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Managing Discharges to Ground for the Industrial Stormwater General Permit

by Brandon Boyd

April 30, 2025

The 2025 reissuance of the Industrial Stormwater General Permit saw a number of critical changes to how stormwater discharges to ground should be managed at industrial facilities. In this post, we will go over everything you need to know to navigate these new challenges, including a recording of the presentation given by the Washington Department of Ecology on this topic.

Discharge to Ground Presentation:

Shannon McClellan-Ecology’s industrial stormwater general permit writer, and John Bhend-Ecology’s Underground Injection Control Coordinator, join us to discuss new ISGP permit requirements regarding discharges to ground.

Download full PowerPoint Presentation

Post Presentation Q&A:

Click to expand the full Q&A from this presentation

1. Is Ecology putting out a guidance document on sampling methodology for PFAS for discharge to ground (ensuring sample integrity clothing of sampler, etc.)

Ecology recommends the ITRC and Michigan DEQ PFAS sampling documents for PFAS sampling and sampling integrity. Ecology is updating and preparing to republish the ISGP Sampling Manual for where and how to sample for a discharge to ground and PFAS sampling.

2. Sounds like foundation drains do not qualify [as UIC] but some French drains could depending on how they discharge?

Foundations drains are not classified as a UIC well. A French drain that is designed in part to discharge surface water into the ground is classified as a UIC well.

3. If a recycling facility has several different discharge locations that discharge to separate bioswales that frequently do not discharge to surface water – and one of those discharge locations / bioswales only handles packaged food waste (for de-packaging) could this discharge point be exempted from pfas testing?

Waste Management and Remediation Services (562xxx), including, but not limited to, landfills, transfer stations, open dumps, and land application sites, except as described in S1.C.6, C.7, or C.9 are subject to PFAS requirements as stated in S5 Table 3 of the ISGP. Materials Recycling Facilities (MRF) fall under NAICS 562XXX and are required to sample for PFAS. However, the other types of recycling facilities that require ISGP coverage, including NAICS 42314x and 42393x (e.g., metal scrap yards, auto recyclers, etc.) are not subject to PFAS sampling.

4. If we use perforated pipes for conveyance, would that be a UIC? Perf pipe can be used to collect water and convey away from the area and not be intended to be used for infiltration.

That type of system would not require registration if the intent is to convey stormwater to a location other than groundwater, via infiltration.

5. If we discharge industrial stormwater solely through an infiltration pond in a sole source aquifer recharge area… would that require a permit? Or since this is 100% infiltration, no permit required?

A site that discharges solely to groundwater (including an infiltration scenario within sole source aquifer recharge area) is categorically exempt from needing coverage under the ISGP, except in the rare case of such an onsite infiltration discharge being the functional equivalent to a point source discharge to surface waters. Ecology does have authority to require site-specific ISGP coverage under Condition S1.B on a case-by case- basis. If Ecology finds the facility a Significant Contributor of Pollutants and/or the discharge is a functional equivalent, then we’d require the facility to gain ISGP coverage per S1.B.

6. if the surface area of the site is larger than the infiltration trench or perforated pipe length then that site would not need an ISGP permit?

The surface area of the site is not relevant to determining if the facility has a discharge to surface waters of the state; the trigger for categorical ISGP coverage. The site needs an ISGP permit if it conducts industrial activity related to those listed in Table 1, is considered a Significant Contributor of Pollutants under Condition G12, or has a functional equivalent point

source discharge to surface water. Whether ISGP coverage is required or not, the stormwater infiltration structure with perforated pipe will need to be registered as a UIC well.

7. If a stormwater basin is already associated with a surface water discharge does it also need a ground discharge sample if we are required to sample PFAS? For example, water that flows through a swale and may or may not discharge to surface water depending on amount of precipitation.

