A Soil Infiltration Cookbook: How Deep Does Your Water Go?

Street Flooding Due to Failing Infiltration Systems – Yakima, Washington (Source: Aspect Consulting)

The Pacific Northwest recognized early on the negative effects of urbanization on receiving waters and has been a leader in the stormwater management field for decades. This was initially driven by concerns about the impacts of urban runoff on threatened and endangered species in the Puget Sound area, and later reinforced by the National Pollutant Discharge Elimination System (NPDES) permitting program.

More recently, a study led by the University of Washington solved the mystery of why Coho salmon are dying in urban streams before spawning. As many suspected, stormwater runoff is the culprit. It was found that runoff from high traffic areas contains a tire-related chemical that is toxic to Coho.

As the science of stormwater management continues to progress, practitioners must learn about and apply the latest techniques, including the use of stormwater infiltration. While infiltration is not really a new concept, widespread infiltration design is new for some areas and some water quality designers. When infiltration systems fail, the consequences can range from increased pollutant loading and erosion in streams to flooded roads, businesses, and homes. 

Infiltration Design For Cleaner Water and Better Habitat

Many agencies operate stormwater programs under NPDES Municipal Separate Storm Sewer System Permits (MS4 Permits). MS4 Permits require development projects within the permittee’s jurisdiction to include permanent Best Management Practices (BMPs) to reduce stormwater pollution and flow-related problems. Permanent BMPs typically include detention vaults, filters, and bioretention basins.

Over the last decade, MS4 Permits have started requiring the use of infiltration BMPs to mitigate the hydrologic and water quality impacts of development. From a surface water perspective, infiltrated runoff does not cause stream erosion, has no pollutant loading, and helps recharge groundwater.

Using Infiltration to Reduce Hydrologic and Water Quality Impacts (Source: Aspect Consulting)

Lessons Learned from the Track Record of Filtration and Infiltration

Much like the early application of other stormwater BMPs, some designers don’t have a technical background in infiltration. Numerical models, simplifying assumptions, and step-by-step procedures make applying infiltration feasible for generalists, but there are some key issues to consider.

Accurately Estimating Infiltration Rates. The starting point for designing an infiltration BMP is determining the infiltration rate your BMP is expected to have. Allowable methods have changed over the years, with some agencies requiring in-situ infiltration testing for all infiltration BMPs, and some allowing “grain size methods” to estimate rates in certain cases. It’s recommended to only use grain size methods for screening purposes when soils meet all criteria for application of the equation being used.

Reliance on grain size method results can backfire, which is why in-situ infiltration testing must be done to support the design process, with knowledgeable analysis of testing results. The number of infiltration tests should scale with the size, complexity, and soil heterogeneity of the project site. Thoughtful testing and analysis methods can also indicate if groundwater was mounding during the test, and whether a formal mounding analysis is needed.

Once correction (safety) factors are applied, there are generally two ways an infiltration rate is used to help model and design infiltration BMPs: (a) assume the infiltration rate is constant during BMP filling and emptying; or (b) assume the infiltration rate varies (typically linearly) with water depth (pressure head) above the infiltration surface. Knowing how the infiltration BMP will be sized is important when analyzing test data because the resulting constant infiltration rate will not necessarily be the same value as the variable rate.

Extreme plugging/sediment loading to a proprietary stormwater filter (Source: Oregon Department of Transportation, Operation and Maintenance Manual, DFI No.: D00183, 2011)

Plan to Manage Plugging. Infiltration BMPs are susceptible to the same plugging by stormwater sediments and biofouling that stormwater filters are. Stormwater filters plug up and infiltrating into soil is like infiltrating into a filter. Therefore, pre-treatment to remove sediment is necessary to prolong the life of infiltration BMPs.

Plugging of the infiltration surface is the main reason safety factors are used when sizing infiltration BMPs. Stormwater manuals often prescribe safety factors, but the safety factor really should scale based on the soil being infiltrated into.

For instance, when infiltrating slowly into a fine soil with a permeability not much higher than stormwater sediment, reducing the measured rate by a factor of 2-4 is acceptable. But when infiltrating into coarser soil with a permeability much greater than stormwater sediment, a larger safety factor is warranted (some jurisdictions do set a maximum design infiltration rate). The level of treatment for sediment removal prior to infiltration should affect the safety factor too.

Check and Address Groundwater Mounding. Infiltrating runoff sometimes causes groundwater mounding. As mounding increases, it can reduce the infiltration rate, cause seepage problems, and raise regulatory compliance concerns. We recommend monitoring groundwater during infiltration testing to see if it rises in response to the test. If so, the data can be used to help calibrate a mounding model. In cases where mounding is an issue, a formal analysis and modeling (such as USGS’s MODFLOW) of the receptor soil/aquifer combination should be done and infiltration BMP flowrates and spacing fine-tuned to avoid mounding problems.

