DDT residues in forest floors and soils of western Oregon, September-November 1966.

Between 1945 and 1965, 1.82 million hectares, or about 17 percent of the total commercial forestland in Oregon, were treated with 2.02 million kg DDT. Detectable residues of this insecticide might be present in forest soils, even those which have never received a direct application of insecticide. Forest floor and mineral soil samples were collected along four east-west transects across the Coast and Cascade Ranges. DDT residues were found in all samples, even though all but one site had never received a direct application of insecticide. In the Coast Ranges, mean concentrations of sigma DDT in forest floor samples were 0.049 ppm at the coast and 0.047, 0.064, 0.075, and 0.119 ppm at 16, 32, 48, and 64 km inland, respectively. Mean residue levels in the surface layers of mineral soil were much lower, 0.009 ppm and 0.006 ppm in the 0 to 7.5-cm and 7.5 to 15-cm depths, respectively. Sampling sites along the Cascade Range transects were selected on the basis of elevation except that the eastern site of each transect was located 16 km east of the crest of the Cascades. Residue concentrations in forest floor samples were three to four times higher than in the Coast Ranges, but were still below 0.50 ppm. In general, sigma DDT levels increased with increasing elevation up to 1,372 meters and then decreased quite sharply east of the crest. Variations can be explained on the basis of total rainfall distribution and by transect location relative to agricultural and metropolitan centers.