Polycyclic aromatic hydrocarbons in hydromorphic soils of the tropical metropolis Bangkok

The tropics are assumed to contribute to global contamination with PAHs. However, little is known about the state of pollution in tropical environments. This study aimed to examine concentrations, compositions, and possible sources of 20 PAHs in Bangkok surface soils. We sampled 30 hydromorphic urban soils at 0-5 cm depth. The sum of the 20 PAH concentrations ranged between 12 and 380 mu g kg(-1); far lower than in soils of temperate cities. The reasons may be a shorter accumulation time, the tropical climate enhancing biodegradation, volatilisation, and photo-oxidation, and leaching into the groundwater. The most abundant compounds were naphthalene (on average 14% of the total PAH concentration) > benzo(b + j + k)fluoranthenes (11%)> perylene (10%) but not fluoranthene, pyrene, and chrysene like in many temperate soils. Based on a principal component analysis it may be distinguished between naphthalene, phenanthrene, and perylene and all other compounds, The former compounds were assigned to the same principal component as the ratio of oxalate- to dithionite-extractable Fe. This ratio is a measure of the crystallinity of Fe oxides and thus an indicator for the frequency of reducing conditions. The result suggested that naphthalene, phenanthrene, and perylene had common sources possibly related with anaerobic conditions. While perylene is known to be produced biologically in reducing soils and sediments, there is no clear evidence for biological formation of naphthalene and phenanthrene. (C) 1999 Elsevier Science B.V. All rights reserved.