The Occurrence Level, Transport And Ecological Risk Assessment Of PAHs In Urban Rainfall Runoff And Dust

Accompanied by the rapid urbanization and modernization processes, large amounts of pollutants produced by traffic and industrial activities were poured into urban environments and thus laid a basis for rainfall runoff pollutions. The augments of urban impervious surfaces alter the hydrological pathway of rainfall into runoff, intensifying the discharge of area-source pollution into aquatic environment. The persistent organic pollutants (POPs) such as polycyclic aromatic hydrocarbons (PAHs) have received an increasingly considerable attention in terms of the prominent toxic, mutagenic and carcinogenic characteristics. Sixteen PAHs identified by the US EPA as priority pollutants were selected as research subjects in this paper. The pollution characteristics in combination with runoff processes, source apportionment and ecological risk assessment of PAHs in urban rainfall runoff were analyzed. Since the surface dust acted as the primary source of rainfall runoff pollution, the distribution and occurrence level of PAHs in size-fractionated dust samples were discussed. The accumulation and transport of PAHs among runoff, dust and rainwater were also studied.The PAHs loading measured in runoff from diverse land use types varied greatly with maximum and minimum concentrations of PAHs showed in Longwu Road and Campus Road, respectively. The values of event mean concentration (EMC) of PAHs and total suspended solids (TSS) in different sampling sites exhibited the similar spatial tendency in the following order:traffic road>residential road> residential roof>campus road. The spatial differentiation of EMC was in response to the buildup of pollutants in the dry period and the characteristics of watersheds. The calculated EMCs from different storm events showed the obviously temporal change with the decline from May to June and the rise in September.The washoff process of pollutants was affected by factors including runoff volume, pollutants accumulation and rainfall characteristics, etc. The higher pollutants loadings were detected to enrich in the early stage of runoff in various underlying surfaces, especially for the particle-bound PAHs, which was closely related to the length of dry period. The pollutants loadings were largely influenced by the variations of watersheds characteristics in different sites, as well. An unimodal pattern from different rainfall events was presented in both dissolved and particle-bound PAHs due to the impact of rainfall characteristics on runoff processes. In terms of first flush effect (FFE), as the result of rainfall intensity and runoff volume, the magnitude of FFE was showed on different levels corresponding to various storm events and sampling sites.As to the management of road runoffs, the relationship between PAHs loading and runoff volume was analyzed and the satisfactory linear relationship was found with a fact that PAHs compositions concentrated in a lesser portion of runoff. Based on the thought that the treatment of entire storm runoff was impossible along with a consideration of the FFE, it could be assumed that the application of FFE was of great significance. And the effectiveness factor E(V) was defined to quantitatively assess the magnitude of the discharge treatment in early runoff. A BaP discharge limit-based model was applied with a finding that the entire runoff needed to be treated in studied storm events. The partition coefficients of PAHs between the aqueous and particulate phases in runoff were highly related with the concentrations of SS and that the partition coefficients decreased with the SS contents of samples.Surface dusts were washed off by the rainfall runoff and became a major source of PAHs in runoff. The total concentrations of PAHs in dust samples varied in different functional area. The maximum and minimum PAHs contents presented in industrial areas and residential areas, respectively. It has been revealed that dust accumulation differed in various sites as well. The transferability and removal efficiency of dust particles were constrained by the particle characteristcs like density and size. As to the mass fractions of road dust particles of four size fractions, particles of 53～150μm account for the largest part, while particles of 250～425 μm account for the least. The PAHs contents of dust samples, which was closely correlated with the property and density of dust, showed that the highest contents occurred in the size-fractioned particles of less than 53μm and lowest in the size of 250～425μm in all investigated areas.The PAHs in runoff and dust samples were detected as petroleum and pyrogenic-derived pollution. According to the correlation analysis, the results based on the correlation coefficient matrix among the individual PAH as well as ∑PAHs differed for dust and runoff samples. The ecological risk assessment indicated that the toxicity of runoff varied greatly in sampling sites. The potential risk was posed to aquatic organisms by calculating the hazard quotient of the 8 selected individual PAH. The values of toxic equivalents (TEQ) were higher in dust samples from industrial area than that in residential area. The differences of TEQ were also observed in particles with diverse sizes. The estimated lifetime cancer risk for exposing to dust due to dermal contact was relatively high. The total cancer risk exceeded 10-5 in traffic and industrial area, which had posed the potential risk.Rainfall events had a great impact on the accumulation of road dusts and PAHs contents. By comparing the distribution of PAHs within the road dusts, rainfalls and surface runoffs, it was found that the PAHs contents in the surface runoff came primarily from the PAHs within dust particles, while wet precipitations only contributed a few.