Partitioning of Mercury into Several Size Fractions in Highway Runoff

Abstract

Due to their large impervious coverage, highways are ideal sites for the transport of pollutants to either treatment units or direct discharge to waterways. Mercury, a highly toxic and ubiquitous environmental contaminant, is rarely studied in stormwater runoff. Removal of particulate-bound mercury in a dry detention basin was analyzed for the five size fractions: <0.45 μm, 0.45–8 μm, 8–20 μm, 20–100 μm, and >100 μm. The highest concentration of mercury for both influent and effluent is observed in the 8–20 μm size fraction. Treatment units are often characterized by suspended solids removal efficiency and particle-bound pollutants correlated to suspended solids removal; however, different suspended solids removal efficiency may occur in specific particle size fractions. Therefore, the benefits of treatment units may be different than originally anticipated, especially if the particle-bound concentrations are greater in fractions more efficiently removed. For sedimentation-based treatment units, the removal is dependent on size fraction, and when one size fraction is captured more efficiently, the removal rate calculated using the entire sample might indicate particle-bound pollutant removal quite different than actually occurring.