Characterizing Solute Movement in Coarse-Textured, Leaching-Vulnerable Soils within Zero-Tension Column Lysimeters
Abstract
Results from field studies utilizing zero-tension column lysimeters that characterize t he fate and transport of pesticides in the soil environment have recently been submitted to DPR in support of pesticide registration in California. In Europe, these studies have been routinely submitted in support of pesticide registration, often substituting for studies that require data from monitoring wells when higher tier assessments of pesticide leaching potential have been necessary. Since it remains unclear if data obtained from lysimeters represents pesticide movement in the natural soil environment, this study compared the soil movement of several pre-emergent herbicides and their degradates in soil confined within lysimeters with soil located outside of the lysimeters, denoted from here on as being lysimeter-confined and unconfined soil, respectively. Movement of bromide in soil also was measured because it is a surrogate for the measurement of water movement. Results indicated that in a coarse-textured, leaching vulnerable soil the existence of a saturated zone at the base of the lysimeters retarded the movement of bromide, bromacil and hexazinone through the lysimeter-confined soil compared to the unconfined soil. For diuron, norflurazon, simazine and degradates that did not encounter the saturated zone at the base of the lysimeters, there was no significant difference in their fate and movement in the lysimeterconfined soil compared to the unconfined soil. The HYDRUS-1D computer model was used to investigate the possibility of simulating residues in lysimeters and relating simulated output to the fate and movement of pesticides in the unconfined, natural soil environment. Good agreement occurred between model-simulated and field-recovered residues in lysimeter-confined and unconfined soil under variable water inputs for several pesticides representing diverse levels of soil adsorption potential and degradation rates. Successful simulation of residue fate and transport between either lysimeter-confined or unconfined soil did not require adjustment or manipulation of any pesticide properties or soil hydraulic model input properties. The sole modification required for simulating pesticide residues within lysimeters was changing the soil profile bottom boundary condition from free-draining, which is applicable to the unconfined natural soil environment, to a seepage face boundary condition with the appropriate pressure head.