Water Quality Assessed by Benthic Macro-Invertebrates: Proposing the WQABI Method

Stormwater experts agree that the currently used percent removal methodology metric has many flaws, and some have suggested utilizing a BMP effluent concentration metric. This paper examines a new method that establishes effluent target concentrations for BMPs based on the health of macro-invertebrates in the receiving water. 193 ambient water quality monitoring stations in North Carolina were paired with benthic macro-invertebrate health ratings collected in very close proximity. Water quality for the sites ranged from Excellent to Poor and was divided into three distinct eco-regions. Median NO₃ + NO₂, TKN, TN, and TP concentrations all increased with poorer benthic ratings; however, there was no such trend with respect to DO, TSS, and Fecal Coliforms. Overall, the new method was most effective in the Piedmont and Coastal eco-regions; however with more data collection, the Mountain eco-region may also benefit. For example, if regulators were to require "Good" water quality in the Piedmont eco-region, designers would have to limit effluent concentrations of TN and TP would be 0.99 mg/L and 0.11 mg/L, respectively, per the method presented herein.