Establishing the Basis for Stormwater Pollutant Trading: The San Diego Water Quality Equivalency Process
The San Diego MS4 stormwater permit allows an offsite alternative compliance program to be implemented by co-permittees. It was first necessary to establish a correlation between the water quality benefits provided by an alternative compliance project and a development project that is in full compliance with onsite BMP performance requirements.. The co-permittees worked with consultants, a technical advisory committee representing a range of interests, and a stakeholder advisory committee representing the broader community to develop equations that could be applied to establish the amount of pollutant control and hydromodification credit an alternative compliance project could provide based on a range of water quality factors. This water quality equivalency has been through two rounds of public review and is being approved by the Regional Water Quality Control Board for adoption by co-permittees. The offsite alternative compliance option in the 2013 MS4 permit allows co-permittees to implement an approach for alternative compliance projects to be developed in lieu of retention or biofiltration on priority development projects. The alternative compliance projects have to achieve greater overall water quality benefits. Priority development projects would still have to implement flow through BMPs in addition to the alternative compliance project. The co-permittees envision a system where pollutant control and hydromodification credits could be banked and sold by developers of alternative compliance projects. The method for establishing equivalency needed to take into account different pollutant loads occurring at different locations; different BMP types for alternative compliance projects: regional projects, retrofits, land purchases to preserve floodplain function, stream restoration, water supply augmentation, and groundwater recharge were listed as example categories; different percent captures occurring based on the alternative compliance project size and tributary area; and different BMP pollutant removal efficiencies based on the BMP implemented in one of the categories listed above. The equivalency method needed to be implementable by developers and plan check engineers while taking into account these factors. For pollutant control, volume was established as the currency for trading between alternative compliance projects and priority development projects. To account for pollutant load differences, event mean concentrations for tributary land uses were used to adjust the volume credit an alternative compliance project can create. Not all project categories could have equivalency established for pollutant control. Regional projects, retrofit projects, land purchase to a limited extent, and water supply augmentation/groundwater recharge could have equivalency established. Stream restoration for pollutant load reduction requires more research to establish equivalency factors. Figure 3 shows the categories for which equivalency equations currently exist and can be implemented by co-permittees. For hydromodification, directly connected impervious surface was established as the basis for currency. Directly connected impervious surface has the most direct effect on flow-duration from a site. HMP equivalency would, for most projects, need to be on the storm drain system that discharges to the same location in the receiving water. This paper will outline the program and how it will work and show the methods for establishing equivalency. This is one of the first such programs in the U.S. to establish stormwater quality project equivalencies between locations and project types that takes into account pollutant load differences, BMP differences, and percent capture differences in a way that allows the use of a single currency – volume. This is also the first program to establish equivalency for hydromodification.
Richard Haimann is a national stormwater technical leader. With over 26 years of experience, he has helped MS4 clients negotiate permits, implement permit conditions, and plan programs to meet TMDL WLAs. He has a B.S. in civil engineering, an M.S. in environmental engineering and an M.B.A. in technology management. He is licensed as a professional engineer in California, Texas, and Washington