Bacteriological Sampling — How to Take Compliant Samples Without Contamination

The bacteriological sample is the most consequential routine sample an operator takes. A single positive coliform result triggers repeat sampling, customer notification, and explanations to your state regulator. A false positive — caused by contamination during collection rather than actual contamination in the distribution system — costs your utility staff time, customer trust, and sometimes a boil-water notice that didn't need to happen. Doing it right the first time is one of the lowest-effort, highest-leverage skills an operator builds.

This guide covers how to collect a compliant bac-T sample under the Revised Total Coliform Rule (RTCR): site selection, tap preparation, sample collection, preservation, chain of custody, and the operator habits that prevent contamination.

TL;DR

• The Revised Total Coliform Rule (RTCR) requires every community water system to take routine bacteriological samples on a schedule based on system size, plus repeat samples whenever a routine sample is positive for total coliform or E. coli.
• Sample sites must rotate through approved locations in your written sampling plan — typically 1 sample per 1,000 customers per month, minimum 1.
• Site selection: prefer interior, non-aerated cold-water taps in commercial or public buildings. Avoid kitchen faucets with screens or aerators, hoses, mixing valves, and outdoor spigots.
• Collection procedure: flush the tap 2–3 minutes, disinfect the spout (flame or hypochlorite), use a sterile bottle with sodium thiosulfate (for chlorinated systems), fill to the bottle's fill line leaving headspace, do not rinse the bottle.
• Hold samples on ice at ≤10°C and deliver to a certified lab within 30 hours of collection.
• Practice with the sampling practice test; related guides include the regulations overview and the SDWA explained guide.

What RTCR actually requires

The Revised Total Coliform Rule replaced the original Total Coliform Rule (TCR) in 2016. The big change: instead of treating any single coliform-positive sample as a violation, RTCR uses a "find-and-fix" framework. Coliform-positive samples trigger an investigation (Level 1 or Level 2 Assessment) and corrective action. Violations come from failing to do the assessment or from E. coli–positive results.

Two indicator organisms matter:

Total coliform — a broad group of bacteria common in soil, vegetation, and the gastrointestinal tract of warm-blooded animals. Their presence in finished water indicates that some pathway exists for environmental contamination to enter the distribution system. Not necessarily a health risk by itself — but a strong signal that something has changed.

E. coli — a specific coliform that comes almost exclusively from the gut of warm-blooded animals (including humans). E. coli presence indicates probable fecal contamination, which means probable pathogen contamination. E. coli–positive samples are an acute health violation requiring Tier 1 public notification within 24 hours.

Sample frequency for community water systems is based on population served:

Population served	Routine samples per month
25–1,000	1
1,001–2,500	2
2,501–3,300	3
3,301–4,100	4
4,101–4,900	5
4,901–5,800	6
5,801–6,700	7
6,701–7,600	8
...	up to 480 for very large systems

Each sample must be collected at an approved location listed in the system's written sample siting plan. The plan rotates samples through representative locations across the distribution system over time — you can't sample the same easy tap every month.

Sample siting plan

Every public water system must have a written sample siting plan on file with the state primacy agency. The plan identifies routine sample locations representative of conditions throughout the distribution system. The plan must:

• List all approved routine sample sites
• Describe the rotation schedule (which sites get sampled in which months)
• Identify representative locations across pressure zones, water ages, and customer types
• Designate repeat sample sites for each routine site (typically 1 upstream of, 1 at, and 1 downstream of the routine site)

Most plans include 10–50+ approved sites depending on system size, so the rotation can cover the distribution system completely over a year. The site list is reviewed during sanitary surveys.

Choosing a good sample site

Within the approved plan, day-to-day site selection — picking which approved tap to use today — is the operator's job. The principles:

Prefer interior, cold-water taps. Cold water from a dedicated tap (not mixed with hot) is best. Hot water can scald, kills bacteria, and doesn't represent typical customer-tap water.

