Calculating stormwater volume and pollutant reductions and credits

1. Credits for Better Site design

• Percent removal should preferably be load-based as opposed to concentration-based. When percent removal is based solely on concentrations, water volume is ignored, when in fact it could be markedly influencing the performance of the BMP. For example, if a large volume of heavily concentrated pollution is entering a bioretention BMP and much of the water is infiltrating into the ground, the overall load will be greatly reduced, yet not show up as such if the outflow concentration remains high.
• Inflow concentration should be examined and considered in the analysis. If inflow concentration is already quite low, and if pollutant percent removal is also low, the low percent removal may partly be due to the fact that additional pollutant removal is not currently technologically possible, a concept known as the “irreducible concentration.” Another important factor that must be incorporated into every BMP performance assessment is the limitation to only water that actually flows into and through the treatment system. Any flow that exceeds the design specifications for the BMP should be by-passed or diverted and not included in the treatment efficiency calculations. Instead, this flow should be routed to a receiving water as “untreated” or preferably routed to another BMP for subsequent treatment.

BMP Performance

This Manual update is intended to address the five structural BMPs included in Chapter 12 of version 1.0; these are bioretention, filtration, infiltration, stormwater ponds and constructed wetlands. The majority of the BMP performance data was taken from the ASCE/EPA database, the CWP report, and the Mn/DOT report. These studies were comprehensive data gathering efforts and analyses with a high level of quality control. However, the data are not all directly comparable due to different statistics being presented in the different sources. Medians and interquartile ranges of outflow concentrations were available in the ASCE/EPA database, and percent reductions (based on event mean concentrations) were calculated using the database. In the CWP report, the medians were presented in tables, but the interquartile ranges were estimated from graphic presentations of the data. Percent removal data in the CWP report are based on either load or concentration, but that distinction for each BMP was not noted. The Mn/DOT study only reported means. When possible, data for outflow concentration or percent load removal are presented as medians and interquartile ranges in this Issue Paper. In the percent removal data, the 75th percentile represents the high tier of BMP performance, the median represents the middle tier, and the 25th percentile represents the low tier (Figure 2). The opposite is true for the outflow concentration data, in that the 25th percentile represents the high tier of expected BMP performance, since low outflow concentration suggests high performance, and high outflow concentration suggests low performance (Figure 2).

The primary method to assess performance is recommended to be the outflow concentrationleaving from a particular BMP, with percent load reduction as a secondary recommended approach. Graphics and tables representing each of these approaches follow. Data from the various sources differed slightly, although they followed similar patterns (Figures 3 through 6). The comparisons presented here are from a visual examination of the data; they do not represent statistically significant differences among the groups, which was not possible to test due to the fact that we did not have all of the raw data. Data from bioretention devices and infiltration practices are not included in the following figures due to a lack of evaluation data, but they are discussed later in the Issue Paper.

Median TSS outflow concentrations ranged from 13 to 26 mg/L, and with median/mean percent removals ranging from 54% to 88% (Figures 3 and 4). The lowest TSS outflow concentration and highest percent removal were achieved by media filters, at approximately 10 mg/L and 85%, respectively (Figures 3 and 4). Percent removal in stormwater ponds was similar to that in media filters, but the interquartile range was larger in the pond data. TSS percent removal in grass filters differed between the CWP and ASCE/EPA databases, with median percent removals at 81% and 54%, respectively, and with interquartile ranges not overlapping at all (Figure 4). However, the TSS interquartile ranges of the two databases showed large overlap in the outflow concentration data (Figure 3). Because of the number of studies and scrutiny placed on the character of the data collection, the ASCE/EPA database should be considered preferentially if a user wonders which value to rely upon more.

Grass filters performed the poorest among the BMP categories for TP treatment, with median outflow concentrations ranging from 0.19 to 0.29 mg/L, and median/mean percent removals ranging from -35% to 41% (Figures 5 and 6). Although grass filters do not show particularly good pollutant removal, they do have their place as a fairly effective BMP in certain conditions for volume reduction (see Chapter 12-FIL, Section VII).

The following sections present the performance data from Figures 3 through 6 for each BMP category as a series of tables. The first table in each section presents the outflow concentrations and the second presents the percent removals. A diagram then presents the design elements that most influence BMP performance in that category.

When using these data to estimate the performance of a specific BMP, the estimate should be selected based on the design elements figures. Figures 7-12 are each adapted from an as yet unpublished Design Point Method developed by CWP, combined with the outflow concentration and percent removal tables (Tables 1-8). If the installation shows neither positive nor negative elements as listed in the design elements figures, the median (50th percentile) should be used.

