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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

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A leaching study was undertaken with $ sp{14}$C radio-labelled atrazine in long, intact, soil columns to investigate the fate and transport of atrazine in a Quebec sandy loam soil and to test two contaminant transport models, namely PRZM and LEACHMP. / Results indicated a large variation in atrazine concentration. The maximum levels in the leachate of the soil columns ranged from 0 to 11.2 $ mu$g/L. Deisopropyl-atrazine, the principal metabolite in the soil columns, was detected near the soil surface and did not leach beyond 15 cm depth. The adsorptive and desorptive capacity of atrazine in the soil columns was mainly attributed to organic matter and moisture content. / LEACHMP was found to be superior in predicting hydrologic characteristics in the soil columns as compared to PRZM which required calibration. Both models underestimated levels of atrazine near the soil surface and in the leachate (115 cm depth). The results from first and second order analysis for PRZM and LEACHMP showed considerable uncertainty in atrazine fate and transport. First and second order analyses were found to be useful tools in indicating where efforts to reduce uncertainty can best be directed.

A field study was undertaken during the growing seasons of 1988 and 1989 to investigate leaching characteristics of metolachlor, atrazine and two atrazine metabolites in two agricultural soils of Quebec, namely a sand and a loam field. In both years, atrazine was detected in higher levels in the groundwater of the loam field than of the sand field. Deethylatrazine was the only metabolite found in any significant quantity in soil in 1988 and in groundwater in 1989. Deisopropylatrazine was detected in groundwater at the beginning and at the end of the 1988 season and levels were similar in both fields. Metolach or was never detected in groundwater. / GLEAMS predictions favourably compared to field results at the 0-20 cm depth in both years. Half-life, however, had to be reduced by half in 1989 since higher temperature had increased dissipation. GLEAMS has not predicted leaching past 20 cm in either field in 1988 and 1989. During a dry summer, such as 1988 and 1989, leaching is mostly due to macropore flow and desorption of residues, two processes which are difficult to predict with existing models.

Hazard assessment of pesticides in aquatic environments requires accurate predictions of persistence and compartmentalization. A strategy for developing confidence in predictive fate models, such as the Exposure Analysis Modeling System (EXAMS) and the Simplified Lake and Stream Analysis model (SLSA), is to test the models using carefully chosen chemicals in semi-controlled and field situations. An objective of this approach would be to isolate the variability in a particular fate process and thereby assess the ability of an algorithm to model the process. For example, endothall, a relatively watersoluble aquatic herbicide, has essentially a sole fate process, biotransformation. Endothall was used to test the predictive capabilities of EXAMS and SLSA and to identify sources of variance in those predictions.

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