Two Coliform Positives in July
You operate a community system that collects 60 routine coliform samples per month. In July the routine sample at site 12, a restaurant on a dead-end main, comes back total-coliform positive with E. coli absent. Your operator collects three repeat samples within 24 hours: the repeat at site 12 is again total-coliform positive, and the two repeats at sites upstream and downstream are negative. No E. coli is found. Follow-up shows 0.15 mg/L free chlorine at the site and heavy biofilm on the restaurant's old galvanized service line. The state has asked for your monthly report.
1. Calculate your coliform percentage for the month, state whether you have an MCL violation, and state whether a Level 1 assessment is required.
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The denominator includes repeat samples. Repeat samples count in the monthly calculation, not just routine samples.
Total samples = 60 routine + 3 repeat = 63 Total-coliform-positive = 2 (one routine, one repeat) = 3.2%
MCL. For a system collecting 40 or more samples per month, the monthly MCL is triggered when more than 5.0% of routine and repeat samples are total-coliform positive. At 3.2% you are under the threshold - no MCL violation.
Level 1 assessment. For a system at 40+ samples per month, the Level 1 trigger is the same: >5.0% total-coliform-positive in a month, or any E. coli-positive routine or repeat sample, or a violation. At 3.2% with no E. coli, no assessment is required.
But note how thin the margin is. With a denominator of 63, three positives would be 4.8% and four would be 6.3%. So the difference between "clean month" and "violation plus assessment" is two more samples. That is the reason to be aggressive on the investigative side even though the arithmetic says you are compliant: you are one bad week from a Level 1, and two triggers in twelve months escalate to a Level 2.
2. Suppose the repeat at site 12 had instead come back E. coli positive. Walk through what changes, including deadlines.
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Everything escalates. E. coli in a repeat sample following a total-coliform-positive routine is an E. coli MCL violation - it is an acute violation, not a monthly-percentage question. The percentage math stops mattering.
Tier 1 public notice within 24 hours. Tier 1 is the immediate-notice category: media broadcast, hand delivery, posting - whatever your state's method requires - with health effects language for E. coli. Do not wait for the state to call back; the clock starts at notification of the result.
Five additional routine samples within 24 hours of being notified, at sites the state approves (typically in the same pressure zone or the area around the positive).
Level 2 assessment within 30 days, conducted by a party with the required qualifications or as your state directs. Level 2 is more rigorous than Level 1 and includes a state review; failure to complete it is itself a treatment technique violation.
Operationally: check the chlorine residual at and around the site, look for a cross-connection at the restaurant (carbon filter, grease trap, ice machine, irrigation), and consider a precautionary boil water notice for the affected area while you sample. Assume a pathway until you have ruled one out - E. coli is not a distribution biofilm organism the way total coliform can be; it points to a fecal source or a cross-connection.
3. What is your investigative and corrective plan for site 12, and what are your reporting deadlines?
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Investigate the pathway, not just the sample. Two positives at one site on a dead-end main with 0.15 mg/L residual and a biofilm-laden galvanized service suggests the service line and the dead end, not the main. Actions:
- Confirm the residual with a second test method and check nearby hydrants - is 0.15 mg/L local to this address or system-wide? A system-wide residual problem is a different diagnosis and a bigger one.
- Inspect the premises. Restaurant connections are classic: carbon filters that grow bacteria, improperly installed ice machine lines, a hose bib without a vacuum breaker, a grease trap with a submerged inlet. Look for a cross-connection before you accept a biofilm explanation.
- Flush the dead end on a scheduled program and add a blow-off or hydrant if the main geometry supports it; dead ends accumulate sediment and lose residual.
- Address the service line. Recommend replacement of the galvanized line; it is a reservoir that will keep seeding positives.
- Sample again at the site after corrective work, and increase routine sampling in that area for the next few months.
Reporting. Submit the monthly monitoring report to the state within 10 days after the end of the month, including the routine and repeat results, the sampling locations and dates, and the chlorine residuals. Document the investigation and the corrective actions in your files - if the next month produces another positive and you do end up in a Level 1 assessment, that record is what shows the state you were already working the problem rather than waiting for a violation to act.