Increasing Focus on Deep Infiltration to Meet Low Impact Development Goals

The use of “deep” infiltration to meet project Low Impact Development and flow control requirements has been increasing, particularly where surface soils have low permeability but overlie an unsaturated higher permeability soil. It can be feasible to bore through the surface soil and install an infiltration well in a more permeable receptor soil. Stormwater runoff is routed into the well and infiltrated into the receptor soil.

Simplified Deep Infiltration Schematic (Source: Aspect Consulting)

The concept of deep infiltration is straight forward but a system that reliably functions for decades requires some careful thought. To prevent plugging, it’s critical to keep sediment out of deep infiltration wells. However, normally some runoff is allowed to bypass treatment BMPs during large storms. Directing untreated flow into infiltration wells will reduce their service life. Another consideration is whether advance BMPs can be a source of initial sediment loading including: Is that drain rock really clean? Should I specify on-site washing of drain rock? Will my bioretention mix initially leach sediment? Should I flush the system before connecting to infiltration BMPs?

We recommend designing deep infiltration wells to:

  • Include treatment to remove sediment for
    all flows to the well

  • Control the rate of flow into the well to
    the design rate

  • Reserve head to allow water levels to rise to counteract plugging

  • Prevent baseflows from entering infiltration wells

  • Ensure the system drains down in a
    reasonable time

Using the ‘Infiltration Cookbook’: Infiltration System SOPs

Consistent and successful stormwater infiltration requires clear and comprehensive guidance for planning through construction. As part of the team preparing an updated LID Manual for the Port of Seattle’s Seattle-Tacoma International Airport (STIA), Aspect developed Standard Operating Procedures (SOPs) for shallow and deep infiltration systems. The detailed SOPs for the Port of Seattle— both for shallow and deep infiltration — are publicly available and are a good example of a successful ‘cookbook’ for creating sustainable infiltration design.

SOPs provide guidance for planning, testing, analysis, design, and construction of infiltration facilities. Recommended SOP steps for deep infiltration are shown here:

For more information about soil infiltration best practices for water quality and stormwater issues, contact John Knutson.

An edited version of this article appeared in the February 2022 version of Stormwater Magazine.

Meet Daniel Chang and Ryan Mullen

Aspect recently welcomed Daniel Chang to our Seattle office and Ryan Mullen to our Bellingham office. Here are Five Questions we asked to get to know them better.

Daniel Chang, Staff Engineer

Yellowstone Grand Prismatic Spring! During my post-graduation road trip in Fall 2020

Yellowstone Grand Prismatic Spring! During my post-graduation road trip in Fall 2020

1. Where are you from? If you’re not from the Pacific Northwest, what brought you here?

I was born and raised in Johnson City, Tennessee, but I knew that I wanted to explore outside of the Southeast post-college. After spending the summer of 2019 in Seattle for an internship, I knew I had to come back to enjoy the city and all the nature the Pacific Northwest has to offer.

 2. What inspired you to pursue water resources engineering? What made you curious about it?

I have always been a swimmer, so I like to think my interest in water came from that experience in the sport. Spending 20+ hours a week swimming is plenty of time to think about where this tank of water is coming from, what is in the water, and the occasional thought of the absurdity of swimming in a massive tank of water in the center of cities facing drought. The interest grew through courses in college learning about global challenges with water access, supply, and quality—all of which I am excited to tackle in my career.

 3. What do you like best about your area of expertise? What excites you and keeps you motivated?

I love the general idea of working “behind the scenes” for a resource that the general population takes for granted in our daily lives. We often have the privilege to not think about where our water comes from, or what is in it, so I enjoy investigating all of that under the surface and at the source.

 4. What do you like to do when you aren’t working?

COVID proved to me that I am truly an extrovert. When I am not working, I love to be with friends doing anything from re-watching guilty pleasure movies/TV shows for the 50th time, cooking (and eating!) massive feasts, or exploring nature in my journey to become a true PNWer. For the immediate though, I am currently training for swimming the Olympic trials in June 2021 for a final swan song to my swim career!

 5. What five people would be your dream dinner party guests?

To get the perfect balance of conversation, comedy, and cuisine, I present my dream dinner party roster: Gordon Ramsey (to roast/maybe compliment my food), Meryl Streep (for the high class feeling), Adele (for some giggles and maybe live performance), Bill Nye (for the fun facts), and Mindy Kaling (for great laughs).

