Pressure Loss, Failed Backflow Assembly, and Positive Coliform Sample
A 6-inch cast-iron water main breaks at 10 a.m. in a commercial district. The crew isolates the break, but pressure in the affected area drops to 8 psi for about five hours while repairs are made. The area serves a restaurant, a dental office, and a three-story building with a fire sprinkler system and a boiler treated with corrosion inhibitors. The building's reduced-pressure-zone backflow assembly failed its annual test eight months ago and was never repaired; the utility has a record of the failure but did not shut off service. After repair and flushing, a sample at the restaurant tap shows free chlorine 0.05 mg/L and total coliform positive. The utility's SOP requires a boil-water notice when pressure falls below 20 psi in an area with a known cross-connection. The state requires public notice within 24 hours and repeat samples within 24 hours.
1. What immediate public health actions are needed, and what would you tell the restaurant?
Read the worked answer
Given pressure below 20 psi for five hours and a known failed RPZ, treat this as a potential contamination event. Issue a boil-water notice immediately for the affected area, including the restaurant, dental office, and building; tell the restaurant to stop using tap water for drinking, food preparation, ice, and handwashing, and to use bottled or boiled water. Flush the main and service lines, but do not assume flushing removes backflow contamination. Shut off or isolate service to the building with the failed RPZ until it is repaired and tested. Collect repeat coliform samples and chlorine residual at representative points. Notify the state and public health within 24 hours and document the notice.
2. How would you investigate whether backflow contamination occurred, and what checks would you require before lifting the notice?
Read the worked answer
Investigate the building: inspect the RPZ, check its test tag and differential pressure, and confirm whether it was bypassed or removed. Check the boiler, fire sprinkler system, and any chemical feed for a physical connection; look for a private well or irrigation cross-connection. Sample at the service line, at the building's internal taps, and at the restaurant; test free/total chlorine, coliform, and if indicated, boiler chemicals. Do not lift the notice until the cross-connection is corrected, the RPZ passes testing, the main and building are flushed, and two consecutive sets of coliform samples are negative with adequate chlorine, following state rules. If chemical contamination is possible, coordinate with health officials before lifting.
3. What utility recordkeeping and cross-connection program changes should follow?
Read the worked answer
Records: the utility had a failed RPZ test and did not shut off service; that is a program failure. Change the cross-connection program to track failed tests, send notices, and shut off or isolate service after a failed test or missed retest. Maintain an inventory of assemblies and test dates. Add a main-break procedure that checks for known cross-connections in the affected area and requires notification before repair. Document pressure loss, notices, samples, and corrective actions. Train staff on backflow risk and public notice.