The Village Borehole and the Nitrate Problem
A village of 900 people draws drinking water from a borehole 40 m deep. A nearby farmer keeps 60 cattle in a kraal 30 m from the borehole and spreads 200 kg of ammonium nitrate fertiliser on a maize field each season. Health workers record that 12 infants under six months old have developed a bluish tinge to the lips after being fed formula made with borehole water. A water sample shows nitrate concentration of 45 mg per litre, above the 10 mg per litre (as nitrogen) guideline. The borehole water is clear, odourless and has a pH of 7.2. Villagers argue the water 'looks clean' and refuse to walk 3 km to the alternative standpipe.
1. Explain, using chemical and physiological reasoning, how the nitrate reached the borehole water and why infants rather than adults are affected.
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Pathway of the nitrate: Ammonium nitrate fertiliser dissolves readily in rain water. Nitrate ions (NO3-) are negatively charged and are not held by the negatively charged clay and humus particles in soil, so they are leached downwards through the soil profile rather than being retained. Cattle urine and dung in the kraal are broken down by bacteria to nitrate in the same way. With the water table within reach of the borehole's 40 m depth, the leached nitrate enters the groundwater and is drawn up through the borehole.
Why the water still looks clean: Nitrate is colourless, odourless and fully soluble, so none of the visible or smell-based tests villagers use would detect it. Clarity is not a test for dissolved salts.
Why infants are affected: In the stomach, nitrate is reduced to nitrite. Nitrite oxidises haemoglobin (which normally carries oxygen as oxyhaemoglobin) to methaemoglobin, which cannot bind oxygen. Adults have enzyme systems that reduce methaemoglobin back to haemoglobin fairly quickly. Infants under six months have much less of this activity and a higher proportion of fetal haemoglobin, which is more easily oxidised. Their blood therefore accumulates methaemoglobin, oxygen transport falls, and the skin and lips appear blue - the symptom described. Formula made with the water gives a concentrated dose relative to the infant's small body mass.
Conclusion: The 45 mg/L reading is a real hazard, and the infants' symptoms match the expected effect of nitrite on haemoglobin.
2. A village committee proposes three actions: (i) fence off the kraal and move it 200 m away, (ii) boil all drinking water, (iii) buy bottled water for the 12 affected infants. Judge which combination is most effective and justify your ranking.
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Boiling (ii) is useless for this hazard. Boiling kills bacteria but does not remove nitrate; it actually concentrates it slightly as water evaporates. It addresses the wrong type of contaminant, so it should not be relied on.
Fencing and relocating the kraal (i) is the best long-term measure. It removes a major source at the point of entry, reducing future leaching. However, it acts slowly: nitrate already in the groundwater may take months or years to decline, and the fertiliser application on the maize field is untouched. So it must be paired with reducing fertiliser use and not applying it just before heavy rain.
Bottled water for the infants (iii) is the best immediate measure. It protects the most vulnerable group at once, at low cost for only 12 infants, and is feasible while the source is fixed. Its weakness is that it does not protect other infants born later or adults who may also be at some risk.
Ranking: Immediate protection (iii), then source control (i) plus fertiliser management, and reject (ii) as a false solution. The committee should also test the alternative standpipe and arrange a safe supply for all infant formula preparation, not just the 12 current cases.