7.3 Bacteriology, Sample Siting Plans & RTCR Compliance Sampling

7.3 Bacteriology, Sample Siting Plans & RTCR Compliance Sampling

The Board's needs-to-know tables score Bacteriology on every surface and well examination, drawing from the Rules Governing Public Water Systems. This section covers what the organisms mean, how the samples are taken, and what happens when one comes back positive.

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1. Indicator organisms

Coliforms are used because they are numerous when contamination exists, survive longer in water than most pathogens, and are inexpensive to detect — the pathogens themselves are too sporadic and too costly to monitor directly.

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2. Sample siting plan and frequency

Every system must maintain a written sample siting plan listing the routine and repeat sampling locations and the schedule. Requirements the plan must satisfy:

• Sites must be representative of water throughout the distribution system, not clustered near the plant.
• The plan identifies repeat sample locations upstream and downstream of each routine site.
• Routine monitoring frequency is set by population served, from once per month for the smallest systems up to many samples per month for large systems.
• Sites must be protected from cross-connection and from sampling errors — no leaking faucets, no swivel or aerator taps, no hose bibs adjacent to hazards.

Distribution residual disinfectant sampling runs alongside coliform monitoring: under 18C .1302(a)(2) Table A, Class D systems collect 1 residual sample weekly, Class C systems 3, and Class A and B systems 5, at locations representing maximum residence time or high water age, with same-day samples taken from different locations.

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3. Correct sampling technique

1. Select an approved sampling tap on a service line — not a hose bib, not a swivel faucet, not a tap with an aerator or screen.
2. Remove the aerator if present; disinfect the tap if the site protocol requires it.
3. Flush long enough to draw water from the main, typically 3 to 5 minutes, then reduce to a smooth pencil-sized stream.
4. Measure the disinfectant residual and record it with the sample.
5. Open the sterile bottle without touching the inside of the cap or bottle neck; do not rinse the bottle — it contains sodium thiosulfate to neutralize the chlorine residual so bacteria are not killed in transit.
6. Fill to the 100 mL fill line, leaving headspace for mixing, and cap immediately.
7. Label completely, complete the chain of custody, and place on ice at below 10 °C without freezing.
8. Deliver so the laboratory can begin analysis within the drinking water holding time — 30 hours from collection (wastewater fecal coliform samples have a far shorter 8-hour limit).

Analytical methods. Membrane filtration passes 100 mL through a 0.45 micrometre filter incubated on a selective medium; m-Endo produces coliform colonies with a green metallic sheen. Defined-substrate tests such as Colilert use ONPG, hydrolyzed by beta-galactosidase to give a yellow color for total coliforms, and MUG, hydrolyzed by beta-glucuronidase to fluoresce blue under 365 nm UV for E. coli. Incubation is 24 hours at 35 °C ± 0.5 °C.

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4. When a sample is total coliform positive

   Routine sample TOTAL COLIFORM POSITIVE
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            +--> Analyze that sample for E. coli
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            +--> Collect REPEAT samples within 24 hours:
                   - at the original site,
                   - within five service connections UPSTREAM,
                   - within five service connections DOWNSTREAM
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            +--> Any repeat positive -> another round of repeats
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            +--> Evaluate against the ASSESSMENT TRIGGERS

Assessment triggers. A Level 1 assessment is required when a system collecting 40 or more samples per month has more than 5.0 percent of samples total coliform positive, when a system collecting fewer than 40 samples has two or more total coliform positive samples in a month, or when a system fails to take every required repeat sample. A Level 2 assessment, performed by an approved party, is required after an E. coli MCL violation or after a second Level 1 trigger within a rolling 12-month period. Sanitary defects identified in either assessment must be corrected, and the assessment form must be submitted to the State.

The E. coli MCL is violated when a routine sample is total coliform positive and a repeat is E. coli positive, when a routine is E. coli positive and a repeat is total coliform positive, when a system fails to take all required repeats after an E. coli positive routine, or when a system fails to test for E. coli after a total coliform positive repeat. That violation is acute and requires Tier 1 public notice within 24 hours.

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5. Investigating a positive — the operator's checklist

• Confirm the sampling technique and site: aerator, hose, unflushed line, non-sterile handling, or an expired bottle explain a great many single positives.
• Check the residual at the site and upstream: a zero or near-zero residual points to water age, a dead end, or a loss of feed.
• Look for pressure loss events — main breaks, hydrant use, pump failures, fires — in the preceding days.
• Inspect storage: hatch seals, vents, overflow screens, and floating covers are the classic intrusion pathways.
• Review cross-connection potential at and near the site.
• Flush, re-disinfect where appropriate, and re-sample; document every step in the log.

[!WARNING] Do not ‘wash out’ a positive. Flushing a line and resampling to obtain a clean result without collecting the required repeat samples is both a monitoring violation and a missed chance to find the real defect. Take the repeats the rule requires first.