Sources of heavy metals and polycyclic aromatic hydrocarbons in urban stormwater runoff

Abstract

Polycyclic aromatic hydrocarbons (PAHs) and heavy metals were measured in road debris collecting in urban areas and in the suspended sediment (SS) component of runoff from two stormwater catchments in Dunedin, New Zealand. Levels in the road debris ranged from 119–527 μg/g for lead, 50–464 μg/g for copper, 241–1 325 μg/g for zinc and 1.20–11.6 μg/g for Σ16PAH. The SS from the largely rural catchment (20% urban) had similar concentrations to the road debris, indicating that this urban material was the main source of the contaminants measured in the stormwater. Similar PAH fingerprint profiles and isomer ratios indicative of dominant pyrogenic (combustion) sources were also found in these two groups of materials. The SS from the 100% urban catchment contained 2- to 6-fold higher concentrations of metals and 10-fold greater levels of Σ16PAH. The higher levels of lead and copper were probably a result of industrial land uses in this catchment, while the additional zinc was linked to an abundance of zinc-galvanised roofing iron in the catchment's residential suburbs. The PAH profiles and isomer ratios were different for this urban catchment and suggested that a disused gasworks was contributing PAHs to the stormwater runoff.

Introduction

Motor vehicle emissions, drips of crankcase oil, vehicle tyre wear and asphalt road surfaces, are all diffuse sources of chemical contaminants in urban environments. During rainfall, these contaminants are washed from roofs, roads and other surfaces into the stormwater system and then discharged into surface waterways and estuarine environments. Heavy metals (copper (Cu), lead (Pb) and zinc (Zn)) and polycyclic aromatic hydrocarbons (PAHs) are of particular concern in such runoff due to their prevalence, toxicity to aquatic organisms and persistence in the environment (Hoffman et al., 1984, Borchardt and Sperling, 1997, Walker et al., 1999). Other diffuse sources of PAHs and heavy metals include domestic fire emissions, the spillage or deliberate dumping of waste oil and the corrosion of roofing materials. Specific point sources, such as electroplating workshops, gasworks and commercial incinerators may also exist in urban catchments. Contaminant “fingerprinting”, involving the use of concentration data or ratios of specific contaminants can allow the potential sources of these contaminants to be identified (Walker et al., 1999, Gonzalez et al., 2000, McCready et al., 2000, Soclo et al., 2000).

Previous work on urban stormwater runoff from two catchments in Dunedin, New Zealand (Brown, 2002, Brown et al., 2003), indicated that significantly higher concentrations of Σ16PAH (sum of the 16 USEPA priority listed PAHs) and heavy metals were present in the runoff from an urban catchment (Portobello Road, see Fig. 1) as compared to a local river (Water of Leith) that has 83% of its catchment in rural land uses but receives stormwater discharges from several small urban sub-catchments. Surprisingly, calculations of the annual contaminant loading (kg/ha/yr) from each catchment into the Otago Harbour, Dunedin, New Zealand, showed that the Portobello Road catchment exported less suspended sediment (SS) but considerably more heavy metals and PAHs (up to 20 times more per hectare), even when the data for the Water of Leith catchment were normalised to the urban area of its small urban sub-catchments.

The present study investigated the sources of the PAHs and heavy metals in the two catchments using “fingerprinting” techniques and the likely cause(s) of the higher pollutant loadings from the Portobello Road catchment. The contaminant content of the stormwater SS was determined and then compared to that obtained from the debris collecting on the city roads and a number of other possible source materials.