One Routine Total-Coliform Positive With a Weak Residual

You have from Tuesday 8 a.m. until Wednesday 8 a.m. The clock is the rule; the cause is your investigation.

1. Collect three repeat samples: one at the original site, one within five service connections upstream, and one within five service connections downstream. Use correct technique: clean tap or screen, sterile bottle, no leaks, chilled, delivered inside the holding time.
2. Measure and record free chlorine at every repeat site, plus pH and temperature, on the sample form. The residual reading is part of the sample's meaning.
3. Examine the site while you are standing there. Is it a hose bib with a swivel, a fouled screen, a dripping packing, a recent repair, a private irrigation connection? If the tap itself is suspect, collect at an alternate site and document why.
4. Tell the lab to run E. coli on the original and on all repeats, and confirm the notification time in writing.
5. Log everything: date, time, site, collector, chlorine reading, condition of the tap. Under the rule, a missed repeat is itself a violation, and your documentation is the only thing standing between a sampling problem and an MCL.

A single total-coliform positive that is E. coli negative is not yet an MCL violation. What you do in the next 24 hours decides whether it becomes one.

• All three repeats negative (total coliform and E. coli): no MCL violation. The routine positive is still a monitoring result and your follow-up sampling obligations continue. Investigate the likely local cause (sampling technique, a stagnant dead end, a compromised tap) and keep the site in the rotation.
• Any repeat is total-coliform positive: the total-coliform MCL is exceeded. That triggers a formal system assessment and corrective action within the required timeframe (Level 1 or Level 2 depending on your sample count and history; confirm which applies to a 40-sample system), increased routine sampling the following month, and a Tier 2 public notice within 30 days.
• Any repeat is E. coli positive: E. coli MCL violation. Tier 1 public notice within 24 hours, a boil water notice coordinated with FDEP, an assessment, and immediate corrective action. Do not wait for the paperwork to catch up with the phone call.

Test the logic against the numbers: one hit, a 0.35 mg/L residual above the floor but trending low, unlined cast iron, no flow, and a construction site nearby all point to a local distribution condition rather than treatment failure. That is a reasonable hypothesis and completely irrelevant to the rule. You collect the repeats and let the results decide which branch you are on.

The 4,000 feet of unlined cast iron is the prime suspect. Tuberculation and sediment in a low-flow dead end shield bacteria from chlorine and consume residual, which is exactly what 0.35 mg/L at the far end looks like.

• Flush unidirectionally to a dechlorinated discharge point, recording flush volume and residual before and after. Re-sample once turbidity clears.
• Hunt the cross-connection. A construction site two streets over is a realistic source: a temporary hose, an unapproved tie, or an irrigation system on the new line. Walk it.
• Check the residual trend in your records for that area over the past month. A slow decline at one site is a distribution problem; a step change at many sites is a treatment problem. They call for different responses.
• If flushing does not restore residual above your target, plan a durable fix: an automatic flusher, looping the dead end, or a rechlorination point. Put it in the capital plan with the data behind it.
• Protect the rest of the sampling program. Do not let flushing or repair work contaminate other sample sites, and note flushing volumes so a later low residual is not misread as a new problem.