For example, in a stormwater pond, the outflow TSS concentration that could be expected under most conditions would be about 15 mg/L. If there are positive design elements, it would be lowered to approximately 11 mg/L, and if there were negative design elements, the expected outflow concentration would be raised to approximately 30 mg/L.

Data from the Mn/DOT report are presented in these tables, even though they present only means and not interquartile ranges. The Mn/DOT study incorporated some of the same studies that the CWP database includes; it is therefore presented simply to show its value compared to the currently reported ASCE/EPA and CWP values.

Bioretention

Performance data from bioretention devices are less available than data for the other BMP types. Since inflow often does not enter bioretention devices through a channel, it is difficult to monitor. More importantly, bioretention devices are often designed to infiltrate stormwater, and therefore do not always overflow.

In a USGS study on the effects on water quality of rain gardens in the Twin Cities metropolitan area, two of the five studied rain gardens did not overflow at all during the study’s time period (Tornes 2005) and therefore retained all of the incoming TSS and TP loads. At sites where overflow did occur, the pollutant concentrations in the outflow (Table 9) were generally lower than the concentrations in the inflow. However, since volumes were not monitored, it was not possible to estimate what percent of the pollutant loads were retained. TSS was not monitored in that study, and TSS data for bioretention devices are not included in the CWP report; therefore only TP values are presented here.

In a study on the Burnsville rain gardens in the Twin Cities Metropolitan area, there was an 82% reduction in annual stormwater runoff over a two-year monitoring period, with a greater than 95% reduction in volume for many storms (Yetka and Leuthold 2005). Other local data, from the H.B. Fuller Company bioretention system, show a 73% reduction in stormwater volume, a 94% reduction in particulates, and a 70% reduction in TP. However, the soluble fraction of phosphorus in the runoff increased by 70% (Langer 1997).

Interpretation of the performance data presented here for bioretention is somewhat inconclusive due to the methods used and the low number of documented studies. Despite the relatively low TP percent removal value presented (65%), observational accounts suggest that bioretention devices are highly effective at removing TSS and TP loads, since they often infiltrate the majority of the volume of stormwater runoff events. Discretion is suggested on whether to increase this value based upon observations that validate high infiltration for the design event.

Infiltration

Due to similar difficulties as those that exist with monitoring bioretention systems, there are few data available demonstrating the load reductions or outflow concentrations of larger-scale infiltration practices such as infiltration trenches. Few sampling programs collect infiltrating water that flows through an infiltration system.

For properly designed, operated, and maintained infiltration systems, all water routed into them should be “removed” from stormwater flow, resulting in 100% efficiency relative to volume and pollutant reduction. For this reason, performance tables similar to those above would only reflect this performance. This logic assumes that stormwater is the beneficiary of any infiltration system, but ignores the fact that pollution, if any remains after the internal workings of the infiltration BMP itself (see Chapter 12-INFIL), is being transferred into the shallow groundwater system. Good monitoring data on the groundwater impact of infiltrating stormwater are rare, but there are efforts underway today to document this, so future Manual revisions should be able to include some data updates.

Properly designed infiltration systems (see Chapter 12-INFIL) will accommodate a design volume based on the required water quality volume. Excess water must be by-passed and diverted to another BMP so that the design infiltration occurs within 48 hours if under state regulation, or generally within 72 hours under certain local and watershed regulations. In no case should the by-passed volume be included in the pollutant removal calculation.

Data that are reported in performance literature for infiltration systems, unless reporting 100% effectiveness for surface water or documenting outflow water downward, are not accurately representing behavior, or are representing the excess flow (overflow) from a system. The performance percentages and effluent concentrations reported in the Version 1.1 Manual will be removed for this reason and replaced at a future date to better reflect the movement of surface water pollutants into the groundwater system. Design specifications (see Chapter 12-INFIL) should prevent putting excess water beyond that which will infiltrate within the given timeframe.

Any excess should be diverted away from the infiltration system and reported as inflow to another treatment device. Both Chapter 12-INFIL and Chapter 13 address the necessity of careful use of infiltration BMPs to make sure they are not transporting highly loaded or toxic contaminants into the groundwater system. These chapters address the pollution remediation processes at work in infiltration systems to reduce or totally remove pollutants that move through them. However, extreme caution must be exercised and serious planning undertaken to assure that no highly contaminating material is routed into these BMPs. Of particular concern are toxic organics (gasoline, solvents) and high levels of chloride.

Other Factors Influencing Performance

Even though the ASCE/EPA database is the most comprehensive collection of BMP performance data, there are not enough data points in the database to statistically examine the effect that design parameters have on BMP performance (ASCE/EPA 2000). The design parameters listed in the BMP performance figures are derived from best professional judgment.

