ARS | Publication request: INTERRELATIONSHIP OF MACROPORES AND SUBSURFACE DRAINAGE FOR CONSERVATIVE TRACER AND PESTICIDE TRANSPORT

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 13, 2004
Publication Date: December 1, 2004
Citation: Fox, G.A., Malone, R.W., Sabbagh, G.J., Rojas, K.W. 2004. Interrelationship of macropores and subsurface drainage for conservative tracer and pesticide transport. Journal of Environmental Quality. 33:2281-2289.

Interpretive Summary: Most herbicide movement to subsurface drains and shallow groundwater is through macropores (e.g., worm burrows, cracks, and root channels). Recent research suggests that a portion of macropores are directly connected to subsurface drains, which affects chemical transport through drains. Also, the current version of the ARS Root Zone Water Quality Model (RZWQM) simulates macropore flow but does not simulate a direct connection of macropore flow and subsurface drains. Therefore, the interrelationship between macropore flow and subsurface drains were investigated with field data and with the RZWQM, and RZWQM was modified to route a portion of macropore flow directly to drain flow. The first measured pesticide peak after application ranged from 0.4 and 5.6 ug/L and the modified model simulated pesticide concentration was 1.0 ug/L. Before modification the predicted peak concentration was 0.2 ug/L and below the field measured range. These results are key to a more thorough understanding of pesticide movement into the environment, confirm that a portion of macropores are directly connected to drains, and suggest that simulating this process more accurately represents field conditions. This work will initially help scientists but will eventually help decision-makers and farmers design farming practices that reduce pesticide concentrations in streams and groundwater and reduce pesticide contamination in the environment.

Technical Abstract: Macropore flow results in the rapid movement of pesticides to subsurface drains, which may be caused in part by a small portion of macropores directly connected to drains. The current version of the Root Zone Water Quality Model (RZWQM) simulates pesticide movement to drains and macropore flow, but water and chemicals stop at the water table and peak concentrations shortly after chemical application are under predicted. This research investigated the interrelationship between macropore flow and subsurface drainage on conservative solute and pesticide transport using the Root Zone Water Quality Model (RZWQM) and field data. Potassium bromide (KBr) tracer and isoxaflutole (IFT), the active ingredient in BALANCE herbicide, were applied to an Indiana corn (zea mays L.) field. Water flow and chemical concentrations emanating from the drains were measured from two samplers. Model predictions of daily drain flow amount after minimal calibration reasonably matched observations (slope = 1.03, intercept = 0.01, R2=0.75). Without direct hydraulic connection of macropores to drains, RZWQM under predicted Br and IFT concentration during the first measured peak after application (e.g., observed IFT concentration was between 0.4 and 5.6 mg L-1, RZWQM concentration was 0.2 mg L-1). This research modified RZWQM to include an express fraction relating the percentage of macropores in direct hydraulic connection to drains. The modified model captured the first measured peak in Br and IFT concentrations using an express fraction of two percent (e.g., simulated IFT concentration increased to 1.0 mg L-1). RZWQM modified to include a macropore express fraction more accurately simulates chemical movement through macropores to subsurface drains.