Construction Dewatering: Pumping Water Off Site Without Polluting

The EPA CGP defines dewatering as the act of draining accumulated stormwater and/or groundwater from building foundations, vaults, trenches and other similar points of accumulation. Uncontaminated dewatering water discharged in accordance with the permit is an authorized non-stormwater discharge. Water that's contaminated, for example with fuel, solvents or chemicals from a contaminated site, needs other handling and possibly other permits.

Treatment requirements

Under Part 2.4 of the CGP, operators must:

• Route dewatering water through a sediment control, such as a sediment trap or basin or a pumped-water filter bag, designed to prevent discharges with visual turbidity. (Visual turbidity means a sediment plume, or a discharge that looks cloudy or opaque.)
• Not discharge visible floating solids or foam.
• Not cause a visible sheen or oily deposits. Use an oil-water separator or suitable filter if the water contains, or is expected to contain, oil or grease.
• To the extent feasible, use well-vegetated upland areas to infiltrate dewatering water before discharge. Receiving waters can't be used as part of the treatment area.
• Discharge onto stable, erosion-resistant surfaces (vegetation, clean filter stone, geotextile), keep filter bags off steep slopes, and provide velocity dissipation.
• Haul away backwash water or return it to the start of the treatment process.
• Replace and clean filter media when the pressure differential reaches the manufacturer's specification.

Inspection

Sites discharging dewatering water must inspect the dewatering discharge once per day on which the discharge occurs (CGP Part 4.3.2). The normal site inspection frequency still applies to the rest of the site.

Turbidity monitoring and the 50 NTU benchmark

The 2022 CGP added turbidity benchmark monitoring for dewatering discharges:

• Collect at least one turbidity sample each day a dewatering discharge occurs, at every point where dewatering water is discharged, after treatment and before it leaves the site or enters a drainage feature or storm drain.
• Measure with a turbidity meter that reports in NTUs using an approved method, with a calibration verification before each day's use.
• Compare the weekly average of results to the 50 NTU benchmark (unless EPA approved an alternate benchmark).

When the benchmark is exceeded

If the weekly average exceeds 50 NTU, or you see a sediment plume, sheen or other listed conditions, the CGP's corrective action steps are:

1. Immediately take reasonable steps to minimize or prevent the pollutant discharge, including shutting off the dewatering discharge as soon as possible, considering safety.
2. Determine whether the dewatering controls are working and whether they're causing the problem.
3. Make adjustments, repairs or replacements to lower turbidity below the benchmark or remove the plume or sheen.

Then you may resume discharging. EPA notes that a single sample of about 355 NTU or higher means the weekly average will exceed 50 NTU no matter what the other days show, so very high single readings call for shutting off the discharge to investigate.

Key takeaways

• Dewatering water must pass through a sediment control designed to prevent visible turbidity.
• No floating solids, foam or sheen; use oil-water separation where needed.
• Inspect daily while discharging, and sample turbidity each day after treatment.
• A weekly average above 50 NTU triggers corrective action, starting with shutting off the discharge.

Last reviewed: October 25, 2026 · Free general education based on the Clean Water Act, EPA NPDES rules and the EPA Construction General Permit. Your governing permit (EPA, state, tribal or local) controls. This is not legal advice, not EPA-approved, and not a California QSP/QSD or other state credential, and it does not issue a certificate. Nothing you enter on this page is saved or sent.