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Title: SOLAR RADIATION, RELATIVE HUMIDITY, AND SOIL WATER EFFECTS ON METOLACHLOR VOLATILIZATION

Author
item Prueger, John
item Gish, Timothy
item McConnell, Laura
item McKee, Lynn
item Hatfield, Jerry
item Kustas, William - Bill

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2005
Publication Date: 6/21/2005
Citation: Prueger, J.H., Gish, T.J., Mcconnell, L.L., Mckee, L.G., Hatfield, J.L., Kustas, W.P. 2005. Solar radiation, relative humidity, and soil water effects on metolachlor volatilization. Environmental Science and Technology. 39(14):5219-5226.

Interpretive Summary: Pesticides are an important part of modern agricultural practices. Realistically potential pesticide losses can occur during and after application. During application, losses can occur in the form of spray drift away from the intended target while after-application losses can occur in the form of surface runoff and volatilization, the passing from a liquid form at a surface to a vapor to the atmosphere above a surface. In this study, focus was on the volatilization of a pre-emergent herbicide (active ingredient metolachlor) applied to bare soil after corn was planted. This study was carried out for five consecutive years in the same field, soil, pesticide and corn crop. The variable in this study was the different local meteorology that was present during each of the five years. Volatilization losses were measured in each of the five years for five consecutive days after application. Local meteorology and soil conditions were variable during the study. Total metolachlor volatilization losses at the end of each year ranged between 4-25% of the applied amount. Nearly 90% of the volatilization losses occurred during the first 72 hours after application. Soil water content and local meteorology were found to significantly affect volatilization rates and losses. This research is important to growers who routinely use pre-emergent herbicides to aid in managing pesticide applications in response to local climate and soil conditions. Additionally, the results from this research can aid in the development of newer chemicals that can be less susceptible to influences of weather and soil water conditions.

Technical Abstract: Pesticide volatilization is a significant loss pathway that may have a detrimental impact on surrounding environments. Fundamental information on the effects of meteorological and soil conditions on volatilization losses is required to advise producers on best management practices. This five-year study was designed to evaluate the impact of meteorological variables and surface soil water on volatilization losses of the pre-emergent herbicide metolachlor. Herbicide vapor concentrations were measured after application using polyurethane foam plugs at five heights above the soil surface. Volatilization fluxes were computed using pesticide concentration profiles with a flux gradient technique using corresponding eddy covariance measurements. In 1999, under very dry soil conditions, metolachlor flux rates increased at night (from 40 to 100 ng m-2 s-1) with increasing relative humidity. The highest flux rates were found during years with wet soil conditions combined with high temperatures and intense solar radiation, especially in 2001 where a maximum flux rate of 1500 ng m-2 s-1 was observed. Cumulative volatilization losses across years varied from 5 to 25% of applied active ingredient, with approximately 87% of losses occurring during the first 72 hours. Results suggest that at conditions >20% surface soil water content, metolachlor volatilization losses can increase exponentially.