Well Water After Heavy Rain
A hand-dug well serves 60 households in a riverbank community. Each person uses about 20 litres per day, and the average household has five people. After three days of heavy rain, the water is visibly turbid. A field test shows pH 6.4, turbidity 18 NTU, and a positive test for faecal coliform bacteria. The nearest latrine is 15 metres from the well, and the water table is about 3 metres below ground. The community has a stock of 5% sodium hypochlorite solution and a bucket, but no electricity and no boiling fuel for more than a day. A protected spring 1.2 km away is shared with two other villages, and queues form before dawn. The clinic has oral rehydration salts but no laboratory. A volunteer proposes adding bleach directly until the water smells strongly of chlorine, then drinking it immediately.
1. The volunteer wants to treat the full 6,000 L daily supply at 2 mg/L. Calculate the volume of 5% sodium hypochlorite needed, showing your working, and explain why this volume may not make the water safe.
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Daily volume = 60 households × 5 people × 20 L = 6,000 L. 5% NaOCl contains about 50,000 mg available chlorine per litre (5 g per 100 mL = 50 g/L). Required chlorine = 6,000 L × 2 mg/L = 12,000 mg. Volume = 12,000 mg ÷ 50,000 mg/L = 0.24 L = 240 mL. But the water has 18 NTU turbidity. Turbidity and organic matter consume chlorine and can shield bacteria, so the free chlorine residual may fall below the needed 0.2–0.5 mg/L after 30 minutes. The calculated dose is a starting point, not a guarantee. The water should be settled or filtered first, and the dose or contact time adjusted, with residual testing if possible. Also, treating all 6,000 L at drinking-water strength is wasteful; only drinking and cooking water needs that level.
2. The volunteer says, 'If it smells strongly of chlorine, it is safe.' Evaluate this claim using the test results and give two specific corrections to the treatment plan.
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The claim is unsafe. A strong smell shows excess chlorine, not the absence of pathogens. It can also make people refuse the water, and it says nothing about the 30-minute contact time or the turbidity. The positive faecal coliform result means the source is contaminated; a smell test cannot confirm that the bacteria have been inactivated. Corrections: first, let the water settle and filter it through clean cloth to reduce turbidity before chlorination; second, add the calculated dose, mix, wait at least 30 minutes, and check for a faint chlorine residual rather than a strong smell. If residual testing is unavailable, boiling drinking water for one minute is safer, but fuel is limited, so prioritize drinking water. The well also needs source protection, because a latrine 15 m away with a 3 m water table is a continuing contamination risk.
3. Rank three actions in order of urgency for the next 24 hours and justify the ranking using the test data and community constraints.
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First, stop untreated well water from being drunk and provide a safe alternative for drinking and cooking. The positive faecal coliform test is the most urgent finding because it indicates a real risk of diarrhoeal disease, and the community has no electricity and only one day of boiling fuel. Second, protect the wellhead: divert runoff, repair the apron, and keep animals and latrine waste away from the 15 m zone. Rain is still affecting the site, and every hour of contamination adds to the load. Third, begin hygiene messaging and arrange a longer-term fix, such as lining the well, relocating the latrine, or developing the protected spring 1.2 km away. The spring is not an immediate solution because queues are already long, but it is a safer permanent source. This ranking puts immediate health protection before engineering, while recognising that treatment alone will not stop recontamination.