Underdrain Configuration to Enhance Bioretention Exfiltration to Reduce Pollutant Loads

Abstract

The bioretention drainage configuration of raising the outlet to create an internal water storage (IWS) layer in the media was originally intended to promote denitrifying conditions. The goal was to reduce nitrate and total nitrogen concentrations in nutrient-sensitive watersheds. Two field studies in the Piedmont region of North Carolina, where the in situ soils typically have high clay content, showed this design feature had potential to enhance exfiltration and reduce drainage from bioretention. Two bioretention cells in Rocky Mount, North Carolina, were monitored for two year-long periods to measure the impact of varying IWS zone depths over sandier underlying soils. Nearly 99% of runoff entering the bioretention cell with sand underlying soil (sand cell) was never directly discharged to the storm water network. However, the hydraulic retention time (contact time) of runoff in the media was less than 3 h, and except for total suspended solids (TSS), minimal pollutant removal was achieved. The other bioretention cell had a sandy clay loam underlying soil (SCL cell); the percentage of runoff leaving via exfiltration and evapotranspiration from this cell was 87% during the monitoring period with a 1.03-m IWS zone depth and 75% when the IWS zone depth was 0.73 m. The underlying soil of the SCL cell had a lower hydraulic conductivity, so water would remain in the IWS zone for up to 7 days. The increased hydraulic retention time in the media resulted in lower outflow concentrations. For events monitored with drainage from the SCL cell, efficiency ratios of all the nitrogen species and TSS exceeded 0.5. As an additional metric of performance, the parking lot runoff and treated runoff from both the SCL and sand cells were compared to concentrations consistent with “good” and “fair” benthic macroinvertebrate health in streams. Using this metric, the parking lot runoff only met the “fair” standard for total nitrogen (TN) and total phosphorus (TP), and treated runoff from the SCL cell achieved the “good” standard for both TN and TP. However, because of the short hydraulic retention time of runoff in the media for the sand cell, this cell only maintained the “fair” standard for TN and did not achieve the “fair” standard for TP.