Evaluating Stormwater Pollutant Removal Mechanisms by Bioretention in the Context of Climate Change

Stormwater runoff is one of the leading causes of water quality impairment in the U.S. Bioretention systems are ecologically engineered to treat stormwater pollution and offer exciting opportunities to provide local climate change resiliency by reducing peak runoff rates, and retaining/detaining storm volumes, yet implementation is outpacing our understanding of the underlying physical, biological, and chemical mechanisms involved in pollutant removal. Further, we do not know how performance will be affected by increases in precipitation, which are projected to occur in the northeastern U.S. as a result of climate change, or if these systems could act as a source or sink for greenhouse gas emissions. This research examines the design, construction, and development of monitoring methods for bioretention research, using the University of Vermont (UVM) Bioretention Laboratory as a case study. In addition, this research evaluates mobilization patterns and pollutant loads from road surfaces during the first flush of runoff, or the earlier part of a storm event. Finally, this research analyzes the comparative pollutant removal performance of bioretention systems on a treatment by treatment basis. At the UVM Bioretention Laboratory, eight lined bioretention cells were constructed with monitoring infrastructure installed at the entrance and at the subterranean effluent. A conventional, sand and compost based, bioretention soil media was compared to a proprietary media engineered to remove phosphorus, called Sorbtive Media™, under simulated increases in precipitation. Two drought tolerant vegetation mixes, native to the northeast, were compared for sediment and nutrient retention. Each treatment was sampled for soil gas emissions to determine if it was a source or a sink. The monitoring infrastructure designs used in this research allowed for the effective characterization of pollutant mass loads entering and exiting bioretention. Cumulative mass loads from stormwater were found to be highest for total suspended solids, followed by total Kjeldahl nitrogen, nitrate, non-labile phosphorus and soluble reactive phosphorus, in descending order by mass. Total suspended solids, total Kjeldahl nitrogen, and non-labile phosphorus mass were well retained by all bioretention treatments. However, the compost amendment in the conventional soil media was found to release labile nitrogen and phosphorus, far surpassing the mass loads in stormwater. When compared with conventional media, Sorbtive Media™ was highly effective at removing labile phosphorus and was also found to enhance nitrate removal. Systems containing deep-rooted vegetation (Panicum virgatum) were found to be particularly effective at retaining both labile and non-labile constituents. Overall, none of the bioretention treatments were found to be a significant source of N2O and were small sinks for CH4 in most treatments

Nutrient Leaching from Compost: Implications for Bioretention and Other Green Stormwater Infrastructure

This work is made available under the terms of the Creative Commons Attribution 4.0 International license,. Compost is often used as a soil amendment in gardens, agricultural fields, and other landscaped systems to alter soil biophysical characteristics and increase availability of valuable nutrients including nitrogen (N), phosphorus (P), and carbon (C). However, leaching of soluble nutrients from compost is of concern, particularly in wet settings, such as within green stormwater infrastructure, riparian areas, and floodplains. This research highlights the importance of saturation as an influencing factor on the nutrient leaching potential of different composts and compost-amended bioretention soils. Nutrient leaching potential was evaluated for five different compost types and two compost-amended bioretention soil mixes under increasing saturation durations, measured at 10 min, 1 day, 5 days, and 10 days of saturation. Results indicated significant increases in NH4+ concentrations in leachate for all composts and bioretention media from 10 min to 10 days. Over the same time period results showed decreases in NO3- concentrations in the leachate from all five composts, but an increase in NO3- concentration for one compost-amended bioretention media and no significant change in the other bioretention media. In response to increased saturation durations, PO43- concentrations in the leachate were found to significantly increase at each stage, from 10 min, to 1 day, to 5 days, to 10 days; overall there were higher PO43- concentrations in the leachate from the five composts than in the leachate from the two bioretention mixes