Comparison of the bulk deposition of PCDD/F in a spruce forest and an adjacent clearing

Atmospheric (bulk) deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) was measured in a spruce forest and in an adjacent clearing for one year. The annual bulk deposition flux of 2,3,7,8-Cl4DD toxicity equivalents (TEs) in the forest was twice that in the clearing. The homolog profile measured in the forest deposition closely resembles the profile that has been reported for German forest soils, while that in the clearing matches the agricultural soil profile. This is strong evidence that the elevated levels of PCDD/F present in forest soils compared to agricultural soils are due to atmospheric deposition processes. The bulk deposition fluxes of the higher chlorinated PCDD/F were similar at the two sites and the flux associated with litter fall was relatively low, indicating that the spruce forest does not significantly increase the deposition of particle bound organic contaminants. However, the annual deposition flux of the lower chlorinated PCDD/F was up to five times higher in the forest than in the clearing. The excess deposition in the forest was due in part to falling needles which had accumulated gaseous PCDD/F from the atmosphere. However, approximately half of the excess deposition of the lower chlorinated congeners could not be accounted for by needle fall. It is proposed that there is a further deposition mechanism whereby portions of the needle cuticle are discarded or eroded, carrying PCDD/F from the atmosphere that have accumulated in the cuticle to the forest floor.