Case Histories


In June 1983, “yellow gushy stuff” poured from some faucets in the Town of Woodsboro, Maryland. Town personnel notified the County Health Department and the State Water Supply Division. The State dispatched personnel to take water samples for analysis and placed a ban on drinking the Town’s water. Firefighters warned residents not to use the water for drinking, cooking, bathing, or any other purpose except flushing toilets. The Town began flushing its water system. An investigation revealed that the powerful agricultural herbicide Paraquat had backflowed into the Town’s water system.

Someone left open a gate valve between an agricultural herbicide holding tank and the Town’s water system and, thus, created a cross-connection. Coincidentally, water pressure in the Town temporarily decreased due to failure of a pump in the Town’s water system. The herbicide Paraquat was backsiphoned into the Town’s water system. Upon restoration of pressure in the Town’s water system, Paraquat flowed throughout much of the Town’s water system.

Fortunately, this incident did not cause any serious illness or death. The incident did, however, create an expensive burden on the Town. Tanker trucks were used temporarily to provide potable water, and the Town flushed and sampled its water system extensively.

On January 30, 1990, authorities closed Overland Middle School in Brighton, Colorado, after an antifreeze-like chemical was found in the school’s potable water system. They sent nine students complaining of flu-like symptoms to an area hospital for treatment. The hospital released the students after treating them for ethylene glycol poisoning. Ethylene glycol had backflowed into the school’s potable water system from the school’s hot-water heating system.

During a routine maintenance check of the Overland Middle School’s hot-water heating boiler, maintenance workers left open a valve on the potable water line feeding the boiler. This allowed boiler water containing the antifreeze ethylene glycol to backflow into the school’s potable water system. There was no backflow preventer on the feed line to the boiler.

The Overland Middle School was closed for an additional day while workers flushed the potable water piping at the school and “repaired the hot-water heating system leak.” Presumably workers installed a proper backflow preventer in the potable water line feeding the hot-water heating boiler.

In August 1982, residents in a Connecticut town reported hissing, bubbling noises coming from washing machines, sinks, and toilets. Faucets sputtered out small streams of water mixed with gas. Propane gas had backflowed into the town’s public water system. Local firefighters and other officials asked hundreds of residents to evacuate their homes and businesses.

The town provided water to a propane storage facility in the area. Water was furnished to the facility for both domestic use and fire protection and entered the facility through a single eight-inch-diameter service connection. The facility included 26 subsurface 30,000-gallon liquid propane storage tanks.

On the day of the backflow incident, workers needed to repair a storage tank at the propane storage facility. Before repairing the tank, workers had to purge the tank of residual propane. There are two common methods for purging liquid propane storage tanks. One method is to use an inert gas such as carbon dioxide. The other method is to use water. The use of water is the preferred method because it is a more positive method and will float out any sludge as well as gas vapors. Accordingly, workers attempted to purge the tank using water in this case. They connected a hose to the tank from one of the two fire hydrants at the facility. Unfortunately, the pressure in the propane tank was about 85 to 90 psig, while the pressure in the town’s public water system was about 65 to 70 psig. Consequently, propane gas backflowed into the town’s public water system. It was estimated that about 2,000 cubic feet of gas flowed into the water system over a period of about 20 minutes. This is enough gas to fill approximately one mile of eight-inch-diameter water main.

Fires were reported at two houses, and fire gutted one of these houses. At another house, a washing machine exploded. Police, propane company workers, and town water works personnel, however, limited damage and injuries by quickly sealing off the affected area. The town flushed fire hydrants and individual building plumbing systems and monitored for gas. The propane company promptly instituted revised propane tank purging procedures at its storage facility.