Performance monitoring of roadside green infrastructure: The different perspectives of alternate water quality metrics.

Green infrastructure (GI) is increasingly used for managing developed watershed stormwater and GI designs are quickly developing. Performance monitoring data to guide GI design are challenging to collect and sparse, especially for multiple design types within the same study area. Evaluating GI performance with monitoring data can be done in multiple ways, with different managers using different metrics for evaluation. In this study, the year-round field performance of four roadside GI practices (bioretention, grass channel [GC], compost-amended grass channel [CAGC], and bioswale) in Northern Virginia were monitored to investigate the effectiveness of different designs in managing stormwater. Stormwater runoff volumes and 12 water quality parameters (including nutrients, carbon, sediment, and metals) were measured year-round during 27 storm events. Multiple metrics were used to assess GI performance, including effluent concentration, concentration reduction, load reduction, effluent load/contributing drainage area, and load reduced/footprint. The four GI designs displayed widely ranging performance, with three out of four primarily acting as pollutant sinks but one (CAGC) primarily as a pollutant source. Results indicate that GI design differences (e.g., compost amendment) can alter GI performance and evaluation varies based on water quality parameter and evaluation metric used. While regionally relevant individual pollutants of concern and regulations typically incentivize managers to design GI based on a single parameter or metric, the use of multiple parameters and metrics may better optimize overall water quality benefits. GI design guidelines based on multiple performance metrics and robust performance databases will improve stormwater management and help avoid unintended performance.