Turbidity Benchmark Monitoring (Dewatering) under the Construction General Permit

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What Are the New CGP Turbidity Benchmark Monitoring Requirements under the 2022 CGP?

Untreated water from construction dewatering activities may contain pollutants that, if discharged without being managed by appropriate controls, would likely exceed applicable water quality standards. For this reason, the 2022 CGP requires turbidity benchmark monitoring for sites discharging dewatering water to sensitive waters (sediment-impaired or designated high quality waters). The turbidity benchmark threshold for the 2022 CGP is 50 Nephelometric Turbidity Units (NTUs) unless an alternative benchmark is approved by EPA. The new turbidity benchmark monitoring requirements are summarized as follows:

Applicability Sampling Requirement Turbidity Benchmark Corrective Action Reporting

Sites discharging dewatering water to a sediment-impaired water or to a water designated as a Tier 2, Tier 2.5, or Tier 3.

Collect at least one turbidity sample per day, from each discharge point, on any day there is a dewatering discharge.

Use turbidity sampling procedures specified in CGP Part 3.3.1.

 Compare the weekly average turbidity monitoring results to the 50 NTU benchmark (or alternate benchmark if approved by EPA). If the weekly average exceeds the benchmark, take follow-up corrective action in accordance with CGP Part 5.2.2. Report all weekly average turbidity monitoring results on a quarterly basis via NeT-CGP no later than 30 days following the end of each monitoring quarter.

Operators may request approval of an alternate benchmark threshold consistent with CGP Part 3.3.2.b. To do so, the operator must submit to EPA:

  1. the current turbidity water quality standard that applies to the receiving water and a citation to the applicable State or Tribal water quality standard document, and
  2. if the applicable turbidity water quality standard is determined by the natural or background turbidity levels in the waterbody, include available, representative data on natural turbidity levels in the receiving water and the source of the data.

Operators can use the following EPA resources for assistance in determining the water quality standards that apply to their receiving waters:

Why Was Turbidity Chosen for Monitoring?

EPA selected turbidity as the monitoring parameter for treated dewatering discharges for several reasons:

  1. Turbidity is a simple measurement that requires only the use of a turbidity meter and can be conducted in the field for instantaneous results. Turbidity measurement also does not require any sample preparation other than shaking the sample bottle before analysis.
  2. Turbidity levels in the aquatic environment have well-studied impacts on water quality and organisms. High levels of turbidity affect aquatic ecosystems by reducing photosynthetic activity, reducing food availability, burying habitat, and directly harming organisms.
  3. Turbidity can be a useful indicator of the effectiveness of treatment controls at construction sites as it is an indirect measurement of the amount of sediment present in water.
  4. EPA found that nine States have NPDES dewatering permits that require turbidity measurement, while a few of these include turbidity discharge limitations. The States requiring some form of turbidity monitoring are Alaska, Arizona, California, Hawaii, Montana, Nevada, Wyoming, and EPA’s dewatering permits for Massachusetts and New Hampshire. 

How Do I Conduct Turbidity Benchmark Monitoring at My Site?

Monitoring and Inspection Guide for Construction Dewatering (pdf) (2.04 MB) - This guide assists operators in complying with the turbidity benchmark monitoring requirements. The guide provides information on how to correctly monitor for turbidity, determine if the weekly average exceeds the benchmark, and, if so, how to proceed with corrective action, as well as how to comply with the permit’s dewatering inspection requirements.

Where there are multiple operators associated with the same site, the operators may coordinate with one another to carry out the monitoring requirements to avoid duplicating efforts consistent with CGP Part 3.3. Such coordinating arrangements must be described in the SWPPP consistent with CGP Part 7.2.8.

What Happens If I Exceed the Turbidity Benchmark?

Turbidity benchmarks are not effluent limitations; rather, they are a numeric measure for assessing whether a site’s dewatering controls are effective in protecting water quality. If the weekly average of your turbidity monitoring results exceeds the standard benchmark (or your approved alternate benchmark), you are required to conduct follow-up corrective action in accordance with CGP Part 5.2.2 and document any corrective action taken in your corrective action log in accordance with CGP Part 5.4.

If there is a benchmark exceedance, there is no permit violation. However, there are still enforceable components of the benchmark monitoring requirements. The operator must comply with all the sampling, reporting, and recordkeeping provisions in CGP Part 3.3, and with the corrective action requirements in CGP Part 5.2.2. Failure to comply with any of these provisions is considered a violation of the permit.

Do I Have to Report My Turbidity Monitoring Results?

You must submit reports of your weekly average turbidity results to EPA no later than 30 days following the end of each monitoring quarter. Operators are required to indicate in the turbidity monitoring report if there are monitoring weeks in which there was no dewatering discharge, or if there is a monitoring quarter with no dewatering discharge. Operators must also indicate in the turbidity monitoring report if another operator associated with the same site is conducting turbidity monitoring on behalf of the permittee pursuant to CGP Part 3.3.

For the CGP, the following monitoring quarters and reporting deadlines apply:

Monitoring Quarter # Months Reporting Deadline
(no later than 30 days after
end of the monitoring quarter)
1 January 1 – March 31 April 30
2 April 1 – June 30 July 30
3 July 1 – September 30 October 30
4 October 1 – December 31 January 30

Use EPA’s NPDES eReporting Tool (NeT) to electronically submit quarterly turbidity data, unless, consistent with CGP Part 1.4.2, the operator receives a waiver from the applicable EPA Regional Office. If the EPA Regional Office grants approval to use a paper turbidity monitoring report form, the operator must use the form in CGP Appendix K. If EPA approves of a request to use an alternate turbidity benchmark pursuant to CGP Part 3.3.2b, EPA will substitute the alternate benchmark in the operator’s NeT account.

For each day in which the operator is required to monitor, they must record the monitoring information required by CGP Appendix G, Parts G.10.2 and G.10.3 and retain all such information for a period of at least three years from the date this permit expires or from the date authorization is terminated.

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Last updated on January 3, 2024