Leaching and preferential flow of clopyralid under irrigation: Field observations and simulation modeling

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 now from a large field integrated bg tile drainage but would not be applicable in soil column studies or small plots.