Moving Beyond the Percent Removal Paradigm: Using Lower Limit Effluent Concentrations in Design Guidance and Evaluation

A modeling application, called the Site Evaluation Tool (SET) was developed to test site designs with stormwater BMPs and verify compliance with pollutant loading rate standards in a water supply watershed. Traditionally BMP performance is assessed using average or median percent removals of pollutant mass based on monitoring studies. However, researchers have recognized for some time that removal is strongly related to pollutant influent concentration, and using mass removal efficiencies alone is likely to overestimate gross pollutant removal when influent concentrations are relatively low. The SET uses median percent removal to assess BMP performance, but with removal capped by lower-limit effluent concentration. This paper focuses primarily on research to support development of the lower limit effluent concentrations, and the application of the SET to development site performance assessment.