoxic waters have dissolved oxygen well below saturation but not zero, commonly 1 to 30% saturation or below about 2 to 3 mg/L, which stresses fish and leads to 'dead zones'.

Anoxic water has no measurable dissolved oxygen, so the 1 to 30% range rules it out. Hyperoxic means supersaturated with oxygen, the opposite. 'Hyper anoxic' is not a standard term.

Answer: D

Coliforms indicate the presence of human faeces.

Coliform bacteria, and especially faecal coliforms such as Escherichia coli, live in large numbers in the intestines of humans and warm-blooded animals. Finding them in water shows faecal contamination, and so the possible presence of pathogens causing typhoid, cholera and dysentery.

They say nothing about radioactive waste or excess fertiliser. Decaying organic matter in general is measured as BOD, not by coliform counts.