Distribution and Transformation of Fenitrothion Sprayed on a Pond: Modeling Under Uncertainty

Abstract

Fenitrothion* was used during the period 1969–1977 in New Brunswick, Canada, to control the spruce budworm (Choristoneura fumiferana [Clemens]) in the province’s forests. Millions of hectares were sprayed annually with 150–300 g active ingredient per hectare. The routes and rates of its environmental transformation and disappearance are subjects of much interest (National Research Council of Canada, 1975, 1977). Maguire and Hale (1980) recently reported on the aquatic fate of fenitrothion. Surface water microlayer, subsurface water, suspended solids, and sediment samples were collected from a small pond in a spruce-fir forest in New Brunswick before and after the aerial spraying of a fenitrothion formulation for spruce budworm control; the samples were then analyzed for fenitrothion and its degradation and transformation products. Fenitrothion concentrations in the surface microlayer, subsurface water, suspended solids, and sediment fell below detectable levels two days after the spray; the only identified products were p-nitro-m-cresol in water, which persisted less than two days, and aminofenitrothion (O,O-dimethyl-O-(p-amino-m-tolyl)phosphorothionate) in sediment, which persisted less than four days. Laboratory experiments showed that chemical hydrolysis of fenitrothion and volatilization of fenitrothion from true solution were both slow processes; however, volatilization of fenitrothion from surface slicks was very fast (t 1/2= 18 min at 20°C). Thus, a large fraction of the fenitrothion that reached the pond surface appeared to volatilize rapidly, while the fraction that remained in the water disappeared or degraded within a few days, largely through photolysis and microbial reduction.