ATRAZINE AND CARBOFURAN TRANSPORT THROUGH THE VADOSE ZONE IN THE CLAIBORNE AQUIFER RECHARGE AREA

A 1-ha field plot with a sandy surface soil, located near Plains, Georgia, was studied for three years (from 1993 to 1995) to evaluate pesticide transport in the vadose zone. Vadose zone soil samples were collected 23 times: prior to the initial 1993 pesticide application, each year at approximately 1, 3, 7, 14, 28, and 44 days after pesticide application, each fall after harvest, and in the spring of 1995 prior to planting. The samples were analyzed for atrazine, carbofuran, deethylatrazine (DEA), and deisopropylatrazine (DIA). Atrazine and carbofuran in the active root zone (< 100 cm) degraded rapidly. Overall, the higher concentration levels of atrazine, DEA, DIA, and carbofuran were limited to the top 25 cm of the profile and to the period from 1 to 30 days after application. On the average, by 30 days after application 83% of the atrazine and 96% of the carbofuran had degraded. By 44 days after application, virtually all of the pesticides in the top 250 cm of the soil had degraded. Atrazine was found to be more persistent than was carbofuran with a half life approximately twice that for carbofuran. A two-stage model with a variable dissipation rate for the period up to 44 days after pesticide application and a second dissipation rate for periods greater than that was found to fit the data better than a single stage model. For the first 44 days after application, the first-order decay rate with a half life of 12 days was found to fit the field data for atrazine within the soil profile. A first-order decay rate with a half life of approximately 6 days fit the observed carbofuran data best. The dissipation rate decreased rapidly after the first 44 days. When a two-stage dissipation process was assumed, the dissipation rate coefficient decreased from 0.059 to 0.006 (days -1 ) for atrazine, while for carbofuran it decreased from 0.110 to 0.018 (days -1 ). Observed levels of the atrazine metabolites DIA and DEA were highest in the top 1 cm of the soil. There appeared to be some movement or creation of the metabolites at lower depths in the profile later in the growing season, but not at large concentrations. Keywords. Soils, Aquifers, Pesticide transport, Water quality, Atrazine, Carbofuran.

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