The stormwater regulatory focus over the past decade has been on reducing pollutant loads from new development; however, new regulatory focus in Massachusetts under the NPDES Small MS4 General Permit, is targeting reduction of load from unmanaged or inadequately managed existing development. Retrofitting existing developments includes installation of new water quality best management practices (BMPs) or upgrades to existing BMPs designed for water quantity control. While the main focus on new development is to achieve pre-development hydrology, retrofits are targeted to improve hydrology of an existing site and reduce discharge of stormwater to the maximum extent practicable.

MS4 communities in Massachusetts discharging to impaired watersheds with approved TMDLs are required to retrofit existing development in order to meet pollutant reduction targets. Retrofitting existing development, especially in urban environments, can be extremely challenging due to highly connected impervious surfaces, aged infrastructure, highly compacted subsurface soils, and limited pervious or opens areas to install BMPs. The cost to retrofit existing impervious area also increases the installation cost of BMPs.

Communities in the Charles River Watershed are required to reduce their annual stormwater phosphorus load by 16-54%. Meeting these load reduction targets will require substantial financial investments to retrofit the watershed; and therefore, an optimized approach to select the most cost-effective BMPs which will have the greatest water quality benefit is paramount. Developing an optimized watershed or municipal scale plan will help communities plan for the type of BMPs, locations and investment needed over the next 20 years.

An optimized approach will consider all the available land area for treatment within a watershed or municipal boundary and their suitability for BMPs based on physical constraints (soils, depth of groundwater, bedrock). Optimization also considers variable sizing of BMPs and treating water quality runoff depth, not water quality volume (typically 1-inch). Approximately 85% of the storm events in Massachusetts are less than 1-inch in rainfall volume and typically 80% of the total pollutant mass is transported in the first quarter inch of runoff. Therefore, when water quality is your design goal, smaller BMPs can achieve large load reductions and have a smaller footprint and cost.

Wright-Pierce is currently working with a number of clients on various stormwater projects to achieve water quality goals. Let us help you formulate an optimized approach to stormwater management.