This project is seeking to achieve credit 6.2 by infiltrating 100% of the first flush of stormwater, using a combination of pervious paving and infiltration trenches. According to the California Stormwater Best Management Practices Handbook - Industrial/Commercial (1983) and the California Regional Water Quality Control Board - Standard Urban Stormwater Mitigation Plans, 80-90% of the TSS and other pollutants are contained in this first flush. The reference standard for LEED" does not quantify the amount of stormwater that must be infiltrated to achieve the stated removal efficiencies of 50-100% for both TSS and TP. Is the above design of infiltrating 100% of the first flush an acceptable approach to earning this credit?
For water quality issues, treatment of the "first flush" or first 1" of rainfall is critical to successful treatment strategies. The TAG has determined that if the project can demonstrate that 100% of the first 1" of rainfall is fully infiltrated on site, the intent of this credit would be achieved. Alternately, the project can follow the requirements in the Reference Guide that for the 1.5 year storm, a treatment system is in place to remove 80% of the TSS and 40% of the TP from the stormwater before it is released from the site. The LEED Reference Guide also suggests that the infiltration basins need to infiltrate all of the water within 72 hours to allow for infiltration in the next storm. Keep in mind however that a shorter infiltration time is better to reduce disease vector issues.