Temporal Variability of Heavy Metals in Suburban Road Runoff in a Rainy Cold Climate

Abstract

Road runoff contains toxic heavy metals that can threaten the water quality of urban water systems. This paper characterizes the time variability of heavy metal pollution in road runoff in a 9 ha, steep (12% slope), suburban, rainy (235 rainy days; 1,120 mm per annum), cold climate catchment. Twenty-three water samples, collected during six discrete runoff events spanning 1 year, were analyzed for total and dissolved heavy metal concentrations. The most abundant pollutants were Zn (46 μg/L), Cu (14 μg/L), and Cr and Ni (2.3–2.5 μg/L). Zn, As and Cd were predominantly dissolved (>70%), Ni was primarily bound to particulates (68%), and Cu and Cr were equally divided between phases. Elevated dissolved and total concentrations were measured during snowmelt in correlation with high electrical conductance reflecting the use of road salt as a deicing agent. Event mean concentrations were positively correlated to wind speed and negatively correlated to rainfall depth in the two antecedent weeks. A mild first flush of dissolved metals was observed in most events, with 40–50% of pollution mass being discharged in the first 30% of runoff volume. Particulate-bound heavy metals, however, only exhibited first flush during intensive snowmelt. A strong linear correlation was observed between particulate-bound heavy metals and total suspended solids (TSS) (R²=0.8–0.9), corresponding to 0.45, 0.22, 0.05, and 0.06 μg/mg TSS slope for Zn, Cu, Cr, and Ni, respectively. Overall, the pollution levels were moderate compared with residential catchments in northern Europe and North America, which may constrain runoff treatment efficiency.