Impact of Periodic High Concentrations of Salt on Bioretention Water Quality Performance

Abstract

Bioretention is a stormwater control measure commonly used to remove pollutants, including nitrogen (N) and phosphorus (P), from urban stormwater. This study evaluates the impacts of episodic high concentrations of sodium chloride (NaCl) on bioretention N, P, and heavy metals (Cu and Zn) removal performances. Bioretention mesocosm studies were conducted to examine N and P removal efficiencies following periodic 2,000-, 5,000-, and 10,000-mg/L NaCl salt applications (every fifth stormwater event). Episodic washouts of particulate matter (PM), N, and P were observed for all three columns, generally occurring in the second stormwater event after the high-salt event. PM and associated particulate P and N releases were likely due to soil deflocculation, as noted by high concentrations of effluent total suspended solids (TSS) and turbidity. Solvation of organic matter in the presence of chloride and ion exchange with the sodium and chloride ions appear to have contributed to dissolved N and P washouts. Mass export of P was observed for the 2,000- and 5,000-mg/L NaCl columns, and some removal of N and metals was noted. Based on mass balances for N, P, and heavy metals, it is recommended to limit the use of NaCl deicers to prevent long-term P export.