MRDL (Maximum Residual Disinfectant Level)

An MRDL is the highest level of a disinfectant allowed in drinking water that EPA will permit while still protecting public health, set because some level of disinfection must remain in water to control microbial contamination (40 CFR § 141.2).

Where you'll see it on a CCR

Your Consumer Confidence Report lists MRDLs in the disinfectant residuals table, separate from the MCL table used for contaminants. Three disinfectants carry federal MRDLs under 40 CFR § 141.65:

| Disinfectant | MRDL | Measurement basis | |---|---|---| | 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 | Daily maximum |

Each reported value is the system's actual measured level. A result at or below the MRDL means the disinfectant residual is within the permitted range. Chlorine and chloramine levels are averaged across the distribution system over a rolling 12-month period; chlorine dioxide is evaluated against a single-day maximum because acute exposure is the primary concern.

Alongside each MRDL, your CCR also lists the corresponding MRDLG (Maximum Residual Disinfectant Level Goal). MRDLGs are non-enforceable health goals set at the level where no known or anticipated adverse effect on human health occurs. Because maintaining some disinfectant residual is mandatory, MRDLGs can be set at zero for some disinfectants even when the enforceable MRDL is higher — the MRDLG reflects an ideal, not an achievable operating target.

How it's measured

Utilities collect distribution-system samples at points throughout their service area — not just at the treatment plant exit. For chlorine and chloramine residuals, the RAA is calculated from those monitoring samples across the preceding 12 months. If a system's RAA exceeds 4.0 mg/L, it is in violation of the MRDL even if individual samples are below the limit on a given day.

The 0.8 mg/L chlorine dioxide limit works differently: a single daily exceedance triggers a violation, with additional monitoring required the following two days. This tighter window reflects chlorine dioxide's potential to form disinfection byproducts and its distinct toxicological profile.

MRDL violations must be reported to customers. The statutory authority for MRDLs is SDWA § 1412, and the specific numeric limits are codified at 40 CFR § 141.65.

Related terms

• MCL (Maximum Contaminant Level) — the enforceable limit for contaminants, as distinct from disinfectants
• Disinfection byproducts — compounds that form when disinfectants react with organic matter in water
• Chlorine residual — how systems measure and report chlorine levels in the distribution system
• CCR glossary — all terms defined

Citations: 40 CFR § 141.2 (MRDL definition); 40 CFR § 141.65 (numeric limits for chlorine, chloramines, chlorine dioxide); SDWA § 1412, 42 U.S.C. § 300g-1 (statutory authority). Cornell LII: law.cornell.edu/cfr/text/40/141.65.