Comparison of Field-Scale Herbicide Runoff and Volatilization Losses: An Eight Year Field Investigation.

An 8-year study was conducted to compare field-scale herbicide volatilization and surface runoff losses, and to better understand field-scale factors influencing year to year variability in these loss pathways.  The 21 ha research site used for this study is located at the USDA-ARS Beltsville Agricultural Research Center, in Beltsville Maryland.  Site location, soil properties, herbicide formulations, and agricultural management remained unchanged throughout the duration of the study.  Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] and atrazine [6-chloro-N-ethyl-N’-(1-methylethl)-1,3,5-triazine-2,4-diamine] were co-applied as a surface broadcast spray, while surface runoff and atmospheric concentration profiles were monitored.  Herbicide runoff monitoring was initiated a month before application with a 46-cm H-flume and continued through harvesting.  A flux gradient technique was used to compute volatilization fluxes for the first 5-days after application using herbicide concentration profiles (four heights monitored) and turbulent fluxes of heat and water vapor as determined from eddy covariance measurements.  Additionally, twenty locations were randomly selected each year and subsequently sampled predawn at 4:30 am (EST) each morning to determine surface soil moisture and herbicide concentrations within the top 5 cm of soil.  Results demonstrate that volatilization for these two herbicides were much greater than runoff losses even though both have low vapor pressures and runoff was thought to be the major loss pathway.  Annual herbicides runoff losses were typically << 1%.  The largest annual runoff loss for metolachlor never exceeded 2.5% while atrazine runoff never exceeded 3% of that applied.  On the other hand, herbicide volatilization losses after 5 days ranged from 5 to 63% of that applied for metolachlor and 2 to 12% of that applied for atrazine.  This research demonstrates that volatilization is perhaps the least understood yet most critical loss pathway governing herbicide behavior in nearly level, sandy loam soils.