Excess of which substance in drinking water causes fluorosis, a disease that mottles teeth and deforms bones?

The correct answer is B, cadmium pollution. Itai-Itai disease was first identified in Japan among people who ate rice grown with water polluted by cadmium from mining waste. Cadmium damages the kidneys and softens the bones, causing fractures and such severe pain that the Japanese name itai-itai means ouch-ouch. Option A is wrong because mercury pollution causes Minamata disease, also first reported in Japan, which attacks the nervous system and causes loss of coordination and speech. Option C is wrong because arsenic in groundwater causes arsenicosis, marked by skin lesions and pigmentation, a serious problem in parts of West Bengal and Bihar. Option D is wrong because excess nitrate in drinking water causes methaemoglobinemia, the blue baby syndrome, in infants. Learn these four pairs together, since examiners almost always place them in the same question.

The correct answer is B, mercury. Minamata disease takes its name from Minamata Bay in Japan, where a factory discharged mercury compounds that were converted by bacteria into methylmercury, concentrated in fish through biomagnification and then eaten by people. The poison attacks the central nervous system, producing numbness, loss of coordination, impaired speech and vision, and in severe cases death and birth defects. Option A is wrong because lead poisoning causes plumbism, damaging the nervous system and blood, but it is not called Minamata disease. Option C is wrong because cadmium causes Itai-Itai disease, a bone and kidney disorder. Option D is wrong because chromium, especially in its hexavalent form from tanneries and electroplating, causes skin ulcers and is carcinogenic, but again under a different name. Minamata is also the name of the international convention on mercury.

The correct answer is B, a heavy load of organic pollution. Biochemical oxygen demand measures the oxygen that micro-organisms need in order to break down the organic matter present in a sample. The more organic waste, such as sewage, the more oxygen the microbes consume, so a high BOD is a direct sign of heavy organic pollution. Option A is wrong because clean water has a low BOD; a river upstream of a city will show a much lower value than the same river below the sewage outfall. Option C is wrong because high BOD in fact goes with low dissolved oxygen, since the microbes are using the oxygen up; the two move in opposite directions. Option D is wrong because BOD depends on the activity of bacteria, so a high value indicates their presence and vigour rather than their absence. Chemical oxygen demand for the same sample is always higher than BOD.

The correct answer is B, nitrate. Nitrate from fertiliser run-off and from sewage seeps into groundwater; in an infant it is converted to nitrite, which binds to haemoglobin and forms methaemoglobin, a compound that cannot carry oxygen. The child becomes short of oxygen and the skin takes on a bluish tinge, which gives the condition its common name of blue baby syndrome. Option A is wrong because excess fluoride causes fluorosis, mottling the teeth and deforming the bones. Option C is wrong because iron in water chiefly causes taste, colour and staining problems, not this disorder. Option D is wrong because calcium makes water hard, which affects soap and boilers but does not produce methaemoglobinemia. The practical lesson is that agricultural run-off is a leading non-point source of water pollution.

The correct answer is B, eutrophication. When fertilisers, detergents and sewage add nitrates and phosphates to a lake or a slow-moving river, algae multiply into a bloom that covers the surface. Light is cut off from plants below, and when the bloom dies the decomposers use up the dissolved oxygen, so fish and other aquatic animals suffocate. Where human activity is the cause it is called cultural eutrophication. Option A is wrong because biomagnification is the increase in concentration of a persistent poison such as DDT at each higher level of a food chain, a different process altogether. Option C is wrong because salinisation is the build-up of salts in soil or water, usually through over-irrigation in dry regions. Option D is wrong because sedimentation is the settling of suspended solids, which is in fact a step in treating sewage rather than a form of nutrient pollution.