Chlorine Residual in Your CCR

Chlorine residual is the concentration of disinfectant that remains in drinking water after treatment — it protects against microbial contamination as water travels through distribution pipes to your tap. Federal law requires utilities to measure it, cap it, and report it in the annual Consumer Confidence Report.

Why chlorine is in your drinking water

Water treatment removes pathogens, but the water still has to travel through miles of pipes before it reaches any customer. Without a disinfectant that stays active in the distribution system, bacteria can regrow between the treatment plant and the tap.

The Surface Water Treatment Rule (40 CFR Part 141 Subpart H) requires systems using surface water or groundwater under the direct influence of surface water to maintain a detectable disinfectant residual throughout the distribution system. Most systems use chlorine (hypochlorous acid or sodium hypochlorite), chloramines (combined chlorine), or in some cases chlorine dioxide to meet this requirement. The requirement isn't optional — it's one of the foundational protections against waterborne illness outbreaks.

The statutory chain runs from SDWA § 1414(c)(4), added by the 1996 SDWA amendments (Pub. L. 104-182), through the 2018 America's Water Infrastructure Act (AWIA, Pub. L. 115-270), to the 2024 CCR Rule Revisions (89 FR 46013, May 24, 2024).

Chlorine, chloramines, chlorine dioxide: the three regulated disinfectants

EPA regulates three primary residual disinfectants under the Disinfectants and Disinfection Byproducts Rule (D/DBPR), each with its own Maximum Residual Disinfectant Level (MRDL) codified at 40 CFR § 141.65.

| Disinfectant | MRDL | Units | Compliance metric | |---|---|---|---| | Chlorine | 4.0 | mg/L as Cl₂ | Running annual average (RAA) | | Chloramines | 4.0 | mg/L as Cl₂ | Running annual average (RAA) | | Chlorine dioxide | 0.8 | mg/L as ClO₂ | Daily measurement |

Chlorine is the most common disinfectant. Utilities add it as gas (Cl₂), liquid sodium hypochlorite, or dry calcium hypochlorite. It is effective against most bacteria and many viruses and is easily measured with standard colorimetric tests.

Chloramines form when utilities add ammonia to chlorinated water. The resulting combined chlorine residual is more stable in long distribution systems and produces lower levels of some disinfection byproducts (though it produces different ones). Systems that switch between chlorine and chloramines must combine monitoring results from both in the RAA calculation, and reports must distinguish which disinfectant was analyzed for each sample. (40 CFR § 141.133(c)(1)(i))

Chlorine dioxide has a lower MRDL (0.8 mg/L) because its health-effects profile differs from chlorine and chloramines. Its compliance is measured daily rather than as an annual average. It is less commonly used for distribution-system residual maintenance but appears in some surface water treatment applications.

The corresponding Maximum Residual Disinfectant Level Goals (MRDLGs) match the MRDLs: 4 mg/L for chlorine, 4 mg/L for chloramines, and 0.8 mg/L for chlorine dioxide. (40 CFR § 141.65)

Health effects

Federal regulations require every utility that uses one of these disinfectants to print the following language verbatim in the CCR, as specified in Appendix A to Subpart O of 40 CFR Part 141 (referenced at 40 CFR § 141.153(d)(6)):

Chlorine:

"Some people who use water containing chlorine well in excess of the MRDL could experience irritating effects to their eyes and nose. Some people who drink water containing chlorine well in excess of the MRDL could experience stomach discomfort."

Chloramines:

"Some people who use water containing chloramines well in excess of the MRDL could experience irritating effects to their eyes and nose. Some people who drink water containing chloramines well in excess of the MRDL could experience stomach discomfort or anemia."

Chlorine dioxide:

"Some infants and young children who drink water containing chlorine dioxide in excess of the MRDL could experience nervous system effects. Similar effects may occur in fetuses of pregnant women who drink water containing chlorine dioxide in excess of the MRDL. Some people may experience anemia."

Utilities may not substitute paraphrased language for these statements unless the state primacy agency has approved alternative text. The standard language applies when the disinfectant is detected at any level — it is not conditional on an exceedance.

