FATE OF A SYMMETRIC AND AN ASYMMETRIC TRIAZINE HERBICIDE IN SILT LOAM SOILS

Authors: Koskinen, William; CONN, JEFFREY - FORMER USDA-ARS; SORENSON, BRENT - UNIVERSITY OF MINNESOTA

Submitted to: American Chemical Society Symposium Series
Publication Type: Book / Chapter
Publication Acceptance Date: 2/5/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: In field studies, some herbicides have been shown to degrade slower at high latitudes than at lower latitudes due to cold soils. As herbicide persistence increases, the potential for herbicide leaching also increases. Thus, the potential for herbicide leaching in the subarctic and North Central United States soils is higher than soils in warmer lower latitude locations with similar soil properties and rainfall. Research was conducted to determine the extent of metribuzin and metribuzin degradation product leaching in two soils under subarctic conditions in Alaska. Studies were also initiated to determine the extent of leaching of major atrazine degradation products in a Minnesota soil. Degradation of metribuzin appeared to proceed rapidly in subarctic soils and does not appear to more readily leach downward. It also appears that breakdown products of metribuzin do not leach in appreciable quantity. Significant levels of atrazine residues remained 16 months after application. Detection of these residues in water collected from 30 inch deep monitoring columns indicates leaching did occur. Even though the amount leaching is extremely small, it appears that contamination of water supplies may occur by leaching through the soil. The extremely low atrazine levels detected in ground water supplies would require leaching of only less than 0.1 percent of applied atrazine. It appears that contamination of ground water by atrazine and one breakdown product can occur in silt loam soils. The question remains in both cases, however, as to whether the bound residues will be slowly available in the future. Additional research is needed on long-term bioavailability of bound residues.

Technical Abstract: The formation of 14C-atrazine and 14C-metribuzin degradation products and their distribution in the top 90 cm of silt loam soils under field conditions was determined over a 16-month period. The atrazine experiment was conducted on a Port Byron silt loam soil (2.4 percent OC, pH 5.4 and 23 percent clay), while the metribuzin experiment was conducted on Tanana (3.8 percent OC, pH 6.5, 14 percent clay) and Beales (6.4 percent OC, pH 4.7, 8 percent clay) silt loam soils. For atrazine, 68 percent of the applied 14C was present 16 months after treatment (MAT). Radioactivity moved to 30-40 cm by 1 MAT. Atrazine accounted for 20 percent of the 14C applied 16 MAT, and was the predominant 14C-compound in soil below 10 cm through 16 MAT. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil, 13 percent of the 14C present 1 MAT. Predominant degradation products at depths greater than 10 cm were HA and deethylatrazine. Deisopropylatrazine and deethyldeisopropylatrazine were also detected 1 MAT. Metribuzin degraded rapidly in both soils: 12 percent remaining at 1 MAT; 2 percent remaining by 16 MAT. The maximum amount of metabolites desamino-, diketo-, or desaminodiketo-metribuzin, found was 4 percent; most of the 14C remaining was in the form unextractable bound residues. The majority of the metribuzin residue remained in the surface soil. Some metribuzin residues leached to 45 cm: 0.1 and 0.3 percent of the applied 14C remained in the 6.4 and 3.8 percent OC soils, respectively.