Polynuclear aromatic hydrocarbons in an agricultural soil: long-term changes in profile distribution.

Soil profile samples collected from the same plot at Rothamsted Experimental Station in southeast England in 1893, 1944 and 1987 have been analysed for polynuclear aromatic hydrocarbons (PAHs). The total PAH burden of the plough layer (0-23 cm) has increased approximately four- to five-fold since the 1890s, with some compounds (notably benzo(b)fluoranthene, benzo(a)pyrene and pyrene) showing substantially greater increases. Average rates of increase in the plough layer for individual PAHs in the Rothamsted plots over the century since c. 1890 vary between 0.01 and 0.67 mg m(-2) year(-1). It is concluded that atmospheric deposition from natural sources has been augmented in recent years by regional fallout of anthropogenically-generated PAHs derived from the combustion of fossil fuels. The total PAH content of the 1893 Rothamsted samples was similar to that observed in soils from contemporary isolated/rural locations in the UK and showed little surface enrichment. By 1987 the surface soil at Rothamsted had been enriched in all PAH compounds measured by a factor of between 1.3 (acenaphthalene) and over 20 (benzo(a)pyrene). Increases in the PAH content of the 23-46 cm subsurface layer indicate migration of PAHs from the plough layer. Net average annual migration rates ranged from 0.01-0.14 mg m(-2) year(-1) for individual PAHs, and the rate appeared to be primarily a function of the plough layer PAH content, rather than physical/chemical properties of the individual compounds. This suggests particle-bound translocation as the dominant mechanism of PAH migration.