Stormwater Action Monitoring - Effectiveness studies

In this project, the Washington State University–Puyallup (WSU-P) determined the effectiveness of three mulch types to reduce weeds, retain moisture in bioretention soils, and capture pollutants. The three types of mulch to be tested were: arborist wood chips, medium bark, and nugget bark.

Study questions

• Which mulch minimizes the need for costly maintenance for weeding?
• What is the capacity of each mulch to improve water retention and release carbon and nutrients?
• While all three of these mulches are commonly available in Western Washington, are there any concerns with regard to maintenance and treatment?

Study findings

All three mulches suppressed weed growth significantly over controls, and no single mulch type was significantly better over the two years of this study. All three mulches preserved more soil moisture than the no-mulch controls. The cells with arborist chips maintained the highest soil moisture readings during the study. However the arborist chips were depleted, presumably due to soil microbe consumption, and were the only mulch replenished during the study. While all of the bioretention cells exported nitrogen and phosphorus, the nitrogen concentrations in bioretention effluent were significantly lower in the presence of mulch compared to the no-mulch controls.

Stormwater maintenance programs using mulches may see reduced O&M costs overall due to reducing watering needs in the summer, improved plant survival, and reduced weeding or the need for herbicides. Bioretention designers, landscape designers, horticulturists and others should limit use of water loving and easily spreading plants such as ‘ninebark’ to minimize maintenance needs in bioretention facilities and rain gardens.

Key project documents

• Scope of Work
• Deliverable 2.2 Final QAPP
• Deliverable 6.2 Final report with Excel spreadsheet of final QA reviewed data
• Deliverable 6.3 Two presentations:  SWG 2/9/2022 and Green Infrastructure Summit 2022
• Deliverable 6.4 SAM Fact Sheet

King County completed the study to evaluate the effectiveness of stormwater treatment by oyster shell. This study compared water quality in two groups of catch basins in the City of Mercer Island — one with crushed oyster shell retrofit and the other without oyster shell. Visit King County's project webpage for more information. 

Study questions

• Does oyster shell provide treatment for dissolved metals?
• Can catch basins with oyster shell retrofits reduce stormwater pollutants significantly more than catch basins without oyster shell?

Study findings

Initial data showed no treatment effect by oyster shell retrofit in catch basins. This is likely due to limited contact time and not enough oyster shell medium used to treat the high volume of stormwater discharge from large contributing basins in the study area. The study was halted because the oyster shell-fitted catch basins in Mercer Island were not significantly improving water quality. 

Retrofitting of existing stormwater infrastructure with oyster shells to improve stormwater quality may potentially still be successful with a greater treatment-media-volume to stormwater-flow ratio that provides sufficient treatment/contact time.

Key project documents 

• Scope of Work, Amendment 1 
• Quality Assurance Project Plan 
• Final Report 
• Presentation to Stormwater Work Group 
• SAM FactSheet #24 Oyster shell retrofits in catch basins for dissolved metals treatment

 

A two-phased study of the bioretention best managment practices (BMP) was completed to assess hydrologic performance to capture infiltrate stormwater runoff in Western Washington using multiple design models. The City of Bellingham led the first phase of the study to evaluate bioretention designed prior to the 2012 Western Washington Hydrologic Model (WWHM) and the City of Olympia led the second phase evaluating those designed using WWHM2012 to the current standard required in the Stormwater Managment Manual for Western Washington (SWMMWW).

Study questions

• Are bioretention facilities being built across Western Washington as designed?
• Once built, are bioretention facilities performing hydrologically as modeled by the Western Washington Hydrology Model (WWHM)?

Study findings

Based on the 20 bioretention sites evaluated across Western Washington, most met the hydrologic performance targets established by the stormwater permits minimum requirements for the LID standard and flow control. Some of the bioretention facilities were built as retrofits, which are not held to the performance thresholds of new and re-development as they are often limited to the size of the available property. The pre-2012 designs were adequate but were often built larger and with coarser substrate than designed. These two factors combined accounted for the more than adequate hydrologic performance to control stormwater flows, design constraints, and exceeded modeled expectations. 

The report summarizes the findings, provides reasons and data for observed differences in performance and expectations, and makes recommendations for engineers, jurisdictions, and model development.

