Fate and behavior of pollutants in a vegetated pond system for road runoff: pollutants in vegetated ponds

Road runoff contains a variety of contaminants that threaten aquatic systems. Pollutant mitigation is therefore an important function of sustainable drainage systems such as vegetated ponds. Vegetated ponds have a variety of treatment mechanisms that potentially can remove different pollutants. However, design is predominantly based on “black box” approaches. This study monitored a swale and vegetated pond receiving road runoff over two years to investigate the interaction of various pollutant removal processes. Storm runoff, pond water, and sediments were monitored for general water quality, metals, hexane extractable hydrocarbons and polycyclic aromatic hydrocarbons (PAHs). A non-exponential “first flush” was apparent for biochemical oxygen demand, chemical oxygen demand, and solids with higher concentrations seen during the early stages of storms; however, for hydrocarbons and PAHs peak concentrations were often seen later in storm events. Due to interference from natural compounds, PAHs are probably more appropriate than hexane extracts for tracking runoff pollutant fate. Pyrene, despite having low concentrations in water, accumulated in soils and sediments (median = 850 µg/g). Naphthalene was more mobile, but was effectively removed in the pond. Metals had similar concentrations to other studies, with large removals of particularly Cu and Zn. However, metals exhibited different deposition patterns, with Zn and Cu accumulating in pond inlet sediments and Ni and Cr accumulating at the outlet. The different behaviors and fates of pollutants means that refining design guidance is difficult and requires consideration of how specific mechanisms may be enhanced for prioritizing removal of pollutants that pose the greatest risk.