Efforts to estimate pesticide degradation rates in subsurface vadose and aquifer materials
Citation:
Washington, J., T. Jenkins, Jack Jones, C. Stevens, AND E. Weber. Efforts to estimate pesticide degradation rates in subsurface vadose and aquifer materials. Environmental Modeling Public Meeting, DC, Washington, June 28, 2017.
Impact/Purpose:
Presentation for the EPA/OPP/EFED convened “Environmental Modeling Public Meeting” scheduled for EPA Headquarters on June 28.
Description:
When pesticides are used in real-world settings, the objective is to be effective in pest eradication at the site of application, but also it is desired that the pesticide have minimal persistence and mobility as it migrates away from the application site. At the site of application, sorption on soil and surface-soil degradation rates both factor into the pesticides' persistence. But once it migrates to the subsurface vadose zone and/or aquifers, subsurface degradation rate is a factor as well. Unfortunately, numerous soil properties that might affect pesticide degradation rate vary by orders of magnitude in the subsurface environment, both spatially and temporally, e.g., organic-carbon concentration, oxygen concentration, redox conditions, pH and soil mineralogy. Consequently, estimation of subsurface pesticide degradation rates and, in tum, pesticide persistence and mobility in the environment, has remained a challenge. To address this intransigent uncertainty, we surveyed peer-reviewed literature to identify > 100 data pairs in which investigators reported pesticide degradation rates in both surface and subsurface soils, using internally consistent experimental methods. These > 100 data pairs represented >30 separate pesticides. When the > 100 subsurface half-lives were plotted against surface half-lives, a limiting line could be defined for which all subsurface half-lives but three fe ll below the line. Of the three data points plotting above the limiting line, two were for pesticides not registered for use in the United States, carbofuran and oxadixyl. The third data point falling above the limiting line was atrazine. When mean half-lives were calculated for each of the > 30 separate pesticides, the mean of every pesticide registered for use in the United States, including atrazine, fell below this upper limiting line. For these data, the subsurface/surface half- life ratio showed no obvious relationship with subsurface depth or organic carbon. These preliminary efforts suggest that potentially subsurface-degradation rates might be conservatively estimated from surface-soi l-degradation rates.