Key project documents of the first phase of study (early designs):

• Scope of Work, Amendment 1, Amendment 2, and Amendment 3
• Quality Assurance Project Plan for monitoring 
• Presentation to SWG 06/06/2018
• Bioretention Hydrogology Performance Phase I Final Report - Sept 2018 and Appendices
• Memo on Study Conclusions and Recommendations
• SAM fact sheet Bioretention Hydrologic Performance Study, Phase I

• Key project documents of the current design (WWHM2012) second phase of study:

• Scope of work, Amendment 1
• Quality Assurance Project Plan for monitoring
• Project findings presentation on Washington Stormwater Center's YouTube channel
• Bioretention Hydrology Performance Phase II Final Report - Sept 2020 and Appendices
• SAM fact sheet #20 Bioretention Hydrologic Performance Study Phase II

U.S. Fish & Wildlife partnered with Washington State University-Puyallup to evaluate improvements in the default bioretention soil mix capability to reduce nutrient, bacteria, and metal pollutants when amended with fungal spores and plants. The default mix is 60 percent sand and 40 percent compost per Ecology's Stormwater Management Manual for Western Washington.

Study questions

• Do fungal and/or plant amendments improve the effectiveness of the default bioretention soil mix to reduce metal, fecal coliform, and nutrient pollution from highway runoff?
• Does the fungal amended mix still prevent toxicity to aquatic organisms?

Study findings

This two-year field scale evaluation gathered new information about how local plants and mulch inoculated with S. rugosoannulata affect water quality and other bioretention performance parameters.

• Although stormwater runoff influent concentrations were low, the bioretention soil mix, regardless of fungal and/or plant amendments, was found to significantly improve water quality by removing metals, bacteria, solids, and aromatics.
• Fungal amended mulch treatments significantly reduced the initial leaching of ortho-phosphorus by 70% during the two-year study compared to the control.
• Whereas plants increased hydraulic conductivity, fungi decreased it when evaluated individually. Treatments with both plants and fungi had no overall change in hydraulic conductivity.
• Fungi are components of healthy soil and became established in all treatments by the end of the study, decomposing nearly all of the alder wood mulch mass. Annual mulch substrate should be resupplied to promote water retention and healthy soil.
• The stormwater influent was inconsistently toxic to zebrafish and made it difficult to interpret the differences between treatments.

Study deliverables

• Scope of Work, Amendment 1 
• Quality Assurance Project Plan: Field test of plants and fungi on bioretention performance over time
• Final report: Plant and fungi amendments to bioretention for pollutant reduction over time
• Fungi amendment to bioretention - SWG presentation Jan 2020
• SAM Fact Sheet #19: Effectiveness of plants and fungi amendments for bioretention treatment of stormwater

King County evaluated the capture and treatment of polychlorinated biphenyls (PCBs) in bioretention mesocosms using stormwater gathered from Interstate 5 near the Ship Canal in Seattle.

Study questions

• Does the default bioretention soil mix to reduce PCB concentrations in highway runoff?
• Is the capture of PCB by the soil mix seasonal or longer?
• Are the amended mixes still capable of preventing toxicity to aquatic organisms?

Scope of Work, Amendment 1

Study findings

This field study provides much needed information on treatment of polychlorinated biphenyls (PCBs) transported by stormwater. PCBs are a significant pollutant to Puget Sound and responsible for many fish consumption advisories. Despite being banned in Washington there are still ongoing PCB sources from wash-off of products or paints, transport of contaminated soils, or low-level arial deposition. Bioretention columns (55-gallon drums) were built using the default bioretention soil mix (60% sand and 40% compost) and tested quarterly for two years of natural rainfall in a Seattle neighborhood. The key findings are:

1. Effluent concentrations of PCBs were approximately 90 percent lower than the PCBs in stormwater influents are removed by filtration thru the bioretention soil mix.
2. PCB mass slightly decreased in the soils across the two years. Losses were slight with no seasonal pattern detected.
3. PCBs did not accumulate in bioretention soils. No special soil management practices need be considered in the short-term (years) with regard to accumulated PCBs. In fact, the concentrations went down over the two year period in the soil mix itself.
4. The study site's watershed contained a highway and a residential neighborhood. The PCBs in stormwater from this watershed were low. The biologically active soil mixture is presumed to have degraded the PCB concentrations at a rate comparable to their input, but this requires confirmation.

• Final QAPP
• Final Report "Bioretention Capture Efficacy of PCBs from Stormwater" and appendices
• Presentation to SWG, 2019
• SAM Fact Sheet #17 Bioretention capture efficacy of PCBs from stormwater

State and local governments, academia, and the private sector worked together to develop and test several alternative bioretention soil mix (BSM) blend that does not export phosphorus, like Ecology's stormwater management manuals default 60/40 BSM, for use in areas near sensitive surface waterbodies.

Study questions

• Are there alternative mixtures for the bioretention soil mix (BSM) that will meet basic, enhanced, and phosphorus treatment objectives?
• Do these alternative mixes prevent toxicity to aquatic organisms?
• Are these mixes affordable and available to permittees in Washington state?

