The RTCR changed how we think about coliform results. Under the old Total Coliform Rule, a positive result was primarily a compliance event. Under the RTCR, a positive result is a signal to investigate. That is a meaningful shift, and the T-5 exam tests whether you understand the difference.
I deal with coliform monitoring on a regular basis at my plant. The thing I wish I had understood earlier: the RTCR is not really about coliforms. It is about distribution system integrity. Total coliforms are the canary in the coal mine. E. coli is the actual threat. The rule uses one to find the other, and the assessment and corrective action process is designed to fix the vulnerabilities that let contamination in, not just to count bacteria.
Let me walk you through how the rule works, what triggers what, and the numbers the exam expects you to know.
What Changed from the Old Total Coliform Rule to the RTCR?
The RTCR shifted from a detect-and-report approach to a find-and-fix approach. Under the old rule, total coliform results above a threshold were MCL violations. Under the RTCR, total coliform results trigger assessments designed to identify and correct the underlying problem.
That is more than a wording change. It means total coliforms are no longer treated as the health threat themselves. E. coli is the sole acute health-based indicator, with an MCL and an MCLG of zero. Total coliforms serve as indicators of distribution system vulnerability, flagging conditions that could allow pathogens to enter the system.
The RTCR applies to all public water systems: CWS, NTNC, and TNC. California implemented the rule through Title 22 CCR sections 64421 through 64426.9, effective July 1, 2021. The federal rule was published in 2013 with a compliance date of April 1, 2016.
The biggest change from the old rule to the RTCR is the purpose of a total coliform-positive result. Under the old rule, it was a potential MCL violation. Under the RTCR, it is an assessment trigger. The exam will test whether you understand this distinction.
What Constitutes an E. coli MCL Violation?
E. coli is the only contaminant with an MCL under the RTCR. The MCLG is zero. An E. coli MCL violation occurs in any of four scenarios, and you need to know all four for the exam.
First, an E. coli-positive repeat sample follows a TC-positive routine sample. Second, a TC-positive repeat sample follows an E. coli-positive routine sample. Third, the system fails to collect all required repeat samples after an E. coli-positive routine sample. Fourth, the system fails to test for E. coli when any repeat sample comes back TC-positive.
That third and fourth scenario catch systems that try to avoid bad news by not collecting the samples. The rule is structured so that failing to look for the problem is treated as seriously as finding it.
When an E. coli MCL violation occurs, the system must notify the State Board by end of day and issue a Tier 1 public notice within 24 hours. No variances or exemptions from the E. coli MCL are permitted.
The E. coli MCL violation triggers are a high-frequency exam topic. Notice that two of the four scenarios involve failure to collect or test, not detection of contamination. The rule penalizes inaction.
How Many Routine Samples Are Required?
The number of routine total coliform samples required each month depends on the population served and the number of service connections. These are the key values for the exam.
| Population Served | Service Connections | Minimum Samples Per Month |
|---|---|---|
| 25 to 1,000 | 15 to 400 | 1 |
| 1,001 to 2,500 | 401 to 890 | 2 |
| 2,501 to 3,300 | 891 to 1,180 | 3 |
| 3,301 to 4,100 | 1,181 to 1,460 | 4 |
| 25,001 to 33,000 | 8,901 to 11,800 | 30 |
| 96,001 to 130,000 | 34,301 to 46,400 | 100 |
| 3,960,001 and above | 1,414,301 and above | 480 |
You do not need to memorize this entire table. The exam typically tests the concept (larger systems sample more frequently) and may give you a specific population to look up. The critical thing to know is that these are minimum requirements. Your state may require more.
What Happens When a Routine Sample Is Total Coliform Positive?
A TC-positive routine sample triggers repeat sampling. This is the first step in the find-and-fix process, and the requirements are specific.
At least 3 repeat samples must be collected within 24 hours of notification. At least one must come from the original tap where the positive was found. The others must be collected within 5 service connections upstream and downstream of the original location. All repeats should be collected within the same 24-hour period.
The catch is what happens next. If any repeat sample comes back TC-positive, the system must collect another full set of repeats. This cycle continues until either a complete set of repeats comes back negative, or a treatment technique trigger is exceeded and an assessment is required.
Every repeat sample that is TC-positive must also be tested for E. coli. Failing to test for E. coli when a repeat is TC-positive is itself an E. coli MCL violation. That is one of the four violation scenarios covered in the previous section.
The repeat sampling requirements are very testable. Remember the numbers: at least 3 repeats, within 24 hours, at least 1 from the original tap, others within 5 service connections upstream and downstream. The exam often presents scenarios and asks whether the system followed proper procedure.
What Triggers a Level 1 or Level 2 Assessment?
