What is a Substantially Identical Outfall for Stormwater Permits [40 CFR 122.21(g)(7)]

May 17, 2018

Federal EPA and state NPDES permitting authorities are authorized at 40 CFR 122.21(g)(7) to allow stormwater permit holders that have two or more outfalls with substantially identical effluents to test only one outfall, and report the quantitative data as applicable for the substantially identical outfalls. EPA’s guidance document titled, Industrial Stormwater Monitoring and Sampling Guide (EPA 832-B-09-003), provides information about how this monitoring exemption applies to industrial facilities where EPA is the NPDES permitting authority.

Facilities permitted under EPA’s 2015 Multi-Sector General Permit (MSGP) for Stormwater Discharges Associated with Industrial Activity, are required to monitor all outfalls that receive stormwater discharges from their industrial activities. The exception to this rule under the 2015 MSGP provides permittees with the flexibility to monitor the discharge at one outfall and consider the results as representative for all substantially identical outfalls—with this exemption applicable for quarterly visual assessments and industry-specific benchmark monitoring requirements, but not effluent limitation guideline monitoring.

The 2015 MSGP defines substantially identical outfalls as two or more outfalls the permittee believes have discharges of substantially identical effluents based upon similarities that include:

  • General industrial activities and control measures,
  • Exposed materials that may significantly contribute pollutants to stormwater, and
  • Runoff coefficients of the drainage areas


The runoff coefficient is the ratio of excess runoff compared to the amount of precipitation for a given time over a given area. This value will range from 0.0 when there is no runoff potential, up to 1.0 if all the precipitation will run off. The value is increased due to impervious surfaces at a site such as buildings, pavement, or sidewalks and is decreased due to pervious surfaces such as grass or graveled areas.

Let’s consider the following scenario to determine if the facility is eligible for the substantially identical outfall exemption under the terms of the 2015 MSGP:

  • A metal recycling facility has a uniform scrap metal pile with three drainage ditches that direct discharge from three areas of the pile to three separate outfalls
  • Runoff from the scrap pile is managed using the same type of control measures in each drainage area


Based on this description, can the substantially identical outfall exemption be used for the three outfalls, or are there any differences in the conditions at the facility—compared to the exemption’s required components—which would prevent the facility’s operator from using the exemption?

The answer is yes, the facility’s operator can use the substantially identical outfall exemption for the three outfalls because all of the required components have been met as follows:

  • The industrial activities and the control measures being used are all the same,
  • It can be assumed the runoff flowing through the exposed areas does contribute the same type of pollutants to stormwater discharge, and
  • The drainage areas have the same or similar runoff coefficients


The 2015 MSGP requires that permittees who use the substantially identical outfall exception document their determinations in their stormwater pollution prevention plan (SWPPP) to include the following:

  • Location of each substantially identical outfall
  • General industrial activities conducted in the drainage area of each outfall
  • Control measures implemented in the drainage area of each outfall
  • Exposed materials located in the drainage area of each outfall that are likely to be significant contributors of pollutants to stormwater discharges
  • Estimate of the runoff coefficient of the drainage areas (low = under 40%, medium = 40–60%, high = above 65%)
  • Why the outfalls are expected to discharge substantially similar effluents


The 2015 MSGP specifies that permittees must perform the quarterly visual assessments for substantially identical outfalls on a rotating basis throughout the period of permit coverage. The permit also specifies that if stormwater contamination is identified in the visual assessment for a substantially identical outfall, permittees must assess and modify the control measures as appropriate for each substantially identical outfall represented by the monitored outfall.

If an event triggers a requirement for a corrective action that is associated with an outfall that has been identified as a substantially identical outfall, permittees must assess the need for corrective action for all related substantially identical outfalls. Any necessary changes to control measures that affect these other outfalls must also be made before the next storm event if possible, or as soon as practicable following that storm event. Any corrective actions must be conducted within the timeframes identified in the permit.

If your facility’s stormwater discharge is under the jurisdiction of a state SPDES permitting authority, you must determine if the state’s stormwater permitting program allows an exemption for substantially identical outfalls and comply with the terms and conditions of your state SPDES stormwater discharge permit that pertain to this exemption.