The fate of terbuthylazine in test microcosms

The fate of 14C-labeled terbuthylazine in several soils from Israel was determined in a microcosm, designed to gather effluent and to trap volatile organic compounds and CO2. The soils studied included a heavy clay vertisol, a loess soil which has received organic amendments periodically over the last 31 years, and a control loess soil from adjacent unamended plots. No [14C]CO2 was produced in any of the soils, thus indicating that mineralization of the terbuthylazine did not occur. The cumulative volatilization of terbuthylazine and its metabolites from the loess soil after 67 days was 4.2% of the amount applied, while only 1.8% volatilized from the amended soil. This difference is attributed to greater sorption of the herbicide in the amended soil. In the vertisol, 5.4% of the amount applied was volatilized. Over 90% of the volatilized material was parent terbuthylazine. A total of 0.2–1.7% of the applied herbicide was leached from the loess soils by the 137 mm of water applied over the course of the experiment. The breakthrough of 14C activity from the non-amended soil was greater and more rapid than from the amended soil. Breakthrough from the vertisol systems was later than from the loess soils. Metabolites of terbuthylazine comprised the bulk of the activity leached from the columns. In the vertisol, terbuthylazine did not leach below 2 cm, whereas, in the loess soils the herbicide was leached to a depth of 5–6 cm. Although the downward movement of parent terbuthylazine was limited in the three soils studied, incomplete mineralization of the parent compound resulted in the formation of metabolites which were much more mobile in the soil column. As such, these metabolites may pose a serious threat to ground water quality if similar behavior is exhibited under actual field conditions.