Stormwater FAQ

Questions from stormwater working group meetings

Stormwater vault inspection

We strive to keep our waterways clean by preventing pollution, educating tenants and implementing new programs and technologies to manage stormwater in an industrial setting.

ISGP permit eligibility and requirements

Most operations are indoors but some materials are outside but wrapped in plastic. Do I qualify for a Conditional No Exposure (CNE) Certificate?

The NWSA encourages you to consider whether the plastic effectively protects the materials from precipitation prior to applying for a CNE Certificate. This evaluation should consider stormwater run-on and other forms of stormwater contact, and the ability of the plastic to withstand storm events. If you feel the plastic is an effective barrier that prevents stormwater contact, application for a CNE Certificate may be appropriate.

However, in the NWSA’s experience, Ecology is becoming less likely to grant CNE Certificates in these situations. In some cases, Ecology has requested sample collection prior to making a CNE determination. The ISGP allows Ecology to require that a facility obtain coverage (e.g., if the operations conducted at the site would generate stormwater with similar characteristics to facilities required to get a permit based on their industrial activities or Standard Industrial Classification (SIC) code listings in Table 1 of the ISGP). Many warehouse operations now have NPDES permit coverage because Ecology determined they should have a permit based on sampling results, even if their SIC code does not qualify

Can I qualify for a CNE Certificate for the part of my facility where all operations are conducted indoors or where there are no potential sources, and only obtain permit coverage for areas where outdoor operations occur?

No. The entire facility is covered by the ISGP or Individual Permit (or by the CNE Certificate). Ecology does not allow for a facility to be segmented into areas covered by the permit and areas not covered by the permit. Differences in site operations or activities may be reflected in different sampling requirements in individual permits; under the ISGP, representative samples must be collected from each area.

I am the operator at a facility that just does maintenance on chassis, with some vehicle traffic for cargo pickup. I was told I need an ISGP because I do maintenance. Should I get one?

Transportation facilities are only required to have an ISGP if they perform mobile fueling or vehicle maintenance. The definition of “vehicle maintenance,” as found in NPDES permits, is:

the rehabilitation, mechanical repairing, painting, fueling and/or lubricating of a motor-driven conveyance that transports people or freight, such as an automobile, truck, train or airplane.

Chassis are not motor-driven, so chassis maintenance does not trigger the permit requirement. However, if you conduct mobile fueling, you may still be required to obtain NPDES permit coverage. Additionally, if you conduct other industrial activities that are similar to industrial activities conducted by other industries required to obtain permit coverage, Ecology may still require you to obtain a permit. The NWSA can help you evaluate whether permit coverage is required before you sign up.

I heard the Port of Tacoma/Port of Seattle is the permit holder for some industrial facilities with operations similar to mine. Can the NWSA take over stormwater management at my site?

The NWSA does not intend to take over any permits currently held by site operators, and would prefer that all operators at NWSA-licensed properties hold their own permits. In the experience of NWSA staff, environmental stewardship is more effective when permits are held by the site operator, because site operations and BMPs become linked to permit compliance, and thus accountability improves. Though the NWSA is available to assist operators with permit compliance and environmental stewardship, the alliance cannot afford the environmental liability of a site that is not incentivized to fully and consistently implement BMPs to reduce pollution.

I read Condition S8 of the ISGP, and I still don’t understand Level 1, Level 2 and Level 3 Corrective Actions. If I exceed benchmarks in Quarter 1, but not in Quarter 2, 3 or 4 (either because there are no qualifying sampling events or because I met bench

* I read Condition S8 of the ISGP, and I still don’t understand Level 1, Level 2 and Level 3 Corrective Actions. If I exceed benchmarks in Quarter 1, but not in Quarter 2, 3 or 4 (either because there are no qualifying sampling events or because I met benchmarks), am I still in “Level 1” at the end of the year? If I exceeded benchmarks in Quarters 1 and 4, do I need to do “Level 1” corrective actions again when I exceed in Quarter 4? How long do I stay in Level 2 or Level 3 corrective action status before it “resets”?


