How contaminants reach drinking water

Contamination enters at four distinct points, and each has a different owner, a different detection method and a different fix.

Why lead is different: Lead almost never comes from the catchment or the works. It comes from the last few metres of pipe, which is why regulatory sampling is done at the consumer tap.
Where responsibility changes: In most jurisdictions the water company is responsible up to the boundary; the property owner is responsible beyond it. Contamination arising inside a building is generally not the company’s to fix.
Why zone results are not your tap: Compliance data describe a supply zone of up to 100,000 people. They cannot tell you what is coming out of your own tap if your building introduces something.

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Four entry routes to drinking water. First, catchment sources such as agriculture, industry and sewage reach the raw water. Second, treatment failures allow contaminants through, or introduce them as by-products. Third, distribution network problems allow ingress through bursts or pressure loss, or release deposits from the mains. Fourth, contamination inside the building from lead pipes, poorly maintained storage or stagnation. Regulation and monitoring differ at each point.