Three Wells, One Lab: Ranking Water Risk
A community health team is assessing three hand-dug wells in a rural district after reports of stomach illness. Samples were tested on the same morning. Well A: pH 6.8, turbidity 4 NTU, nitrate 3 mg/L as NO3, E. coli 0 CFU/100 mL. Well B: pH 7.1, turbidity 12 NTU, nitrate 8 mg/L, E. coli 45 CFU/100 mL. Well C: pH 8.9, turbidity 2 NTU, nitrate 1 mg/L, E. coli 0 CFU/100 mL. The team's reference sheet lists: E. coli must be absent in 100 mL; turbidity should not exceed 5 NTU; pH should be between 6.5 and 8.5; nitrate as NO3 should not exceed 10 mg/L. The Well B sample was drawn from a public tap fitted with a loose, cracked rubber seal, two days after heavy rain. The district has one laboratory, and results take four days. About 300 people use Well B.
1. Rank the three wells from lowest to highest health risk and justify the ranking with the guideline values.
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Ranking: Well A (lowest) < Well C < Well B (highest).
Well B - highest and urgent. E. coli at 45 CFU/100 mL violates the 'absent' guideline outright, indicating faecal contamination and immediate risk of diarrhoeal disease. Turbidity at 12 NTU also exceeds 5 NTU; turbidity both signals contamination and shields microbes from disinfection. Nitrate at 8 mg/L is within the 10 mg/L limit but leaves little margin and merits monitoring, especially for infants.
Well C - moderate. pH 8.9 exceeds the 6.5–8.5 range, but pH is mainly an aesthetic and operational problem - taste, scaling, reduced disinfection efficiency - rather than an acute hazard. Microbial and nitrate results are clean, so immediate risk is lower than B's.
Well A - lowest. All four values are within guidelines; routine testing is enough.
The ranking should be driven by acute hazard first - pathogens - then by indicators that undermine treatment, not by counting how many parameters fail.
2. The Well B sample came from a tap with a cracked seal after heavy rain. How does this affect your interpretation, and what should the team do?
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What the seal and rain do to the inference. The sample may not represent the water in the well. A cracked seal is a plausible entry point for bacteria at the tap, and heavy rain raises the chance of surface contamination entering the wellhead or the line. The 45 CFU therefore cannot be attributed confidently to the aquifer or the well itself.
What does not change. The tap is where people actually collect drinking water. Even if contamination is confined to the fitting, users are exposed. E. coli is a faecal indicator, not a harmless nuisance organism, and 45 CFU is not a borderline result.
Action under uncertainty. Do not wait four days for confirmation. Advise boiling, chlorination, or use of an alternative source for Well B; replace the seal; disinfect the tap; and record who drew water in the preceding week. Refer any reports of diarrhoea to the clinic.
Principle: a confounded sample lowers confidence in the source attribution, not in the exposure risk. Precaution should scale with hazard severity and the number of people exposed - here about 300.
3. The team can afford one resampling round. Which wells should be resampled, at which points, and what result would change the recommendation?
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Priority: Well B, multiple points.
- Wellhead before the tap, after purging, to test whether contamination originates in the well.
- Tap after the seal is replaced and the line flushed and disinfected.
- One household storage container, because contamination often occurs after collection.
Interpretation rule. If the wellhead sample is also positive, the source is contaminated and the well needs protection, repair, and treatment. If only the tap or storage samples are positive, the problem is the fitting or household handling, and the fix is local.
Secondary: Well C. Retest pH with a freshly calibrated meter and check whether a nearby source - concrete lining, industrial discharge - is raising alkalinity. One high reading is not enough to act on.
Well A needs no special round; keep it on the routine schedule.
What would change the recommendation: a clean wellhead result would downgrade B from 'source unsafe' to 'tap unsafe'; a persistently positive wellhead would justify closing the well until treatment is installed.