What must a public water system do to meet coliform sampling requirements under 40 CFR 141.21? Answer in under 100 words.
Details / How it works
Public water systems must collect total coliform samples at sites representative of water throughout the distribution system, as specified in a written sample siting plan subject to State review and revision. Routine monitoring frequency for community water systems depends on the population served, with minimum monthly samples ranging from 1 for systems serving 25 to 1,000 persons, up to 480 for systems serving 3,960,001 or more persons. Non-community water systems must monitor each calendar quarter they provide water, with specific rules for systems using ground water, surface water, or ground water under the direct influence of surface water.
Numbers & thresholds
| Population Served | Minimum Samples per Month |
|---|---|
| 25 to 1,000 | 1 |
| 1,001 to 2,500 | 2 |
| 4,101 to 4,900 | 5 |
| 96,001 to 130,000 | 100 |
| 3,960,001 or more | 480 |
When a routine sample is total coliform-positive, systems collecting more than one routine sample per month must collect no fewer than 3 repeat samples within 24 hours. Systems collecting one routine sample per month or fewer must collect no fewer than 4 repeat samples.
Exceptions & edge cases
A system serving 25 to 1,000 persons with no history of total coliform contamination and a satisfactory sanitary survey within 5 years may have its monitoring frequency reduced by the State, but not to less than one sample per quarter. Systems using surface water or ground water under the direct influence of surface water that do not practice filtration must collect at least one sample near the first service connection each day source water turbidity exceeds 1 NTU. Special purpose samples taken for disinfection checks after pipe repairs cannot be used for compliance determination, but repeat samples taken per paragraph (b) must be used.
Sources
- [1]U.S. National Archives, Electronic Code of Federal Regulations — 40 CFR 141.21 — Coliform sampling · as of 2026-08-26
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