Effect of Residue Removal On Movement of Metolachlor and Its Metabolites to Subsurface Drains - RZWQM Simulations.

Removal of crop residues for bioenergy production can alter the hydrologic properties of soil. In particular, residue removal can negatively impact earthworm populations, which can contribute to reduced macropore formation and increased surface runoff. While the impact of residue removal on erosion has been measured and modeled, there is little information on the long-term impact on the movement of herbicides to subsurface drains. Therefore, we used the Root Zone Water Quality Model (RZWQM) to investigate the effects of corn stover removal on the movement of metolachlor and its metabolites metolachlor ethane sulfonic acid (ESA) and metolachlor oxanilic acid (OXA) to subsurface drains. The model was applied to continuous corn drainage plots in Nashua, IA and the effects of 0, 50, and 100% stover removal were investigated. The model was previously calibrated using measured atrazine concentrations in drainage water from these plots. Using the calibrated model and site specific measurements of metolachlor and OXA sorption to soil, we tested RZWQM with measured metolachlor concentrations in subsurface drainage. Based on the previous studies that indicate that residue removal significantly reduces surface  saturated hydraulic conductivity and active soil macroporosity, our preliminary modeling results suggest that this greatly increases metolachlor losses in subsurface drains.