Atrazine and carbofuran transport through the vadose zone in the Claiborneaquifer 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 trans port 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 anal yzed for atrazine, carbofuran, deethylatrazine (DEA), and deisopropylatrazi ne (DIA). Atrazine and carbafuran in the active root zone (< 100 cm) degrad ed 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 per iod from 1 to 30 days after application. On the average, by 30 days after a pplication 83% of the atrazine and 96% of the carbofuran had degraded. By 4 4 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 wa s carbofuran with a half life approximately twice that for carbofuran. A tw o-stage model with a variable dissipation rate for the period up to 44 days after pesticide application and a second dissipation rate far periods grea ter than that was found to fit the data better than a single stage model. F or the first 44 days after application, the first-order decay rate with a h alf 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 (da ys(-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 h ighest in the top 1 cm of the soil. There appeared to be some movement or c reation of the metabolites at lower depths in the profile later in the grow ing season, bur not at large concentrations.