Biodegradation rates of polycyclic aromatic hydrocarbons (PAHs) in spilled oil stranded on tidal flats were studied using model reactors to clarify the effects of NAPL on the biodegradation of PAHs in stranded oil on tidal flat with special emphasis on the relationship between dissolution rates of PAHs into water and viscosity of NAPL. Biodegradation of PAHs in NAPL was limited by the dissolution rates of PAHs into water. Biodegradation rate of chrysene was smaller than that for acenaphthene and phenanthrene due to the smaller dissolution rates. Dissolution rates of PAHs in fuel oil C were smaller those in crude oil due to high viscosity of fuel oil C. Therefore, biodegradation rates of PAHs in fuel oil C were smaller than those in crude oil. Biodegradation rates of PAHs in NAPL with slow decrease rate like fuel oil C were slower than those in NAPL with rapid decrease like crude oil. The smaller decrease rate of fuel oil C than crude oil was due to higher viscosity of fuel oil C. Therefore, not only the dissolution rate of PAHs but also the decrease rates of NAPL were important factors for the biodegradation of PAHs.

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