EW-3: Runoff Quality

Goal

Improve water quality of stormwater runoff leaving the roadway Right-of-Way (ROW)

Requirements

1. Develop a stormwater management plan for the site using stormwater best management practices (BMPs) for water quality treatment. Explicitly state the goals of this plan and how performance will be measured.
2. Use low-impact development (LID) BMPs to the maximum extent feasible as determined in Project Requirement PR-8 by a licensed professional.
3. Compute the 90th percentile average annual rainfall event post‐construction runoff volumes (Vtotal) for two areas as follows:
   • Vrunoff: the total pollution generating surface (PGS) area of the project ROW 
   • Vrun-on: The total PGS area outside the ROW that may generate untreated stormwater which runs into the ROW BMPs, if any. 
   • Vtotal = Vrunoff + Vrun-on 
   • Vtreated = Vrunoff-treated + Vrun-on-treated 
   • % of Total Post-Construction Runoff Volume Treated = Vtreated/Vtotal x 100% 
   • Compute a weighted average of volumes treated for the total volume managed in the project where more than one BMP is used.
4. Provide treatment for a desired percentage of the total computed runoff volume for either of the areas noted in Table EW-3.1. List the types, manufacturers, treatment levels, and total volumes treated in BMPs..
5. Demonstrate that the planned BMPs meet the following quality criteria: 
   • BMPs reduce sediment loads to total suspended solids (TSS) concentrations of 25 mg/L or less, as an indicator of overall treatment level. See Table EW-3.2. 
   • BMPs conform to all applicable minimum water quality standards for all effluent leaving the ROW set by the governing jurisdiction for contaminants, such as heavy metals, hydrocarbons, pathogens, water temperature and turbidity. State the minimum requirements, including critical erosive flow criteria, and provide referenced document or policy.

Documentation

A copy of the executive summary of the project drainage design report that includes, at minimum, the following:

• Short narrative that addresses each of the credit requirements.

As needed, supporting calculations for runoff areas, runoff volume (output from any rainfall modeling software used is adequate), and treatment levels may be requested.

Public commentary on credit EW-3