Quantitation of bound pesticide residues in field and wetland soils

A sequential extraction scheme was developed for the quantitation of extractable and bound pesticide residues in field and wetland soils. Extractable and bound fractions of pesticides in soil were measured in spiked field and wetland soils that were stored for periods of up to ninety days. Treatment conditions included microbiological control through the use of irradiated soil and biocide. Bound pesticide residue formation was also studied as affected by moisture level, dark and light storage, and concentration. The water column of wetland microcosms was dosed with pesticides to study sorption behavior of pesticides in intact wetland systems. Extractable pesticide degradation in soils could be described by first-order rate kinetics. Dissipation curves of extractable residues in soil and from the water column of wetland microcosms exhibited biphasic kinetic behavior. Bound pesticide formation was greater in soils exposed to light than samples stored in the dark. Bound pesticides in spiked wetland soils after ninety days of storage accounted for nearly all the pesticide present in the soil and represented a major fraction of the amount of initially added pesticide. The formation of bound pesticide residues occurs during the first few days of contact with the soil with a substantial amount of instantaneous formation of bound residues. The instantaneous formation of bound residues in soil favors a model of energetically favorable entrapment of pesticides within voids and micropores arising within aromatic rich portions of the structure of aggregated humic acids. A triangular model of pesticides in soil was developed between those pesticides in the aqueous soil solution phase, pesticides partitioned to the soil phase, and those pesticides in the bound fraction. Equations were developed that describe the concentration of bound pesticides in field and wetland soils upon initial soil contact and also the maximum concentrations of bound pesticides in the soil as a function of contact time