Leaching and Preferential Flow of Clopyralid under Irrigation: Field Observations and Simulation Modeling

Abstract

Pesticide leaching is a potential problem on irrigated land in Saskatchewan. This study examined a worst case scenario for leaching of the herbicide, clopyralid. Herbicide concentrations were measured in water samples from suction lysimeters and tile drain effluent during and after a leaching irrigation to remove salts. Clopyralid was found throughout the soil profile within 20 d of application and was detected in the first water flowing in the tile drains. Approximately 1.5% of the applied clopyralid was lost in the tile drain effluent. The presence of higher concentrations of clopyralid in tile drain water samples at 2 m than in lysimeter water samples taken from 1.5 and 1.8 m was attributed to preferential transport. Another indication of preferential flow was the disparity between measured soil hydraulic characteristics and the timing of the arrival of water in the tile drains. Good agreement between observed and simulated clopyralid distributions was obtained using a bimodal flow scenario with the LEACHM model. It assumed that 40% of the applied water moved through preferential flowpaths with saturated hydraulic conductivities two orders of magnitude greater than the bulk soil. Chemical moving with this water did not penetrate the soil matrix and was not accessed by suction lysimeters. The remaining 60% of applied water moved through the soil matrix according to measured hydraulic characteristics. The modeling approach described herbicide concentrations in the soil matrix and preferential flow from a large field integrated by tile drainage but would not be applicable in soil column studies or small plots.

Using Lysimeters To Evaluate the Relative Mobility and Plant Uptake of Four Herbicides in a Rye Production System

“…Some leaching of 3,6-dichlorosalicylic acid was observed, but its contribution was smaller than that of the parent compound (Figures 2A and 3A). Similar clopyralid concentrations in leachate and cumulative mass loss (1.5% of applied) have been observed following fall application of clopyralid with frequent irrigation (16).…”

Is tile drainage water representative of root zone leaching of pesticides?

“…Leaching of sorbing pesticides in tile‐drained loamy and clayey soils will mainly occur through macropores 12, 36–41. If the water containing them is flowing in the soil matrix, the residence time and the sorption capacity will be sufficient to prevent most pesticides from leaching to any great extent.…”

Dynamics of clopyralid herbicide during composting in small composting experiment units

“…41) In a field experiment, clopyralid was the only herbicide detected in quantifiable amounts from a no-tillage field when the leaching rates of 7 herbicides, including clopyralid, were compared between conventional tillage and notillage. 42) In irrigated land in Saskatchewan, Canada, clopyralid was found throughout the soil profile within 20 d of application and was detected in the first water flowing in the tile drains, 43) and as much as 0.31% of the applied clopyralid was carried in drainage water from the irrigation district and released into the South Saskatchewan River. 44) It was subsequently detected in 15 reservoirs in the Northern Great Plains of North America, and was identified as a contaminant in drinking water drawn from these reservoirs.…”