Page 293 - Water Engineering Hydraulics, Distribution and Treatment

Automobile antifreeze 9.2 Public Health Significance of Cross-Connections 271
added to boiler water
Backsiphonage
(reverse flow)

Normal flow

Curb stop with stop Water main Figure 9.3 Heating system antifreeze into
and waste drain potable water.

was only 65–70 psi (451–486 kPa), propane gas backpres- Repeated efforts to clean and flush the lines were not sat-
sure backflowed into the water main. It was estimated that isfactory, and a decision was eventually made to replace the
the gas flowed into the water mains for about 20 min and water line and all the plumbing that was affected. There were
3
3
that about 2,000 ft (57 m ) of gas was involved. This was no reports of illness, but residents of the housing authority
approximately enough gas to fill 1 mi of an 8 in. (1.61 km of were told not to use any tap water for any purpose and they
a 200 mm) water main. were given water that was trucked into the area by volunteer
fire department personnel. They were without their normal
water supply for 27 days.
9.2.7 Chlordane and Heptachlor at a
Housing Authority 9.2.8 Boiler Water Entered High School
Drinking Water
The services to 75 apartments housing approximately 300
people were contaminated with chlordane and heptachlor in A high school in New Mexico was closed for several days
a city in Pennsylvania. The insecticides entered the water when a home economics teacher noticed the water in the
supply system while an exterminating company was apply- potable system was yellow. City chemists determined that
ing them as a preventive measure against termites. While the samples taken contained levels of chromium as high as
pesticide contractor was mixing the chemicals in a tank truck 700 mg/L, “astronomically higher than the accepted levels
with water from a garden hose coming from one of the apart- of 0.05 mg/L.” The head chemist said that it was miracu-
ments, a workman was cutting into a 6 in. (150 mm) main lous that no one was seriously injured or killed by the high
line to install a gate valve. The end of the garden hose was levels of chromium. The chemical was identified as sodium
submerged in the tank containing the pesticides, and at the dichromate, a toxic form of chromium used in heating system
same time, the water to the area was shut off and the lines boilers to inhibit corrosion of the metal parts.
were being drained prior to the installation of the gate valve. No students or faculty were known to have consumed
When the workman cut the 6 in. (150 mm) line, water started any of the water; however, area physicians and hospitals
to drain out of the cut, thereby setting up a backsiphonage advised that if anyone had consumed those high levels of
condition. As a result, the chemicals were siphoned out of chromium, the symptoms would be nausea, diarrhea, and
the truck, through the garden hose, and into the system, con- burning of the mouth and throat. Fortunately, the home eco-
taminating the 75 apartments (see Fig. 9.7). nomics teacher, who first saw the discolored water before