The herbicide, Dacthal (dimethyl 2,3,5,6,-tetrachloroterephthalate) is hydrolyzed to give the corresponding diacid and this derivative is a common contaminant in ground water. In the Ontario region of eastern Oregon the use of this herbicide on onions has resulted in the contamination of an aquifer with this derivative. Since movement to groundwater is determined by the hydrolysis of the parent compound to a more soluble derivative. The rate at which this hydrolysis reaction occurs and the stability of the metabolite must be defined. These parameters have been determined using soils from Ontario in which onions had been raised. At room temperature and 50% field capacity, the parent was hydrolyzed rapidly (half-life 16 days) to the diacid derivative. An increase of the incubation temperature to 38°C reduced the hydrolysis rate significantly (half-life 86.8 days). It is assumed that this response reflects reduced microbial activity at the higher temperature. At both temperatures only small amounts of the monoacid intermediate were detected suggesting that the Dacthal monoacid was being hydrolyzed at a faster rate than the parent. It was established that at room temperature, the half-life of the monoacid was only 2.8 days. Over the 300 days the experiments were carried out, little if any degradation of the diacid metabolite could be detected. There was virtually no degradation of the parent over a 60 day period in sterilized soil, suggesting that microbial activity is primarily responsible for this step. By contract, the monoacid was hydrolyzed at comparable rates in sterilized and nonsterilized soil. This study explains why the Dacthal metabolite is a common contaminant in groundwater. The parent is rapidly hydrolyzed to the diacid which is much more water soluble. More important, however, is the persistence of the diacid metabolite in the environment. / Graduation date: 1992