This presentation will detail field investigations that were conducted on the performance of small capacity urban retrofit stormwater control measures (SCMs). 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. In many states performance credits are not granted to SCMs that are not designed to manage regionally derived water quality volumes. In retrofit applications there may exist numerous limitations to conventionally sized systems such as limited rights of way, existing utilities, property ownership, etc. This study introduces data on an innovative bioretention design with a water treatment residual (WTR) admixture filter media and an internal storage reservoir and an undersized linear subsurface gravel wetland with an internal storage reservoir sized to treat as much of the water quality volume (WQV) as possible given existing retrofit constraints. The bioretention system was constructed in the town of Durham, NH in summer 2011 and sized to treat a 0.3 inch rainfall event from a 0.69 acre commercial area with 92% impervious cover (IC). The subsurface gravel wetland system was constructed in a linear drainage right of way in a residential neighborhood of Durham, NH in the fall of 2013 and sized to treat a 0.1 inch rainfall event from a 6 acre residential neighborhood at 30% IC.
Sediment and metal removals for both undersized systems were high with median removal efficiencies (RE) at 86% and 75% for both TSS and TZn in the Durham Bioretention (Durham Bio) and Subsurface Gravel Wetland system (SGW) respectively. Total Phosphorus (TP) RE were higher than conventional Bioretention systems studied elsewhere with the Durham Bio achieving a median RE of 40% for TP and the SGW system achieving a median RE of 53%. Both systems reduced total nitrogen by approximately 20% (21% for Durham Bio, and 23% for SGW and) with median effluent concentrations of 1.4 mg/L. Reduction in nitrate was limited to storms that were at or below the design storm event in the SGW only, still, median effluent concentrations for the Durham Bio and the SGW were generally low at 0.2 mg/L and 0.3 mg/L respectively.
Performance for all pollutants with the exception of dissolved nitrogen species approached performance expectations for conventionally sized systems studied elsewhere despite being “undersized” by 70-90%.