T he southern Coastal Plain of the U.S. is a physiographic region with a wide range of soil types and crop management systems. The region’s abundant rainfall and long growing season are conducive to intense crop production. Studies conducted by scientists at the Southeast Watershed Research Laboratory (SEWRL) Tifton, Georgia, and the USGS, Georgia District, have revealed significant pesticide and pesticide metabolite concentrations in area aquifers (Bosch et al., 1997; Leonard et al., 1979; Leonard et al., 1988). The Clayton and Claiborne aquifers of southwestern Georgia are important sources of groundwater, supplying the majority of municipal, industrial, agricultural, and domestic water for the area (McFadden and Perriello, 1983). In this area, the sedimentary deposits in which these aquifers reside gently dip to the southeast from the central portion of Georgia. These aquifers outcrop and are recharged in a belt trending southwest-northeast through the center of the state (Beck et al., 1985). Because of this, the land use in this area is of great importance to the citizens of South Georgia and North Florida. Losses of agricultural chemicals from the root zone are important relative to both crop production and environmental quality. When agrichemicals leave the root zone they are no longer available for crop protection and growth and may negatively impact surface or subsurface water quality. Transport of agrichemicals depends on rainfall amount, intensity, and duration, chemical solubility and dissipation properties, soil properties, biological processes, and management practices. A scientific understanding of these processes requires complex well defined research which quantifies each of these factors. Two pesticides of particular interest are atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methyl carbamate). Atrazine is widely and extensively used in many parts of the world for the control of a variety of weeds. Carbofuran is an insecticide used for insect, mite, and nematode control in maize and other crops. Reported soil organic carbon sorption coefficients for atrazine range from 38 to 174 (mL g –1 ) while measured field dissipation half lives range from 18 to 120 days (Hornsby et al., 1996). Field dissipation includes volatilization, degradation, and transformation, and is a function of annual variations in management and climate. Carbofuran is generally A TRAZINE AND CARBOFURAN TRANSPORT THROUGH THE V ADOSE ZONE IN THE CLAIBORNE AQUIFER RECHARGE AREA D. D. Bosch, C. C. Truman, R. A. Leonard ABSTRACT. 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. Article was submitted for publication in March 2000; reviewed and approved for publication by the Soil & Water Division of ASAE in September 2000. Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the products listed by USDA. All programs and services of the U.S. Department of Agriculture are offered on a nondiscriminatory basis without regard to race, color, national origin, religion, sex, age, marital status, or handicap. The authors are David D. Bosch, ASAE Member Engineer, Research Hydraulic Engineer, Clinton C. Truman, Soil Scientist, and Ralph A Leonard, Retired Soil Chemist, USDA-ARS, Southeast Watershed Research Laboratory, Tifton, Ga. Corresponding author: D. D. Bosch, Southeast Watershed Research Laboratory, PO Box 946, Tifton, GA 31793, phone: 912.386.3899, fax: 912.386.7294, e-mail: <dbosch@tifton.cpes.peachnet.edu>. Transactions of the ASAE 2000 American Society of Agricultural Engineers 1609 VOL. 43(6): 1609-1620 sw 3546.ms 7/9/01 3:22 PM Page 1609