The Washington State Department of Ecology has released the draft 2024 Stormwater Management Manual (SWMM) for both Western (SWMMWW) and Eastern Washington (SWMMEW). These manuals serve as a reference document for local jurisdictions, land developers and designers, and businesses developing stormwater pollution prevention plans. In addition to providing guidance on stormwater control measures, the SWMM is incorporated and referenced in National Pollutant Discharge Elimination System (NPDES) stormwater permits. The 2024 revisions focus on providing consistency in stormwater guidance across the state and increasing the usability of the SWMMs by maintaining the online format of the documents and revising text for clarity and accessibility. For facilities in Washington operating under the Industrial Stormwater General Permit (ISGP), it is particularly important to explore the significant revisions within the SWMMs. These changes could have a significant impact on compliance and stormwater management practices. Read on for a concise overview of the major revisions affecting ISGP-covered facilities in Washington.
New Development and Redevelopment Project Thresholds:
The thresholds for the minimum requirements of these projects have been updated both at the “Project Level”, “Threshold Discharge Area (TDA) Level” (SWMMWW), and ”Core Element Level” (SWMMEW). For example, in the draft 2024 SWMMWW, the revised threshold in Minimum Requirement 6 now specifies that Runoff Treatment BMPs must be implemented for TDAs containing a total of 2,000 square feet or more of pollution-generating hard surface (PGHS). The prior version (2019 SWMMWW) of this threshold had set the threshold at 5,000 square feet of PGHS. In the draft 2024 SWMMWW, the revised thresholds can be found in the same chapters as the 2019 SWMMWW, Chapters I-3.3 and I-3.4.6. Previously, the minimum requirements were detailed in Chapters 2.5, 2.6, and 2.7 of the 2019 SWMMEW but have been moved to Chapters 2.3, 2.4.5, and 2.4.6 of the draft 2024 SWMMEW.
Nutrients and Toxic Organics:
New guidance for nutrients (nitrogen and phosphorous) and toxic organic compounds (PAHs, PCBs, and pollutants from rubber preservatives) has been added to Chapter I-1.5 of the SWMMWW and Chapter 1.1.6 of the SWMMEW and. The revised guidance incorporates background information regarding the land uses and activities that can introduce pollutants into stormwater, along with suggestions for the types of BMPs that can effectively offer either Source Control or Runoff Treatment for these pollutants. Refer to these chapters for more information on source control BMP recommendations for these pollutants.
Source Control BMPs – PCB Edits:
Guidance for preventing pollution of PCBs from building materials has been added to several source control BMPs in both the SWMMWW and SWMMEW. Most notably for facilities covered under the ISGP, S424 BMPs for Roof/Building Drains at Manufacturing and Commercial Buildings and S431 BMPs for Washing and Steam Cleaning Vehicles/Equipment/Building Structures in both have been updated to include guidance for pollution prevention of PCBs from these activities. For more information on how to identify PCBs at your facility, Ecology published How to Find and Address PCBs in Building Materials (Ecology, 2022).
Bioretention:
Guidance for the design of bioretention stormwater management practices now includes High Performance Bioretention Soil Mix (HPBSM), which has advantages over other BSMs for use near phosphorus-sensitive waterbodies. Ecology approved the use of HPBSM in 2021 and published Guidance on Using New High Performance Bioretention Soil Mixes (Ecology, 2021). Guidance for compost-amended biofiltration was previously included in BMP T7.40 of the SWMMWW and BMP T5.60 of the SWMMEW. This guidance can now be found in BMP T7.30 of the SWMMWW and BMP F6.23 of the SWMMEW.
UIC Program Guidelines:
The program guidelines for Underground Injection Control (UIC) wells have been revised to include new guidance for deep UIC wells within sensitive groundwater areas and wellhead protection areas. According to the revised manuals, an IUC well that extends below an upper confining layer (dense clay for example) and discharges into the underlying vadose zone is designated as a deep UIC well. See Chapter I-4.15 of the SWMMWW and Chapter 5.15 of the SWMMEW for further guidance on registration requirements for deep IUC wells.
Pavement Maintenance Project and Underground Utility Project Exemptions:
Project scope descriptions have been updated in the Exemptions chapter to ensure categorical exemptions are clearly defined. Key updates include the following: timberland conversion projects are not exempt from the Minimum Requirements, pavement maintenance projects qualifying for exemption do not include changes to the traffic or function of a paved area, and exemptions for underground utility projects can only be applied to projects wherein the scope only includes utility work. See Chapter I-3.2 of the SWMMWW and Chapter 2.2 of the SWMMEW.
If you would like to learn more about the 2024 SWMM revisions and how they could impact your facility, please contact Trinity’s Seattle office at 253.867.5600.
