TRANSPORT AND PERSISTENCE OF PESTICIDES IN ALLUVIAL SOILS - II - CARBOFURAN

Carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate), a soil-applied insecticide, is a potential leaching risk when applied to permeable soils overlying shallow, unconfined aquifers. We conducte d this project to evaluate the long-term movement, persistence, and le aching risk of carbofuran applied to perennial small fruit crops grown on alluvial soils with shallow (<3 m deep) groundwater. Carbofuran wa s applied to strawberry (Fragaria x ananassa Duch.) and red raspberry (Rubus idaeus L.) in paired plots at two sites in western Washington. Strawberry received carbofuran at 2.2 kg ha(-1) in a 30-cm wide band e ach August, and raspberry received 3.1 kg ha(-1) in a 90-cm band each November. We applied carbofuran yearly from 1986 to 1989, and sampled soil (to 180 cm) and shallow groundwater monthly until April 1991, and at one site again in March 1994, Carbofuran was initially mobile, and significantly more carbofuran moved below 60 cm in raspberry than in strawberry, Differences in carbofuran leaching between sites correspon ded to differences in soil-binding constants. The disappearance half-l ife of carbofuran decreased from 110-221 d in 1986-1987 to 69-100 d in 1989-1990, evidence of enhanced degradation. Enhanced degradation app eared to affect freshly added carbofuran, but not residual carbofuran that had become more tightly bound to the soil over time. Carbofuran w as found in shallow groundwater beneath both crops and sites the first winter. The leaching risk declined after the first winter, which is p robably a result of enhanced degradation and nonequilibrium binding.