Washington · small ground water systems
Washington chlorine residual requirements
Short answer
Washington requires ground water disinfection only where the Department of Health determines it's needed. Systems that chlorinate the distribution system keep a detectable residual, defined as 0.2 mg/L, in all active parts. Residuals are checked once a day, five days a week, reported monthly by the 10th, and chlorine residual records are kept three years.
- Minimum residual
- Detectable = 0.2 mg/L in all active parts of distribution (when chlorinating)
- How often
- Once a day, 5 days/week (7 days for 4-log systems), plus with coliform samples
- Report
- DOH 331-430 / 331-593 monthly by the 10th
Who has to chlorinate
- Disinfection is required when the Department determines source or distribution conditions call for it. WAC 246-290-451
Minimum residual
- Continuous distribution chlorination: a detectable residual in all active parts of the distribution system. WAC 246-290-451
- Detectable means 0.2 mg/L free, total or combined chlorine (or chlorine dioxide). WAC 246-290-010
- Where source disinfection is required, a CT (concentration × contact time) of at least 6. WAC 246-290-451
Maximum
- 4.0 mg/L chlorine. WAC 246-290-310
How often to test
- Source (CT6) and distribution residuals: at least once a day, five days a week. WAC 246-290-451
- Anyone adding chlorine for any reason measures the residual with routine and repeat coliform samples. WAC 246-290-451
- Systems doing 4-log treatment measure and record the entry-point residual 7 days a week. WA DOH
Test kits and methods
- Source disinfection compliance can't use a DPD kit that relies on a visual color comparison; a digital colorimeter with an EPA-approved method is allowed. WAC 246-290-451
- Distribution compliance may use a DPD colorimetric field test kit unless the Department says otherwise. Test strips may not be used. WAC 246-290-451
Reporting
- Monthly water treatment reports are due by the 10th of the following month. WAC 246-290-451
- The Department's forms are DOH 331-430 (source disinfection) and DOH 331-593 (distribution). WA DOH
How long to keep records
- Chlorine residual records: at least 3 years. WAC 246-290-480
Parks, campgrounds and seasonal systems
- These are the Group A rules (WAC 246-290). Smaller Group B systems have separate rules.
Keeping the log in Washington
Five days a week, every week, adds up to a lot of rows. Keep them in one place, with the date, time, location and result for each, so the monthly form is a copy job rather than a hunt through scraps of paper.
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
- WAC 246-290-451 — Disinfection of ground water
- WAC 246-290-010 — Definitions (detectable residual)
- WAC 246-290-310 — Maximum contaminant and residual disinfectant levels
- WA DOH — Chlorination monitoring and reporting
- WAC 246-290-480 — Recordkeeping
Last reviewed September 28, 2026. Rules change and states add their own; confirm with your state drinking water program.