[Residues of organochlorine insecticides in sediments of a high estuary of Fleuve St-Laurent].

In order to determine the distribution and levels of organochlorine insecticide residues in sediments of the upper estuary of the St. Lawrence River, we have carried out the first analyses in Quebec of these compounds in samples from different parts of this region. The recovery of these residues was conducted by continuous Soxhlet extraction of the samples with 100 ml of a mixture of acetone and n-hexane (50:50). The insecticide-containing extract was cleaned up by column chromatography on activated florisil. Elution with 100 ml of n-hexane and then 150 ml of a mixture of n-hexane and ether (85:15) gave two fractions. The analyses were performed by gas liquid chromatography using an electron-capture detector with a radioactive 63Ni source. In the two series of samplings (June and July 1973), DDT and its metabolites were detected in appreciable amounts at nearly all sampling sites while the other insecticides: alpha-BHC, gamma-BHC, heptachlor and dieldrin, were found only in a few and at very low concentrations: beta-BHC, aldrin and heptachlor epoxide were absent at all sites. In general, the insecticide levels at the various sites show a slight variation in time. However, in some, the concentration of organochlorine insecticides is much lower than in others; in these cases it is presumed that this is due to a greater rate of sedimentation in the absence of current and also to local agriculture and sediment texture.