9VAC25-870-96. Water quality.
A. Compliance with the water quality criteria may be achieved by applying the performance-based criteria or the technology-based criteria to either the site or a planning area.
B. Performance-based criteria. For land-disturbing activities, the calculated postdevelopment nonpoint source pollutant runoff load shall be compared to the calculated predevelopment load based upon the average land cover condition or the existing site condition. A BMP shall be located, designed, and maintained to achieve the target pollutant removal efficiencies specified in Table 1 of this section to effectively reduce the pollutant load to the required level based upon the following four applicable land development situations for which the performance criteria apply:
1. Situation 1 consists of land-disturbing activities where the existing percent impervious cover is less than or equal to the average land cover condition and the proposed improvements will create a total percent impervious cover that is less than the average land cover condition.
Requirement: No reduction in the after disturbance pollutant discharge is required.
2. Situation 2 consists of land-disturbing activities where the existing percent impervious cover is less than or equal to the average land cover condition and the proposed improvements will create a total percent impervious cover that is greater than the average land cover condition.
Requirement: The pollutant discharge after disturbance shall not exceed the existing pollutant discharge based on the average land cover condition.
3. Situation 3 consists of land-disturbing activities where the existing percent impervious cover is greater than the average land cover condition.
Requirement: The pollutant discharge after disturbance shall not exceed (i) the pollutant discharge based on existing conditions less 10% or (ii) the pollutant discharge based on the average land cover condition, whichever is greater.
4. Situation 4 consists of land-disturbing activities where the existing percent impervious cover is served by an existing stormwater management BMP that addresses water quality.
Requirement: The pollutant discharge after disturbance shall not exceed the existing pollutant discharge based on the existing percent impervious cover while served by the existing BMP. The existing BMP shall be shown to have been designed and constructed in accordance with proper design standards and specifications, and to be in proper functioning condition.
C. Technology-based criteria. For land-disturbing activities, the postdeveloped stormwater runoff from the impervious cover shall be treated by an appropriate BMP as required by the postdeveloped condition percent impervious cover as specified in Table 1 of this section. The selected BMP shall be located, designed, and maintained to perform at the target pollutant removal efficiency specified in Table 1 or those found in 9VAC25-870-65. Design standards and specifications for the BMPs in Table 1 that meet the required target pollutant removal efficiency are available in the 1999 Virginia Stormwater Management Handbook. Other approved BMPs available on the Virginia Stormwater BMP Clearinghouse Website may also be utilized.
| Table 1* | ||
| Water Quality BMP* | Target Phosphorus Removal Efficiency | Percent Impervious Cover |
| Vegetated filter strip | 10% | 16-21% |
| Grassed swale | 15% | |
| Constructed wetlands | 20% | 22-37% |
| Extended detention (2 x WQ Vol) | 35% | |
| Retention basin I (3 x WQ Vol) | 40% | |
| Bioretention basin | 50% | 38-66% |
| Bioretention filter | 50% | |
| Extended detention-enhanced | 50% | |
| Retention basin II (4 x WQ Vol) | 50% | |
| Infiltration (1 x WQ Vol) | 50% | |
| Sand filter | 65% | 67-100% |
| Infiltration (2 x WQ Vol) | 65% | |
| Retention basin III (4 x WQ Vol with aquatic bench) | 65% | |
| *Innovative or alternate BMPs not included in this table may be allowed at the discretion of the local program administrator or the department. Innovative or alternate BMPs not included in this table that target appropriate nonpoint source pollution other than phosphorous may be allowed at the discretion of the local program administrator or the department. | ||
Statutory Authority
§§ 62.1-44.15:25 and 62.1-44.15:28 of the Code of Virginia.
Historical Notes
Former 4VAC50-60-96 derived from Virginia Register Volume 27, Issue 26, eff. September 13, 2011; amended, Virginia Register Volume 29, Issue 4, eff. November 21, 2012; amended and renumbered, Virginia Register Volume 30, Issue 2, eff. October 23, 2013.