Mitigation of nutrient leaching from bioretention systems using amendments

Bioretention systems have been showcased to be effective in reducing stormwater quantity and improving stormwater quality. However, the systems can also leach nutrients especially during their initial operation, which needs to be mitigated to optimize their benefits. To examine the efficiency of six amendments selected primarily for reducing phosphorus (P) leaching, six amended and two control cells were constructed and monitored soon after their construction in the field of the 2020 growing season. The effects of the amendments on both the hydrological performance (in terms of water retention rate (WRR)) and water quality performance (in terms of the event mean concentration and pollutant removal rate) were investigated. The results showed that all amendments had the capability of preventing or mitigating P leaching from bioretention systems to varying degrees, with the water treatment residual (WTR) outperforming all other amendments, followed by the activated aluminum (AA) and sorptiveMEDIA (SM) amendments. In addition, some of the amendments (i.e., drywall (DRY), WTR, and SM) were also found to be beneficial in reducing the nitrogen (N) leaching to a slight degree, whereas eggshell (EGG) introduced an extra source of N leached. Furthermore, the temporal evolution of the P leaching of the amendment cells was found to be different from that of the control cells. The same result was not observed for the temporal evolution of the N leaching, implying that the amendments (except EGG) did not largely affect the N leaching. Among the amendments’ effect on WRR, there was no obvious difference. Whereas the observed differences between the control cells and some amendment cells in vegetation growth and antecedent media moisture condition might imply their potential impacts on the hydrologic performance. Overall, other than P leaching, the use of amendments affected other functions of bioretention systems, which should be taken into consideration when selecting an amendment for practice.