Polycyclic aromatic hydrocarbons in San Francisco Estuary sediments

The objectives of this study were to examine surface sediments in the San Francisco Estuary for PAH composition over a range of spatial and temporal scales to determine distributions, trends, and possible sources. Surface sediments (top 5 cm) were collected at 26 sites from 1993 to 2001. The mean total PAH (∑PAH) concentration in sediments was spatially distributed as Central Bay (230 mg/kg TOC), South Bay (217 mg/kg TOC), North Estuary (96 mg/kg TOC), Extreme South Bay (87 mg/kg TOC), and Delta (31 mg/kg TOC). Overall, the mean ∑PAH concentrations were significantly higher in the Central Bay and South Bay segments compared to the North Estuary, Extreme South Bay and Delta segments, and the Delta was significantly lower than all other segments (Kruskal–Wallis, H=156.94, df=4, p=0.000). In addition, no significant difference in ∑PAH concentration was found between the Central Bay and South Bay. This distribution reflects the large amount of urbanized and industrialized areas that surround the Central Bay and South Bay compared to the less urbanized and rural areas surrounding the Delta. Temporal trend analysis showed a statistically significant temporal trend in ∑PAH concentration at only 1 of the 26 sampling sites located throughout the estuary (San Pablo Bay, significant decrease, p=0.024, r 2=0.314, n=16), which suggests that ∑PAH concentrations in the estuary generally remained constant from 1993 to 2001. Source analysis using PAH isomer pair ratios as indicators showed that PAH are derived primarily from combustion of fossil fuels/petroleum (gasoline, crude oil, and coal) and biomass (wood and grasses), with minor amounts of PAH derived from direct petroleum input.