Hexazinone Persistence and Mobility of a Steep Forested Watershed

Abstract

AbstractThe mobility and persistence of hexazinone, 3‐cyclohexyl‐6‐(dimethylamino)‐1‐methyl‐1,3,5‐triazine‐2,4‐(1H,3H)‐dione, applied to a forest floor with a spot‐gun applicator was monitored for 43 months. A 15‐m buffer zone separated the steeply sloping (40%) treated area from a continuously flowing stream directly below. Automated in‐stream samplers revealed that the highest hexazinone concentrations in water routinely occurred during storm events, even when stream flow increased tenfold. More hexazinone ran off the treated watershed during winter months after tree leaves had fallen. During the first 24 months after treatment, 4.7% of total applied was detected in the stream sampled. Initially, forest litter was highly effective in adsorbing surface applications of hexazinone; however, after 5 months less than 0.2% of the original amount was retained in the litter. Over 99% of the hexazinone had dissipated from the top 30 cm of soil within 18 months after application. After 36 months, trace levels of hexazinone were detected in water samples collected soon after storms. However, no hexazinone was present in samples collected 43 months after application. Distribution coefficients for hexazinone adsorption revealed that this compound was retained most readily by the litter layer and decreasing adsorption was shown for soil deeper in the profile. Mean concentrations of hexazinone in green leaves of trees in the treated area ranged from 0.5 to 1.0 kg−1. Rapid dissipation of hexazinone in leaves lying on the floor suggested the possibility of recycling into existing vegetation.