Michigan · small ground water systems
Michigan chlorine residual requirements
Short answer
Michigan's rules require disinfection for supplies that use phosphate or certain iron removal treatment, and its numeric entry-point minimum is written for surface water systems. Community and non-transient systems that add chlorine measure the residual with every coliform sample and stay at or under 4.0 mg/L, and supplies that treat file monthly operation reports.
- Minimum residual
- Set for each system (the numeric rule minimum is for surface water); 4.0 mg/L maximum
- How often
- With each coliform sample; daily at peak flow for 4-log systems
- Report
- Monthly operation report the following month (treating systems); quarterly residual report
Who has to chlorinate
- Disinfection is required for supplies that use phosphate treatment or certain iron removal treatment (R 325.10604a). Mich. Admin. Code R 325
- Michigan's numeric entry-point minimum (0.2 mg/L) applies to surface water systems (R 325.10611a). Mich. Admin. Code R 325
Maximum
- 4.0 mg/L chlorine (R 325.10610a). Mich. Admin. Code R 325
How often to test
- Community and non-transient non-community supplies adding chlorine measure the residual at the same point and time as total coliform samples (R 325.10719e). Mich. Admin. Code R 325
- 4-log systems take a daily grab sample during the hour of peak flow (R 325.10739a). Mich. Admin. Code R 325
Reporting
- A community supply that treats, or a non-community supply that treats for public health, prepares a monthly operation report on an EGLE form, including residual disinfectant concentration, and submits it the following month (R 325.11502). Mich. Admin. Code R 325
- Supplies that don't file monthly reports file an annual report by March 31 (R 325.11504). Mich. Admin. Code R 325
- Disinfectant residual results are reported within 10 days after each quarter (R 325.10719f). Mich. Admin. Code R 325
How long to keep records
- Bacteriological records at least 5 years; chemical records at least 10 years (R 325.11506). Mich. Admin. Code R 325
- Lowest daily residuals for 4-log systems: at least 5 years (R 325.11509). Mich. Admin. Code R 325
Parks, campgrounds and seasonal systems
- EGLE's reporting forms include a stipulation form for supplies licensed by the state, such as mobile home communities. EGLE
Keeping the log in Michigan
For most ground water systems, the range you work to comes from your system's approvals and your operator's judgment. Write it at the top of your log and flag readings outside it. A trend you can see is worth more than a single number.
Print a dated month with the chlorine residual log sheet, or build a sheet for your whole round with the daily water system log sheet.
Other states
For federal rules and every state's program, see chlorine residual requirements.
Sources
- Mich. Admin. Code R 325 — Safe Drinking Water Act rules (R 325.10101 to R 325.12820)
- EGLE — Community water supply reporting forms
Last reviewed September 28, 2026. Rules change and states add their own; confirm with your state drinking water program.