Imidacloprid Miscible Displacement in SW Florida Flatwoods Soil Columns.

Imidacloprid (IMD) breakthrough curves experiments (BTC) were conducted on samples from three diagnostic horizons from SW Florida Flatwoods soils from Immokalee Fine Sand (IFS) series. We analyzed the A (surface or plow layer), E (albic horizon), and Bh horizon (spodic horizons). Each horizon was packed in a column of 7.5 cm in diameter, and 15 cm in height. Florida rain water was used as the carrier, and pore water velocity was maintained between 32 and 39 cm h^-1 using a peristaltic pump (Darcy flux between 12.7 and 13.7 cm h^-1). One pulse was injected in a fertilizer mixture containing IMD with an approximate concentration of 70 μg mL^-1. Bulk densities for the A, E, and Bh soil columns were 1.47, 1.72, 1.64 g cm^-3, respectively. Pore volumes were 285, 232, 246 cm³, respectively. The pulse length was 4.7, 2.8, and 3.8 pore volumes, respectively.  Cl^- was used as the tracer solute. The effluent was collected, filtered, and analyzed in an Agillent HPLC with UV detection. Previous studies we have conducted showed that IMD is a weakly sorbed and persistent chemical in IFS. As expected, the E horizon BTC showed almost piston displacement behavior, confirming its characteristic low sorption capacity for charged and nonpolar solutes. The BTC for A and Bh horizons showed similar shapes which were properly fitted by the two-site nonequilibrium model. This data suggests that IMD has both instantaneous and rate-limited partitioning with IFS colloidal phase (mostly organic matter). This characteristic described in the model explained the long tailing in the BTC for the A and Bh horizons. Also, the retardation factor (RF) was highest for the A horizon (4.26) compared to the Bh horizon (RF = 3.12) even though the latter has a higher organic matter content. The nature of the organo-mineral complexes in IFS spodic materials may not be consider suitable for higher sorption and retardation of the IMD leaching in Florida Flatwoods soils used for citrus production. The data obtained in these studies will improve on the correct modeling of fate and transport processes of IMD SW Florida Flatwoods soils. The governing advective-dispersive transport equation for IMD in IFS conditions should include terms for: linear-sorption equilibrium, zero-order degradation, two-site sorption nonequilibrium, and a sink term for plant uptake.