Assessment of vadose zone sampling methods for detection of preferential herbicide transport

Abstract. Accurate soil water sampling is needed for monitoring of pesticide leaching through the vadose zone, especially in soils with significant preferential flowpaths. We assessed the effectiveness of wick and gravity pan lysimeters as well as ceramic cups (installed 45–60 cm deep) in strongly-structured silty clay loam (Hudson series) and weakly-structured fine sandy loam (Arkport series) soils. Simulated rainfall (10–14 cm in 4 d, approximately equal to a 10-yr, 24 h storm) was applied following concurrent application of agronomic rates (0.2 g m⁻²) of atrazine (6-chloro-N²-ethyl-N⁴-isopropyl-1,3,5-triazine-2,4-diamine) and 2,4-D (2,4-dichloro-phenoxy-acetic acid) immediately following application of a chloride tracer (22–44 g m⁻²). Preferential flow mechanisms were observed in both soils, with herbicide and tracer mobility greater than would be predicted by uniform flow. Preferential flow was more dominant in the Hudson soil, with earlier breakthroughs observed. Mean wick and gravity pan sampler percolate concentrations at 60 cm depth ranged from 96 to 223 μg L⁻¹ for atrazine and 54 to 78 μg L⁻¹ for 2,4-D at the Hudson site, and from 7 to 22 μg L⁻¹ for atrazine and 0.5 to 2.8 μg L⁻¹ for 2,4-D at the Arkport site. Gravity and wick pan lysimeters had comparably good collection efficiencies at elevated soil moisture levels, whereas wick pan samplers performed better at lower moisture contents. Cup samplers performed poorly with wide variations in collections and solute concentrations.