Long-term in situ leaching and degradation of six herbicides aged in subsoils

Only limited information exists concerning the fate of aged herbicide residues in subsurface soils. To assess the importance of degradation and leaching in the persistence of aged herbicide residues in subsoils, formulations of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine), metribuzin (4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one), metolachlor (2-chloro-6'-ethyl-N-(2-methoxy-1-methylethyl)acet-o-toluidide), picloram (4-amino-3,5, 6-trichloropyridine-2-carboxylic acid), 2,4-D) (2,4-dichlorophenoxyacetic acid), and alachlor (2-chloro-2',6'-diethyl-N-methoxyethylacetanilide) were applied separately to two moist Arkansas subsoils and buried for up to 3 yr. The treated subsoils were placed in one of two storage container types, buried in situ at 30, 90, or 150 cm with the soil profile, and retrieved at 6-mo intervals. Amounts of herbicide remaining in the subsoils at two study locations were determined chromatographically. Use of two different types of storage containers allowed for comparison of the relative degradation and leaching that occurred with time. Statistical evaluation of these data found that significant early (less than 6 mo after treatment) degradation occurred in 80% of 36 possible herbicide x soil depth x location combinations. Early leaching occurred between 30 and 50% of the time. Long-term degradation (greater than 6 mo after treatment) occurred in 23 of 28 cases, while long-term leaching occurred in 7 cases. Slow degradation and notable leaching of picloram occurred at 90 and 150 cm at the loamy fine sand location. In addition, prolonged persistence of alachlor relative to metoachlor was observed in subsoils at both study locations.