Prefer non-aerated taps. Faucet aerators harbor biofilm and can contaminate a sample even if the distribution water is clean. Many sample sites use a dedicated, capped sample tap with no aerator.

Avoid kitchen sinks with attached hoses, sprayers, or filters. These accumulate microbes. The water in the line is fine; the contamination comes from the device itself.

Avoid outdoor spigots and hose bibs. They're exposed to weather, soil, insects, and frequent low-velocity backflow events.

Avoid mixing valves and shower-only taps. Mixing valves can entrain hot water and skew results.

Look for locations representative of typical customer exposure. A school cafeteria's main cold-water tap is good. A janitor's mop sink is bad.

Avoid sites with known recent disturbance. A neighborhood that just had a water main repair will have transient quality issues — sampling there gives unrepresentative results.

For repeat sampling after a positive routine, the standard pattern is to sample three sites: one upstream of the positive site, the positive site again, and one downstream. This locates the contamination source.

Sample collection procedure

A standard procedure looks like this:

1. Prepare the tap. Remove any aerator, screen, hose, or attached filter. If the tap has a swivel head, lock it straight down. Inspect the spout for grime or biofilm.

2. Flush. Open the cold water full and let it run for 2–3 minutes. The goal is to clear the standing water in the building's service line and bring distribution water to the tap. For long, oversized service lines this might take longer — flush until water feels at distribution-system temperature.

3. Disinfect the tap spout. Two acceptable methods: - Flame disinfection (preferred for outdoor taps and metal spouts): briefly heat the spout with a propane torch or alcohol flame for 5–10 seconds. Let it cool before sampling. - Hypochlorite disinfection (for plastic or sensitive fixtures): wipe the spout with a 100 ppm chlorine solution, let it air-dry for at least 30 seconds.

Skip disinfection on shut-off-style sample taps designed to be permanently sterile — overzealous disinfection can damage them.

4. Reduce flow. Throttle the tap to a steady, pencil-thin stream — about 1/8 inch in diameter. A high-velocity stream aerates the sample and can introduce contamination.

5. Open the sample bottle. Use a sterile, pre-prepared sample bottle from your certified lab. The bottle contains a measured amount of sodium thiosulfate (for chlorinated samples — it quenches residual chlorine that would otherwise kill bacteria in the bottle during transit).

6. Fill the bottle. Hold the bottle under the stream without letting the spout touch the bottle's neck or rim. Fill to the bottle's marked fill line (typically 100 mL). Do not rinse the bottle first — that washes out the thiosulfate. Leave airspace at the top (an inch or so) so the lab can mix the sample properly.

7. Cap the bottle. Without touching the inside of the cap, screw it on tight.

8. Label. Mark the bottle with system name, sample location, date, time, collector's name, and free chlorine residual measured at the same tap.

9. Preserve. Place the bottle on ice or in a refrigerated cooler. Hold at ≤10°C until delivered to the lab.

10. Deliver. Get the sample to a certified lab within 30 hours of collection. Some states require <30 hours; many require <8 hours for chlorinated samples specifically — check your state rule.

Chain of custody

Every sample must have documentation that follows it from collection to lab analysis. The chain-of-custody form includes:

• System name and PWSID
• Sample location (specific address, not just "system distribution")
• Sample type (routine, repeat, special)
• Date and time of collection
• Collector's name and signature
• Sample preservation method (sodium thiosulfate + ice)
• Free chlorine residual at the tap
• Lab receiving the sample
• Any unusual conditions during collection (e.g., "tap was running before I arrived")

If the chain of custody is broken — sample arrives late, packaging is compromised, label is missing — the sample is invalid and you have to re-sample within a specified window.

What contamination looks like

The most common operator-caused contamination patterns:

Touching the bottle interior or cap interior. Your fingers carry coliforms. Any contact with the bottle's interior surfaces contaminates the sample.

Spout touching the bottle. Even briefly, even barely. The spout has whatever biofilm survived disinfection; transferring it to the bottle puts it directly into the sample.

