The Ground Water Rule: what it requires of ground water systems.
The Ground Water Rule (40 CFR 141 Subpart S) is the federal rule that protects public water systems using ground water from fecal contamination. Instead of a single limit, it uses four risk-targeted parts: periodic sanitary surveys, triggered source-water monitoring after a coliform-positive, corrective action, and — for systems that disinfect — 4-log virus treatment with compliance monitoring.
The triggered-monitoring path after a coliform-positive.
The Ground Water Rule (40 CFR 141 Subpart S) does not set a single limit. It reduces the risk of fecal contamination through sanitary surveys, source-water monitoring, and corrective action — and for a system that does not already treat to 4-log, a routine total coliform-positive starts a 24-hour clock at the source.
- 1 · Routine total coliform-positive under the RTCR, at a system that does not treat to 4-log
- 2 · One source sample within 24 hours 24 h clock from each ground water source in use
- 3 · Test for a fecal indicator — E. coli, enterococci, or coliphage
- 4 · Fecal indicator-positive → corrective action required
A fecal indicator-positive — or a significant deficiency found in a sanitary survey — means the system must choose one corrective action:
- Correct the significant deficiency
- Provide an alternate source
- Eliminate the source of contamination
- Provide 4-log virus treatment → then compliance monitoring (§ 141.403(b))
Source: 40 CFR §§ 141.400 · 141.402 · 141.403 (Subpart S)
The four parts of the Ground Water Rule
Periodic sanitary surveys
The state primacy agency inspects each ground water system on a set cycle, reviewing eight elements from source to operator compliance and flagging significant deficiencies to correct (40 CFR § 141.401). A significant deficiency is a defect in the source, treatment, storage, or distribution that could let contamination reach customers.
Triggered source-water monitoring
A system that does not already treat to 4-log for viruses must, after a routine total coliform-positive under the RTCR, collect at least one ground water source sample within 24 hours and test it for a fecal indicator — E. coli, enterococci, or coliphage (40 CFR § 141.402(a), (c)).
Assessment source-water monitoring
Where the state directs it, systems conduct additional source-water monitoring on a schedule the state sets — a targeted look at sources the state considers vulnerable, separate from the coliform trigger (40 CFR § 141.402(b)).
Corrective action
When a fecal indicator comes back positive or a sanitary survey finds a significant deficiency, the system must correct it — one of four options, on the state's timeline (40 CFR § 141.403(a)).
The four corrective-action options
Correct the significant deficiency
Fix the defect the survey or monitoring exposed — for example, reseal a well casing, screen a storage hatch, or restore a disinfection process (40 CFR § 141.403(a)(6)(i)).
Provide an alternate source
Switch the affected supply to a different, uncontaminated source of water (40 CFR § 141.403(a)(6)(ii)).
Eliminate the source of contamination
Remove what is contaminating the source — for example, relocate or repair a nearby septic system or sewer line driving the fecal contamination (40 CFR § 141.403(a)(6)(iii)).
Provide 4-log virus treatment
Install treatment that reliably achieves at least 4-log (99.99%) inactivation or removal of viruses before or at the first customer — then demonstrate it with compliance monitoring (40 CFR § 141.403(a)(6)(iv), (b)).
One coliform-positive can start two clocks.
At a system that does not treat to 4-log, a routine total coliform-positive is not just an RTCR repeat-sampling event — it also starts a 24-hour clock to collect a source-water sample and test it for a fecal indicator (40 CFR § 141.402(a), (c)). Miss that window and the monitoring violation stands on its own, separate from the coliform result.
1water’s CCR agent reports your ground water monitoring today, and a coliform-response workflow that tracks the RTCR repeat samples and the § 141.402 source-water trigger side by side is rolling out — so one positive result doesn’t become two missed deadlines. Tell us your state →
Ground Water Rule — common questions
- U.S. EPA — Ground Water Rule, 40 CFR Part 141 Subpart S (§§ 141.400–141.405): applicability and GWUDI exclusion (§ 141.400), sanitary surveys (§ 141.401), source-water monitoring (§ 141.402), corrective action and compliance monitoring (§ 141.403).
- U.S. EPA — Ground Water Rule overview and quick-reference guidance (risk-targeted approach; four components; December 1, 2009 compliance date), 2006.
- U.S. EPA — Revised Total Coliform Rule, 40 CFR Part 141 Subpart Y, 2013 (the total coliform-positive that triggers § 141.402 source-water monitoring).
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Don't let one positive become two violations.
1water helps small and mid-size water systems stay ahead of coliform, the Ground Water Rule, nitrate, lead, PFAS, and CCR requirements — self-serve, and priced for the small end.