December 8th of 1980 Residents in Southern Section of Pittsburgh in particular Beechview area started noticing a very strong chemical smell in there home water supply's.
After a quick investigation by authorities warnings where sent out thru the news and door to door not to use the water because it some how had been contaminated with Pesticides specifically Chlordane.
https://www.epa.gov/sites/default/files/2016-09/documents/chlordane.pdf
Supposedly this was seen as an act of Vandalism when they believed it was injected into the system in the woods off Saw Mill Run Road Pa 51 near Crane Ave.
But the true cause was never determined or individuals prosecuted, but it caused misery for many in and around the Beechview area. for several days while it was flushed out of the water system.
At the time of the incident Back Flow Preventers where not required on most water system supply's to homes and businesses
https://todayshomeowner.com/plumbing/guides/backflow-preventer/
Which help prevent water in the structure from going backwards into the water supply mainline.
The incident could have been as simple as some one with a sprayer on a hose spraying the chemical on their lawn and a water main break in street causing low enough pressure it sucks what ever water is in the home back into the street main.
The Late Judson Hamil a Carrick Plumbing business owner I knew and Allegheny County Master Plumber was working in the area that day .
He observed a lawn service type truck at the top of Dagmar Ave a very hilly and steep area of Beechview and down near west liberty ave there was a water break that day and he believed at the time this truck was possible cause which he reported to authorities but was ignored. When you look at a map of area the scenario is very possible .
A true cause was never determined.
Ultimately this incident saw the enforcement of all businesses and eventually all new homes to have to have back flow preventers installed and yearly inspected for businesses