Water quality criteria
Computing water quality volume
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.
The state has defined how to compute the water quality volume for projects that must meet the requirements in the Construction General Permit. The current MPCA water quality volume criteria is referred to as the “hybrid rule” because it encompasses different rules depending on the type of BMP used.
Designers in the state have traditionally relied on ponds for water quality treatment, so the first water quality rule applies to ponds that drain to regular waters. The total storage volume (Vts) has two additive components:
- dead (or permanent) storage of 1/2 inch per acre (also stated as 1800 cubic feet per acre); and
- live (or temporary) water quality storage of 1/2 inch times the fraction of new impervious cover (IC) for the site. Mathematically, the acre feet of storage needed for basic pond sizing in regular waters is computed as
\( V_{ts} = (V_{pp} + V_{wq})\)
\( V_{pp} = 1815 A\)
\( V_{wq} = 1815 IC\)
where
- Vts = total storage volume in acre feet;
- Vpp = required permanent pool dead storage in acre feet or = 1800 cubic feet of storage per acre draining to pond;
- Vwq = water quality volume live storage in acre feet;
- IC = new site impervious cover, in acres;
- A = total watershed area, in acres, draining to pond; and
- 1815 = a conversion factor (to cubic feet)
The relationship of watershed inches to impervious cover can be established for the two water quality storage components, dead and live storage, in a pond sized according to Rule 1. In addition, ponds are also required to have a live storage release rate no greater than 5.66 cubic feet per second (cfs) per surface acre of pond area (as measured from the top of the live water quality storage bounce above the permanent pool). For example, if the maximum surface area of the pond created by the Vwq is three acres, the allowable maximum discharge rate from the pond would be 16.98 cfs (3 * 5.66). It is important to note that this is a geometrical requirement to achieve an overflow rate that ensures that a five-micron (5µ) sediment particle can be effectively settled within the pond, based on prior work by Pitt (1989). Designers are encouraged to ensure that at least 12 hours of extended detention are provided for the live storage in the pond BMP (using an acceptably sized and protected outlet at the orifice) to ensure an acceptable level of pollutant removal.
The second water quality sizing rule pertains to ponds located within the special waters of the State as defined in the CGP. These ponds must have a greater live storage component (Vwq): 1 inch times the fraction of new IC for the site. The required acre-feet of total storage volume (Vts) needed for ponds draining to special waters is computed as
\( V_{ts} = (V_{pp} + V_{wq})\)
\( V_{pp} = 1815 A\)
\( V_{wq} = 3630 A\)
where
- Vts = total storage volume in acre feet;
- Vpp = required permanent pool dead storage in acre feet -or- = 1800 cubic feet of :storage per acre draining to pond;
- Vwq = water quality volume live storage in acre feet;
- IC = new site impervious cover, in acres;
- A = total watershed area in acres) draining to pond; and
- 1815 and 3630 = conversion factors (to cubic feet)
The live storage in ponds draining to special waters must also conform to the maximum 5.66 cubic feet per second required release rate and should allow for a recommended minimum 12 hour extended detention time.
A third water quality sizing rule contained in the 2003 GCP applies to non-pond BMPs such as infiltration, bioretention and filtering practices. These practices are not explicitly required to have permanent pool storage, although some dead sediment storage is recommended for pre-treatment before discharging into the practice. The basic sizing equation for non-pond BMPs is shown below (Rule 3)
\(V_{wq} = 3630 IC\)
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.
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.