Suitability of hardwoods treated with phenoxy and pyridine herbicides for use as firewood.

Potential exposure to pesticide residues resulting from burning wood treated with phenoxy and pyridine herbicides was assessed. Wood samples from trees treated with 2,4-D [2,4-dichlorophenoxy acetic acid], dicamba [3,6-dichloro-oanisic acid], dichlorprop [2-(2,4-dichlorphenoxy) propionic acid], picloram [4-amino-3,5,dtrichloropicolinic acid], and triclopyr (3,5,6-trichloro-2-pyri dinyl)oxy acetic acid contained variable amounts of parent compound residues at 4, 8, and 12 months after application. At the time of the latter sampling, residues of 2,4-D, dicamba, and picloram were ~2.1 mg/kg on a fresh weight basis. Mean residue concentrations of triclopyr and dichlorprop were somewhat higher at 3.5 and 13.0 mg/kg, respectively. In a laboratory experiment, samples with known amounts of herbicide residue were subjected to either slow or rapidly burning conditions in a tube furnace. During slow combustion, relatively stable compounds such as 2,4-D, dicamba, and dichlorprop were released in significant amounts. Rapid combustion greatly enhanced decomposition of 2,4-D, dicamba, dichlorprop, picloram, and triclopyr. A well-developed fire in a wood stove or fireplace, with active flaming combustion, where temperatures commonly reach 800l,OOO”C, should result in greater than 95% thermal decomposition of the herbicides examined in this study. Burning of herbicide-treated wood under smoldering conditions could result in very low levels of herbicide residue in ambient indoor air. However, the exposure levels are less than * Address correspondence to: Parshall B. Bush, Extension Pesticide Residue Laboratory, 110 Riverbend Road, Athens, GA 30605. 0.3% of the threshold limit value for 2,4-D and triclopyr. The exposure is also more than 3 orders of magnitude lower than the established acceptable daily intakes for these products. In the late 1970’s, rising fossil fuel costs created a large demand for alternative home heating sources. Many households in the United States turned to wood as a primary or supplementary energy source. In metropolitan areas, wood typically sells for up to $200 per cord. One common source of firewood has been poor quality hardwoods left over from forest management practices, especially hardwoods killed by herbicides during site preparation or release projects. Although there are several comprehensive summaries on the fate of forestry pesticides in the environment, there is little information on the fate of these chemicals when they are burned (Ghassemi et al. 1981; USDA Forest Service 1984). One reason for this lack of information is that data on residue fate after burning has not been required in the pesticide registration process, since burning of pesticide treated trees has not been considered a normal end use. Limited information is available on the fate of wood preservative chemicals after burning. Much of the published data is on inorganic chemicals (McMahon et al. 1985a; Peters et al. 1983; Dobbs and Grant 1976; Watson 1958). The combustion of woody material is a complex process, whose end products are affected by factors such as fuel moisture, fuel chemistry, temperature, atmospheric gases, and ignition source and burning rate. Under normal fireplace and woodstove conditions, combustion products are 334 P. B. Bush et al. Table 1. Herbicide application rates and relative concentration factors in turkey oak stems, Ocala National Forest, June 1, 1982