The Corrective Action (CA) Levels 1, 2 and 3 are indicators of the degree and type of corrective action that must be implemented. They do not indicate a permit “status.” Level 1 is Operational Source Control BMPs, Level 2 is Structural Source Control BMPs, and Level 3 is Treatment BMPs. Once the CA is completed, you are no longer at that “Level.”

ISGP Condition S8.B.3 states, the Level 1 CA must be initiated within 14 days of receipt of the sampling results and the deadline for implementing additional Operational Source Control BMPs is “no later than the Discharge Monitoring Report (DMR) due date for the quarter the benchmark was exceeded.” Level 1 CAs are specifically related to the implementation of additional operational BMPs only, which are identified based on the results of an inspection. After completing a Level 1 CA (and if you do not exceed benchmarks for that parameter in any other quarter), note that you exceeded a benchmark in one quarter and summarize the actions taken to correct the exceedance in your annual report, as required by ISGP Condition S8.B.2.

All Level 1 CAs must be completed by the DMR due date for the same quarter that the result exceeded the benchmark. You must complete a Level 1 CA for every quarter you exceed a benchmark. If you exceed the benchmark for a particular parameter in two quarters in a year, you must also complete a Level 2 CA, which requires implementation of additional Structural Source Control BMPs and which must be completed by Aug. 31 of the following year (ISGP Condition S8.C). If you exceed the benchmark for the same parameter in a third quarter, you must complete a Level 3 CA instead of a Level 2 CA. A Level 3 CA requires implementation of additional Treatment BMPs, which must be operational by Sept. 30 of the following year (ISGP Condition S8.D).

Notably, once you reach a Level 2 or Level 3 CA for a parameter, any additional exceedances for that parameter in the entire calendar year following the year that you began a Level 2 or Level 3 CA do not count towards additional Level 2 or Level 3 CAs (ISGP Conditions S8.C.4.e and S8.D.5.e). For example: if you exceeded benchmarks for zinc in the first and third quarter of 2016, then exceeded benchmarks for zinc in the first, second, and fourth quarters of 2017 before completion of your Level 2 CA, you are not required to install treatment as a result of the three benchmark exceedances in 2017. Exceedances in 2018, however, would count towards additional Level 2 or Level 3 CAs.

Ecology’s ISGP Fact Sheet is a good resource for additional clarification on Level 1, Level 2, and Level 3 CAs.

Pollutant Source Identification, Sampling, and Best Management Practices

How does having acres of containers present on my facility affect stormwater quality? Are there any BMPs that are designed to address container storage?

Containers are usually coated with a galvanized paint that contains zinc. Rain can mobilize the zinc in the paint into the stormwater system. The best way to determine whether and if there is a problem, is to sample water in catch basins that take water runoff from the container storage area. Once you know whether you have an issue, then you can look at the Western Washington Stormwater Management Manual (SWMMWW), Volume 4 – Source Control BMPs that will help manage zinc.

How does having heavy industrial equipment present on my facility affect stormwater quality? Are there any BMPs that are designed to address parking and use of heavy industrial equipment?

The answer here is similar to the answer above. Sample to determine if there is an issue and then implement best practices to handle the actual issue. The SWMMWW, Volume 4 – Source Control BMPs is the authority for approved BMPs.

What items can terminal operators use as a “checklist” when reviewing their terminals for possible stormwater issues?

The NPDES permit held for your facility will identify some BMPs that are applicable to all permittees under that permit. For marine terminal operators, Appendix A of the Washington State Marine Terminal AKART and ISGP Corrective Action Guidance Manual presents a “master list” of mandatory BMPs that operators falling into each of four different categories of marine terminal facilities must implement. Additional BMPs to minimize stormwater impacts from common sources of pollution that are applicable to any industrial facility are listed in Volume 4 of the SWMMWW. Therefore, facility operators should also review the SWMMWW’s Table of Contents to identify any additional activities conducted at the facility, or conditions present at the facility, that are associated with applicable or recommended BMPs. Finally, the NWSA is available to help you perform site-specific source evaluations.

Also, Ecology has a template monthly inspection form on their website: Industrial Permit, on the right-hand side, just above “contact us.” The template is a useful check sheet for any facility; you can customize to fit your specific operations.

