Jan
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IMPACT Article: Presenting Urban Stormwater BMP Performance Data to a Broad Audience
January 7, 2009 | Posted by admin
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Authors
John Kosco and Nikos Singelis
INTRODUCTION
Accurate data on stormwater best management practice (BMP) performance is important for many reasons. Engineers use BMP performance data to estimate effectiveness and meet design requirements. Municipal officials use BMP performance data to assess whether a particular design will meet local requirements. State and local water quality professionals use BMP performance data to estimate whether selected BMPs will remove enough discharged pollutants that impair waterbodies. National Pollutant Discharge Elimination System (NPDES) permit writers use BMP performance data to require more effective controls in permits.
However, stormwater BMP performance information has been difficult to summarize and present to these audiences because of the complexity associated with monitoring stormwater BMPs. Unlike the monitoring of a traditional point source such as a wastewater treatment plant, which has a fairly consistent flow and quality, a stormwater BMP is monitored only during or just after a storm. The intermittent discharges associated with storms introduce wide variability in water quantity and quality making accurate sampling of the BMP’s effectiveness more difficult.
The U.S. Environmental Protection Agency (USEPA) has created a web-based tool, the Urban BMP Performance Tool (available at www.epa.gov/npdes/urbanbmptool), to provide stormwater professionals with easy access to research studies on the performance of stormwater BMPs. This tool can be used to search BMPs to identify pollutant removal and stormwater volume reduction information. However, in order to effectively use the tool, stormwater professionals must have a basic understanding of the factors that influence BMP performance. This article describes those factors, including stormwater pollutant load reductions, stormwater volume reductions, and why percent removal is a misleading indicator of BMP performance.
WHAT FACTORS INFLUENCE THE PERFORMANCE OF BMPS IN THE REAL WORLD?
Defining the performance of BMPs is not an exact science. BMPs, unlike other treatment technologies – such as those used in wastewater treatment systems – are not static systems that deliver constant or even predictable results. The performance of stormwater BMPs is significantly influenced by such factors as design, installation, rainfall patterns and characteristics, soil types, slopes, land use in the drainage area, and age of the system, to name a few. Figure 1 illustrates the variability of BMP performance in removing total suspended solids. For example, detention ponds achieve a median effluent quality of 26.7 mg/l, but effluent quality can range from 16.7 to 44.9 mg/l depending on various factors.
Choosing effective BMPs is a significant challenge for all of us in the stormwater community … fortunately, tools, backed by reliable scientific studies, are emerging that will help us improve the process of choosing better BMPs
One important piece of advice is … Don’t latch on to one number and expect a stormwater BMP to deliver that level of performance each and every time. While we can’t now, and might not ever, be able to predict BMP performance with 100% accuracy, we do have – as a result of the diligent efforts of hundreds of researchers and practitioners – information that will help us detect trends in BMP performance.
“STORMWATER URBAN MYTH:” PERCENT REMOVAL
Spend any time at all discussing BMP performance or looking at articles and studies and you are soon going to encounter the concept of “percent removal.” This widely (mis)used concept, which may also be called percent effectiveness or other names, uses an influent and effluent concentration to obtain the percent reduction between the two.
Experts in the field of BMP performance, including those at USEPA, find this measure more misleading than helpful. Among the many reasons to avoid this measure is that by relying on just one percentage value, we lose the context of the conditions from which it was derived. For instance, the influent in one study may have been particularly dirty, in another significantly less so. Looking at the percentage alone does not provide us with that information. In fact, percent removals are highly influenced by how dirty the site (influent) is and don’t tell us much about how well the BMP actually worked. For example, Figure 2 shows that a BMP receiving 100 mg/l influent that removed 80% of the pollutant will achieve 20 mg/l effluent concentration. However, a second BMP with a cleaner influent (20 mg/l), and lower percent removal (50%), could achieve a better water quality result (10 mg/l). If decisions are based only on percent removal, an engineer may end up choosing the wrong BMP for their site. You can find an excellent article on this subject by Jonathan Jones, Jane Clary, Eric Strecker, and Marcus Quigley in the January/February 2008 issue of Stormwater magazine (www.stormh2o.com).
TOTAL LOAD REDUCTION
Rather than relying on a percentage to describe BMP performance, you should consider several factors, including event mean concentrations (EMCs), volume reduction information (discussed next), and total load reduction. If you would like to compare the performance of BMPs across types, one of the better ways to do this would be to compare total load reduction. A load reduction measure takes into account both the volume of stormwater coming into and leaving the BMP as well as the concentration of pollutants. Using a load reduction measure (usually pounds or kilograms) becomes particularly relevant when examining BMPs that reduce the volume of stormwater. Figure 3 illustrates the concept of examining pollutant reduction potential by looking at a load reduction measure. In this example, you can see that calculating a simple “percent removal” using the concentration numbers (e.g., beakers) would result in a 50% reduction of the pollutant while using the total load results in a 75% reduction. The 50% removal number is misleading because it does not account for the large volume of stormwater infiltrated by the BMP. It is only when we look at total load reduction that we see the true performance of the BMP.
