Water System Logbook

Guide · federal and state rules

Chlorine residual requirements for drinking water

Reviewed September 28, 2026 against the eCFR and state codes · Sources below

Short answer

  • Maximum: 4.0 mg/L chlorine, as a running annual average, for community and non-transient systems that add chlorine (federal).
  • Minimum: set mostly by states. 0.2 mg/L free chlorine in the distribution system is common; some states are higher.
  • Surface water systems: at least 0.2 mg/L entering the distribution system, and a detectable residual throughout (federal).
  • Ground water systems with 4-log treatment: a state-set minimum, checked daily if you serve 3,300 or fewer (federal).

Federal rules by system type

Three federal rules set chlorine residual numbers, and which ones apply depends on your water source and system type (see the public water system checker).

RuleApplies toRequirementMonitoring
Disinfectant residual maximum (Stage 1 D/DBP rule)Community and non-transient systems adding a chemical disinfectant4.0 mg/L chlorine or chloramines, as a running annual averageAt the same time and place as each total coliform sample; may not be reduced
Surface Water Treatment RuleSurface water and ground water under the influence of surface waterAt least 0.2 mg/L entering the distribution system (not below it for more than 4 hours); undetectable in no more than 5% of distribution samples a month, for two months runningContinuous at the entry point, or 1–4 grab samples a day for systems of 3,300 or fewer
Ground Water RuleGround water systems that provide 4-log virus treatmentThe minimum residual your state specifies3,300 or fewer: a daily grab sample at peak flow, with follow-ups every 4 hours if low. Over 3,300: continuous

Transient non-community systems (campgrounds, restaurants, rest areas) are outside the federal maximum and its residual monitoring, except for chlorine dioxide (40 CFR 141.130). The Ground Water Rule still applies to every ground water system, and states often add their own rules.

State requirements compared

Most small systems answer to their state first, and states differ more than you'd expect. Here are ten states' rules for small ground water systems, each with its own page and citations:

StateMinimum residualHow often (small ground water systems)Report
Texas0.2 mg/L free (0.5 total as chloramine), entry point and distributionWeekly in distribution (<250 connections and <750 people), plus with each coliform sampleDLQOR (TCEQ-20067), quarterly by the 10th
Pennsylvania0.40 mg/L free at the entry point; 0.2 mg/L in distributionEntry point daily (≤3,300 people); distribution weekly plus with coliform samplesResults within 10 days after the month; monthly operational report kept on file
Ohio0.2 mg/L free or 1.0 mg/L combined, throughout distributionDaily at each entry point and in distributionMonthly operating report (Form 5002) via eDWR by the 10th
IllinoisCommunity: 0.5 mg/L free or 1.0 mg/L combined. Non-community (IDPH): 0.1 mg/L free where requiredNot set as a number in the community rule (forms have daily rows); with each coliform sampleMonthly operating reports within 30 days after the month
IndianaIDEM form: monthly average ≥0.2 mg/L free or ≥0.5 mg/L totalWith each coliform sample; daily at peak flow for 4-log systemsMonthly Report of Operation (State Form 34609) by the 10th
MichiganSet for each system (the numeric rule minimum is for surface water); 4.0 mg/L maximumWith each coliform sample; daily at peak flow for 4-log systemsMonthly operation report the following month (treating systems); quarterly residual report
MissouriRequired systems: 0.5 mg/L free at entry; 0.2 mg/L total in distributionRequired: daily at entry. Voluntary: Mon–Fri. All: with coliform samplesMO 780-2094 by the 10th (4-log systems); others keep records on file
Florida0.2 mg/L free or 0.6 mg/L combined, throughout distribution<3,300 people: daily or 2 days/week, whichever is lessForm 62-555.900(3) monthly operation report within 10 days
WashingtonDetectable = 0.2 mg/L in all active parts of distribution (when chlorinating)Once a day, 5 days/week (7 days for 4-log systems), plus with coliform samplesDOH 331-430 / 331-593 monthly by the 10th
CaliforniaGuidance target 0.2–1.0 mg/L free in distribution; 4.0 mg/L maximumWith each coliform sample; daily recommended in guidanceQuarterly residual report within 10 days after the quarter

