Comparison of the heavy metal content of motorway stormwater following discharge into wet biofiltration and dry detention ponds along the London Orbital M25/ motorway

The Surrey section of the London Orbital M25 motorway uses mainly detention pond facilities for the treatment of stormwater runoff. A majority of these implement the use of dry detention basins. However, in a few locations biofiltration facilities operate through the use of reed bed systems. An assessment of the removal efficiencies for both wet biofiltration and dry pond treatment facilities was undertaken. Motorway-derived contaminants, including V, Cr, Mn, Co, Ni, Cu, Zn, Mo, Cd, Sb and Pb, were measured in unfiltered stormwater collected during the initial stages of a storm event using inductively coupled plasma mass spectrometry ICP-MS.. Results suggest that a higher level of motorway-derived heavy metal contamination exists in stormwater runoff from a road section with a higher average daily traffic density. In addition, a comparison of both sites shows a higher percentage removal efficiency of heavy metals in stormwater from the biofiltration facility.