Field research studies on the movement and degradation of thiodicarb and its metabolite methomyl

Abstract

The movement and degradation of thiodicarb and its metabolite methomyl were measured at three locations in the United States. At a site near Clayton, North Carolina, soil samples were used to determine the degradation rate of thiodicarb in surface soils and methomyl in both surface and subsoils. At sites near Palermo, New York, and Oviedo, Florida, the shallow groundwater was monitored following multiple foliar applications of thiodicarb. These groundwater monitoring studies were conducted under conditions favorable to movement of pesticides to groundwater since surface soils were loamy sand or sand, subsoils were sand, water tables ranged between 0.1 and 1.8 m, and rainfall was supplemented by irrigation to ensure that the sum of irrigation and rainfall was at least 1.5 times the normal monthly rainfall. In these studies the half-life of thiodicarb was only a few hours in surface soils, while the half-life methomyl was about two days in surface soils and about 0.5 to 1.6 months in subsoils. Although methomyl residues were detected in shallow groundwater in the Oviedo and Palermo studies (designed to be representative of worst case situations), these residues were present in only one well cluster and one sampling interval, and at concentrations substantially below the established health advisory level.