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306 Chapter 9 Cross-Connection Control
9.2.8 Boiler Water Entered High School Drinking Water
A high school in New Mexico was closed for several days when a home economics teacher
noticed the water in the potable system was yellow. City chemists determined that samples
taken contained levels of chromium as high as 700 mg/L, “astronomically higher than the
accepted levels of 0.05 mg/L.” The head chemist said that it was miraculous that no one
was seriously injured or killed by the high levels of chromium. The chemical was identi-
fied as sodium dichromate, a toxic form of chromium used in heating system boilers to in-
hibit corrosion of the metal parts.
No students or faculty were known to have consumed any of the water; however, area,
physicians and hospitals advised that if anyone had consumed those high levels of chromium,
the symptoms would be nausea, diarrhea, and burning of the mouth and throat. Fortunately,
the home economics teacher, who first saw the discolored water before school started, imme-
diately covered all water fountains with towels so that no one would drink the water.
Investigation disclosed that chromium used in the heating system boilers to inhibit
corrosion of metal parts entered the potable water supply system as a result of backflow
through leaking check valves on the boiler feed lines (see Fig. 9.8).
9.2.9 Car Wash Water in the Street Water Main
This car wash cross-connection and backpressure incident, which occurred in the state of
Washington, resulted in backflow chemical contamination of approximately 100 square
blocks of water mains. Prompt response by the water department prevented a potentially
hazardous water quality degradation problem without a recorded case of illness.
Water cooler
High school
Bubbler
Bubbler
Recommended installation
of backflow preventer
Street Leaky check valves
Toxic rust inhibitor and
defoamant containing
sodium dichromate
Pump
High school boilers
Figure 9.8 Boiler Water Enters High School Drinking Water