LID Treatment Train: Pervious Concrete with Subsurface Storage in Series with Bioretention and Care with Seasonal High Water Tables

Two infiltrating low-impact development (LID) practices configured in-series, pervious concrete and bioretention (PC-B), were monitored for 17 months to examine the hydrologic and water quality response of this LID treatment train design. For the first LID practice, 0.53 ha of pervious concrete was installed to treat direct rainfall and run-on from 0.36 ha of asphalt parking lot. The pervious concrete was installed over a gravel subsurface storage basin, which was designed to store 25 mm (1 in.) of runoff from the parking lot before draining into the second LID practice, which was a 0.05 ha bioretention cell. The bioretention cell was conventionally drained, had a media depth of 0.5 m (1.6 ft), and was constructed at a location with a high water table. Outflow was only generated in 33 out of 80 monitored events, and over the course of the entire monitoring period, the total outflow volume reduction was 69%. The large outflow reduction subsequently led to high pollutant load reductions for total nitrogen (49%), total phosphorus (51%), and total suspended solids (89%). However, when the contribution of base flow was included in the calculation, the total nitrogen load discharged from the bioretention cell was 64% higher than that of the runoff load because of nitrite and nitrate, ${\mathrm{NO}}_2^{-}$ and ${\mathrm{NO}}_3^{-}$(${\mathrm{NO}}_{2,3}\mathrm{\text{-}}\mathrm{N}$), which were present in the base flow. The total nitrogen (TN) loads of runoff, storm flow (total outflow minus base flow), base flow, and outflow (total) were 7.70, 3.94, 8.69, and $12.64\mathrm{kg}/\mathrm{ha}·\mathrm{year}$, respectively. Of the $8.69\mathrm{kg}/\mathrm{ha}·\mathrm{year}$ TN in the base flow, 92% was in the form of ${\mathrm{NO}}_{2,3}\mathrm{\text{-}}\mathrm{N}$. This study demonstrated the hydrologic benefits (peak flow and outflow reduction) gained by having two infiltration LID practices in-series. When compared with a single treatment practice (bioretention) that was monitored at the same site, the two LID practices in-series treated an additional 10% of annual runoff volume, discharged approximately one-half as much outflow volume, and discharged significantly lower peak outflow rates. However, the water quality results were not as promising because of the influx of groundwater in the bioretention cell and the lack of denitrifying conditions in either the bioretention cell or pervious concrete system. This study also quantified increased TN and ${\mathrm{NO}}_{2,3}\mathrm{\text{-}}\mathrm{N}$ export to surface waters from a bioretention cell that was situated in an area with a high water table.