Repeat Coliform Positive on a Long Dead-End Main
You manage a 2,400-connection community system (population about 6,200) with free chlorination. Last year you extended an 8-inch PVC main 2,300 feet as a dead end to serve 12 homes; the main was not looped and has no blow-off except a single fire hydrant at the end. Chlorine at the plant is 1.05 mg/L, but your last two quarterly measurements at the end hydrant were 0.12 and 0.10 mg/L, and the hydrant was last flushed nine months ago. Average use on the extension is about 1,100 gpd. This month a routine sample from the last house on the main was total coliform positive; the repeat sample at the same tap, collected the next day, was also total coliform positive, with E. coli absent. There is no history of main breaks on the extension, and the service line is 1-year-old copper.
1. Explain what the repeat positive means under the Revised Total Coliform Rule and list the required actions and deadlines.
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Two positives in the same month change the classification. A routine positive requires repeat samples at the original tap plus upstream and downstream locations. When the repeat at the same tap is also total coliform positive with E. coli absent, two things follow:
- Treatment technique trigger. A repeat total coliform positive is a trigger requiring a Level 1 assessment within 30 days, conducted by the state or by the system using the approved form. A second trigger within 12 months escalates to a Level 2 assessment and state-approved corrective action.
- MCL determination. For a system collecting fewer than 40 samples per month, more than one total coliform-positive sample in a month (routine plus repeats combined) is an MCL violation. With a routine and a repeat both positive, that threshold is met, so the system is in violation and a Tier 2 public notice is due within 30 days.
Because E. coli was absent, there is no Tier 1 notice and no boil water advisory on this basis. I would still notify MassDEP immediately, document the sampling, and make sure the assessment looks for the actual cause rather than treating the notice as the end of the job. The assessment must result in corrective action - flushing alone, without a residual and water-age fix, will not close it.
2. Calculate the average detention time in the main at average demand and explain how it informs your corrective action.
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Volume of an 8-inch main: area = π/4 × (8/12)² = 0.349 ft²; 0.349 × 2,300 ft = 803 ft³; 803 × 7.48 = about 6,010 gallons.
At 1,100 gpd average use, detention = 6,010 ÷ 1,100 = 5.5 days. Even allowing for peaking, water sits in this main for days, not hours, and free chlorine decays with time, temperature, and pipe surface. A residual of 0.10–0.12 mg/L at the end hydrant is consistent with that water age, and it is not enough to keep a biofilm suppressed.
The corrective action follows from the number. Options in order of practicality: establish a written flushing program on the extension (monthly or biweekly at first) until residuals recover; install an automatic flusher or a blow-off at the end so flushing is not dependent on someone remembering; add a booster chlorination point if flushing cannot hold residual; and plan to loop the main into the adjacent street, which solves the problem permanently. I would also inspect the sample tap and the home's plumbing - a 1-year-old copper service with a screen aerator is a plausible sampling artifact - and check for cross-connections among the 12 homes. But a 5.5-day water age with 0.1 mg/L residual is a real system problem regardless of what the tap contributes.
3. You decide to shock chlorinate the main. How much 12.5% sodium hypochlorite is needed to reach 50 mg/L, and what safeguards apply?
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Dose calculation:
- Water mass: 6,010 gal × 8.34 lb/gal = 50,090 lb.
- Chlorine needed at 50 mg/L: 50,090 × (50 ÷ 1,000,000) = 2.50 lb as Cl2.
- 12.5% hypochlorite at about 10.7 lb/gal contains 1.34 lb active chlorine per gallon.
- Volume: 2.50 ÷ 1.34 = 1.87 gal, so about 2 gallons.
Safeguards matter as much as the number. Notify every affected customer in advance and tell them not to drink or use the water during the hold. Coordinate with MassDEP on the procedure and on discharge. Keep the chlorinated water in the main for the required contact time, then dechlorinate before discharge - sodium thiosulfate or ascorbic acid - because 50 mg/L hitting a storm drain or stream is a fish kill. Use PPE, add the hypochlorite slowly with the main isolated, and verify strength with a test kit rather than trusting the label.
After contact time, flush until residual falls below about 4 mg/L, then collect coliform samples and confirm satisfactory results plus a stable residual before returning the main to service. Record the volumes, concentrations, contact time, and sampling results; they are the evidence the assessment and the public notice will rely on.