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.