Yes, you are required to sample surface water and discharges to ground for PFAS. PFAS sampling is required at designated engineered groundwater discharge points, even if some of the stormwater from the drainage area also discharges to surface waters in certain circumstances. Please see definition in Permit. “Groundwater Discharge Point: means the location where stormwater associated with industrial activity enters a stormwater infiltration structure that is used, intended or designed to infiltrate water into the ground.” Sampling surface water and discharges to groundwater follows the requirements of the permit to meet waste discharge permit and NPDES permit water quality standards.

8. Would ponding/ a puddle in at a facility that may eventually infiltrate need to sample? (This was asked in terms of functionally equivalent discharge to surface waters and PFAS sampling requirement)

No, a puddle at a facility does not require sampling as it would not meet the definition of a “Groundwater Discharge Point: means the location where stormwater associated with industrial activity enters a stormwater infiltration structure that is used, intended or designed to infiltrate water into the ground.

9. Would the piezometer/ sampling option Shannon mentioned be considered a UIC (It is a hole deeper than its largest surface dimension)

No. It is a sampler and not a UIC well.

10. What would be the mechanism of Ecology determining a groundwater discharge was a functionally equivalent discharge to surface water.

Ecology would use the guidance located on the EPA NPDES website, site specific factors, and inspections (as needed) to determine functional equivalency.

11. What due diligence would an air transportation facility need to go through to determine there is no known historical use of AFFF and it wouldn’t need to sample PFAS.

Air transportation facilities that have ISGP coverage are required to sample for PFAS, if they have had known historical use of AFFF at the facility The Permittee is responsible for the depth and breadth of the site-specific assessment which may include a review of site historical documents, historical and current emergency training activities, emergency system flushing, or other source of information.

12. If an air transportation facility has no known use of AFFF and doesn’t need to sample, how do they mark that on their DMR, or get that removed from their DMR?

If a facility truly doesn’t have known use of AFFF, the Permittee may use qualifier code “M- Monitoring is Conditional/ Not required this MP” on their quarterly DMRs.

13. If we are a construction equipment rental facility and discharge 100% to a biopond and infiltration pond (no discharge to surface waters), then we don’t need a permit?

Correct. Please see the response to Question 5.

Key Presentation Takeaways:

Who has to monitor their discharges to ground?:

Fire Fighters putting out an airplane fire with foam

Ecology clarified that in most cases, facilities won’t have to sample their onsite discharges to ground unless they meet one of the following three criteria:

  1. The facility is subject to PFAS sampling per condition S5B (Waste management and air transportation facilities with known historical use of AFFF)
  2. The facility is specifically required to sample the discharge to ground per condition G12
  3. The discharge point to groundwater is determined by Ecology to be functionally equivalent to a point source discharge to surface water

Determining if a discharge point to groundwater is a functional equivalent to a point source discharge to surface water :

barton creek with bright green dye coming from a cliff face

Where to sample your discharge to ground:

water flowing from outlet pipe into sample bottle

Facilities that are required to sample their discharge to ground have a couple of options for where to take their sample:

  1. Facilities can sample the water as it enters their groundwater discharge point (i.e., sample the water as it goes into your infiltration BMP)
  2. Facilities can utilize subsurface sampling techniques (such as the piezometer example Shannon uses in the presentation) to sample after the water has infiltrated to get the benefit of the infiltration BMP.

What is a UIC well?:

graphic showing a basic UIC well infiltrating stormwater above the water table

  • A man-made structure designed to discharge fluids into the subsurface
  • A bored, drilled, or driven shaft, or dug hole whose depth is greater than the largest surface dimension; an improved sinkhole; or a subsurface fluid distribution system
  • Examples of class V UICs include:
    • Drywells, drainfields, infiltration trenches with perforated pipe, storm chamber systems with the intent to infiltrate, French drains designed in part to discharge surface water into the ground, and bioretention systems intending to infiltrate water below the treatment zone via perforated pipe.

For more information on the Industrial Stormwater General Permit, Contact Brandon Boyd at [email protected].