Contamination of ponds by fenitrothion during forest spraying

Six ponds in areas of New Brunswick sprayed with fenitrothion for control of spruce bud worm were monitored for subsequent contamination. Spraying was carried out twice, at 5-10 d intervals and it was observed that the aircraft did not stop spraying when approaching the ponds so that all ponds received direct application of the insecticide. Tabulated data are given on the characteristics of the ponds and environmental conditions at the time of spraying and on the fenitrothion concentrations in pond outflow at periods of 15 minutes, 30 minutes, 1 h and 2 h after spraying. The maximal mean surface-water concentrations were within the range of 96 h LC50 values for salmonid fish, but were rapidly attenuated by dilution and degradation. Although the concentrations in the outflow were much lower, in some cases they exceeded the 20 ug per litre threshold value for benthic invertebrates. There was also evidence that residual concentrations in streams receiving pond outflow remained high for period longer than normal in lotic systems.