Comparison of Herbicide Loss At the Field–Scale a Multi-Year Investigation.

Once lost to the atmosphere, herbicide transport can result in unintended re-deposition to inhabited areas, streams, rivers, and lakes. To better understand factors governing herbicide volatilization and to determine its impact relative to other loss pathways, field-scale turbulent volatilization fluxes of two herbicides have been conducted annually since 1998, and represent perhaps the most detailed and longest herbicide volatilization database in existence.  Herbicide runoff and turbulent vapor fluxes were simultaneously monitored on the same site located at the OPE3 Field site in Beltsville, Maryland.  Site location, herbicide formulations, and agricultural management have remained unchanged during the 13 years.  Metolachlor and atrazine were co-applied as a surface broadcast spray.  Herbicide runoff was monitored a month before application through harvest using H-flumes.  For the first 10 years, flux gradient technique was used to compute volatilization fluxes for only the first 5 days after application using herbicide concentration profiles and turbulent fluxes of heat and water vapor as determined from eddy covariance measurements.  For the last three years, herbicide volatilization has been monitored for the first 10 days after application.  Various results and future research will be discussed.  For example, when averaged over all years and both herbicides, off-site transport was at least 25 times larger for volatilization than surface runoff.   Additionally, herbicide volatilization losses were strongly influenced by soil moisture.  Temperature increased herbicide volatilization when soils were moist, but had little impact on herbicide volatilization when soils were dry.  This research confirms that vapor losses for some commonly used herbicides far exceeds runoff losses and that herbicide vapor losses will need to be quantified and evaluated if management practice and formulations are to be developed which reduce herbicide loads on surrounding environments.