DISSIPATION OF ATRAZINE ON A CLAY LOAM SOIL, ONTARIO, CANADA, 1986-90

Dissipation of atrazine and its metabolite, desethylatrazine (DEA), was followed and a partial mass balance was calculated, following applications to a clay loam soil at the Animal Research Centre Farm, Ottawa, Canada between 1986 and 1990. The decline in residue in the upper 15 cm of the soil was calculated following the applications on 30 September 1986, 6 June 1987 and 10 May 1989. Residues that leached down the soil profile and appeared in tile drainage water, and the watertable were determined. Atrazine and DEA declined rapidly over the summer months with a half-life disappearance of 37 and 64 days for the parent and 42 and 68 days for the parent plus metabolite, respectively following the 1987 and 1989 applications. When the dissipation period was extended to cover the winter months (22 months) the half-life disappearance increased to 125 days for atrazine alone and 137 and 141 days for the parent plus metabolite. Applied in the fall, September 1986, the half-life disappearance was 198 days for atrazine and 327 days for atrazine plus DEA. An estimated minimum of 3.2 to 6.5% of the applied atrazine leached below 15 cm and was detected in the soil profile to a depth of 60 cm. Atrazine and DEA were found in all samples of the tile drainage water collected from the 1.0 m depth and in all groundwater samples beneath the field between a depth of 1.2 and 4.6 m, the maximum depth sampled. It was estimated that 1.9% of that applied was lost in the tile drainage water following a fall application and up to 0.2% from spring applications. A further 0.3 to 0.6% of that applied was present in the water table at any one time. Losses in surface runoff waters were minimal (< 0.01%) because of the flat topography and conservation tillage.