Polynuclear aromatic hydrocarbons in a River Thames sediment core

The abundance of 14 polynuclear aromatic hydrocarbons in a sediment core from Tilbury docks on the River Thames are reported. Twenty one samples in the depth range -4.3m to -8.4m OD were analysed. PAH concentrations were determined by HPLC after extraction from sediment with supercritical carbon dioxide modified with toluene. PAH extraction efficiencies achieved by supercritical fluid extraction were shown to be comparable to those for an established Soxhlet extraction method. Total concentrations of the 14 PAH in the core showed a sharp increase in sediments below -5.8 m OD (average 24.4 mu g g(-1) sediment dry weight) compared to sediments above -5.8 m OD (average concentration 12.0 mu g g(-1)). Similar increases have previously observed in polychlorinated biphenyl, organochlorine insecticide and metal concentrations in the same sediment samples. These trends have been interpreted to result from reduced contaminant input into the River Thames due to improvements in sewage treatment works discharging into the river in the late 1950s and early 1960s. Significant changes in the ratios of some individual PAH (B[ghi]P/B[e]P; Py/B[e]P; Fl/Py) were observed in the upper and lower sections of the core. These changes may reflect a decreased contribution of PAH derived from coal combustion in sediments deposited after 1966, however, effects due to improvements in sewage treatment cannot be discounted.