How Drinking Water Gets Contaminated

Challenges to clear

• Judge all four cases correctly.
• Answer the clear-well-water question — clear is not the same as safe.

The task

Water can look perfectly clear and still be dangerous. Think each case through.

What this lesson covers

• Water drawn from a village well is completely clear and has no smell. Is it safe to drink untreated? Best choice: No — disease germs are invisible, so it must still be boiled or treated — Cholera, typhoid and hepatitis germs are undetectable by eye. Boiling or chlorination is the only assurance.
• A drinking-water pipe runs alongside a leaking sewage drain. The tap water looks normal. What is the risk? Best choice: Serious — sewage can be sucked into the pipe whenever water pressure falls — This back-siphoning is a classic cause of urban cholera and typhoid outbreaks.
• A household stores boiled water in a wide, open pot and dips a cup in to serve it. Is the water still safe? Best choice: No — dipping hands and cups reintroduces germs after boiling — Recontamination during storage is extremely common. Use a covered container with a tap or a ladle kept only for that purpose.
• After a flood, cases of diarrhoea rise sharply across a town. What is the most likely explanation? Best choice: Floodwater mixed sewage with the drinking-water supply — Flooding breaks the separation between sewage and drinking water, which is why water-borne outbreaks follow floods so reliably.

Hint

Water safety turns on one question: has this water met sewage at any point, including after it was treated?