Calculating stormwater volume and pollutant reductions and credits

1. Credits for Better Site design

The Minnesota Stormwater Manual version 1.0 did not contain sufficient information to allow application of pollutant removal effectiveness for TSS and TP across a range of performance levels. Version 1.0 contained average performance numbers and “typical BMP outflow concentrations.” Input from users asked for more detail on how performance data could be applied across a wider range of conditions with more accuracy. The purpose of this version 2.0 addition is to gather more information about system performance, re-define what that actually means, and report on a three-tiered (high, medium, low) method for using the performance data.

Approach

Several databases and data compilations were reviewed and compared to develop expected performance measures for TSS and TP of the five following categories of BMPs contained in the current Manual: bioretention, filtration, infiltration, stormwater ponds, and stormwater wetlands. The following studies were used:

• International Stormwater Best Management Practices (BMP) Database: http://www. bmpdatabase.org/index.htm. This project began in 1996 under a cooperative agreement between the American Society of Civil Engineers (ASCE) and the U.S. Environmental Protection Agency (USEPA). It now has additional support and funding from the Water Environment Research Foundation (WERF), ASCE Environmental and Water Resources Institute, Federal Highway Administration, and the American Public Works Association (APWA). Wright Water Engineers, Inc. and GeoSyntec Consultants maintain and operate the database clearinghouse and web page. Data from well screened BMP studies from throughout the world continues to be entered. Data were downloaded from the website in February 2007 and were summarized for this current evaluation. This database will hereafter be referred to as the ASCE/EPA database.
• Stormwater Assessment Monitoring and Performance Program (SWAMP): http://www.trca.on.ca/water_protection/hydrology/old%20hydrology/stormwater_management/default.asp#swamp. SWAMP operated from 1995 to 2003. This program was an initiative of the Government of Canada’s Great Lakes Sustainability Fund, the Ontario Ministry of Environment and Energy, the Toronto and Region Conservation Authority, and the Municipal Engineer’s Association. This program is attractive to Minnesota because of the attention paid to cold climate conditions. The SWAMP report (SWAMP 2005) summarizes BMP monitoring data, but does not contain raw data. This program will hereafter be referred to as the SWAMP program.
• National Pollutant Removal Performance Database for Stormwater Treatment Practices, 2nd Edition, March 2000, Center for Watershed Protection (CWP). This report (Winer 2000) summarizes BMP monitoring data, but does not contain raw data. This report will hereafter be referred to as the CWP report.
• The Cost and Effectiveness of Stormwater Management Practices: Report prepared by authors from the University of Minnesota and Valparaiso University and published by the Minnesota Department of Transportation (Mn/DOT) in 2005. This report uses several BMP studies and examines performance relative to costs. This report cited as (Weiss et al. 2005) will be referred to as the Mn/DOT report.

Recommended Performance Measures

BMP performance can be evaluated in several ways, the most common of which are pollutant outflow concentration and pollutant percent removal:

• Pollutant outflow concentration: expressed simply as the pollutant concentration in the BMP outflow (primary recommended performance measure); this is a measure of the outflow only, not an inflow/outflow comparison.
• Pollutant percent removal (or removal efficiency): expressed as a percent change in either pollutant load (secondary recommended performance measure) or pollutant concentration of the outflow relative to the inflow (not recommended when expressed without a consideration of volume).

Although percent removal is often used in describing the performance of a BMP, it is a biased indicator of BMP performance (Strecker and Quigley 1999, SWAMP 2005). This bias results from the fact that percent removal is usually higher in situations where the inflow concentration is high, and influent concentration and percent removal are often correlated (SWAMP 2005). Judging performance on inflow versus outflow is not recommended as the primary performance indicator for this reason. To illustrate this concept, using concentration data from stormwater ponds from the ASCE/EPA database, at low influent TSS concentrations (less than 100 mg/L), percent removals range from 0% to almost 100% removal (Figure 1). However, at higher influent concentration (greater than 100 mg/L), percent removals are greater than 80% (except for one outlier at approximately 65%). This is because, as a pollutant in stormwater becomes less concentrated, it becomes harder to remove that pollutant, due to the pollutant’s physical and chemical properties and the BMP’s removal mechanisms; there is very little more a BMP can do to improve water quality. The percent removal is highly variable and therefore not the best indicator of performance.

As an example, if the inflow TSS concentration to a BMP is 40 mg/L and the outflow is 20 mg/L (assuming that the flow-through volume remains constant), then the BMP is performing at only 50% removal efficiency, even though the outflow has good water quality. On the other hand, if the inflow TSS concentration is 500 mg/L, and the outflow is 100 mg/L, the BMP is performing at a much higher efficiency, 80%, but the outflow is of poorer water quality than the outflow in the first example.