10/25/2011 -

In August 1982, 500 New Englanderswere evacuated after residents' reports of bubbling and hissing sounds coming from their toilets and faucets were followed by two explosions. A propane and water mixture seeping from their pipes had contaminated the entire town. It wasn’t a terrorist attack, but the same process that prompted hundreds of New Englanders to evacuate could be used to launch a widespread terrorist attack, according to the Department of Homeland Security (DHS).

For months DHS has warned about possible attacks on utilities infrastructure. Earlier this month, DHS and the Interagency Threat Assessment and Coordination Groupissued a bulletin advising local agencies to be aware of a terrorist tactic aimed at contaminating drinking water. The bulletin was published online Monday by Public Intelligence. Through a relatively simple process, a chemical or biological agent could be deposited into the water supply and spread over long distances without immediate detection.

“Over the last decade, violent extremists have expressed aspirational interest in contaminating unspecified water supplies, and as recently as July 2011 specifically raised backpressure as a means of contamination," the bulletin states. But while experts agree that the potential for a backpressure attack exists, existing measures may already be enough to thwart a potential attack.

Backpressure contamination comes from the reversal of the normal flow of water systems, with the water picking up contaminants as it flows backward. Unintentional backpressure incidents can occur for a number of reasons.

“When you lose pressure in the public water system, like if a water main breaks, anything that is in that line will get siphoned back into the water line…so it’s not an uncommon thing,” said Roy Dillard, a director at the American Backflow Prevention Association. “Backpressure happens when there is a pressure, higher than the city water pressure, exerted on the water systems and it causes the water to flow backward.” 

It’s exactly what happened to the town in New England that August. A narrative of the incident is featured in the most recent edition ofStop Backflow News: Case Histories and Solutions, a report published by Watts Water Technologies.

Repairmen had connected an industrial propane tank to a fire hydrant to purge the defective tank before starting repairs. Because of the tank’s higher pressure, propane vapor forced itself into the pipes and contaminated the whole town’s water supply.

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