Impact of de-icing salt runoff in spring on bioretention efficiency

We investigated the effect of de-icing salt in stormwater runoff on bioretention system hydrology and filtration of contaminants. Salt runoffs during the snow melt period were simulated in 20 mesocosms planted with 1 of 3 plant species (Cornus sericea, Juncus effusus and Iris versicolor) or left unplanted, and then watered with semi-synthetic stormwater runoffs supplemented with 4 NaCl concentrations (0, 250, 1,000 or 4,000 mg Cl/L). All bioretention mesocosms, irrespective of treatment, were efficient in reducing water volume, flow and pollution level. There was no phytotoxic effect of NaCl on plants, even at the highest NaCl concentration tested. Water volume reduction and flow rate were influenced by plant species, but salt concentration had no effect. Salt runoffs significantly increased the removal of some metals, such as Cr, Ni, Pb and Zn, but had no effect on nutrient removal. Because snowmelt laden with de-icing salt is of short duration and occurs during plant dormancy, plants in bioretention may be less affected by de-icing salt than previously thought, provided that salinity decreases rapidly to normal level in the soil water. The long-term effects of de-icing salt and general performance of bioretention should be further studied under full-scale conditions.