Two Total Coliform-Positive Routines, a Missed Repeat, and the Assessment Question
A 12,000-population community water system collects 10 routine total coliform samples per month under the Revised Total Coliform Rule. In June, two routine samples are total-coliform positive by the presence/absence method: one at the elementary school tap and one at a dead-end hydrant that has been a sampling site for 15 years. The system collects three repeat samples at each positive site (the site, one upstream, one downstream) within 22 hours; all six repeats are negative. A seventh required repeat, upstream of the school site, was never collected because the sampler ran out of bottles. The system uses chloramine, and there are no E. coli-positive samples. The June total chlorine residual at the school site was 0.15 mg/L, and a hydrant flush the week before produced rusty water. The last sanitary survey, 14 months ago, found no significant deficiencies.
1. Is this an MCL violation, a monitoring violation, or both? Determine the assessment level the system must complete and show your counting.
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Both, and the assessment level is the part most people get wrong.
MCL counting. The rule sets the percentage threshold by monitoring frequency, not by population. A system collecting fewer than 40 samples per month exceeds the MCL with more than one total-coliform-positive sample; a system collecting 40 or more uses 5%. This system collects 10 routine samples, so any two positives exceed the MCL. Two positives here, even though all repeats were negative. Repeat samples do not erase routine positives, so the MCL violation stands.
Assessment level. An MCL violation triggers a Level 2 assessment, not a Level 1. Level 2 must be conducted by the state or a state-approved party and completed within 30 days. A Level 1 would apply to a system collecting 40 or more samples per month that exceeded 5% through routine and repeat samples, or that had other Level 1 triggers.
Monitoring violation. The missing upstream repeat is a separate RTCR monitoring violation. It does not change the MCL count, but it is reportable and belongs in the record.
2. List what the system must do, and by when, including public notification.
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- Level 2 assessment: completed within 30 days of the triggering sample, by the state or a qualified third party. It is more detailed than a Level 1 and must identify the likely cause and any needed corrective actions.
- Corrective actions: complete any required actions identified by the assessment, on the state's schedule. If the state requires corrections and they are not made, that becomes a treatment technique violation.
- Public notice: an MCL violation for total coliform is Tier 2, issued within 30 days. Monitoring violations are Tier 3, reported within a year but usually best handled immediately in the CCR and on the website.
- Records: retain the sampling plan, the repeat results, the missed-sample documentation, and the assessment report.
Do not wait for the assessment to fix obvious causes. A 0.15 mg/L total chlorine residual at the school site, in a chloraminated system, is a residual failure that explains a positive sample better than anything random. Flush and restore residual at that site now, and document that you did it.
3. How would you evaluate the sampling site selection and the chloramine residual problem, and what would you change?
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The hydrant that has served as a site for 15 years is a candidate for replacement, not out of superstition but because a dead-end hydrant with historic rusty water is a poor representative of distribution water quality and is prone to artifacts from corrosion and stagnation. Sample sites should be spread across the distribution system, reflect different hydraulic conditions, and be chosen from the sampling plan rather than by convenience.
The stronger signal is the school tap at 0.15 mg/L total chlorine. In a chloraminated system that indicates either stagnation in the building plumbing, a long dead leg, or nitrification in that area. Check whether the school tap is at the meter or after building plumbing, run the line before sampling, and add free ammonia and nitrite at that site to test for nitrification.
Going forward, update the sampling plan, rotate sites so the system is characterized rather than one hydrant, and link coliform site selection to places where residual is weakest. That is the corrective action most likely to prevent a repeat MCL violation.