Attenuation of Roadway-Derived Heavy Metals by Wood Chips

Wood chips were evaluated for their ability to attenuate heavy metals in roadway runoff. Column experiments with controlled synthetic runoff composition and flow rate were used to assess effects of flow rate (intercepted sheetflow from a 3-m wide roadway section), runoff salt concentration, wood exposure to alternating wetting and drying cycles, wood aging, competition among dissolved heavy metals, and removal of particle-associated heavy metals. Overall, wood chips damped the “pulse” of copper in the synthetic runoff such that the effluent was characterized by lower concentrations (3–25% of input) over longer periods of time, but with little retention of the total copper mass. The most effective treatment was wood chips aged up to 9 months. Increased aging and chip water content reduced effluent concentrations, relative to no treatment. Flow rate had no effect on effluent concentrations. The presence of salt (>2?mS/cm) or dissolved lead (500?μg/L) in the runoff caused greater copper effluent concentrations than the no treatment case. Removal of suspended particles (and associated contaminants) was greater than 85% with an estimated capacity of 0.16?g/gwood. Field evaluation with concentrated flow to a gutter containing a wood chip treatment showed little effect on total or dissolved copper and zinc runoff concentrations and indicated that wood chips may be a source of contaminants in subsequent storm events. Applications of wood chips to treat roadway runoff would not provide a significant decrease in total maximum daily load contributions (e.g., kg/d); however, there may be some scenarios for which wood chip treatments to decrease peak storm water concentrations of dissolved heavy metals in sheetflow runoff is desirable.