Effect of formulation upon movement and dissipation of 14C-metolachlor and atrazine in field lysimeters

Abstract

One set of 12 lysimeters (75 cm × 15 cm, packed with 70 cm of Plainfield sand) was treated with 14C-labelled metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methyl ethyl)acetamide] in an emulsifiable concentrate (EC) while a second set was treated with a 25% granular (25G) formulation. Atrazine [2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine] was applied to all lysimeters as an internal reference. Duplicate soil cores from each treatment were removed and frozen at 1, 2, 4, 8, 12, and 21 wk following application. All lysimeters received supplementary water throughout the study, simulating 25-yr return storms on days 2 and 9, followed by a simulated irrigation practice thereafter. Soil cores were cut into seven 10-cm segments to obtain persistence data and concentration profiles. No metolachlor and only a few traces of atrazine appeared in the effluent throughout the study. Metolachlor EC disappeared more rapidly than metolachlor 25G, with 50% disappearance times of 2.2 and 4.0 wk, respectively. After 1 and 2 wk, 14.0 and 28.7% more metolachlor 25G than EC, respectively, remained in the top 10-cm layer. No metolachlor was detected below 40 cm in any soil core. Up to 22.3% of applied 14C accumulated in the effluent after 21 wk, with traces appearing in the effluent on day 2, following a 50-mm watering. 14C recovery in the lysimeter systems decreased to 78% after 21 wk, which may be partially ascribed to volatilization losses. Key words: Leaching, degradation, radioisotope, mobility, herbicide