Fourteen cases after the rain
A residential school draws its water from a hand pump located 8 m downslope of a cattle shed and 10 m from a soak pit. Two days after heavy rain, 14 of the 60 hostel students develop loose motions and vomiting within 36 hours. All 14 drank the pump water; the 60 day scholars, who bring boiled water from home, have no cases. The pump water is clear and smells normal. A student adds soap solution to a sample, shakes it and gets very little lather; she reports: 'The water is only hard, so it is safe to drink.' The science teacher asks the class to treat the report as a scientific investigation - a hypothesis, a control, a record of observations, and a plan for making the supply safe until the pump is repaired.
1. Evaluate the soap test and the reasoning 'clear water is safe'. What can the soap test actually tell you?
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The soap test detects hardness, not germs. Poor lather means the water contains dissolved calcium and magnesium ions, which form scum with soap. That is a chemical property with no bearing on bacteria, viruses or protozoan cysts.
Why 'clear and odourless' proves nothing: disease-causing organisms are microscopic and usually do not produce colour, turbidity or smell. Water can look perfectly clean and still carry faecal contamination.
What would actually be informative:
- A bacteriological test for coliforms (MPN method or a hydrogen-sulphide strip test) on the pump water, plus a sample from a known safe source for comparison.
- The epidemiological pattern, which is far stronger evidence than any field test here: 14 of 60 exposed students ill (about 23%) against 0 of 60 unexposed.
Judgement: the student's test is irrelevant to the question being asked. Reporting 'hard, therefore safe' is a non sequitur and could delay proper action.
2. State a testable hypothesis, identify the control group, and name one confounder that could undermine the conclusion.
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Hypothesis: the pump water was contaminated by faecal bacteria when rainwater carried material from the cattle shed and soak pit into the shallow aquifer or the well head; drinking it caused the illness.
Prediction: if true, students who drank the pump water should be ill far more often than students who did not.
Control group: the 60 day scholars who drank boiled water from another source - 0 cases. (A comparison group drawn from the same population is what makes the 14 cases meaningful; 14 cases alone, without a denominator, says nothing.)
Confounders to check:
- Food: hostel students also share a common kitchen. If the illness came from a hostel meal, water would be wrongly blamed. Check whether any hostel student who drank only boiled water fell ill, and whether any day scholar ate hostel food.
- Person-to-person spread: a dormitory outbreak can pass from student to student, so early cases may not all be water-borne.
- Recall bias: students may misremember what they drank after they fall ill.
Judgement: the association is strong but not yet proof; the food route and person-to-person spread must be excluded before the pump is declared the source.
3. Recommend what should be done until the pump is repaired, and say how the class would know the measures had worked.
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Immediate measures:
- Stop using the pump for drinking; use an alternative safe source.
- Boil drinking water and let it cool in the same covered vessel (or use chlorine tablets at the dose on the packet). Boiling kills bacteria, viruses and cysts; chlorine is easier but is less effective against some cysts and needs contact time and low turbidity.
- Store treated water in covered containers with a tap or ladle, never by dipping hands or cups.
- Treat patients with oral rehydration solution; refer anyone with blood in the stool, persistent vomiting or severe dehydration.
Repair and protection of the source: disinfect the well/pump with bleaching powder, seal the head against surface runoff, raise the platform, and relocate the cattle shed and soak pit so that drainage does not move towards the pump.
How the class knows it worked:
- A repeat bacteriological test shows no coliforms, and chlorinated water shows a faint residual chlorine smell at the tap.
- No new cases appear after about two incubation periods (roughly a week of daily recording).
- The class keeps a dated table of cases per day and per group - evidence, not opinion, is what closes the investigation.