Nursery Runoff Treatment by Novel Biochar-Amended Bioretention Systems

Extensive fertilization and irrigation in commercial plant nurseries generates runoff containing high levels of nutrients. Biochar-amended bioretention systems with internal water storage zones (IWSZs) have been shown to enhance total nitrogen removal from urban runoff. However, their effectiveness for treatment of nursery runoff and the role of biochar in the IWSZ remain understudied. The goal of this research was to investigate nitrogen transformations in pilot-scale bioretention systems treating nursery runoff with varying biochar-amendment strategies: (a) throughout both the unsaturated zone and the IWSZ (CBA), (b) only in the unsaturated zone (PBA). Variables investigated included hydraulic loading rate (HLR: 0.11, 0.22, and 0.55 cm/min), IWSZ depth (44 and 69 cm), and the presence of plants (Muhlenbergia capillaris). The presence of biochar in the IWSZ (CBA) enabled significantly greater nitrogen removal (p = 0.031) compared to PBA. CBA had improved hydraulic efficiency by mitigating short-circuiting (34% increase in mean retention time) and likely enhanced performance by promoting nutrient uptake and microbial activity. Three times the above ground plant biomass was observed in CBA vs. PBA (0.73 kg in CBA vs. 0.23 kg in PBA). The highest nitrogen removal efficiency (84%) was achieved in the planted CBA unit at an HLR of 0.22 cm/min and IWSZ depth of 69 cm. A spreadsheet-based tool, utilizing a logarithmic regression model for CBA (R2 = 0.88 for TIN, 0.86 for NOx) and PBA (R2 = 0.50 for TIN, 0.60 for NOx), was developed for system design to achieve nitrogen removal targets. The greater variability in the PBA-fitted model (lower R2) compared to CBA (higher R2, better fit) suggests biochar&rsquo:s ability to mitigate short-circuiting and improve hydraulic performance.