Mass budget and dynamics of polycyclic aromatic hydrocarbons in the Mediterranean Sea

Abstract

A mass budget was constructed to examine the status and dynamics of polycyclic aromatic hydrocarbons (PAHs) in the western Mediterranean Sea. Using gas chromatography-mass spectrometry, 11 PAHs have been quantified in atmospheric aerosols, rivers and seawater, sediment cores and sediment trap samples. Total PAH concentrations in Mediterranean aerosols range from 0.2 to 2 ng m−3, with 50–70% associated with the sub-micron particles. Maximum PAH concentrations were observed in winter when the concentrations were double those recorded in the spring. Total PAH inputs from the atmosphere were estimated to be from 35 to 70 t year−1 with a mean value of 47.5 t year−1 (wet/dry mean ratio of ∼2–3). Atmospherically-deposited PAH are dominated by the benzofluoranthenes. The total PAH riverine inputs amount to about 5.3−33 t year−1 from the Rhone river and 1.3 t year−1 from the Ebro river. The difference in these riverine fluxes is due to differences in annual water discharges and upstream land use. The total PAH accumulation rate in surficial sediments in the whole basin is estimated at 182 t year−1. Nearly 50% of the total PAHs accumulate in the 0–200 m water depth area supporting the importance of the coastal zone as a trap of terrigenous material and associated contaminants. Sediment trap experiments gave a mean residence time in the water column of total PAH (considering only particle settling) of 11 years, with higher residence times for high molecular weight PAHs. This supports the hypothesis that lower molecular weight PAHs are more efficiently removed from the water column. Water exchange resulted in a net outflow of 20 t year−1 and 2 t year−1 through the Gibraltar and Sicilian Straits, respectively. Atmospheric deposition and the Rhone River are the major contributors of PAH in the western Mediterranean. Sedimentation was identified as the major net output of PAH.