Difference between revisions of "Water quality criteria"
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| + | 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. | ||
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==Computing water quality volume== | ==Computing water quality volume== | ||
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The state has defined how to compute the water quality volume for projects that must meet the requirements in the [http://www.pca.state.mn.us/index.php/view-document.html?gid=18984 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. | The state has defined how to compute the water quality volume for projects that must meet the requirements in the [http://www.pca.state.mn.us/index.php/view-document.html?gid=18984 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. | ||
[[File:Schematic of total storage volume.jpg|thumb|300px|alt=schematic showing total storage volume|<font size=3>Illustration of total storage volume. The total storage volume includes two components - dead storage and live storage</font size>]] | [[File:Schematic of total storage volume.jpg|thumb|300px|alt=schematic showing total storage volume|<font size=3>Illustration of total storage volume. The total storage volume includes two components - dead storage and live storage</font size>]] | ||
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| − | 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 (V<sub>ts</sub>) has two additive components: | + | Designers in the state have traditionally relied on ponds for water quality treatment. The water quality volume is given by |
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| + | , so the first water quality rule applies to ponds that drain to regular waters. The total storage volume (V<sub>ts</sub>) has two additive components: | ||
*dead (or permanent) storage of 1/2 inch per acre (also stated as 1800 cubic feet per acre); and | *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 | *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 | ||
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'''[[Unified sizing criteria#Computing water quality volume|Rule 3]]''' | '''[[Unified sizing criteria#Computing water quality volume|Rule 3]]''' | ||
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<math>V_{wq} = 3630 IC</math> | <math>V_{wq} = 3630 IC</math> | ||
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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. | 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. | ||
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| + | For more information on pond design, [https://stormwater.pca.state.mn.us/index.php?title=Design_criteria_for_stormwater_ponds link here]. | ||
<!--[[File:Plot impervious vs watershed inches.png|thumb|300px|alt=graph showing a plot of impervious cover versus watershed inches|<font size=3>Recommended adaptation for Non-Pond Treatment under MN CGP (Note that the storage for areas with less than 20 percent imperviousness is proposed to achieve a minimum amount of treatment from small areas).</font size>]] | <!--[[File:Plot impervious vs watershed inches.png|thumb|300px|alt=graph showing a plot of impervious cover versus watershed inches|<font size=3>Recommended adaptation for Non-Pond Treatment under MN CGP (Note that the storage for areas with less than 20 percent imperviousness is proposed to achieve a minimum amount of treatment from small areas).</font size>]] | ||
Revision as of 16:12, 24 July 2017
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.