Imidacloprid Miscible Displacement In Soil Columns Of Immokalee Fine Sand During Saturated Flow.

Imidacloprid (IMD) miscible displacement in saturated soil columns were carried out using soil samples from three diagnostic horizons of Immokalee Fine Sand (IFS) series: Ap (plow layer), E (albic horizon), and Bh (spodic layer). The soils were packed in a 7.5 cm diameter and 15 cm long column. Bulk densities for Ap, E, and Bh columns were 1.47, 1.72, 1.64 g cm^-3, respectively. The columns were initially saturated with simulated Florida rain until steady state water flow. Then about 3 to 4 pore volumes pulses of IMD (and Cl^- and NO₃^- as tracers) prepared in fertilizer mixture were applied at Darcy flux between 12.7 and 13.7 cm h^-1. After the pulse application IMD and tracers were eluted with Florida rain. IMD in the effluent was analyzed by HPLC with UV detection. The Cl^- and NO₃^-  breakthrough curves (BTCs) for Ap, E, and Bh were described by the convective-dispersive equilibrium (CDE) model implying no physical non-equilibrium in the columns. However, BTCs for IMD from Ap and Bh columns showed similar shapes with considerable tailing, and were described by the two-site non-equilibrium model (TSNE). The retardation factor (R) for IMD in the Ap column was higher (R=3.83±0.07) than the Bh column (R=3.20±0.08) even though the latter has a higher organic matter content. The BTCs of IMD and NO₃^- in the E column showed almost piston displacement (Peclet Number=91) with R=1.20±0.02 for IMD, confirming its very low sorption. Under IFS field conditions, IMD will be lost to leaching once it passes the Ap horizon where most feeder roots for citrus (and other crops) are concentrated. For modeling IMD fate and transport in the field, the TSNE model seems appropriate once a zero-order degradation term and a sink term due to IMD plant uptake are included.