Persistence of Fluridone in Small Ponds

Fluridone {1‐methyl‐3‐phenyl‐5‐[3‐(trifluoromethyl)phenyl]‐4‐[1H]‐pyridinone} was applied to three small ponds (5 by 3.5 m2) to give water concentrations of 5,000, 700, and 70 µg/liter, respectively. Water, hydrosoil (0 to 3 and 3 to 6 cm depth), duckweed (Lemna minor), and minnows (Pimephales promelas) were collected for residue analysis over a 70‐week period following herbicide application. The half‐life of fluridone in the water column (0.5 m depth) ranged from 4 (at 700 µg/liter) to 7 days (70 µg/liter). Upon retreatment of one of the ponds the following year at 100 µg/liter, a half‐life of 4 days was observed. Fluridone persisted in hydrosoils with an apparent half‐life of 1 year or more at all treatment levels. Retreatment of one of the ponds resulted in a buildup of herbicide residue which dissipated more rapidly (half‐life of 20 weeks) than after the first treatment. Bioassays indicated that fluridone residues in hydrosoil from the ponds treated at 700 and 5,000 µg/liter, taken 42 to 70 weeks after treatment, were phytotoxic to duckweed. Fluridone levels in minnows were very low (< 0.02 to 0.14 µg/g) throughout the sampling period with bioconcentration factors ranging from 0 to 64. Fluridone levels in duckweed were proportional to the herbicide concentrations in the pond water with bioconcentration factors ranging from 19 to 85.