There are many other factors that affect the performance of BMPs that have not been discussed in the previous sections. There is little in-depth statistical analysis that can be done for these other factors due to the lack of reported information on them. Nevertheless, it can be stated with certainty that they do have an impact on BMP performance and should, therefore, be considered when designing a practice. A discussion of these related factors follows. Manual users are urged to review the referenced chapters for the information needed.

The effects of geographic location

Chapter 2 and Appendix A go into detail on the geographic variation within Minnesota and the impact that this has on stormwater behavior and BMP selection. These parts of the Manual present information on such variables as soil, geology, temperature, precipitation and land use.

Watershed configuration

Watershed configuration factors such as size, slope, shape and depth to bedrock/water table are presented in the Chapter 12 BMP design sheets for each of the five BMPs considered in detail, as well as the Fact Sheets that summarize BMPs in less detail.

Uncertainty of hydrologic measurements

The proper design, installation and operation of BMPs rely on good hydrologic information for the drainage area within which the BMP occurs. Such factors as initial losses due to landscape retention, variable runoff coefficients, location of rainfall data recorders and extent of connected imperviousness have a major impact on BMP effectiveness. Chapter 3 contains information and internal links within the Manual that describe the importance of these factors.

BMP design criteria

Variations in the design specifics of an installed BMP can vary substantially from one location or jurisdiction to another. The information presented in this Manual attempts to describe methods to assess those variations, but in the end the designer will need to account for variation and customize the application. Among many factors to consider in typical BMP implementation are:

• Ratio of permanent and temporary storage to drainage area
• Storage configuration
• Drawdown time
• Length-to-width and bathymetric ratios
• Soil permeability

Chapter 12 of the Manual discusses the specifics of these factors as they apply to the featured BMPs.

BMP maintenance

The most common failures that occur with BMPs happen when maintenance is ignored. Each of the Chapter 12 discussions of BMPs contains information on maintenance. Appendix D also contains maintenance checklists for the major BMPs covered in Chapter 12.

Climate

Chapter 9 and several narratives scattered throughout the Manual describe the particular issues that arise in Minnesota because of the large variation experienced in seasonality and temperature. Minnesota has the largest difference between summer maximum temperature and winter minimum temperature of any state in the U.S. This variation and the substantial precipitation difference from west to east present a wide array of design considerations that must be incorporated into any BMP. Chapter 2 provides some insight and further information links to use in BMP selection and design.

References

• ASCE/EPA Database. International Stormwater Best Management Practices (BMP) Database: http://www.bmpdatabase.org/index.htm.
• ASCE/EPA 2000. Determining Urban Stormwater Best Management Practice Removal Efficiencies EPA Assistance Agreement Number CX 824555-01-Task 3.4.
• Langer, R. 1997. 1997 Twin Cities Water Quality Initiative Grant Report. Ramsey-Washington Metro Watershed District.
• Schueler, T. 1996. Irreducible Pollutant Concentrations Discharged from Stormwater Practices. Article 65: Technical Note #75 from Watershed Protection Techniques 2(2):369-372. Center for Watershed Protection.
• Strecker, E, and M. Quigley. 1999. Development of Performance Measures, Task 3.a – Technical Memorandum: Determining Urban Stormwater Best Management Practice (BMP) Removal Efficiencies. Prepared by URS Greiner Woodward Clyde, Urban Drainage and Flood Control District, Urban Water Resources Research Council (UWRRC) of ASCE, and Office of Water, US Environmental Protection Agency.
• SWAMP (Stormwater Assessment Monitoring and Performance Program) 2005. Synthesis of Monitoring Studies Conducted Under the Stormwater Assessment Monitoring and Performance Program. Prepared for Great Lakes Sustainability Fund of the Government of Canada, Toronto and Region Conservation Authority, Municipal Engineers Association of Ontario, and Ontario Ministry of the Environment.
• Tornes, L. 2005. Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04. U.S. Geological Survey Scientific Investigations Report 2005-5189.
• Yetka, L. and K. Leuthold. 2005. Burnsville Rainwater Garden Retrofit Project. Presentation at 2005 Annual Minnesota Erosion Control Association Conference.
• Weiss, P.T., J.S. Gulliver, and A.J. Erickson. 2005. The Cost and Effectiveness of Stormwater Management Practices. Minnesota Department of Transportation, St. Paul, MN.
• Winer, R. 2000. National Pollutant Removal Performance Database for Stormwater Treatment Practices, 2nd Edition. Center for Watershed Protection, for EPA Office of Science and Technology.

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