PAHS IN STREAM SEDIMENTS FROM RIO BRANCO URBAN AREA (SãO FRANCISCO STREAM - AMAZON BASIN, BRAZIL)

The scientific research on stream sediments from riverine systems has long been used as an exploration tool and also for environmental purposes, providing information on weathering and transport processes, pollutant sources, emissions pathways, distribution, and fate of contaminants. In recent sediments, hydrocarbons may derive from natural (produced during thermal maturation of the organic matter – petrogenic source) and anthropogenic sources (domestic and industrial waste, by-products of incineration of industrial and urban wastes, oil spills, combustion of fossil fuels and biomass, among others) and therefore, can enter the aquatic systems directly by deposition of airborne particles, leaching, spillage, accidental release and natural oil seeps, or indirectly through sewers, urban and highway runoff. Once in the aquatic environment, hydrocarbons rapidly associate with particulate matter and then deposit in bottom sediments of surface water bodies and groundwater systems.

The main objectives of the present study are: (i) to identify and quantify the 16 priority PAHs in the stream sediments from São Francisco stream in the area of Rio Branco city (State of Acre, Brazil); and (ii) to evaluate the environmental risk of contamination with PAHs.

In the studied samples, the most abundant PAHs are fluoranthene and benzo[a]anthracene, followed by minor proportions of chrysene, benzo[ghi]perylene and anthracene. The highest values are reported for the right bank of downstream point of Rio Branco city, where the anthropogenic influence is more significant. The concentration of PAHs in the sediments does not represent environmental concerns when considering the Canadian sediment quality guidelines and the Brazilian legislation. However, the predominance of high molecular weight PAHs comparatively with the low molecular weight PAHs and the ratio benzo[a]anthracene/ chrysene indicate that PAHs in sediments come from combustion sources (combustion of biomass in forest and farmers fires and/or combustion of fossil fuels). The industrial activities, mainly those related with wood, should also be considered as a source of PAHs.