Changes in the polynuclear aromatic hydrocarbon content of wheat grain and pasture grassland over the last century from one site in the U.K.

Crops harvested and stored from two long-term agricultural experiments started in the 1840-1850s at Rothamsted Experimental Station (U.K.) have been analyzed for polynuclear aromatic hydrocarbons (PAHs) because PAHs have increased four-to-five fold in the underlying 0-23 cm depth of soil over the last century due to atmospheric deposition. Bulked samples of herbage and wheat grain were analyzed for 16 compounds by gas chromatography-mass spectrometry for groups of years between 1860 and 1986. The sigma PAH (16 PAH) concentrations in herbage ranged between 110 and 6900 micrograms kg-1 dry weight, with fluoranthene, pyrene, benzofluoranthenes (b + j + k), chrysene and triphenylene consistently the most abundant. Benz[a]anthracene, benzo[ghi]perylene, benzo[a]pyrene, benzo[e]pyrene and indeno(1,2,3-cd)pyrene were also all important constituents. PAH concentrations in wheat grain ranged from 4 to 46 micrograms sigma PAH kg-1; the mixture of individual compounds was similar to that in herbage. Correcting for yield differences these corresponded to 37-1500 and 2.4-8.8 micrograms sigma PAH m-2 for herbage and wheat grain respectively. The lowest values were found in the most recent samples (1980s) and the highest for samples collected in 1879-81. Generally, there was a decline in the PAH burden of the vegetation with time. The temporal trends of PAHs in the samples are discussed in the context of possible sources of PAHs. Various lines of evidence indicate that the PAH concentrations in vegetation are an indication of atmospheric deposition directly onto the above-ground plant portions, and that uptake and translocation of soil-bound PAHs is of minor importance.