Abstract
This thesis examines the geochemistry of polycyclic aromatic hydrocarbons (PAHs) in urban stormwater. The manner in which PAHs partition between the particulate, colloidal and truly dissolved phases is likely to affect their transport and fate in the environment and the degree to which they exhibit toxicity to aquatic organisms. Stormwater samples were collected from an urban catchment during eight rainfall events and from a river draining a largely rural catchment. During rainfall, the levels of PAHs and the heavy metals Pb, Cu and Zn (also important contaminants in stormwater) rose significantly and were correlated with the suspended sediment (SS) concentrations. PAH fingerprint ratios confirmed the debris collecting on urban streets as the principal source of the contaminants, which exhibited a signature typical of combustion-derived particles and motor vehicle crankcase oil. An old gasworks was an additional source of PAHs in the urban catchment. The quantities of PAHs and heavy metals discharged into the local harbour were much higher from the urban catchment, whereas the rural catchment contributed more SS. The partitioning of the PAHs between the SS and the water was similar to that previously reported for other aquatic systems. However, particulate organic carbon exerted little influence, suggesting that combustion-derived soot particles may play an important role. This study is the first to have investigated the in-situ partitioning of PAHs between the truly dissolved and colloidal phases (that bound to dissolved organic carbon (DOC)) in urban stormwater. The separation of the two phases, using a new C18 disk-based method, showed a small percentage of the total concentration of each PAH was bound to DOC. Differences in the partitioning between the urban and rural catchments were not explained by estimating the degree of aromaticity of the DOC, suggesting that aliphatic groups may also be involved in the PAH binding. Naphthalene bound to the DOC more strongly than expected, possibly because this smallest PAH was binding to sites inaccessible to the larger PAHs. The influence of the hydrophobicity of the PAHs was clearly evident when considering the partitioning between all three phases. The truly dissolved phase was dominated by the soluble low molecular weight (LMW) PAHs, whereas the particulate phase was enriched in the less soluble high molecular weight (HMW) PAHs. Contrary to what was expected based on the hydrophobicity of the PAHs, the importance of the colloidal phase decreased for the HMW PAHs because the particulate phase was a better sorbent. While the contaminant concentrations in the stormwater frequently exceeded relevant water quality guidelines, the levels in the bioavailable truly dissolved phase indicated that acute toxic impacts were unlikely. However, PAHs and zinc may lead to chronic effects in some organisms. The presence of moderate amounts of LMW PAHs in the truly dissolved and colloidal phases is likely to result in the addition of moderate quantities of PAHs to the harbour water column. Those bound to SS settle out rapidly once discharged into the harbour, resulting in localised sediment contamination which is likely to be exhibiting chronic effects on benthic organisms.