Runoff and Leaching of Metolachlor from Mississippi River Alluvial Soil during Seasons of Average and Below-Average Rainfall

Authors: SOUTHWICK, LLOYD - ARS-RETIRED; Appelboom, Timothy; Fouss, James

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2008
Publication Date: 2/25/2009
Citation: Southwick, L.M., Appelboom, T.W., Fouss, J.L. 2009. Runoff and Leaching of Metolachlor from Mississippi River Alluvial Soil during Seasons of Average and Below-Average Rainfall. Journal of Agricultural and Food Chemistry. 57(4):1413-1420

Interpretive Summary: After a herbicide is applied to soil at the time of crop planting, it is susceptible to removal from the site of application upon the occurrence of rainfall in surface runoff and soil leaching. During periods of reduced rainfall, runoff may be much reduced, but leaching through the soil may still occur to a significant amount. Auch movement of metolachlor applied to the soil at the time of planting to corn was studied over a six year period (1995-2000). The first three years were characterized by normal rainfall amounts, but the last three years were characterized by reduced rainfall. During the periods of normal rainfall, 4.1-6.1 % of the metolachlor application left the field in surface runoff, while 0.10-0.18 % of the application leached through the soil. During the last three years of the study, reduced rainfall led to appearance of 0.066-0.37 % of the metolachlor applied in runoff, whereas 0.22-0.27 % of the application leached through the soil. A 35 % reduction in the occurrence of rainfall led to a 97 % reduction in runoff loss of the metolachlor and to a 79 % increase in the amount of the herbicide leaving the site of application in leachate. Therefore, concerns about the removal of the application into waterways through surface runoff would be ameliorated in those periods of reduced rainfall leading to reduction in surface runoff, but concerns about the effect of soil leachate processes might still be an issue. Since the volume of rainfall during the dry period of the investigation was much reduced, the lateral transport into adjoining waterways of the metolachlor after leaching through the soil may have also been reduced. This flow pathway was not studied.

Technical Abstract: The movement of metolachlor via runoff and leaching from plots planted to corn on Mississippi River alluvial soil (Commerce silt loam) was measured for a six-year period, 1995-2000. The first three years were characterized by normal rainfall volume, the second three years by reduced rainfall. The four-month periods prior to application plus the following four months after application were characterized by 1039 ± 148 mm of rainfall for 1995-1997 and by 674 ± 108 mm for 1998-2000. During the normal rainfall years 216 ± 150 mm of runoff occurred during the study seasons, accompanied by 76.9 ± 38.9 mm of leachate. For the drier years these amounts were 16.2 ± 18.2 mm of runoff (92 % lower than the wet years) and 45.0 ± 25.2 mm leachate (41 % less than the wet seasons). Runoff of metolachlor during the normal rainfall seasons was 4.1-6.1 % of application, whereas leaching was 0.10-0.18 %. For the dry periods, these losses were 0.066-0.37 % in runoff and 0.22-0.27 % in leachate. Averaging over the wet and dry seasons, a 35 % reduction in rainfall was characterized by a 97 % reduction in runoff loss and a 79 % increase in leachate loss of metolachlor. The data indicate an increase in preferential flow in the leaching movement of metolachlor from the surface soil layer during the reduced rainfall periods. An added observation in the study was that neither runoff of rainfall nor runoff loss of metolachlor was influenced by the presence of subsurface drains, compared to the results from plots without such drains that were described in an earlier report.