Ryan Mulllen, Staff Geologist 

Mountain biking the 100-mile White Rim loop in Canyonlands National Park, UT

Mountain biking the 100-mile White Rim loop in Canyonlands National Park, UT

1. Where are you from? If you’re not from the Pacific Northwest, what brought you here?

I am a Pacific Northwest native. I moved away from Washington after high school and spent the last seven years in southwest Colorado. I recently moved back here to be closer to family, water, and bigger mountains. 

2. What inspired you to pursue hydrogeology? What made you curious about it?

I grew up spending summers swimming on Lake Washington and skiing down the glaciers of Mount Rainier and Mount St. Helens. Spending time on water was always a big part of my life. It was not until I moved to Colorado that I began to understand how important (and scarce) water can be.

I studied physical and historical geology during my undergraduate degree and was fortunate to have gone to school in a unique geographic environment where 500 million years of geologic time was on display in the cliffs and valleys across from campus. I witnessed the distribution and movement of water across these landscapes and saw how seasons of extreme drought and catastrophic flooding impacted the community. 

3. What do you like best about your area of expertise? What excites you and keeps you motivated? 

The work is always interesting and varied, allowing me to be involved in many different types of projects. Hydrogeology encompasses a range of disciplines and each project has its own set of challenges that require innovative solutions. 

4. What do you like to do when you aren’t working? 

Generally, just about anything besides relaxing. I typically try to pack in as many adventures as I can after work hours and on the weekends. Lately I have been pursuing trail running, climbing, paragliding, skiing, and mountain biking. My wife and dog often get persuaded into joining my overambitious exploits. When I do find some downtime, I enjoy cooking, photography, reading, and sipping on a tasty sour beer while planning for the next adventure. 

5. Where in the world would you like to travel next?

My wife and I dream of touring Europe by paraglider someday. On the more practical side, we are looking forward to exploring the mountains of the North Cascades, BC, and Alaska, hopefully this summer. 

The Future of Stormwater Management: MuniCon April 5-8 2021 - Part 2

Aspect is looking forward to discussing key stormwater management issues at this year’s Municipal Stormwater Conference April 5-8, 2021. Please join us in four (virtual) sessions where our stormwater team helps lead the discussion of the future of stormwater management in the Pacific Northwest. Please join us for Aspect presentations three and four (the first two Aspect presentations are summarized here)

 Stormwater Infiltration at Seattle-Tacoma International Airport – April 6

Protecting water quality is a key goal for SeaTac Airport’s stormwater program.

Protecting water quality is a key goal for SeaTac Airport’s stormwater program.

Join Tom Atkins, Principal Engineer, for a discussion on SeaTac Airport’s stormwater infiltration program that extends across 1,600 acres  of drainage area flowing into the Puget Sound and three local streams. The Port of Seattle –the steward for SeaTac Airport -- is interested in stormwater infiltration at STIA to achieve NPDES permit LID and flow control requirements along with GSI sustainability goals. Over the past four years infiltration has been investigated through shallow and deep infiltration feasibility assessments to guide the testing, analysis and design of BMPs for future development. This presentation will summarize the challenges, outcomes and tools that this work has produced, and will describe the deep infiltration testing planned for 2021.

How Shoreline Is Integrating ‘Salmon-Safe’ and the NPDES Phase II Permit – April 7 

In 2019, the City of Shoreline was the first in the state to achieve Salmon-Safe certification.

In 2019, the City of Shoreline was the first in the state to achieve Salmon-Safe certification.

Join Bryan Berkompas, Associate Hydrologist, to learn how Shoreline became the first city in Washington to achieve Salmon-Safe certification and how they are incorporating its requirements in City operations. Salmon-Safe guidelines are rigorous but also complimentary with many of the requirements of the NPDES Phase II permit. This presentation will discuss the process of achieving Salmon-Safe certification, the benefits of Salmon-Safe certification, and designing programs that satisfy the requirements of both the Permit and Salmon-Safe.

Aspect Welcomes Hydrogeologist Jay Pietraszek to Water Resources Team

Senior Hydrogeologist Jay Pietraszek joins Aspect’s Pacific Northwest-leading water resources team. Jay is a hydrogeologist with over 15 years of experience focusing on process-based assessments of water quality and water quantity for water supply, resource evaluations, and water management planning for commercial and industrial projects. He is based in Seattle, supporting Aspect’s Puget Sound clients as well as expanding Aspect’s water supply services throughout the Pacific Northwest.  

