Mobility and Persistence of Hexazinone in a Forest Watershed

The concentration of hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1H,3H)-dione] in soil, water, and plant tissue was monitored following hexazinone application (2.0 kg a.i./ha) to an 11.5-ha watershed in northwestern Arkansas. The hexazinone concentration in the top 10 cm of soil on the watershed decreased to approximately 10% of the initial concentration in 42 d. Hexazinone dissipation rate on the watershed was more rapid than could be accounted for solely by degradation. Leaching of hexazinone to lower soil depths probably was important to hexazinone dissipation. Hexazinone degradation in soil incubated at 10 and 30°C followed first-order kinetics and had a half-life of 77 d at 30°C. Hexazinone was stable in incubated stream water requiring several years for 50% disappearance of the compound at 30°C. The maximum hexazinone concentration in the stream that drained the watershed was 14 µg/L, and hexazinone residues (< 3 µg/L) were detected in stream discharge over 1 yr after application. The amount of hexazinone transported from the watershed in stream discharge represented 2.0 to 3.0% of the amount of initially applied. Analyses of oak foliage and leaf litter collected from the forest floor indicated that < 0.10% of the hexazinone applied was returned to the forest floor in leaf deposition.