Can the Northwest Seaport Alliance (NWSA) provide assistance with stormwater sampling?

The NWSA cannot assist customers with sampling requirements under their Industrial General Stormwater Permit (ISGP). However, the NWSA can assist with facility inventories of pollutant sources and site characterization sampling. The NWSA can also provide strategic insight into best practices for sample collection and good housekeeping practices to incorporate into your daily operations.

The NWSA can provide technical assistance to improve sampling techniques employed by sampling personnel. The NWSA can also arrange group training provided by third-party groups. This provides customers two benefits: (1) arranging a training for a group is more cost-effective for customers than signing up individually, and (2) signing up as a group would allow course content to be tailored for sampling stormwater in the maritime environment.

What are some BMPs for over-the-road (OTR) queuing areas, drive aisles and other areas that experience heavy traffic?

Heavy traffic areas often experience high pollutant loading for particulates and metals. BMPs for heavy traffic areas include vacuum sweeping (required a minimum of once a quarter per permit requirements), catch basin cleaning and monthly vehicle inspections (for leaking fluids such as oil, antifreeze, etc.). Catch basins receiving flow from these areas can also be retrofitted with catch basin inserts to improve stormwater quality at the end of the pipe where discharge is monitored for permit compliance.

The NWSA is working on a pilot project to design a new insert that would fit within the catch basin and that would not fall into the catch basin when the lid is lifted. The NWSA’s goal is to create a product that is an improvement upon other products on the market today.

What’s a best practice for getting copper and zinc out of stormwater?

Zinc and copper are found in water in a particulate fraction and a dissolved fraction. Compliance with National Pollutant Discharge Elimination System (NPDES) permits is based on total zinc and total copper, which are the sums of particulate and dissolved fractions. The most effective form of treatment will depend on whether the particulate fraction or the dissolved fraction makes up the bulk of a customer’s total. Before selecting or evaluating treatment, it is best to collect several samples and analyze them for both total and dissolved zinc and copper. In addition to helping guide treatment selection and design, this may help in identifying potential sources of copper and zinc at your site. Once the relative proportion of dissolved versus particulate metals is known, the following treatments can be applied.

For stormwater with a high particulate fraction, passive techniques, like filtering stormwater through sand or soil, work best. If stormwater samples from your site are high in total suspended solids and/or turbidity, a membrane filter may be required to get the dirt out of the water.

To treat the dissolved fraction with a passive system, some form of adsorptive treatment media (e.g., biochar) must be included. Alternatively, active treatment may be required.

Does the NWSA have or recommend a decant facility for solids removed during facility sweeping?

No. Most vactor truck and sweeper vendors already have agreements in place with preferred decant facilities. The decant facility nearest to most NWSA properties at the Port of Tacoma is PRS Group, located at 3003 Taylor Way in Tacoma. The decant facility nearest to most NWSA properties at the Port of Seattle is the King County Road Services, located at 155 Monroe Avenue NE in Renton. Unlike PRS Group, this facility will only accept stormwater catch basin clean out material from private operators and agencies after they have completed a 2.5-hour training and other financial requirements. If you own and operate your own vactor truck or sweeper, a full list of decant facilities in Pierce and King Counties can be found at the Washington Stormwater Center.

Downspout Treatment Boxes for Roof Runoff

Can the NWSA provide downspout treatment boxes to operators at port facilities?

The NWSA may be able to provide downspout treatment boxes and other forms of stormwater treatment as part of pilot programs available to customers and operators at Port of Tacoma‑owned or NWSA-managed properties. The location of the treatment and the area treated may not include privately-owned contributing areas. However, the NWSA can share the “bill of materials” and construction diagrams so you can build and install your own.

What makes the media in the downspout treatment boxes particularly effective?

The media in the downspout treatment boxes offered by the NWSA are tailored specifically to treat stormwater runoff. In most cases, the media we use are a blend of gravelly sand and compost. The Port of Tacoma performed column tests of six different compost and media options. The results of these tests showed that in order for the boxes to function as designed, the compost mixture must be 95 percent free of household-derived yard waste, which can contain high amounts of metals. Influent samples from your facility may be collected to determine the relative fraction of metals in stormwater coming from your roof runoff (total vs. dissolved). If the effluent contains high concentrations of dissolved metals, the gravelly sand and compost mixture may be amended with adsorptive media.

