Road sediment: characterization and implications for the performance of vegetated strips for treating road run-off.

Increasing pressure to mitigate pollution from urban stormwater and road run-off is generating interest in vegetative treatment systems. The treatment performance of these systems depends on the characteristics of the pollutants entering them--for example, whether metals are dissolved or particle bound, and the particle size. This study sought to provide information on the characteristics of sediment derived specifically from road-use activities. A length of roadside gutter was vacuumed repeatedly at 2-day intervals. The samples obtained were found to contain predominantly fine particles (52% <250 microm). Particles <250 microm had the highest metal contents: 181-212 mg Cu/kg, 1073-2080 mg Zn/kg and 251-334 mg Pb/kg. A high percentage of the total metal load was associated with particles smaller than 125 microm (64% of Zn, 57% of Cu and 46% of Pb). These <125-microm particles are generally predicted to be poorly trapped by vegetation. In this study, these smaller particles were also found to have a lower density (<2200 kg/m3) than normally modelled for sediment (>2600 kg/m3), and this may further reduce predicted trapping efficiencies. The impact of sediment entering vegetative treatment systems via aerial deposition is also discussed in terms of evaluating such systems for treatment performance.