Total Coliform Sampling Frequency Under the RTCR

How many total coliform samples your system must collect each month is one of the most common compliance questions an operator has, and the answer is not a single number. The Revised Total Coliform Rule (RTCR) in 40 CFR Part 141 Subpart Y sets your minimum routine sample count by the population you serve, with separate tracks for community, non-community, and seasonal systems. Missing the count is a monitoring and reporting violation even when the water is clean, and it lands on the same EPA SDWIS record your state reviews at every sanitary survey. Match the table below to your system, then check what your own record already shows.

The RTCR minimum by population served

For systems serving more than 1,000 people, the minimum number of routine total coliform samples per month is set by population in 40 CFR 141.857:

• 1,001 to 2,500 people: 2 samples per month
• 2,501 to 3,300: 3
• 3,301 to 4,100: 4
• 4,101 to 4,900: 5
• 4,901 to 5,800: 6
• 5,801 to 6,700: 7
• 6,701 to 7,600: 8
• 7,601 to 8,500: 9
• 8,501 to 12,900: 10
• 12,901 to 17,200: 15
• 17,201 to 21,500: 20
• 21,501 to 25,000: 25
• 25,001 to 33,000: 30
• 33,001 to 41,000: 40
• 41,001 to 50,000: 50
• 50,001 to 59,000: 60
• 59,001 to 70,000: 70
• 70,001 to 83,000: 80
• 83,001 to 96,000: 90
• 96,001 to 130,000: 100

Above 130,000 people the count continues to climb, reaching 120 samples per month at 130,001 to 220,000, then 150, 180, 210, 240, 270, 300, 330, 360, 390, 420, 450, and 480 as population bands step upward, with 480 per month applying to systems serving more than 3,960,000 people. The full table is in 40 CFR 141.857(b), and your state's approved monitoring schedule is the controlling document for your system.

Small community systems: one per month as the baseline

A community water system serving 1,000 or fewer people must take at least one routine total coliform sample each month. Your state can approve a less frequent schedule for a small system that meets the RTCR criteria for reduced monitoring, but the default is monthly, and a missed month is a violation. Systems that use only ground water and serve 4,900 or fewer people may collect all of the month's required samples on a single day, provided the samples come from different sites, under 40 CFR 141.853(a)(2).

Non-community systems: quarterly as the baseline

Non-community water systems using only ground water and serving 1,000 or fewer people monitor at least once each calendar quarter the system provides water to the public, under 40 CFR 141.854. A state may reduce a well operated system to annual monitoring when it meets three conditions: a clean compliance history for at least 12 months, a recent sanitary survey showing no uncorrected sanitary defects and a protected source, and an annual site visit by the state within the last 12 months. Seasonal systems monitor every month they operate unless they qualify for a less frequent schedule, and they must complete a state approved start up procedure each season.

When monitoring jumps to monthly

Small systems on quarterly or annual monitoring move back to monthly the month after any of these events: a level 2 assessment trigger or two level 1 assessment triggers in a rolling 12 month period, an E. coli MCL violation, a coliform treatment technique violation, or two monitoring violations in a rolling 12 month period. A system that collects a total coliform positive sample while on quarterly or annual monitoring must also take at least three routine samples the month after the positive, unless the state waives it under the conditions in 40 CFR 141.854(j).

What a positive sample triggers

A routine total coliform positive sample starts the clock. Within 24 hours you must collect repeat samples: one from the original tap, one within five service connections upstream, and one within five service connections downstream, under 40 CFR 141.853(a)(5). The repeats are tested for both total coliforms and E. coli. A total coliform positive with a negative repeat set triggers a level 1 assessment. An E. coli positive result, or a repeat sample that is positive for total coliforms, triggers the more serious level 2 assessment under 40 CFR 141.859, and an E. coli positive repeat sample is an E. coli MCL violation with its own public notice and corrective action requirements. See how to respond to an E. coli or total coliform violation.

Why the sample count is a compliance risk

Monitoring violations are the most common violation type in state SDWIS records, and they are almost always preventable. The sample schedule is set by your system's population and source type, your lab must be state certified, results must be reported by the state's deadline, and the paperwork must be retained for five years under the record schedule in 40 CFR 141.33. A system that misses a month, uses an uncertified lab, or submits results late earns a violation without any water quality failure. See how long to keep compliance records and how to build a compliance calendar.

Match the rule to your exact system

The RTCR minimums are the federal floor, and your state primacy agency sets the schedule that actually applies to your system, including your sample siting plan. The fastest way to see what your system already owes is the free lookup at orevant.com, which reads your EPA record by PWSID. For the full picture, including every monitoring deadline on your specific calendar, run the compliance scan.

Get Your Compliance Roadmap for $199. See what your system already owes free first: check your water system's record at orevant.com.

FAQ

How many coliform samples per month does a small community water system need?

At least one routine total coliform sample each month for a community system serving 1,000 or fewer people, unless your state has approved a reduced schedule under the RTCR criteria. Systems serving more than 1,000 people follow the population table in 40 CFR 141.857.

Can my state reduce my total coliform monitoring frequency?

Yes, for small ground water systems that qualify. Non-community systems serving 1,000 or fewer can move from quarterly to annual monitoring with a 12 month clean history, a sanitary survey free of uncorrected defects, and an annual state site visit. Reduced schedules are never automatic; your state must approve them in writing.

What happens the month after a total coliform positive sample?

You collect repeat samples within 24 hours and, if you are on quarterly or annual monitoring, at least three additional routine samples the following month unless the state waives the requirement. The results also determine whether you owe a level 1 or level 2 assessment under 40 CFR 141.859.

Is a missed total coliform sample a violation?

Yes. A missed or late routine sample is a monitoring and reporting violation under the RTCR, recorded on your SDWIS history even if your water quality is perfect. Two monitoring violations in a rolling 12 months also move a small system off its reduced schedule and back to monthly monitoring.

Sources: 40 CFR Part 141 Subpart Y (Revised Total Coliform Rule), sections 141.853, 141.854, 141.855, 141.857, 141.858, 141.859; EPA SDWIS public records. Orevant provides compliance information tooling, not legal advice.