Water quality performance of eight roadside bioretention cells in their third and fourth years of implementation were evaluated in Burlington, Vermont. Bioretention cells received varying treatments: (1) vegetation with high-diversity (7 species) and low-diversity plant mix (2 species); (2) proprietary SorbtiveMedia™ (SM) containing iron and aluminum oxide granules to enhance sorption capacity for phosphorus; and (3) enhanced rainfall and runoff (RR) to certain cells (including one with SM treatment) at three levels (15%, 20%, 60% more than their control counterparts), mimicking anticipated precipitation increases associated with climate change. A total of 121 storms across all cells were evaluated in 2015 and 2016 for total suspended solids (TSS), nitrate/nitrite-nitrogen (NOx), ortho-phosphorus (Ortho-P), total nitrogen (TN) and total phosphorus (TP). Heavy metals were also measured for a few storms, but in 2014 and 2015 only. Simultaneous measurements of flow rates and volumes allowed for evaluation of the cells’ hydraulic performances and estimation of pollutant load removal efficiencies and EMC reductions. Significant average reductions in effluent stormwater volumes (75%; range: 48–96%) and peak flows (91%; range: 86–96%) was reported, with 31% of the storms events (all less than 25.4 mm (1 in.), and one 39.4 mm (1.55 in.)) depth completely captured by bioretention cells. Influent TSS concentrations and event mean concentrations (EMCs) was mostly significantly reduced, and TSS loads were well retained by all bioretention cells (94%; range: 89–99%) irrespective of treatments, storm characteristics or seasonality. In contrast, nutrient removal was treatment-dependent, where the SM treatments consistently removed P concentrations, loads and EMCs, and sometimes N as well. The vegetation and RR treatments mostly exported nutrients to the effluent for those three metrics with varying significance. We attribute observed nutrient exports to the presence of excess compost in the soil media. Rainfall depth and peak inflow rate had consistently negative effects on all nutrient removal efficiencies from the bioretention cells likely by increasing pollutant mobilization. Seasonality followed by soil media presence, and antecedent dry period were other predictors significantly influencing removal efficiencies for some nutrient types. Results from the analysis will be useful to make bioretention designers aware of the hydrologic and other design factors that will be the most critical to the performance of the bioretention systems in response to interactive effects of climate change.
Bibliographical noteFunding Information:
This work was supported through a combination of support from University of Vermont’s College of Agriculture and Life Sciences, Lake Champlain Sea Grant (Award #NA10OAR4170063 ), and the Lintilhac Foundation. The authors extend thanks to Joel Tilley for technical support in the laboratory for water analysis. This work would not have been possible without the help from undergraduate interns: Anna Levine, Iliansherry Santiago, Sam Wooster, Lindsay Cotnoir, Danya AbdelHameid, Jelissa Reynoso, Hannah Klein, Lauren Jenness, Nichole Montero, Wileyshka M. Rivera, Maxwell Landsman-Gerjoi, Brad Hansen, and Jacob Woodworth. Additional thank-you is extended to Alan Howard and Dr. Josef Görres for statistical counseling, Nelish Pradhan, Gabriela Buccini and Vanesa Perrillo for providing assistance with R software, and Jason Kokkinos for assistance with field and lab work. Lastly, we thank Amanda Cording for her time and efforts spend during construction phase of the bioretention cells.
© 2017 The Authors
- Stormwater management
- Urban road runoff