What Qualifies as a Storm Event for Stormwater Sampling [40 CFR 122.21(g)(7)(ii)]

June 07, 2018
When sampling is required by a National Pollutant Discharge Elimination System (NPDES) stormwater discharge permit, samples must be obtained during a storm event. EPA defines a storm event at 40 CFR 122.21(g)(7)(ii) as a rainfall event with greater than 0.1 inch of rainfall and at least 72 hours from the previously measurable—greater than 0.1 inch rainfall—storm event.
Although the definition of a storm event described above dates back to when the federal regulations were published in 1983, the required measurement of 0.1 inches or greater is not a requirement in EPA’s 2015 Multi-Sector General Permit (MSGP) for Stormwater Discharges Associated with Industrial Activity.
Section 6.1.3 of EPA’s 2015 MSGP states that for all required monitoring:
  • The monitoring must be performed during a measurable storm event that results in an actual discharge of stormwater from a site that follows the preceding measurable storm event by at least 72 hours
  • In the case of snowmelt, monitoring must be performed at a time when a measurable discharge occurs at the site
  • The 72-hour requirement is waived where the permittee documents that less than a 72-hour interval is representative for local storm events during the sampling period
  • For each monitoring event, except snowmelt monitoring, you must identify:
    • The date and duration (in hours) of the rainfall event,
    • Rainfall total (in inches) for that rainfall event, and
    • Time (in days) since the previous measurable storm event
  • For snowmelt monitoring, you must identify the date of the sampling event
Learn more about the stormwater management requirements applicable to your facility by attending Environmental Resource Center’s Stormwater Regulations – Webcast.