Assessments are the core of the RTCR's find-and-fix approach. They are triggered by specific conditions, and the exam expects you to know exactly what those conditions are.
| Assessment Level | Trigger Conditions |
|---|---|
| Level 1 | Systems collecting 40 or more samples per month: more than 5.0% TC-positive in a month |
| Level 1 | Systems collecting fewer than 40 samples per month: 2 or more TC-positive in the same month |
| Level 1 | System fails to collect every required repeat sample |
| Level 2 | E. coli MCL violation |
| Level 2 | Second Level 1 trigger within a rolling 12-month period |
The progression matters. Level 1 is the first response. Level 2 is more serious, triggered either by an E. coli MCL violation or by a pattern of recurring problems (second Level 1 trigger within 12 months). Think of Level 1 as "something might be wrong, investigate." Level 2 is "something is definitely wrong, the State Board gets involved."
A Level 2 Assessment is triggered by either an E. coli MCL violation or a second Level 1 trigger within 12 months. The exam frequently tests this escalation pathway. A system that has one Level 1 trigger in January and another in October is now facing a Level 2.
What Do Assessments and Corrective Actions Involve?
Both assessment levels follow a similar review process, but they differ in who conducts them and how thorough they are.
A Level 1 Assessment is conducted by the system operator or owner. It requires reviewing sample sites and protocols, identifying atypical events, examining changes to the distribution system, evaluating source water and treatment, and reviewing existing water quality data. The goal is to identify sanitary defects and monitoring deficiencies. The completed assessment must be submitted within 30 days.
A Level 2 Assessment is arranged with and conducted by the State Board. It covers the same review elements as Level 1, but more thoroughly. For E. coli MCL violations, expedited actions are required. The completed assessment is also due within 30 days.
For both levels, every sanitary defect identified must be corrected. The system must notify the State Board within 5 business days when each corrective action is completed. Failing to complete an assessment or failing to correct identified sanitary defects is a treatment technique violation.
That last point deserves emphasis. The RTCR does not just require you to find problems. It requires you to fix them. The assessment without the corrective action is itself a violation.
What Are the Sample Siting Plan Requirements?
Every system must maintain a written sample siting plan that identifies the specific locations and collection schedule for total coliform monitoring. The plan must be representative of the entire distribution system, covering each pressure zone, each source area, and each reservoir.
The plan must include regular sites, alternative sites (used when a regular site is unavailable), dual purpose sites (used for both coliform and disinfectant residual monitoring), and special purpose sites (used during assessments or investigations).
Plans must be updated at least every 10 years, or within 30 days if the existing plan is no longer compliant with current requirements.
Two additional requirements show up on the exam. Only the State Board can invalidate a coliform sample, and a documented reason is required (such as improper collection technique or lab error, not just an inconvenient result). Seasonal systems that shut down and restart must develop a State Board-approved start-up procedure, complete it, and receive written approval before serving water. Failure to complete the start-up procedure is a coliform treatment technique violation.
The RTCR is one of the more nuanced rules on the T-5 exam, and it rewards understanding over memorization. If you grasp the find-and-fix philosophy, the E. coli MCL triggers, and the Level 1 to Level 2 escalation pathway, the specific numbers and timelines will fall into place around that framework.
I will continue adding T-5 study guides on H2oCareerPro.com as I work through each regulation. If this guide helped clarify the RTCR for you, share it with a colleague who is also preparing. We are all in this together.
This guide is for educational purposes and reflects federal and California regulations as of April 2026. Always verify current requirements with your state regulatory agency. Regulations and their implementation may have changed since publication.
Frequently Asked Questions
What triggers a Level 1 Assessment under the RTCR?
A Level 1 Assessment is triggered when a system collecting 40 or more samples per month has more than 5% total coliform-positive results, when a system collecting fewer than 40 samples per month has 2 or more TC-positive results in the same month, or when a system fails to collect all required repeat samples. The assessment must be completed and submitted within 30 days.
What is the difference between a Level 1 and Level 2 Assessment?
A Level 1 Assessment is conducted by the system operator or owner and reviews sample sites, atypical events, distribution changes, and water quality data to identify sanitary defects. A Level 2 Assessment is more comprehensive, arranged with and conducted by the State Board. Level 2 is triggered by an E. coli MCL violation or a second Level 1 trigger within a rolling 12-month period.
How many repeat samples are required after a TC-positive result?
At least 3 repeat samples must be collected within 24 hours of notification of a TC-positive routine sample. At least 1 must come from the original tap, and the others must be collected within 5 service connections upstream and downstream. If any repeat is TC-positive, another full set must be collected. All TC-positive repeats must be tested for E. coli.
What constitutes an E. coli MCL violation under the RTCR?
An E. coli MCL violation occurs when an E. coli-positive repeat follows a TC-positive routine, when a TC-positive repeat follows an E. coli-positive routine, when a system fails to collect all required repeats after an E. coli-positive routine, or when a system fails to test for E. coli when any repeat is TC-positive. Any violation requires Tier 1 public notification within 24 hours.
What is a sanitary defect under the RTCR?
A sanitary defect is any condition that could provide a pathway for microbial contamination to enter the distribution system, or that indicates a failure or imminent failure in a barrier designed to prevent contamination. Sanitary defects are identified during assessments and must be corrected. Failure to correct identified defects is a treatment technique violation under the RTCR.