MRDL vs MCL: how a residual is regulated differently

Most contaminants in drinking water have a Maximum Contaminant Level (MCL) — a ceiling the utility must stay below. Chlorine residual flips that logic. Utilities need enough disinfectant to protect the distribution system, but not so much that they exceed the MRDL.

An MRDL is the highest permissible level of a disinfectant added to water, recognizing that there is convincing evidence that addition of a disinfectant is necessary to control microbial contaminants. (40 CFR § 141.2) Because the disinfectant is intentionally added — and because some residual is required — the MRDL sits at a manageable upper bound rather than functioning as a hard prohibition.

The corresponding MRDLG is a non-enforceable health goal. For chlorine and chloramines, the MRDLG matches the MRDL at 4 mg/L; for chlorine dioxide it also matches at 0.8 mg/L. (40 CFR § 141.65) That alignment is unusual — for most regulated contaminants, the MCLG is set lower than the enforceable MCL because technology or cost prevents reaching the goal. For these disinfectants, the MRDLG reflects a judgment that the health risk from residual exposure at those levels is acceptable given the protection they provide.

How chlorine residual interacts with disinfection byproducts

Maintaining a residual creates a tradeoff. Chlorine reacts with natural organic matter (humic and fulvic acids) already present in source water to form disinfection byproducts (DBPs), including trihalomethanes (TTHMs) and haloacetic acids (HAA5s). Higher residuals and longer contact time generally produce more DBPs. Lower residuals reduce DBP formation but increase microbial risk if the distribution system is long or has dead ends.

EPA regulates this tension through two parallel limits: the MRDL caps the disinfectant; the MCLs for TTHMs (0.080 mg/L, LRAA) and HAA5s (0.060 mg/L, LRAA) cap the byproducts. Utilities must hit both targets simultaneously, which sometimes requires balancing booster chlorination against DBP formation across monitoring locations.

For a detailed breakdown of how TTHMs and HAA5s are reported and calculated, see the disinfection byproducts contaminant page.

How it's measured: RAA at the system level

For chlorine and chloramines, compliance is based on a running annual arithmetic average, computed quarterly, of monthly averages of all samples collected by the system. If the RAA across any consecutive four-quarter period exceeds 4.0 mg/L (as Cl₂), the system is in violation and must notify the public and report to the state. (40 CFR § 141.133(c)(1)(i))

The RAA calculation works as follows:

  1. Samples are collected in the distribution system throughout the year (at the same points and times as total coliform samples, per 40 CFR § 141.132(c)(1)).
  2. Monthly averages are computed from all samples in that calendar month.
  3. At the end of each quarter, the four most recent monthly averages are averaged together.
  4. If that four-quarter rolling average exceeds the MRDL, the system is out of compliance.

This averaging approach means a single high reading does not immediately trigger a violation — but sustained elevated levels do. It also means a utility cannot average away a consistent pattern of excess: if every month runs above 4 mg/L, the RAA will exceed it by year's end regardless of quarterly timing.

Chlorine dioxide uses a different compliance trigger: if any daily sample exceeds 0.8 mg/L, the system must take action and notify the state within 24 hours if a second consecutive sample also exceeds the MRDL.

What utilities must disclose on the CCR

Every system using one of these three disinfectants must include the following in its annual CCR under 40 CFR § 141.153:

The CCR table for disinfectant residuals follows the same column structure as the regulated contaminants table — contaminant name, MRDL, units, highest level detected, range, and a source/health-effects note — but it reports the disinfectant added by the utility, not a contaminant it is trying to remove.

If a system uses both chlorine and chloramines during the reporting year (a common practice during annual "chlorine burn" periods to reduce nitrification), both disinfectants must appear in the CCR table separately.

The 2024 CCR Rule Revisions (89 FR 46013) did not change the MRDL values or the Appendix A health-effects language for disinfectants. The substantive changes in those revisions affected delivery requirements, the mandatory summary section (40 CFR § 141.156), and LSL inventory disclosures, not the disinfectant residual reporting requirements.

Last reviewed: 2026-05-03. Next scheduled review: every 6 months.