Jay Pietraszek, LHG

Jay Pietraszek, LHG

“As a firm founded by hydrogeologists, Jay’s background and expertise are an ideal match with Aspect’s water resources client base,” said Dan Haller, Principal Engineer for Aspect’s Water Resources Practice. “We’re excited to continue to build Aspect’s water resources team to be one of the strongest in the entire Pacific Northwest.”

Jay has performed extensive hydrogeologic analyses including site characterizations, water quality source assessments, evaluation of surface-groundwater interactions, pumping test analyses, well installations in complex hydrogeologic systems, production well design, and dewatering assessments for public agency, tribal, and mining clients. Pairing with his deep scientific know-how is Jay’s ability to translate complex ideas into clear communication and engage with regulatory and permitting bodies to move projects towards milestone goals.

“I’m excited to join the Aspect team and have the opportunity to work and collaborate with such a talented group of consultants,” said Jay.  “Aspect is one of the premier hydrogeologic consultancies in the region, with an excellent reputation for developing innovative and practical approaches to solve complex water resource challenges. I’m confident that Aspect’s core values and brand will provide a foundation for future success and opportunities, particularly as the demand for water resource-related services continues to grow.”

Visualizing Stormwater Infiltration + Visualizing the Story

Aspect recently led a first-of-its-kind approach to help the City of SeaTac (City) understand water quality requirements at the land use planning stage. Aspect, along with Robin Kirschbaum, developed publicly-available webmaps that visualize stormwater infiltration potential at a parcel level across the City’s 10 square miles. These maps will help both City planners as well as developers screen development options with infiltration requirements and make this step of the land use planning process much more efficient.

Take a look at the interactive Story Map for this exciting new tool here: https://maps.aspectconsulting.com/lidmapjournal/index.html.

Read Emelie’s article in the Daily Journal of Commerce about this pioneering project.

The Story in the Sediment: Tracing Stormwater Pollution Sources at Superfund Sites

Since 2001, the lowest five miles of Seattle’s Duwamish River (known as the Lower Duwamish Waterway or LDW) has been designated as a 412-acre Environmental Protection Agency (EPA) Superfund site. The LDW’s Superfund status results from decades of historical industrial activity. On top of the historical contamination, the LDW has ongoing issues with contaminated stormwater runoff. Rain hits the abundance of impervious surfaces – e.g., asphalt roads and lots, building roofs – in the industrial areas next to the river, collects contaminants from those surfaces, and carries it to the nearest storm drain and into the river.

Sediment sampling in Seattle's Lower Duwamish Superfund area helps identify contaminates and cleanup strategies to improve water quality. 

Among the many tasks in cleaning up Superfund sites is the ongoing detective work to sleuth what contaminants are there and where they came from. In the LDW cleanup, one of the key clues isn’t even in the river water itself, but in the sediment carried by stormwater. 

Aspect staff have sampled stormwater sediments across the LDW Superfund site – previously for the City of Seattle and King County and currently for the Port of Seattle at Harbor Island. Our efforts studying these solids in stormwater runoff provide key information about the recent history at a site and the extent of contamination.
 

The Benefits of Sediment Sleuthing: Unlike Water, It Accumulates

Unlike stormwater, which runs through and beyond an outfall to receiving waters, heavier sediments and other settleable solids (relatively heavy substances that sink in water) carried by runoff drop out and accumulate. This accumulation, which occurs in key locations such as stormwater catch basins, vaults, and low-gradient pipes, provides a rich environment for valuable leads on water quality contaminants that may eventually end up in streams, rivers, lakes, wetlands, and Puget Sound. Sediment monitoring often provides a more comprehensive historical picture of pollutants compared to instantaneous or short-term sampling of water alone.

At Superfund areas, and many other sites challenged by stormwater runoff, sediment monitoring benefits clients by: 

  • Providing a historical picture of pollution—through accumulated sediment analysis--associated with stormwater runoff and industrial discharge.
  • Identifying chronic types of pollution that may deteriorate water quality and habitat.
  • Tracing the sources of pollution to their origin for purposes of management, treatment, or elimination.
  • Determining sediment accumulation rates in sewers and catch basins to improve maintenance and operation needs and to anticipate and prevent flooding.
  • Complying with permits, records of decision, and other legal requirements for preventing environmental degradation or requiring cleanup of polluted sites.
  • Measuring the effect of land use activities and stormwater treatment best management practices (BMPs).
     

Tracking Hot Spots Across 600 Acres of Pavement and Buildings

Boeing Field (aka King County International Airport or KCIA) is one of the nation’s busiest primary non-hub airports and covers over 630 acres of mostly impervious surface. Managing stormwater runoff over this much area and with many industrial tenants is a challenge, especially because KCIA faces the challenge of being responsible for all discharge to the LDW from its property, even runoff or discharge in tenant-operated areas. 

