Stormwater control measures like bioretention are a critical part of meeting hydrologic and water quality goals. However, there are concerns that de-icing salts applied for public safety may negatively impact the function of bioretention systems. The objective of this work was to conduct an experiment with bioretention mesocosms to explore the interactions of vegetation, salt loading, and hydraulics in impacting water quality outcomes. 48 mesocosms were established in 2021 and dosed with synthetic stormwater throughout 2022 and early 2023, including added NaCl in the late winter. Treatments included high or low salt concentration; the presence of Panicum virgatum, Eupatorium purpureum, or no vegetation; and the presence of an internal water storage (IWS) zone or free-flowing design. Water quality evaluation revealed generally good removal of nitrogen and total suspended solids (TSS) and excellent removal of total phosphorus, copper, and zinc throughout the experiment; however, deicing salt application significantly reduced the spring removal of phosphorus, copper, TSS, and zinc, resulting in major, though brief, export of zinc. IWS and vegetation were generally found to positively affect or have no influence on performance, varying with analyte, although both were important in nutrient removal. Good annual removal was demonstrated despite the effects of salt, especially when vegetation and IWS were included. This indicates that bioretention can successfully operate in cold climates, although reduced salt application, specific design considerations, and regular maintenance can all facilitate more resilient bioretention systems.
