Pesticide Sorption and Transport in Variable Landforms.

We observed that in eroded landforms, the spatial variation in herbicide retention and rate of dissipation is compound-specific. We conducted a number of laboratory and field studies measuring the retention and dissipation of herbicides across landscapes that had large differences in soil properties. These landscapes are affected by soil formation processes, but also by extensive erosion that has translocated topsoil from the upper slope to the lower slope. Thus, soils in the lower slope are higher in organic carbon and tend to have higher soil moisture than soils in the upper slope. Some herbicides, such as S-metolachlor and isoxaflutole, demonstrated essentially no difference in sorption/dissipation throughout these landscapes. Sorption of glyphosate was 10% higher in upper slope soils. Sorption of most herbicides, including saflufenacil, aminocyclopyrachlor, picloram, 2,4-D, nicosulfuron, rimsulfuron, and sulfonylurea herbicide metabolites, was 1.5 to 6 times higher in the depositional lower slope than in the eroded upper slope. This research shows that the potential for offsite transport of new and traditional herbicides is generally greater from eroded upper slopes as compared to depositional lower slope areas, underscoring the need for field studies to better predict water and contaminant transport in hilly and eroded landscapes. Results challenge the commonly accepted approach of normalizing pesticide retention parameters to the organic carbon content of the soil, which we agree is appropriate in some cases, but not in others. Rather, in many cases, compound- and soil-specific information is needed for accurate predictions of pesticide fate. These studies provide critical information for the development of management practices that protect water quality.