Because stormwater infiltration and filtration projects can be complex, it’s critical that they be effectively planned, designed, and constructed to operate correctly over the long term with reasonable ongoing operations and maintenance (O&M).

At MuniCon 2025, Geosyntec’s Aaron Poresky and John Knutson, along with two of our clients, will lead a panel discussion “Stormwater Infiltration and Filtration Lifecycles: Lessons from Municipalities and Applied Research Findings.” The panel will present real-world examples of infiltration system problems and discuss applied research, and offer practical ways to prevent problems and control risks during planning, design, and construction.

About the Speakers

Aaron Poresky, PE, is a Principal Water Resources Engineer focused on urban stormwater and watershed management. As a leader in integrated planning and design of stormwater best management practices (BMPs), Aaron leads research on BMP performance and emerging stormwater pollutants, helping municipalities apply findings to watershed management.

John Knutson, PE, PMP, is a Senior Principal Water Resources Engineer focused on stormwater planning and design, floodplain management, and development of local and regional stormwater manuals. John contributed to Washington’s original NPDES II permits, the revised Washington State Underground Injection Control Rule, and the technical guidance for design and construction of underground infiltration systems.

Q&A

What’s your background in stormwater management?

Aaron Poresky

My career has mainly focused on stormwater BMPs, particularly their effectiveness in controlling hydrology and stormwater pollution. Lately, I’ve been more interested in the long-term O&M elements of stormwater BMPs. As cities assign the O&M of BMPs to their public works departments, the focus shifts to understanding what it takes to maintain them: How are they managing their overall stormwater conveyance and stormwater quality protection systems?

I’ve also been fortunate to participate in applied research with Geosyntec, some of which has been funded by the National Cooperative Highway Research Program. Although driven by the US Department of Transportation’s research priorities, many of these align with municipal stormwater managers’ priorities, like infiltration and filtration. I’ve also had the chance to perform local applied research under the Ecology SAM program, and I’ve translated research findings into project designs and stormwater technical guidance in Washington, California, and Texas.

John Knutson

My career has encompassed all aspects of stormwater management, with an emphasis on the application of filtration- and infiltration-based stormwater BMPs. I started doing research and development on stormwater filtration BMPs right out of graduate school, where my research and thesis was on vadose-zone hydraulics. After working in stormwater R&D and consulting, I joined local government and served on the Eastern Washington Stormwater Steering Committee that oversaw the development of the Stormwater Management Manual for Eastern Washington and the Model Municipal Stormwater Program for Eastern Washington. I co-chaired the subcommittee that developed the model program, which became the template for the first NPDES II Permit for eastern Washington. I also served on the committee that revised Washington’s UIC Rule and helped develop siting and design guidance for new stormwater UICs like drywells and infiltration trenches.

What are some of the challenges that municipalities face when planning new or retrofitting stormwater filtration and infiltration projects?

Aaron Poresky

The two main challenges that come to mind are how to get a good characterization of the site, and system maintenance as it relates to clogging.

First, the performance of these systems depends on subsurface conditions, which can be hard to characterize due to variability below the ground. You’re never going to have a perfect characterization. So, the question is, what’s good enough when it comes to site characterizing testing?

Second, filtration or infiltration systems will inevitably experience clogging. So, how often is that going to happen? What can you do that’s economically appropriate to prevent that? Should you implement high-level pretreatment to delay clogging, or opt for low-level pretreatment and build a system that’s easier to maintain when it gets clogged?

The challenges are really on both the front end (understanding conditions before you go into the design) and the back end (understanding how to keep the system maintained). The design process needs to deal with those two tricky questions.

John Knutson

I want to echo what Aaron said. Stormwater infiltration depends heavily on the site. For a successful long-term infiltration facility, site characterization is critical. It’s often a tradeoff between risk and cost during site characterization and evaluation.

Some clients choose to accept more risk and spend less on site characterization. However, during construction, they might find they don’t have as much good permeable material as they thought they did, which could result in a mid-construction redesign. For deep stormwater infiltration, they might initially think a couple infiltration wells are enough, only to find out during installation and testing that the wells are not infiltrating water as expected, which means more will be needed.

What are some of the tools that can help support life-cycle design decisions?

Aaron Poresky

One tool is a set of fact sheets on infiltration testing approaches from NCHRP research. This can help municipal stormwater managers understand the pros and cons of the different infiltration testing methods. For example, the more rigorous methods like full-scale pilot infiltration tests can be necessary when you’re designing a significant system. However, lighter methods could be used for prescreening when developing an understanding of a broader site. The toolbox of different infiltration testing methods can help everybody understand what’s available and appropriate for their needs.

Another tool is the clogging risk assessment tool we developed for NCHRP. We looked at research on how much sediment it takes to clog a filter and how sediment buildup reduces infiltration rate. That tool helps answer the key questions about BMP lifespans and how much improvement can we gain from advanced pretreatment to remove sediment. Work began with a meta-analysis of existing research, then involved compiling existing research into a consistent framework that can be put into a practitioner’s hands to help support life-cycle decisions.

John Knutson

I would recommend that designers carefully review and understand the current design guidance in relevant stormwater manuals and site suitability assessments. Then answer the question: Is this a good BMP to apply on your site specifically for stormwater infiltration? There are clear guidelines for site suitability – for example, you don’t necessarily want to install your BMP right next to a slope.

We can share our experience creating standard operating procedures (SOPs) for shallow and deep infiltration systems. I worked on infiltration SOPs for the Port of Seattle’s Low Impact Development Manual rewrite, creating comprehensive procedures from planning to construction for both shallow and deep stormwater infiltration facilities. This work was featured in Stormwater Solutions in 2022.

How can municipalities integrate these tools and guidance into the design for their projects?

John Knutson

Because we’re doing the planning and design for both stormwater retrofit and compliance projects, we’re directly using our expertise to help clients integrate these recommendations and tools into their processes.

A recent example is a large infiltration retrofit project for a municipal entity. We advised on stormwater infiltration and UIC compliance issues. We found that this unique facility was going to have so much gravel in it, on top of the native receptor soil, that the minor amount of fines (sediment) in the gravel, once mobilized, was enough to potentially clog the infiltration receptor surface. We helped the prime consultant and client assess and evaluate the potential plugging issue and develop a solution to mitigate the risk.

Aaron Poresky

Along the same lines as John, we help private and public clients understand where they need to focus their efforts. Some of the smaller, low-risk projects may not need detailed analysis, but large infrastructure projects require the right expertise to address complex questions upfront because these issues can be extremely costly to fix in the long run.

For example, we’ve worked with a major industrial client on engineered media filtration systems for various sites. These large basins require careful attention to media specifications and hydraulics to prevent premature clogging and maintain the right balance between treatment and lifespan.

The main point is that larger systems justify the extra attention to detail to make sure the right value-based and risk-based decisions are made in the design. Is this a design that we will want to own and maintain? Then, the right construction oversight should be chosen to make sure the materials specified are the ones being installed.