Stormwater BMP implementation and maintenance demand substantial municipal budgets to comply with NPDES permits. Pollutant treatment and runoff reduction are primary goals of BMPs; however, few BMPs can treat all concerns equally, and maintenance condition influences BMP functionality. Interpreting the success or failure of a BMP using common metrics such as %- removals requires subjective judgement of the engineer, manager, or regulatory reviewer.
Since Dr. Fassman-Beck’s last seminar in Minnesota, she has led the development of new BMP performance indices for water quality treatment and hydrology (runoff capture volume) to convert quantitative field monitoring data to simple, unbiased categorical performance determinants with clear management implications, including corrective actions. The index benchmarks performance against specific water quality and runoff capture volumes design goals, while considering operating conditions on a storm-by-storm basis. The index value enables objective assessments across a range of pollutant types, water quality goals, design specifications and/or implementation across a wide variety of landscape settings. The unbiased and transferable metrics enable post-construction performance evaluation of a unique BMP, or amongst BMPs of the same or different types, while deconstructing metrics provide information to support maintenance and planning, for example by tracking performance over time. An open-source web application and calculator have been developed to implement and explore the BMP Performance Index.
Applying the BMP Performance Index to field monitoring data from the Southern California Stormwater Monitoring Coalition’s Regional BMP Monitoring Network, this seminar will show how the Performance Index is informing regional management decisions in relation to TMDL compliance.
Dr. Fassman-Beck is the Engineering Department Head at SCCWRP, responsible for developing and implementing a research program on BMP effectiveness. Research areas range from site-scale BMP evaluations, to integrating BMP effectiveness into watershed models, to advancing stormwater capture and use, and advancing sensing technology and applications in urban water quality. Dr.
Fassman-Beck is has extensive experience in field monitoring and laboratory studies evaluating the performance of urban stormwater runoff quantity (hydrologic) and quality (contaminant) best management practices (BMPs), with emphasis on bioretention systems, infiltration technologies (dry wells, galleries), permeable pavement, and swales, as well as non-structural controls such as street sweeping and turf replacement BMPs. She is the co-lead for the US EPA Southwest Stormwater Center of Excellence and manages a Regional BMP Monitoring Network in southern California. She has co-authored over 45 peer reviewed journal articles, and contributed to BMP design manuals in New Jersey, North Carolina, and Auckland (New Zealand).
Dr. Fassman-Beck currently serves as a Technical Reviewer for the National Municipal Stormwater Alliance Stormwater Testing and Evaluation for Products and Practices, and on the Advisory Board for the State of Oregon Stormwater Technology Testing Center. She has recently served on a National Academies of Science, Engineering and Medicine Consensus Committee on Managing Pollutant Loads in Highway Stormwater Runoff (2025-2027) and the US EPA Board of Scientific Counselors, Safe and Sustainable Water Resources Committee (2017-2021). For over 10 years she has been an associate editor of Journal of Sustainable Water in the Built Environment. Prior to joining SCCWRP in 2019, she had a 15-yr academic career between the University of Auckland and Stevens Institute of Technology. She was the recipient of the American Society of Civil Engineers NJ Educator of the Year (2018), Wesley W. Horner Award (2014) and Outstanding Reviewer (2012).
Offline Q and A
Panelists
- Tim Sundby, Water Resources Supervisor, Carver County Watershed Management Organization.
- Mike Kinney, District Administrator, Comfort Lake Forest Lake Watershed District
- Emily Resseger, Monitoring, Assessment, and Research Program Manager, Mississippi Watershed Management Organization.
Q1
Andy Potts, Jacobs (11:41am): Regarding the WQ index and interested in panel thoughts too. If I understand the scoring, orange scores are worse (higher) than yellow. Orange seemed to be BMPs with significant removal but they were just missing the threshold limit whereas yellow was no change or exporting pollutants. Why would the orange get a lower score? Seems like the orange should be scaled based perhaps on a percent removal or another metric.
- Elizabeth Fassman-Beck: The weighting between yellow and orange was determined from answering the question: Where would you direct resources (between those categories)? Data points in the yellow show BMPs where the effluent is meeting the downstream water quality goals. Data in the orange indication conditions where additional treatment is going to be required in order to meet in-stream targets. The more pressing need for taking corrective action is orange, in order to meet in-stream goals, thus the weighting factor is “worse”. Check out the interpretation guide too - the scores give pathways for “what do I do with this information”.
- Elizabeth Fassman-Beck: Why consider EMC rather than mass? Investigating changes in EMCs between inlet and outlet gives a direct indication of treatment. We have a separate index investigating retention (based on hydrology). Treatment and retention are the two expected functions - the design or functional elements in most BMPs that contribute to treatment are different than those that contribute to retention. Since the overall goal of the index is to diagnose BMPs that are working well, those that can be improved, and those that need to be fixed, the index helps point the user directly to BMPs with functions to replicate in the future vs those that need attention, potentially immediately (issues could be design, construction or maintenance). If you were looking at mass, you’d still need to disaggregate the mass terms to figure out what is going on (or not).
- Elizabeth Fassman-Beck: On the index overall — we were asked “Why does my event-to-event BMP monitoring suggest “good” %-removal, but we’re still not meeting compliance?”. De-listing of waterways means that it is no longer impaired for its beneficial use. At the end of the day, that de-listing depends on in-stream water quality. The implication with the index is that evaluating a BMP’s effluent quality directly in that context gives a permittee actionable information regarding the potential for eventually meeting in-stream water quality conditions.
- Emily Resseger: Elizabeth will have the final word, but my understanding is that orange has a higher weighting and that makes sense to me because that represents influent higher than the threshold (so high concentrations coming in) that remove some pollution but not enough. These higher concentrations are where the biggest potential for reduction occurs, and they aren’t meeting that expectation. The yellow may be exporting pollutants but the concentrations are low enough that the effluent is still below the threshold, so these BMPs are not doing damage to receiving waterbodies.
Q2
Rich Fisher (11:49am): Low-Flow Diversions (LFDs)—diverting dry-weather runoff and first-flush flows into the sanitary sewer system for treatment—are widely used in places like Southern California to manage indicator bacteria (E. coli). Given that green infrastructure often struggles to consistently hit high log-reductions for bacteria, why isn't dry-weather diversion utilized more in other regions? Is it primarily driven by wastewater capacity limits, regulatory barriers between separate stormwater/sanitary utilities, or cold-climate operational constraints?
- Tim Sundby: First flush capture is critical for meeting phosphorus goals, which has been the leading cause for nutrient impairments in Minnesota. Also, sanitary sewers have been separated from storm sewers in Minnesota based on the amount of rain that we can get and overwhelming the wastewater treatment plant.
- Emily Resseger: My understanding is that this is primarily driven by wastewater treatment design- our wastewater plants are designed for normal flows and peak flows, and since higher than normal flows are already entering the WWTP during events due to I&I, there is a risk of higher CSOs if first flush was diverted to treatment plants.
- Elizabeth Fassman-Beck: SoCal suffers from “urban drool” - that is, dry weather season flow in the MS4 is substantial. Dry weather flows are considered urban runoff, and thus the stormwater permittees are on the hook for the loads. Diversions were identified as a viable mechanism for providing that treatment during the dry season only. The POTWs (wastewater treatment plants) have excess capacity during dry season, and also have the potential to reclaim the water for water supply (through partnerships with the water districts). There are some concerns about reducing the flow back to the streams to support ecology. We use the term ecohydrology, and that is also being addressed through research and the California Environmental Flows Framework (CEFF).