N treatment performance in stormwater biofilters: Relationships between sequestered pollutants, environmental conditions, and N cycling soil bacteria.
Stormwater runoff imposes large hydrologic and nutrient imbalances in urban areas through the delivery of large pollutant loads to surface waters. To mitigate this pollution, green stormwater infrastructure (GSI) approaches are increasingly used. Nitrogen (N) removal in GSI varies, depending on soil characteristics and plant species. However, GSI may often leach nitrate and export N. Removal of N is mediated by nitrifying and denitrifying bacteria; whether these bacteria are affected by retained soil contaminants, such as trace metals, is unknown. If accumulated soil metals reach levels that inhibit N cycling microorganisms, N treatment may be reduced. Further N treatment uncertainty arises due to limitations from prior studies, which have been mostly performed under steady-state conditions, in controlled lab environments, or have insufficiently considered transient flow conditions. A detailed understanding of the timing and magnitude of N processes across transient storms is needed to examine the role of biofilters as sources or sinks of aqueous and gaseous N, and to determine how biofilters should be managed to mitigate N export.
This doctoral research aims to address these knowledge gaps by 1) evaluating total and bioavailable metal concentrations in soils of field-scale GSI, and how accumulated metals may be predicted from drainage area characteristics, 2) assessing how soil properties, as well as total and bioavailable metal concentrations, influence nitrifying and denitrifying bacteria across representative GSI, and 3) comprehensively assessing N fates and N transformation processes within and in between storms in a field-scale GSI receiving high-flow storm events. Results show that metals can accumulate in GSI soils, and that total metals are significantly correlated to the ratio of impervious drainage area to GSI area. Thus, monitoring efforts may prioritize soils with highest impervious ratios. Results from representative GSI show that linear regression models including soil properties and metal concentrations provide good estimates of nitrifying and denitrifying gene abundances in soils. Bioavailable fractions of Cd and Pb seem to reduce gene abundances of denitrifying microorganisms (nirS, nosZ), with implications for N2O release. In contrast, total Cu, Ni and V appear to exert a positive influence on functional gene abundances, suggesting metal limitation in soils. Results reinforce including bioavailable metal fractions in metal risk assessments. In the final study, chemical, bacterial, and stable isotope data show that denitrification is limited even for high-frequency, large storms, and that GSI systems perform poorly, in terms of N removal, when challenged with a large transient storm, behaving as persistent N sources in subsequent storms. I propose an alternate design consisting of a treatment train of a real-time control stormwater capture system, sequentially followed by a fast-draining cell, and a slow-draining cell.
This dissertation has advanced the understanding of N processing in GSI; the potential interactions between soil nitrifying and denitrifying bacteria and accumulated soil metals has also been evaluated. Recommendations were provided to prioritize metal risk assessments, improve N treatment in GSI, and minimize N export and undesirable environmental consequences.