Modeling Parent and Metabolite Fate and Transport in Subsurface Drained Fields With Directly Connected Macropores¹

Abstract

Few studies exist that evaluate or apply pesticide transport models based on measured parent and metabolite concentrations in fields with subsurface drainage. Furthermore, recent research suggests pesticide transport through exceedingly efficient direct connections, which occur when macropores are hydrologically connected to subsurface drains, but this connectivity has been simulated at only one field site in Allen County, Indiana. This research evaluates the Root Zone Water Quality Model (RZWQM) in simulating the transport of a parent compound and its metabolite at two subsurface drained field sites. Previous research used one of the field sites to test the original modification of the RZWQM to simulate directly connected macropores for bromide and the parent compound, but not for the metabolite. This research will evaluate RZWQM for parent/metabolite transformation and transport at this first field site, along with evaluating the model at an additional field site to evaluate whether the parameters for direct connectivity are transferable and whether model performance is consistent for the two field sites with unique soil, hydrologic, and environmental conditions. Isoxaflutole, the active ingredient in BALANCE® herbicide, was applied to both fields. Isoxaflutole rapidly degrades into a metabolite (RPA 202248). This research used calibrated RZWQM models for each field based on observed subsurface drain flow and/or edge of field conservative tracer concentrations in subsurface flow. The calibrated models for both field sites required a portion (approximately 2% but this fraction may require calibration) of the available water and chemical in macropore flow to be routed directly into the subsurface drains to simulate peak concentrations in edge of field subsurface drain flow shortly after chemical applications. Confirming the results from the first field site, the existing modification for directly connected macropores continually failed to predict pesticide concentrations on the recession limbs of drainage hydrographs, suggesting that the current strategy only partially accounts for direct connectivity. Thirty‐year distributions of annual mass (drainage) loss of parent and metabolite in terms of percent of isoxaflutole applied suggested annual simulated percent losses of parent and metabolite (3.04 and 1.31%) no greater in drainage than losses in runoff on nondrained fields as reported in the literature.

Numerical Simulation of Flow Dynamics during Macropore–Subsurface Drain Interactions Using HYDRUS

“…In general, the numerical simulations suggested an even greater contributing area with directly connected macropores than suggested by Akay and Fox (2007) and Fox et al (2004) The need for calibration observed by Fox et al (2007) is supported by these numerical simulations that show that the contributing area depends on the site‐specific macropore depth of penetration.…”

“…C oncerns exist about the rapid transport of contaminants, such as pesticides (Fox et al, 2004), pathogens (Joy et al, 1998; Geohring et al, 1999; Jamieson et al, 2002; Shipitalo and Gibbs, 2005), and nutrients from the soil surface to groundwater through macropores (Magesan et al, 1995; Kladivko et al, 1999; Shipitalo and Gibbs, 2000). Commonly observed short‐circuiting in many subsurface drainage field studies has been hypothesized to be due to direct hydrologic connectivity between macropores and subsurface drains (Fox et al, 2004, 2007; Shipitalo and Gibbs, 2000, 2005). Directly connected macropores can result in the rapid transport of contaminants from the soil surface into the subsurface drains and then into adjacent receiving streams and channels, bypassing the soil's filtering capacity.…”

Effectiveness of Vegetative Filter Strips in Reducing Pesticide Loading: Quantifying Pesticide Trapping Efficiency

“…Also, the empirical model was evaluated against measured Δ P using linear regression and also based on the STDD: where x m and x p are the i th observed and predicted values of n observations, respectively. This function has been used in the past for quantitative evaluation of pesticide fate and transport models (Pennell et al, 1990; Fox et al, 2004; Fox et al, 2006; Fox et al, 2007). This research used a criterion that the STDD between measured and predicted Δ P should be less than 15%.…”

Corn stover harvest increases herbicide movement to subsurface drains – Root Zone Water Quality Model simulations

“…The Root Zone Water Quality Model (RZWQM) incorporates these elements and has been successfully tested and used to simulate pesticide and pesticide metabolite fate and transport in numerous field and laboratory studies . Furthermore, the RZWQM has recently been used for accurate simulation of metolachlor and OXA fate in widely different soil and climatic conditions in Maryland and Nebraska .…”