Why the maximum is an average

The 4.0 mg/L maximum residual disinfectant level (40 CFR 141.65) is met when the running annual average of monthly averages, computed each quarter, stays at or below 4.0 mg/L (40 CFR 141.133). A single reading above 4.0 isn't automatically a violation, and operators may raise chlorine temporarily to deal with contamination after a main break. A consistently high residual will push the average up, and it usually brings taste and odor complaints long before that.

On safety, the CDC says chlorine or chloramine up to 4 mg/L is considered safe in drinking water (CDC).

Setting your working range

The legal minimum is a floor, not a target. Most operators set a working range with room above the minimum, so a low reading prompts a check of the feed pump before it becomes a violation. They also keep the top of the range well under 4.0 mg/L. Write that range on your log: the chlorine residual log sheet has a row for it under the column headings.

The Water System Logbook app works the same way. You set a minimum and maximum for each reading, and a value outside them is flagged “Outside configured range”. The app never supplies limits of its own, because the right ones come from your state and your system.

Find your state drinking water program

Your state program (EPA in Wyoming and the District of Columbia) sets your minimum, your sampling schedule and your forms. The list follows EPA's primacy agency directory.

Questions

What is the minimum chlorine residual in a distribution system?

It depends on your state and water source. Many states use 0.2 mg/L free chlorine in the distribution system (Texas, Florida and Ohio, for example). Illinois community systems must keep 0.5 mg/L. Surface water systems must keep a detectable residual under federal rules. Check your state's page or program.

What is the maximum chlorine residual allowed in drinking water?

4.0 mg/L (as Cl2) for chlorine and chloramines. This federal maximum residual disinfectant level is measured as a running annual average of monthly averages, computed quarterly. It applies to community and non-transient non-community systems that add a chemical disinfectant.

Is 4 ppm of chlorine safe to drink?

The CDC says chlorine or chloramine levels up to 4 mg/L (4 ppm) are considered safe in drinking water. EPA lists eye and nose irritation and stomach discomfort as possible effects above the maximum.

Do campgrounds and other transient systems have to keep a chlorine residual?

Not under the federal maximum and residual monitoring rule, which covers community and non-transient systems. But the Ground Water Rule applies to all ground water systems, and many states set their own requirements. Texas, for example, requires residual records from transient systems too.

Should I measure free or total chlorine?

For systems that add chlorine, free chlorine is what most state minimums are written in. Federal rules allow free or total chlorine to show compliance with the chlorine maximum, and total (or combined) chlorine for chloramines.

Sources

  1. 40 CFR 141.65 — Maximum residual disinfectant levels (chlorine 4.0 mg/L as Cl2)
  2. 40 CFR 141.54 — Maximum residual disinfectant level goals
  3. 40 CFR 141.133 — Compliance requirements (running annual average)
  4. 40 CFR 141.132 — Monitoring requirements (residual with total coliform samples)
  5. 40 CFR 141.130 — Disinfectant residuals and byproducts: who the rule covers
  6. 40 CFR 141.131 — Analytical requirements for disinfectant residuals
  7. 40 CFR 141.72 — Disinfection criteria (surface water systems)
  8. 40 CFR 141.74 — Analytical and monitoring requirements (DPD methods, entry-point sampling)
  9. 40 CFR 141.403 — Ground water source treatment and compliance monitoring
  10. CDC — Water disinfection with chlorine and chloramine
  11. EPA — National Primary Drinking Water Regulations (chlorine MRDL)
  12. EPA — Primacy agency drinking water data (state programs)

Last reviewed September 28, 2026. Rules change and states add their own; confirm with your state drinking water program.