Two contaminated sources, ₹40,000 and a bag of bleaching powder
Two drinking-water sources serve a village of 2,400 people. The borewell near the school tests at 1.8 mg/L fluoride and 0.9 mg/L iron. The open well near the tank tests at 12 NTU turbidity with coliform bacteria present in 100 mL. The applicable acceptable limits are 1.0 mg/L fluoride (1.5 permissible only in the absence of an alternative source), 0.3 mg/L iron (1.0 permissible), 1 NTU turbidity (5 permissible), and no coliform in 100 mL. Several children in the village have mottled teeth. The Panchayat has ₹40,000 for water works and a stock of bleaching powder labelled 'available chlorine 4%'. One member proposes simply boiling the borewell water and distributing it; another proposes pouring bleaching powder into the open well 'until it smells of chlorine'.
1. Rank the two sources by health risk and state what each actually needs.
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Open well - acute risk, rank 1. Coliform presence means faecal contamination and the immediate possibility of diarrhoea, typhoid and hepatitis. Turbidity of 12 NTU is itself a health problem: particles shield micro-organisms from chlorine and consume the disinfectant. Sequence: settle or filter first, then chlorinate, then re-test. If a protected alternative is available, advise boiling or chlorinated supply in the interim.
Borewell - chronic risk, rank 2 but non-negotiable. Fluoride at 1.8 mg/L exceeds even the permissible 1.5 mg/L, and the mottled teeth already show dental fluorosis; prolonged use risks skeletal fluorosis. This water must not remain the sole source. Iron at 0.9 mg/L is within the permissible 1.0 mg/L but above the acceptable 0.3 mg/L: it is chiefly aesthetic - taste, staining, discoloured clothes - and is removed by aeration followed by sand filtration, not by fear.
So: fix the well immediately for acute risk; fix the fluoride by dilution or defluoridation for chronic risk. Iron is third.
2. Using the 4% bleaching powder, calculate the dose for a 5,000-litre tank at 2 mg/L of available chlorine, and state the precautions.
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Chlorine required: 5,000 L × 2 mg/L = 10,000 mg = 10 g of available chlorine.
Powder required: the powder is only 4% chlorine, so 10 g ÷ 0.04 = 250 g of bleaching powder.
Precautions:
- Dose after turbidity is reduced; chlorine is wasted on mud, not on microbes.
- Mix the 250 g to a paste in a bucket, then dilute, then add - never tip dry powder into a full tank.
- Contact time of about 30 minutes, then test free residual chlorine of 0.2–0.5 mg/L; below that, re-dose; far above it, the water is unpalatable and forms more disinfection by-products.
- Check that the powder is fresh and has been stored dry and closed; damp bleaching powder loses strength and the label figure becomes fiction, so re-dose on measured residual, not on the label.
- Use gloves and keep the solution away from eyes; never store the solution in a drinking-water container.
3. Evaluate the two suggestions - boiling the borewell water, and dosing the well until it smells - and give the realistic ₹40,000 plan.
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Boiling the borewell: wrong tool. Boiling kills microbes but does not remove fluoride; evaporation concentrates it, so the fluoride rises above 1.8 mg/L. It also costs fuel and labour for 2,400 people, and the water was not the microbiological problem.
Dosing 'until it smells': unsafe. Smell detects gross over-dosing, not adequacy. Over-dosing gives a bad taste, encourages people to use the unprotected source again, and raises disinfection by-products; under-dosing leaves pathogens alive. Chlorine demand must be met by measurement of residual, not by nose. Dosing before settling is wasted on turbidity.
₹40,000 plan:
- Shock-chlorinate and clean the open well, then test - about ₹3,000 including a field test kit.
- Repair/protect the well head and apron - ₹10,000.
- Aeration and sand filter for iron on the borewell - ₹15,000.
- Dilute the borewell water with the low-fluoride source, or start rainwater harvesting for drinking - ₹8,000.
- Balance ₹4,000 for repeat testing and a school dental screening.
Defluoridation (activated alumina or the alum-lime Nalgonda technique) is the fallback if no low-fluoride source exists.