TACOMA, Wash. — Last year, environmental engineers working for this city had an unnerving worry. After a decade and tens of millions of dollars devoted to a fastidious cleanup of its corner of Puget Sound, Tacoma’s shore front was in danger of being contaminated with a highly toxic chemical.

Working their way upstream, the engineers discovered the source of the problem: a storm drain in a residential neighborhood. Black goo stuck to its sides was old grout that contained large concentrations of polychlorinated biphenyls, or PCBs. Used in local road building in 1975, PCBs, odorless and carcinogenic, were outlawed soon thereafter. Yet they were swept up and sent mostly through drain pipes on their way to Puget Sound.

More than four decades after the 1972 passage of the federal Clean Water Act and after years of expensive cleanups like the one here, industrial water pollution from factory pipes is closely monitored and, by many measures, on the way to being vanquished.

But a menace is coming from a surprising source: rain, and the storm water it produces. It scours all corners of a watershed, carrying with it things like leftover industrial pollutants, airborne toxins, automotive oil and grease, pesticides from lawns, and pharmaceuticals from leaky septic tanks.