Inadequate flush. Standing water in the customer's plumbing (especially galvanized or copper pipe) can support biofilm growth. Sampling without flushing pulls in that biofilm.

Aerator on the tap. Aerators are notorious harborage sites. Even one cell can grow to detectable levels in the bottle if the thiosulfate has neutralized the chlorine.

Sample sitting unrefrigerated. Coliforms multiply in 24-30°C water within hours. A sample left in a hot truck for an afternoon may be useless by the time it reaches the lab.

Sample older than 30 hours. Lab will reject and you have to re-collect.

What to do when a sample is positive

A total coliform–positive routine sample triggers:

1. Immediate repeat sampling — within 24 hours of being notified by the lab. Collect: - One sample at the same location as the positive - One sample at a connection within 5 service connections upstream - One sample at a connection within 5 service connections downstream - (Some states also require additional samples in different parts of the distribution system)

2. Check for E. coli. If the original or any repeat is E. coli–positive, this is a Tier 1 public notification — boil-water notice within 24 hours.

3. Level 1 Assessment — if two or more total-coliform-positive samples in a month, or if the system fails to take all required repeat samples. The system inspects itself and identifies the likely cause (open hatch on storage tank, recent main break, low chlorine residual, etc.) and documents corrective action.

4. Level 2 Assessment — for E. coli-positive samples or for repeated Level 1 triggers. A more rigorous review by a certified third party (typically the state primacy agency or a registered engineer).

5. Documentation — every step, including assessments and corrective actions, must be reported to the state primacy agency within specified timeframes.

A clean repeat sample doesn't end the investigation — the assessment process still proceeds based on the routine sample's results.

Common exam pitfalls

Confusing TCR and RTCR. The Revised TCR (2016+) is current; the original TCR is historical. Don't use 1989-era violation rules.

Forgetting the sodium thiosulfate purpose. It quenches chlorine in chlorinated samples so the chlorine doesn't keep killing bacteria in the sample bottle on the way to the lab.

Thinking E. coli and total coliform are the same. Total coliform is a broad group (environmental + fecal); E. coli is specifically fecal. E. coli is an acute health violation; total coliform alone usually isn't.

Skipping the disinfection step. Flame or hypochlorite disinfection of the tap is required by most state procedures, even though it's tempting to skip on familiar taps.

Using kitchen-faucet aerators or hose bibs. Both are notorious contamination sources and excluded from approved sample sites.

Holding samples longer than 30 hours. The lab will reject. Plan delivery windows around the lab's hours.

Quick reference

• Rule: Revised Total Coliform Rule (RTCR), effective 2016
• Frequency: 1 sample per 1,000 customers per month, minimum 1
• Sites: rotating, listed in your written sample siting plan
• Flush: cold water full open, 2–3 minutes minimum
• Disinfection: flame (5–10 sec) or hypochlorite wipe (30+ sec air dry)
• Bottle: sterile, sodium thiosulfate preservative, fill to 100 mL with airspace
• Preservation: on ice at ≤10°C
• Hold time: 30 hours max to certified lab
• Repeat samples: 3 samples (upstream, at, downstream) within 24 hours of a positive
• E. coli positive: Tier 1 public notification within 24 hours

Practice and next steps

• Free sampling practice test — 50 questions on collection procedures, preservation, chain of custody, and RTCR compliance.
• Free regulations practice test — broader regulatory framework around sampling.
• Safe Drinking Water Act overview — the law authorizing RTCR.
• MCLs explained — coliform and E. coli compliance in context of MCL framework.
• DBP sampling — where, when, and how — companion guide for Stage 2 D/DBPR sampling.
• Lead and Copper Rule guide — the parallel sampling protocol for lead/copper compliance.

Bacteriological sampling is the operator's most public regulatory moment. The lab result is reported to the state, sometimes to customers, and always to your sanitary survey file. Good technique makes the result mean something. Bad technique makes the result mean nothing — or worse, mean the wrong thing.