IMPORTANCE OF VOLUME REDUCTION
As we learn more about the relationship between urban and suburban development and watershed health, it becomes clear that one of the major factors affecting the condition of our rivers, lakes, coastal waters and wetlands is the increased volume and velocity of stormwater runoff reaching these waterbodies. As we add impervious surfaces (including roads, driveways, homes, shopping malls and all of the other facets of urbanization) to our watersheds, we create changes in how these watersheds function hydrologically. Increases in impervious surfaces result in increases in stormwater flowing off the land and corresponding reductions in the rain and snow melt that would otherwise soak into the ground. These changes can have profound impacts on many of our waters, particularly sensitive headwater streams, wetlands, and small lakes. Much has been written elsewhere on this topic. The key message is that when planning and designing systems to manage urban stormwater, we need to carefully weigh options that encourage infiltration of stormwater and help to maintain or restore the natural hydrologic balance of the land that is being developed. Another important point to keep in mind (and can clearly been seen in the Figure 3) is that when we choose stormwater BMPs that reduce the volume of stormwater we also reduce the load of the pollutants entering surface waters.
OTHER FACTORS TO CONSIDER WHEN SELECTING BMPS
Obviously, pollutant and volume reduction potential are two key considerations we should always keep in mind when selecting or approving stormwater BMPs. There are many other considerations which also have a role in BMP selection. Costs are always a significant factor. Total capital costs (e.g., cost of installation), as well as factors such as cost per acre treated, can provide valuable information. Another cost-related factor that is frequently overlooked is the cost of maintenance of the BMP over the long term. All BMPs require regular, and sometimes frequent, maintenance. Considering ease of maintenance and overall maintenance costs should help inform our choices. Other factors to consider include the function of the BMP in regional drainage and flood management efforts and whether the BMP has “secondary” benefits, including habitat values, carbon sequestration, and water conservation, among others. Aesthetics and public acceptance are also very important, as is the longevity or useful life of the BMP. Of course, everyone involved in stormwater management also needs to consider and mitigate any potential safety issues.
USEPA’S URBAN BMP PERFORMANCE TOOL
So, how do you select a “better” BMP for your site? In January 2008, USEPA released the Urban Stormwater BMP Performance Tool (available at www.epa.gov/npdes/urbanbmptool) which presents BMP performance data from more than 220 scientific studies. This tool contains many of the studies collected by the International Stormwater BMP Database (www.bmpdatabase.org) and allows a user to search BMP performance data by their pollutant of interest, by the type of BMP they may want to install, or by the volume of stormwater reduction they would like to achieve.
The Urban BMP Performance Tool results are displayed showing both the influent and effluent concentrations of each BMP, with the effluent concentrations sorted from low to high. Volume reduction information is also displayed, where available.
There are many ways the Urban BMP Performance Tool can be used. However, two common approaches include: (1) finding the best BMP for controlling a specific pollutant, or (2) determining how a specific BMP performs across a range of pollutants and parameters.
Users who are concerned about a specific pollutant, for example, those who are in an impaired watershed with a TMDL, should select “search studies by the pollutants that were measured.” This allows users to identify a specific pollutant or pollutant group and see the most effective BMPs. Users who are interested in how effective a particular BMP is in controlling different pollutants should select “search studies by the BMPs examined.” Users then select the BMP and can further narrow their search by selecting an appropriate pollutant group.
This tool provides valuable information to help users select a better BMP for their site. As discussed above, however, there are many factors that go into proper BMP selection; therefore users should not just select the first BMP on the list. USEPA will continue to update the Urban BMP Performance Tool and add additional studies and BMPs as the data become available.
SUMMARY
Choosing effective BMPs is a significant challenge for all of us in the stormwater community. As discussed in this article there are still many unknowns, and the many considerations that we need to balance can make the task challenging. Fortunately, tools, backed by reliable scientific studies, are emerging that will help us improve the process of choosing better BMPs.
Author Link:
John Kosco, P.E.
Tetra Tech, Inc
10306 Eaton Place, Suite 340
Fairfax, VA 22046
(703) 385-6000
john.kosco@tetratech.com
singelis.nikos@epa.gov
John Kosco has over 15 years of experience working on stormwater programs, including stormwater program development, implementation, and compliance. He currently manages Tt’s stormwater support to USEPA and has extensive experience with the Phase I and Phase II stormwater regulations. John holds a B.S. degree in agricultural engineering from the Pennsylvania State University and an M.S. degree in Civil/Water Resources Engineering from George Washington University.
Editor’s Note: Additional articles may be found in AWRA’s bi-monthly magazine IMPACT.
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