Pressure Loss and Coliform Positive After Main Break
A 6-inch unlined cast-iron main in a commercial district failed at 7:40 a.m. The crew isolated it by 8:10 a.m.; pressure in the affected zone fell to 16 psi for about 2.5 hours and the zone lost chlorine residual (0.05 mg/L at a nearby hydrant). Repair was completed at 2:30 p.m. and service was restored without a documented flush. Two days later, a routine total-coliform sample at a restaurant tap is positive; E. coli is absent. The system has 1,800 connections, 60-year-old pipe, and several cross-connections, including a car wash with a submerged inlet and a fire sprinkler system without a backflow assembly. The next routine sampling date is three weeks away. The operator wants to wait for the repeat sample before doing anything else.
1. What immediate actions and sampling should the system take?
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Treat the pressure loss and residual loss as a contamination risk, not just a repair. The total-coliform-positive routine sample must be followed by repeat samples within 24 hours: the original site plus upstream and downstream sites (or as required by the TCR). Analyze for total coliform and E. coli, measure chlorine residual and turbidity at each site, and flush the area to restore residual. Because the zone had pressure below 20 psi and lost residual, the system should issue a precautionary boil water advisory for the affected area while results are pending. Notify MDE within the required timeframe. Do not wait three weeks for the next routine sample. The positive result may be a treatment or distribution problem, but the pressure event makes intrusion the leading hypothesis.
2. Should the system issue a boil water advisory before the repeat results are available? Explain the decision and the notice pathway.
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A precautionary boil water advisory is appropriate even without E. coli because pressure below 20 psi and chlorine residual of 0.05 mg/L indicate a pathway for contamination. The advisory should tell customers to boil water for one minute or use bottled water, and it should be issued through local media, door hangers, and the system's notification plan. If repeat samples are negative and chlorine residual and pressure are restored, the advisory can be lifted after flushing and two consecutive negative sets, with MDE concurrence. If any repeat is positive, the system has a total coliform MCL and must follow Tier 2 public notice requirements; if E. coli is confirmed, Tier 1 notice is required within 24 hours. Keep records of the advisory, sampling, and rescission.
3. What cross-connection and long-term follow-up should be pursued?
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The car wash's submerged inlet and the fire sprinkler connection without a backflow assembly are potential cross-connections and should be investigated immediately. The main break may have created negative pressure that pulled contaminated water into the main. Inspect the repair site for soil or groundwater entry, check for illegal connections, and test backflow assemblies. Long term: conduct a cross-connection survey, require reduced-pressure-zone assemblies at the car wash and fire line, flush and disinfect the repaired main, and increase chlorine residual to the low end of the target range. Review the main break response to require flushing and bacteriological sampling after every repair where pressure is lost.