Performance analysis of two relatively small capacity urban retrofit stormwater controls

Abstract

The objective of the two-year study (2013-2015) was to provide performance data on stormwater retrofits that could not be sized according to conventional standards. Sediment and metal removals for both undersized systems were high with median removal efficiencies (RE) in the Subsurface Gravel Wetland (SGW) system (SGWSC#1) of 75% for both Total Suspended Solids (TSS) and Total Zinc (TZn). The Durham Bioretention (Durham Bio) (IBSC#2) recorded median RE of 86% for TSS and TZn. Total Phosphorus (TP) RE were higher than conventional Bioretention systems with the SGW system achieving a median RE of 53% and the Durham Bio achieving a median RE of 40% for TP. Both systems reduced total nitrogen by approximately 20% (23% for SGW and 21% for Durham Bio). Performance for all pollutants with the exception of dissolved nitrogen species approached performance expectations for conventionally sized systems despite being "undersized" by 90% for the SGW and by 70% for the Durham Bio as compared to conventional sizing methods.