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.
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