Total Coliform Positive After a Main Repair
A crew repaired a 10-inch ductile main break on a residential street, replacing a 4-foot section and a 6-inch service tee. The main was shut down, depressurized, and the trench was dewatered with a trash pump; the repair was made open-trench with the pipe ends exposed to the air for about 3 hours. The crew flushed the repaired reach at a hydrant until clear, then put the main back in service the same afternoon. Standard practice at this utility is to collect one bacteriological sample per repaired reach the next morning. That sample came back total-coliform positive, fecal-negative. The lab has not yet run the repeat set. A neighboring utility had a similar positive last year and the state required a boil-water notice.
1. List the likely causes of the total-coliform positive and rank them, given the repair conditions described.
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Ranked by fit to the described conditions:
- Inadequate disinfection of the repair area. Open-trench exposure for 3 hours with dewatering means soil and groundwater contacted the pipe interior. Flushing removes turbidity but does not disinfect. The reach should have been swabbed/sprayed with a strong chlorine solution (typically 100-200 mg/L on the repaired surfaces) before backfill, and the reach should have been chlorinated and held before sampling.
- Insufficient contact/flush before sampling. If the main was returned to service and sampled the next morning, residual may have been low at the sample point, or the sample tap itself was not flushed/sterilized. A single sample from a hydrant or a poorly flushed tap can pick up biofilm or sediment.
- Sample collection error. Unsterile bottle, unflushed service line, or a sample point downstream of a dead leg.
- Pre-existing biofilm in the reach disturbed by the shutdown and flow reversal.
Fecal-negative argues against gross sewage contamination but does not clear the result. The immediate action is the repeat set (and any required additional samples) with residual checks; do not wait on the neighbor-utility precedent to drive decisions - follow this utility's and the state's coliform rule triggers.
2. The repeat set comes back clean. The operations manager wants to close the incident with no further action. State whether you agree, and what documentation and follow-up you would require.
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A clean repeat set does not automatically close the incident. Under the total-coliform rule, a positive followed by clean repeats resolves the compliance trigger, but the cause is unresolved and the repair conditions were a known contamination pathway.
I would not close with 'no further action.' Require:
- Documented repair disinfection - confirm whether the reach was chlorinated and held; if not, note the gap.
- A residual profile across the repaired reach and the affected services over 2-3 days, to prove chlorine is holding and there is no dead leg.
- Targeted follow-up samples at the repair location and downstream, beyond the minimum repeat set, plus a check of the service tee and any low-flow branches.
- A written incident record tying the positive to the repair, the corrective actions, and the results - this is what a state inspector will ask for.
- A procedural review: the standard 'one sample next morning' practice is too thin for open-trench, dewatered repairs. Recommend a disinfection-and-hold step and a defined sample plan for repairs.
If any follow-up sample is positive again, escalate to the full repeat/assessment process and consider a precautionary notice; a clean repeat set is evidence, not a closure.