218 Dissipation of Isoxaben in Simulated Gravel-based Nursery Runoff Retention Basins

Abstract

Recent research indicated that the herbicide simazine dissipated quickly in gravel-based subsurface-flow constructed wetlands. This indicates that retention areas at nurseries may be developed to facilitate pesticide remediation and reduce offsite movement. A site to simulate runoff retention areas at a containerized nursery was established with troughs containing pea gravel and controls containing no gravel in an open field. Irrigation water was applied daily to replace half of the capacity of the trough, simulating daily irrigation and runoff at a nursery. A study was conducted to determine the effects of this system on isoxaben (a pre-emergent herbicide) concentrations in the water leaving the troughs and the change in microbial organisms associated with the gravel. Initially, 19 L of a dilute isoxaben solution (1.3 μg/L) was added to each tank. Drainage was collected and assayed for isoxaben concentration over a 40-day period. Isoxaben was detected in troughs containing gravel through 14 days while isoxaben was detected in troughs containing no gravel through only 4 days. Microbial analysis of the gravel showed a variety of microorganisms initially, but, by day 14, Pseudomonas spp. became the dominant genus present. Preliminary analysis revealed that the isoxaben binds to the gravel, and is then desorbed over time. Further investigations will include the abilites of Pseudomonas and other isolated organisms to metabolize isoxaben as the sole carbon-source in the laboratory.