Residues from Organic Arsenical Herbicides in Chemically Thinned Forests

Conifers in four Pacific Northwest locations and forest types were stem‐injected with the organic arsenicals cacodylic acid and monosodium methanearsonate (MSMA), herbicides used for forest thinning and insect control. Concentrations and locations of As residues in tree stems, twigs, foliage, and in litter and soil were determined. Within each forest type, spring treatment led to somewhat higher concentrations in crowns than did fall application; differences were larger with cacodylic acid than with MSMA. Most residue concentrations were in the range of 20 to 60 mg As/kg (dry‐weight basis). Trees killed 5 yr earlier showed As concentrations of 122 to 670 mg/kg in the phloem just above the injection sites, 10.9 to 25.0 mg/kg in the xylem, and 34.7 to 77.8 mg/kg in upper lateral twigs and terminal leader shoots. Residues in litter were lowest (5–9 mg/kg) in Douglas‐fir [ Pseudotsuga menziesii (Mirb.) Franco] stands and highest (27–46 mg/kg) in lodgepole pine ( Pinus contorta Engelm.) stands. Surface soil concentrations below the litter traps increased 0 to 3.7 mg As/kg over native soil concentrations of 1 to 7 mg/kg. Most observations were within the range of naturally occurring As in these sites. Persons using fuel from treated stands probably would not receive measurable exposure to toxic forms of As. Residues in fuelwood would be minimized if wood at the point of injection and bark on the lower stem were excluded from the fuel. Bark removal is made easy by injecting arsenicals in the fall.