RESIDUAL AND CONTACT HERBICIDE TRANSPORT THROUGH FIELD LYSIMETERS VIA PREFERENTIAL FLOW

Authors: Malone, Robert - Rob; Shipitalo, Martin; Wauchope, Robert - Don; Sumner, Harold

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2004
Publication Date: 12/1/2004
Citation: Malone, R.W., Shipitalo, M.J., Wauchope, R.D., Sumner, H.R. 2004. Residual and contact herbicide transport through field lysimeters via preferential flow. Journal of Environmental Quality. 33:2141-2148.

Interpretive Summary: Post-emergence herbicides such as glyphosate (Round-Up) and glufosinate (Liberty) are becoming more common and may reduce some of the negative environmental impact of agriculture because they are considered less likely to move to groundwater and may be less toxic than pre-emergence herbicides (e.g., atrazine, alachlor). Few published field leaching studies, however, have been conducted that compare the subsoil transport of post- and pre-emergence herbicides. One of the reasons for the lack of studies is that the risk of leaching is generally regarded to be low. But preferential flow through earth worm channels, root channels, soil cracks, etc. complicates the picture because more of a chemical can leach than expected from a herbicide's laboratory measured chemical properties (e.g., sorption to soil). Therefore, the objectives were to investigate the field transport of pre- and post-emergence herbicides under a worst case scenario (heavy rainfall shortly after herbicide application) and investigate the preferential flow characteristics of herbicides with different laboratory measured sorption to soil. The results support that rapid transport of herbicides to a depth of 2.4 m can occur through preferential flow, that the arrival time of herbicides in percolate at 2.4 m is independent of the sorptive properties of the chemicals, but that glyphosate and glufosinate are not likely to be transported at concentrations that are of environmental concern. This study will be useful to scientists trying to understand preferential flow and decision-makers designing herbicide management practices that reduce the environmental impact of herbicides.

Technical Abstract: The post-emergence herbicides glyphosate and glufosinate are becoming more commonly used and may reduce some of the negative environmental impact of agriculture because they are more strongly sorbed to soil and may be less toxic than pre-emergence herbicides. Preferential flow complicates the picture, however, because even strongly sorbed chemicals can move quickly to ground water. Therefore, four monolith lysimeters (8.1 m2 by 2.4 m deep) were used to investigate transport of pre- and post-emergence herbicides under a worst case scenario. Glufosinate, atrazine, alachlor, and linuron were applied in 1999 prior to corn planting and glyphosate, alachlor, and metribuzin were applied in 2000 prior to soybean planting. A large volume, high intensity simulated rainfall was applied shortly after herbicide application both years, and chemical concentrations in percolate were monitored for 5-d in 1999 and 148-d in 2000. In 1999, only one sample contained glufosinate above the detection limit (5 ug/L) while 10 of 24 samples contained atrazine above the Maximum Contaminant Level (atrazine MCL = 3 ug/L). In 2000, glyphosate was not detected in percolate (detection limit = 5 ug/L, MCL = 700 ug/L) while the average alachlor concentration within 1.1-d of rainfall initiation was 1.9 ug/L (MCL = 2.0 ug/L). Most of alachlor, metribuzin, atrazine, and linuron loss occurred within 1.1-d of rainfall initiation and the peak concentration of the herbicides coincided (within 0.1-d of rainfall initiation in 2000), suggesting little difference in breakthrough time among these herbicides. Transport quantity in percolate, however, was dependant on sorption because significantly more of the applied metribuzin leached compared to alachlor during the first 1.1-d after rainfall initiation (2.2% vs. 0.035%, P < 0.05). These results indicate that rapid transport of herbicides to a depth of 2.4 m occurred through preferential flow, that the breakthrough time of herbicides in percolate was independent of the sorptive properties of the chemicals but the transport amount was dependent on chemical properties, and that glyphosate and glufosinate are not likely to be transported to 2.4 m at concentrations of environmental concern.