Sediment traps in a storm sewer manhole

Aspect staff previously performed inexpensive but high-resolution sediment monitoring throughout KCIA’s storm drainage infrastructure that ranged from shallow old brick manholes to deep new stormwater treatment vaults. The results from the sediment monitoring provided a finer-grained picture of accumulated sediment quality than had ever been collected at KCIA. This allowed King County to identify hot spots of likely pollution sources coming from both individual tenants and from legacy airport infrastructure and helped prioritize an action plan to address these areas.

Using Sediment Data to Track Down Drainage Ditch Polluters

In a different area of the LDW, sediment data helped the City of Seattle identify the source of intermittent toxic metal pollutants from a far upland drainage area to the LDW, despite having outdated drainage maps. 

Sediment collection in a storm sewer manhole

Because the area was previously in unincorporated Seattle, sewer records were incomplete. With the assistance of an Ecology inspector who knew the area and businesses well, Aspect staff collected sediment samples from both the public and private drainage systems. The sediment samples helped both Ecology and the City efficiently trace the source of the metals pollution to a business that had a previously unknown illicit connection from its industrial waste drainage system to the ditches outside, which served as the public storm drainage.

Long-term Sediment Monitoring at Harbor Island to Support Environmental Compliance

Aspect is currently assisting the Port of Seattle with sediment monitoring at a 15-acre marine terminal on Harbor Island, a discrete Superfund site located downstream of the LDW Superfund area. The sediment monitoring supports the Port in demonstrating compliance with a Record of Decision (ROD) to rehabilitate the site. As a site that drains directly to Puget Sound, the objective of the cleanup (which included dredging and removing contaminated soil) is to reduce concentrations of hazardous substances in runoff to levels that will have no adverse effect on marine organisms. 

Sediment traps mounted on the side of a storm sewer manhole

Sample bottles with accumulated sediment at the bottom

To evaluate this over the required 10-year monitoring period, Aspect is monitoring accumulated sediments in the new stormwater drainage system at the terminal for metals, tributyltin, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs). The results from the sediment monitoring are compared to target concentrations in the Washington State Sediment Management Standards and show the Port’s commitment to compliance with the ROD and to ensuring that the site rehabilitation was successful. 

Sediment Sampling Provides Key Historical Context to Water Quality Evaluation

Measuring sediment quality is an excellent – and affordable – complement to measuring water quality. Aspect’s sediment sleuthing has helped clients in the LDW create a more holistic picture of both historical and ongoing stormwater pollution, as well as flooding potential. From this picture, they are better able to identify sources of contamination and create specific plans to address them—leading to a healthier LDW for all. 

Looking Forward to StormCon – August 27-31

Aspect is excited to attend and present at the 16th Annual StormCon, August 27-31 in Bellevue, WA this year. This national conference, organized by Forester Media, offers a vast curriculum of workshops, certifications, and presentations focused on surface water quality. A diverse range of topics will be available over six tracks ranging from cutting edge research and technologies to lessons learned managing stormwater in various settings. Aspect’s Tom Atkins, Senior Associate Engineer, and James Packman, Senior Hydrologist, will be presenting on three topics at this year’s event. 

On Tuesday, August 29th, James Packman will be presenting with Beth Schmoyer from the City of Seattle on the design and testing results of an R&D pilot project to develop a new suspended solids fluvial sampling device (a.k.a. sediment trap). Later in the day, Tom Atkins will be presenting on the systematic approach and successful strategies that were used to achieve stormwater regulatory compliance at Maxum Petroleum’s diesel fueling and petroleum fuel/lubricant shipping and receiving facility located on Harbor Island in Seattle.

During the Wednesday, August 30th sessions, James will be presenting again, this time alongside Greg Vigoren from the City of Lakewood on the results of a regional evaluation of municipal stormwater source control inspection data. The project is part of the western Washington Stormwater Action Monitoring program and is the first time a regional evaluation of this type of data has been done in Washington.

Dan Haller Speaking on the Policy Implications of Climate Change on Water Supply Management, January 26th

Aspect’s Principal Water Resource Engineer, Dan Haller, will be discussing climate impacts to water on January 26th in Stevenson, Washington.  As future food production and processing systems in the region are expected to be challenged by water supply, the conference aims to create a dialogue among the communities that use and value the regions water supply and water quality. Dan will join a group of water resource experts to discuss policy implications of climate change on water supply management. Learn more about projected climate impacts on water accessibility in the Pacific Northwest and the sustainable management decisions HERE.