Online

Ament, Michael Rick

Plant and Soil Science

2021

Ph. D.

Bioretention systems can reduce stormwater runoff volumes and filter pollutants. However, bioretention soil media can have limited capacity to retain phosphorus (P), and can even be a P source, necessitating P-sorbing amendments. Drinking water treatment residuals (DWTRs) have promise as a bioretention media amendment due to their high P sorption capacity. This research explores the potential for DWTRs to mitigate urban P loads using a combination of lab experiments, field trials, and an urban watershed model. In the laboratory portion of this research, I investigated possible tradeoffs between P retention and hydraulic conductivity in DWTRs to inform bioretention media designs. Batch isotherm and flow-through column studies demonstrated that DWTRs have high but variable P sorption capacities, which correlated inversely with hydraulic conductivity. Large column studies showed that when applied as a solid layer within bioretention media, DWTRs can restrict water flow and exhibit only partial P removal. However, mixed layers of sand and DWTRs were shown to alleviate flow restrictions and exhibit complete P removal. These results suggest that mixing DWTRs with sand is an effective strategy for achieving stormwater drainage and P removal goals. In the field portion of this research, I assessed the capacity of a DWTR-amended media to remove different chemical species of P from stormwater in roadside bioretention systems. I also explored whether DWTRs affect system hydraulics or leach heavy metals in the field. Significant reductions in dissolved P and total P concentrations and loads were observed in both the Control and DWTR media. However, the removal efficiency percentages (RE) of the DWTR cells were greater than those of the Control cells for all P species, and this difference increased substantially from the first to the second monitoring season. Furthermore, the DWTR used in this study was not shown to affect bioretention system hydraulics or to significantly leach heavy metals. These results indicate that DWTRs have potential to improve P retention without causing unintended consequences. In the third phase of this research, I used the EPA - Storm Water Management Model (SWMM) to assess the impacts of different bioretention P removal performances and infiltration capacities on catchment-scale P loads, runoff volumes, and peak flow rates. Model outputs, which measured the cumulative effects of widespread bioretention use, showed that both P removal performance and infiltration capacity (i.e., presence or absence of an impermeable liner) have major impacts on watershed P loads. Infiltrating bioretention systems showed the capacity to reduce urban P loads and stormwater volumes, even with media that exhibited low P removal. Notably, P-sorbing amendments can be a limited resource and infiltration is not feasible in all locations. These results therefore suggest that water quantity and quality goals can be effectively achieved through a mixture of infiltrating bioretention and strategic use of P-sorbing amendments. Together, this research shows that DWTRs have significant potential to improve P removal within bioretention systems, but that fine-scale processes (e.g., P sorption capacity, hydraulic conductivity) must inform media designs if bioretention systems are to effectively reduce catchment-scale P loads and eutrophication risks.