Scope of Work

Study findings

This bench-scale study successfully tested the performance of eight experimental BSM treatments over five storm events using highway runoff stormwater. Influent stormwater was compared to effluent from each BSM blend for total suspended solids (TSS), total and dissolved copper (Cu), lead (Pb) and zinc (Zn), total phosphorus (TP), ortho phosphorus (ortho-P), nitrate+nitrite, fecal coliform bacteria, polycyclic aromatic hydrocarbons (PAHs), and total petroleum hydrocarbons. Pollutant reduction targets were defined by Ecology's TAP-E treatment goals for TSS, metals, and phosphorus.

One experimental BSM (treatment #4) accomplished all the experimental pollution reduction goals including export of phosphorus, but was able to reduce phosphorus carried in the influent. Treatment 4 consisted of several layers (primary, polishing, and compost mulch) that when used in combination, could be anticipated to meet basic, enhanced, and phosphorus reduction goals. The study recommendations provide the specifications for Ecology to adopt the combination or component parts of the treatment 4 experimental blend as a new Washignton bioretention media. A bioretention media specification that meets Ecology’s basic, enhanced and phosphorus treatment objectives will greatly increase the surface water settings where designers and jurisdictions can confidently apply bioretention systems to manage stormwater runoff.

• Final QAPP
• Presentation to SWG, 2019
• Final Report "Bioretention Media Blends to Improve Stormwater Treatment: Final Phase of Study to Develop New Specifications"
• SAM Fact Sheet #16 Bioretention Media Blends to Improve Stormwater Treatment

State and local governments want to learn and share effectiveness of catch basin cleaning programs to prevent pollution in stormwater. King County led an effort to collect, assemble and analyze local government catch basin cleaning data.

Study questions

• What can municipalities learn from existing catch basin cleaning programs?
• Which alternative inspection schedules and frequencies are most efficient?
• What is a standard list of data to be collected across programs?

Scope of work

Study findings

The high variability in not only the data that is collected across the many jurisdictions in Puget Sound, but also the lack of a standard catch basin definition precluded accomplishing the original goals for this project.  With assistance from the project technical advisory committee, alternate goals were established based on the information collected on catch basin cleaning from municipal stormwater permittees. Recommendations on the type and quality of data collection and efficiencies to reduce costs are made. Based on data from seven permittees, it appears that over 80 percent of catch basins do not require more frequent cleaning than the standard inspection schedule. Revisit the definition of a circuit to consider if the circuit option will work either alone or in combination with other schedules.

• Survey and data request of municipal catch basin maintenance programs
• Survey responses technical memorandum
• Data analysis plan
• CB programs summary and costs
• Final report "Using Western Washington Catch Basin Inspection and Maintenance Data to Predict Maintenance Schedules and Identify Cost-Efficiencies"
• Final database (available upon request)
• Presentation to SWG November 2018
• SAM Fact Sheet #015 Western WA catch basin inspection and maintenance data review

The City of Puyallup partnered with Washington State University Extension and Stewardship Partners to create an easy-to-use field protocol to assess rain gardens and bioretention facilities function and maintenance needs.

Study questions

• What attributes can be measured by volunteers and staff through visual observation or simple tests that reliably predict the functional success of rain gardens and bioretention facilities?
• What construction and maintenance activities contribute most to the overall success of a rain gardens and bioretention facilities and can they be easily measured?
• What attributes of rain garden/bioretention facilities contribute most to a landowner's perception of functional success?

Scope of work

Study findings

The Raingarden and Bioretention Assessment Protocol was developed that provides replicable results and gives an overall indication of the current state of a rain garden or bioretention facility. Training is not necessary, but is suggested (even if self-directed). Data collected will provide sufficient detail to indicate if a site needs further actions for the functional aspects related to maintenance, hydrology, and vegetation.

Protocol Development:

• Literature review for protocol creation
• Final Report on social science results and recommendations
• Summary of first two versions's experiences

Final Protocol:

• Rain Garden & Bioretention Assessment Protocol, functional assessment form, data input spreadsheet, training webinar, and handouts
• Protocol assessment findings
• Presentation to SWG Jan 16, 2019 on protocol development and implementation
• Proposal for regional implementation
• SAM Fact Sheet #014 Rain garden and bioretention assessment protocol

King County evaluated how effectively a large improvement project treats stormwater at a regional water detention facility in Federal Way.

Study question

• How well does each individual practice treat stormwater in this large retrofit?
• What about combinations of treatments?
• What is the overall flow control and pollutant reduction to the stream?