What do I need to do to maintain the downspout treatment boxes to ensure they are performing properly?

Maintenance requirements are minimal and typically include deadheading plants within the boxes to keep the top layer of soil free of plant debris, and monitoring the plants during summer months to keep them from drying out. Operators that installed downspout treatment boxes at the Port of Tacoma more than three years ago have found that the boxes are still effectively reducing metals concentrations to levels below ISGP benchmarks even without changing media within the boxes. If your facility achieves consistent attainment (i.e., meets benchmarks for eight consecutive quarters) for copper and/or zinc after installing the downspout treatment boxes, the NWSA recommends that you continue sampling effluent from the treatment boxes twice per year to ensure that they are still performing as expected.

I collected effluent samples from my facility’s downspout treatment boxes, and I found that metals concentrations are above benchmarks. What should I do next?

If this happens, begin sampling the influent before it enters the box, and then compare it to your effluent results. If the comparison shows little or no difference, or if the influent concentrations are lower than the effluent, it is time to change the top layer of media.

I decided to build downspout treatment boxes for my facility. Does it matter how water enters the boxes from the downspout?

We have learned the boxes perform better when stormwater through the downspouts is directed into the boxes using two 45-degree angle joints, particularly for multi-story buildings. This allows for a more gradual transition in stormwater flow velocity before reaching the media. This has several benefits: it increases the time that stormwater stays in contact with the treatment media, helps prevent preferential pathways or channels from forming in the treatment media and reduces pressure buildup in the downspout during heavy rains. If the downspouts cannot be modified with PVC pipe, a splash block will work to dissipate the flow energy.

In the Port of Tacoma’s initial installations, the roof gutters leading into the downspout treatment boxes were connected with a single 90-degree elbow to a horizontal PVC pipe with drain holes that allowed water to percolate out of the pipe and onto the surface of the media. The end of the pipe has a clean-out to accommodate maintenance and prevent plugging. This works well for one-story buildings.

Are the downspout treatment boxes going to overflow during heavy rains?

The boxes are sized to handle the flows specific to the roof area that drains from a particular downspout. One standard treatment media unit can treat 5,000 square feet of roof. Overtopping of the treatment boxes during excessively heavy rains is managed by drilling half-inch drain holes around the upper area approximately four inches above the media. The NWSA can help you size the downspout treatment boxes relative to your roof size, ensuring sufficient capacity to handle most storms.

Other Treatment Technologies

Are there any other low-tech, cost-effective alternatives to planter boxes for the treatment of roof runoff?

Some operators at NWSA-managed properties have reported the successful treatment of runoff by using oyster shells in lieu of the compost/sand/biochar mix. Oyster shells can be sourced locally through Taylor Shellfish Farms. For this treatment method, the oysters are placed in mesh sacks within catch basins or in drums below roof downspouts to filter roof runoff. Some operators have crushed the shells prior to placement in the mesh sack. The NWSA is currently conducting testing of oyster shells.

Are there any benefits to using a “low tech” treatment media like oyster shells or downspout treatment boxes instead of proprietary treatment media?

Using a non-proprietary media allows you to blend the media to match your needs, and is generally lower cost than proprietary media. Site-specific factors (pH, hardness, the nature of other pollutants in the water, etc.) affect which treatment media will work best at your site. The NWSA recommends pilot testing different media and/or making iterative system improvements to identify the best match for your site’s stormwater chemistry.

I’m thinking about trying a new/novel treatment media at my facility. Can the NWSA help pay for it?

The NWSA is interested in partnering to conduct pilot studies with customers that install treatment systems to facilitate better assessments of these treatment systems and to generate a more robust dataset that can be used by other customers when selecting appropriate treatment for their facilities in the future. Contact NWSA staff for more information.

Does the NWSA have any experience with MetalZorb or other proprietary treatment media?

