Treatment of stormwater runoff is needed to meet in-stream water quality standards and protect aquatic life and water resources. Extensive monitoring has revealed high concentrations of sediments, nutrients, bacteria, metals, oxygen-demanding substances, hydrocarbons and other pollutants in untreated stormwater runoff (Pitt et al., 2004) and demonstrated their impact on stream and lake quality (CWP, 1999 and CWP, 2003). A range of BMPs can provide a high degree of removal for stormwater pollutants (ASCE, 2004 and Winer, 2001). The 2000 state manual (MPCA, 2000) established a performance goal that BMPs provide a minimum degree of pollutant removal for a defined fraction of stormwater runoff events, which has been operationally defined as 90 percent sediment removal. A 50 percent total phosphorus removal can be assured to accompany this removal. Parts of the state CGP reference the 80 percent Total Suspended Solids (TSS) standard.
Computing water quality volume
Designers in the state have traditionally relied on ponds for water quality treatment. The water quality volume is given by
\(V_{wq} = 3630 IC\)
where
- Vwq = required water quality volume live storage in acre feet;
- IC = new site impervious cover, in acres; and
- 3630 = conversion factor (to cubic feet).
The minimum pre-treatment volume recommended (not required in CGP) to protect non-pond BMPs from clogging and increase their longevity is 0.20 watershed inches.
For more information on pond design, link here.
Modifications to water quality criteria
Most communities do not allow many exemptions to their basic water quality sizing criteria, although they may choose to reduce or exempt certain redevelopment and infill projects.
Water quality sizing criteria can be modified upward or downward. The first occurs when stormwater credits are offered to reduce water quality sizing when acceptable better site design techniques are applied on the site. The second occurs when sizing criteria are increased to provide an enhanced level of treatment to protect special waters, such as a nutrient sensitive lake or when local criteria exceed the state minimum.