A source mixing model to apportion PAHs from coal tar and asphalt binders in street pavements and urban aquatic sediments

Abstract

Present-day and more than 30 years old road and footpath pavements from Auckland, New Zealand were analysed for PAHs to test the hypothesis that coal tar based pavement binders contribute to unusually high PAH concentrations in adjacent stream and estuarine sediments. Total PAH (∑ 28PAH) concentrations in the dichloromethane-soluble fraction (“binder”), comprising 5–10% of pavement mass, were as high as 200 000 mg kg −1 (10 000 mg kg −1 in binder + aggregate). Older and deeper pavement layers were strongly pyrogenic, whereas pavement layers from recently sealed roads had a more petrogenic composition and more than 1000 times lower ∑ 28PAH concentrations. Source identification analysis using three PAH isomer ratio pairs (benz(a)anthracene/(benz(a)anthracene + chrysene); benzo(a)pyrene/(benzo(a)pyrene + benzo(e)pyrene); and indeno(1,2,3-cd)pyrene/(indeno(1,2,3-cd)pyrene + benzo(g,h,i)perylene) revealed low PAH (bitumen) pavements to have consistently lower isomer ratios than high PAH (coal tar) samples. Moreover, pavement data for one isomer ratio (e.g. benzo(a)pyrene/(benzo(a)pyrene + benzo(e)pyrene) were highly correlated with those of another isomer ratio (e.g. benz(a)anthracene/(benz(a)anthracene + chrysene) and were bounded at their lower and higher extremes by the characteristics of pure bitumen and coal tar, respectively, suggesting that PAH composition of a given pavement sample could be accounted for by conservative mixing between coal tar and bitumen as source materials. A concentration-weighted mixing model, with coal tar and bitumen as source materials, explained more than 80% of the variance in isomer ratios and enveloped the entire PAH compositional and concentration range encountered. PAH composition and concentrations in adjacent stream sediments (> 15 mg kg −1 dry weight) were consistent with diluted coal tar material as a principal PAH source. Due to the very high PAH concentrations of coal tar, a coal tar content of as little as 0.01% of total sediment mass can account for more than 90% of PAH concentrations in adjacent stream sediments.