The NWSA uses bulk soil, sand, gravel, glass, compost and carbon components in treatment systems installed at NWSA properties at the Port of Tacoma and Port of Seattle. Some NWSA customers have reported that proprietary media such as MetalZorb works at their facilities, but, as noted above, the NWSA recommends pilot testing at your facility before committing to any particular treatment media. Even though proprietary media are formulated to work over a broad range of conditions, pilot testing is still beneficial to ensure the media and the system used to hold it in place work for your site-specific stormwater chemistry, volume and velocity. Also, it is important to note that most proprietary treatment media are sold in cartridges or bags that are designed to fit only within the vendor’s specific treatment system, and many vendors will not sell replacement media without an initial purchase of their system.

Does the NWSA have any experience, comments or input on the Enpurion stormwater treatment system?

The Enpurion system is a stackable electrocoagulation treatment system that requires pumping stormwater to aboveground treatment. Compared to other aboveground systems, its stackable design economizes space. It is more expensive than passive treatment systems, but may be cost‑effective when compared to other forms of active treatment systems depending on the nature of the site and influent stormwater quality. At least one NWSA customer at the Port of Seattle or Port of Tacoma has installed an Enpurion system at its facility and reports its benefits include consistent performance and a high degree of adaptability.

I heard about a stormwater treatment project at the Port of Tacoma at Terminal 4. Can you tell me about that project and its costs?

Project Land Use

Cost per acre constructed
in 2014 dollars

Operations & Maintenance/yr

Type of technology installed

Rail Yard 1

$48,100 (12 acres)


Modular wetland technology for dissolved metals

Rail Yard 2

$37,100 (22 acres)


Membrane filtration, solids management system

Strad Container Terminal

$35,900 (56 acres)


Media filtration for dissolved metals

Hostler / Chassis Container Terminal

$29,630 (135 acres)1

$$; mostly media replacement and sweeper operations

Media filtration for dissolved metals



Treatment was installed as part of a larger redevelopment project, rather than an isolated treatment system installation. Costs may vary depending on the characteristics of the stormwater, the scope of construction and other site-specific considerations. The approximate costs are provided for reference and may be useful for planning purposes.

Miscellaneous Questions

I saw that the Port of Tacoma recently published a Port Stormwater Management Guidance Manual (SMGM), which has more than 100 pages. What is it, and what is its relevance to me as an operator at a port facility?

The Port of Tacoma’s SMGM governs the port’s management of stormwater under its Phase 1 Municipal Stormwater (MS4) Permit. The port’s MS4 permit covers port-owned properties that are not permitted separately under ISGPs or Individual Permits under the NPDES program. As such, customers and operators at Port of Tacoma facilities that hold individual or ISGP permits do not need to become familiar with the contents of the SMGM as part of their routine operations. However, any customer or operator at a Port of Tacoma facility that would like to renew its lease, change operations at its facilities or complete site improvements may be subject to requirements described within the SMGM. Port of Tacoma or NWSA staff can help you determine if the stormwater requirements within the SMGM are triggered by any proposed changes.

The SMGM describes low-impact development (LID) best management practices (BMPs) that are suitable for a port environment and that satisfy MS4 permit requirements to use LID principals during redevelopment projects. Due to the location of port-owned properties, infiltration is not a viable technique for most areas. The SMGM also defines BMPs that are viable at port facilities; over time, as more data are collected and the manual is developed, the SMGM may help the port establish what qualifies as “all known, available and reasonable methods of prevention, control and treatment (AKART) for stormwater treatment at port facilities.

Note that the Port of Seattle also has a SMGM with which customers that don’t have an ISGP must comply.

Does following the Port’s Stormwater Management Manual meet the City’s requirements for individually-held stormwater permits?

No. The requirements described within the SMGM do not replace the requirements of ISGPs, Individual Permits, or the City’s Stormwater Management and Site Development Manuals. This is true in the City of Tacoma and the City of Seattle.

I’m considering attending a stormwater workshop hosted by the Environmental Coalition of South Seattle (ECOSS). What is ECOSS, and would the workshop be useful?

ECOSS is a non-profit training organization whose mission is to provide education to businesses to improve their environmental permit compliance and environmental stewardship. Its stormwater workshops are described and advertised on its website. Several NWSA customers have taken its beginner and advanced workshops and found them to be useful.