Hydrological Conditions for Contaminant Leaching Through Highway Swales

Abstract

Highway runoff contains pollutants in concentrations that can harm ecosystems of receiving freshwaters. To suggest control measures the hydrological conditions for pollutant transport from road edge to receiving water, and the associated retention mechanisms must be identified. Often, the transfer of highway runoff to receiving water bodies is accomplished by means of roadside swales equipped with gullies and underdrains. It is well documented that infiltration into the swale constitutes the primary runoff removal mechanism, while surface flow across and along the swale is mainly restricted to heavy storm episodes. Dye tracing at four Danish highways was used to describe the hydrological conditions for leaching of highway runoff pollutants through highway swales. At all sites the surface layer of the exposed swale profiles consists of a homogeneous sandy material deposited after construction of the highway, reaching a thickness correlated with, among other things, the road age. The dye solution infiltrated these deposition layers uniformly, replacing all antecedent soil water. The below-swale soil profiles were percolated through more preferential pathways, but contrary to the expectation no earthworm burrows or other biopores played a part. At two sites the preferential flow patterns resembled macroporous flow through physical voids in heterogeneous construction materials (stony loam) and at borders between construction materials with different texture (gravel, clayey till, fine and coarse sand). At the other two sites, which were built almost exclusively from homogeneous sand, the flow patterns resembled fingered flow initiated around thin, deep-running roots from the surface vegetation. The contaminant transport consequences of these flow patterns are discussed.