|Southwick Jr, Lloyd|
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/30/2003
Publication Date: 7/31/2003
Citation: SOUTHWICK JR, L.M., GRIGG, B.C., FOUSS, J.L., KORNECKI, T.S. ATRAZINE AND METOLACHLOR IN SURFACE RUNOFF UNER TYPICAL RAINFALL CONDITIONS IN SOUTHERN LOUISIANA. JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY. 2003. Interpretive Summary: In a three-year study of the herbicides atrazine and metolachlor applied to fields planted to corn, atrazine in the top inch of soil (a practical depth collected for analysis of soil pesticide residues) showed half lives of 10-17 days, and metolachlor persisted with soil half lives of 16-28 days. The residue that is available for removal in surface runoff after rainfall is revealed by residue concentrations in the runoff over time after application. The persistence in this runoff-available zone of the soil (possibly as thin as the top 1/8th inch) was shorter than in the soil sampling zone of the top inch: atrazine in the runoff available zone was only 10-30% as persistent as in the top inch, and metolachlor lasted only 10-40% as long as in the top inch. That is, atrazine in the runoff-available zone showed half lives of 1.4-5.7 days, and metolachlor in this zone showed half lives of 1.9-6.4 days. Our data are an especially good illustration of the difference that has been theoretically known for many years between soil pesticide residues and runoff active zone residues. Only a few reports have actually shown this difference. Soil pesticide persistence data, which are readily available in the literature, may overestimate by as much as 10-fold the persistence of the material available to be removed in a runoff event after rainfall.
Technical Abstract: We report here runoff of atrazine and metolachlor from 0.21 ha plots laid out on Mississippi River alluvial soil. In the study, conducted over a three-season period characterized by rainfall close to the 30-year average, we have gathered data on persistence in the top 2.5 cm layer of soil and in the runoff active zone of the soil as measured by decrease in runoff concentrations with time after application. We developed regression equations that allow an estimate of the runoff extraction coefficients for each herbicide, and regression equations that permit an estimate of the decrease in these extraction coefficients with time after application. In these studies, atrazine and metolachlor have shown half lives in the top 2.5 cm layer of soil of less than 30 days. Persistence in the runoff active zone of the soil, as measured by decrease in runoff concentrations with time after application, was 0.09-0.41 as long as persistence in the top 2.5 cm layer. The equations relating runoff concentrations of atrazine to soil concentrations contain extraction coefficients in the range 0.05-0.11. For metolachlor these equations contained extraction coefficients from 0.02 to 0.08. Persistence of these herbicides in the runoff active zone varied: metolachlor in this layer of soil lasted 2.5 times as long as did atrazine. Comparing percent of application losses in runoff, those for atrazine were 1.23 to 1.52 times as great as those for metolachlor. The relative trends that we report here for runoff extraction coefficients, runoff active zone persistence, and percent of application losses are all consistent with the difference in Koc's of the two compounds.