Scope of Work and Amendment 1

The study evaluated two new bioretention facilities, an expanded and new combined detention stormwater treatment wetlands (wetland complex), and the regional facility as a whole, for their ability to improve water quality and to reduce peak flows of stormwater runoff. The study also evaluated the in-stream water quality and benthic macro-invertebrates in North Fork West Hylebos Creek before and after the retrofit.

Study findings

Eighteen storms between March 2016 and April 2017 were sampled using compositors. Flows were measured continuously. Both bioretention facilities, the expanded wetland complex, and the system as a whole reliably attenuated stormwater flows by reducing and delaying the timing of peak flows. The bioretention facilities and the wetland complex had mixed water quality treatment results, they were able to treat some targeted pollutants but not others. The system as a whole reduced total suspended solids, organic contaminants, and total metals (zinc, lead, copper and cadmium). However, the system increased concentrations of nutrients and dissolved lead. The bioretention facilities were a source of nutrients.

Water quality and benthic community data from the North Fork of West Hylebos Creek were collected downstream of the retrofit and expansion before and after the project to assess overall performance of the system. Turbidity improved, though the data were not significant, and no changes in the benthic community were not observed. It is likely too early to detect a change in the benthic communities in the creek.

• Final Quality Assurance Project Plan (QAPP)
• Final project report and appendices
• Presentation to SWG Jan 16, 2019
• King County's website on this Regional Retrofit Project
• SAM Fact Sheet #013 Regional stormwater facility study in Federal Way

Study questions

• How effective is each individual best management practices (BMP) at treating stormwater under this large retrofit project?
• What well does the entire retrofit protect Echo Lake from pollutants?
• What is the potential protection to the lake itself?

Scope of work; Amendment 1

Study findings

Individual BMPs — bioretention and proprietary devices — provided significant reductions in common pollutants carried by stormwater. A system-wide analysis showed the detection tank provided additional pollutant treatment, but flow control and pre- vs. post- retrofit comparison were not possible due to challenges discovered during the study. 

• The sampling phase was completed in early 2017, see Quality Assurance Project Plan (QAPP)
• Retrofit Preliminary Results at Echo Lake SAM Symposium (June 2017)
• Short Report: Monitoring Stormwater Retrofits in the Echo Lake Drainage Basin  and Final Report: Appendices
• SAM Fact Sheet #008 Stormwater Retrofit Monitoring in the Echo Lake Drainage Basin

King County's website for this project.

The City of Lakewood and consultants lead this SAM study (Scope of Work, Amendment 1, and Amendment 2) to look at permittees efforts for source control. This study gathered survey responses in 2016 from NPDES stormwater permittees. State and local governments want to learn about source control undertaken by small business owners to prevent pollutants from entering stormwater runoff.

Source control activities — such as common practices, techniques, or devices —  are collectively termed as best management practices (BMPs) and are employed at small businesses across the region. 

The final report for Business Inspection Stormwater Source Control Effectiveness Study (Aspect, 2017) summarizes responses from Phase I and II permittees on the types of businesses, frequencies of inspections, and source control BMPs employed. Recommendations for municipal stormwater permittees, Ecology's permit writers, and future study questions are provided. 

• SAM Fact Sheet #007 Business inspection stormwater source control effectiveness study

Study questions

• Does the default bioretention soil mix reduce Polycyclic Aromatic Hydrocarbons
  (PAH), metal, fecal coliforms, and nutrient pollution from highway runoff?
• Does the default bioretention soil mix reduce toxicity to adult and embryonic Coho salmon?

Study findings

This SAM Effectiveness study funded ongoing work to evaluate the effectiveness of bioretention soils to reduce the toxicity of urban stormwater to Coho. 

State and local governments are relieved to learn that the default 60:40 bioretention soil mix given in the Stormwater Management Manual for Western Washington is capable of removing acute toxicity to adult and embryonic Coho salmon. U.S. Fish and Wildlife partnered with Suquamish Tribe and Washington State University-Puyallup to specifically test the default mix (60 percent sand and 40 percent compost) for toxicity treatment.

What does it tell us?

This new information on the default bioretention soil mix helps inform stormwater managers across Washington as they adaptively manage stormwater runoff in urbanizing watersheds. The study also found that the default soil mix we specified for bioretention is effective at reducing metals, PAHs and fecal coliform concentrations.

Ecology’s stormwater management manual will continue to specify the 60:40 mix as the standard BSM for bioretention. 

• SAM Fact Sheet on Bioretention toxicity prevention
• SAM Final Report
• Media coverage on this ongoing research topic​: WSU press release and NOAA Fisheries. 

This new information on the bioretention soil mix informs stormwater managers across Washington as they adaptively build bioretention facilities to manage stormwater runoff in urbanizing watersheds.