The Construction General Permit (CGP) requires sites discharging dewatering water to sensitive waters to perform turbidity benchmark monitoring. If there are multiple operators associated with one site, the operators may coordinate together to fulfill the monitoring requirements to avoid duplicating efforts. However, each operator is responsible for ensuring compliance with these requirements.

Turbidity benchmark monitoring requirements for applicable sites are as follows:

• Sampling. Collect at least one turbidity sample per day, from each applicable discharge point, on any day that a dewatering discharge occurs, following the sampling procedures specified in the CGP. Samples must be taken after the dewatering water has been treated by installed treatment devices and before it is discharged off-site into a receiving water, constructed or natural site drainage feature, or storm drain inlet. Samples must be representative of the dewatering discharge for any given day.
• Turbidity benchmark. Compare the weekly average of the sampling results with the standard turbidity benchmark value of 50 Nephelometric Turbidity Units (NTU) or an alternate EPA-approved benchmark.
• Corrective action. If the weekly average of turbidity monitoring results exceeds the 50 NTU or applicable benchmark, follow-up corrective action is required.
• Reporting. Operators must report all weekly average turbidity monitoring results on a quarterly basis. See the REPORTING subtopic within the STORMWATER—CONSTRUCTION ACTIVITIES topic for more information.

Note: If an operator conducts turbidity sampling for more than one dewatering discharge point, a weekly average turbidity value must be calculated for each discharge point, and each must be compared with the turbidity benchmark.   

In addition, some specific states, Indian country lands, and territories covered by the CGP have their own monitoring requirements. See the CGP for more information about the requirements in your area.

Governing laws and regulations:

• 33 United States Code (USC) 1251 through 1389
• 40 Code of Federal Regulations (CFR) 122
• 40 CFR 124