Assessing the Accuracy of Bioretention Installation in North Carolina

Abstract

The performance of bioretention cells (BRCs) depends upon their capacity to store an adequate amount of runoff such that the desired stormwater volume and pollutant load are retained. Performance is also dependent upon the use of a suitable soil media that can provide storage within the void space, adequate infiltration between storm events, and substrate for plant and microbial interactions. Widespread construction of improperly designed cells could have substantial environmental consequences because of frequent overflow events. To determine if this is indeed a problem, 20 BRCs throughout North Carolina were assessed. Topographic surveys were performed and the as-built surface storage (ponding within the basin below the outlet elevation) was calculated using CAD software. Visual inspections of the soil profile were used to identify hydric soils and moisture regime. Particle-size distribution of the soil media at various depths was determined using the hydrometer method and permeability was estimated with constant-head saturated hydraulic conductivity tests. Results were compared to the intended storage volumes and soil specifications from original design calculations and construction plans. Preliminary findings show that 82% and 44% of BRC soil media failed to satisfy particle-size distribution and permeability specifications, respectively. Additionally, 50% of surveyed BRCs were either moderately or severely undersized. Continuing research will confirm whether BRCs throughout North Carolina are improperly installed and maintained. Research findings will be used to evaluate state requirements and provide recommendations to ensure that installed BRCs satisfy water quality and flood control regulations.