The Village Water Pump
A rural community draws drinking water from a tube well. Health workers report that 18 of 120 children under five have had recurring diarrhoea in the past month, versus an expected baseline of about 4%. A field test shows the well water is clear and odourless, but the well is 12 m from a pit latrine, and the concrete apron around the casing is cracked. The tube well is 30 m deep and fitted with a hand pump. Villagers say the water "looks clean," so some families have stopped boiling it.
1. Calculate the observed attack rate and compare it with the baseline. What does the comparison suggest?
Read the worked answer
Observed rate = 18 / 120 = 0.15 = 15%. Baseline ≈ 4%. The observed rate is 15 / 4 = 3.75 times the baseline - a substantial excess. This does not by itself prove waterborne transmission (confounders like nutrition or sanitation exist), but a nearly fourfold excess clustered around one source is a strong signal warranting investigation. The reasoning point: quantify the excess before jumping to a cause.
2. Explain, using the distances and physical features given, why "clear and odourless" is not evidence of safety.
Read the worked answer
Pathogens (bacteria, viruses, parasites) are invisible and mostly odourless - clarity reflects suspended solids, not microbial contamination. The well is 12 m from a pit latrine; if the latrine is uphill or the aquifer is shallow/fractured, contamination can migrate. The cracked apron is critical: it lets surface runoff and wastewater flow down the outside of the casing directly into the water, bypassing soil filtration. A 30 m depth gives some protection but not guaranteed. Therefore the sensory test is invalid; a microbiological test (e.g., faecal indicator bacteria) is needed.
3. Propose the two highest-priority interventions and justify the ordering.
Read the worked answer
Priority 1 - stop the contamination pathway: repair/replace the cracked apron and improve drainage, and relocate or line the latrine if feasible. This addresses the source, which is more effective than treating water after contamination. Priority 2 - household water treatment and safe storage: resume boiling or use chlorination/filtration plus covered storage, and run a health-education campaign explaining why clear water can still be unsafe. Justification: engineering the source is a permanent fix benefiting everyone; treatment is a stopgap that depends on daily compliance. Both together, with retesting, is the sound public-health answer.