title: Quantifying soil surface photolysis under conditions simulating water movement in the field: a new laboratory test design.
abstract: Soil surface photolysis can be a significant dissipation pathway for agrochemicals under field conditions, although it is assumed that such degradation ceases once the agrochemical is transported away from the surface following rainfall or irrigation and subsequent drainage of soil porewater. However, as both downward and upward water movements occur under field conditions, relatively mobile compounds may return to the surface, prolonging exposure to ultraviolet light and increasing the potential for degradation by photolysis. To test this hypothesis, a novel experimental system was used to quantify the contribution of photolysis to the overall dissipation of a new herbicide, bicyclopyrone, under conditions that mimicked field studies more closely than the standard laboratory test guidance. Soil cores were taken from 3 US field study sites, and the surfaces were treated with [(14) C]-bicyclopyrone. The radioactivity was redistributed throughout the cores using a simulated rainfall event, following which the cores were incubated under a xenon-arc lamp with continuous provision of moisture from below and a wind simulator to induce evaporation. After only 2 d, most of the test compound had returned to the soil surface. Significantly more degradation was observed in the irradiated samples than in a parallel dark control sample. Degradation rates were very similar to those observed in both the thin layer photolysis study and the field dissipation studies and significantly faster than in the soil metabolism studies conducted in the dark. Thus, for highly soluble, mobile agrochemicals, such as bicyclopyrone, photolysis is not terminated permanently by rainfall or irrigation but can resume following transport to the surface in evaporating water.
authors.affiliation: Product Safety Department, Jealott's Hill International Research Centre, Syngenta, Bracknell, United Kingdom.
authors.forename: Laurence H
authors.initials: LH
authors.lastname: Hand
authors.affiliation: Product Safety Department, Jealott's Hill International Research Centre, Syngenta, Bracknell, United Kingdom.
authors.forename: Carol
authors.initials: C
authors.lastname: Nichols
authors.affiliation: Product Safety Department, Jealott's Hill International Research Centre, Syngenta, Bracknell, United Kingdom.
authors.forename: Sui F
authors.initials: SF
authors.lastname: Kuet
authors.affiliation: Product Safety Department, Jealott's Hill International Research Centre, Syngenta, Bracknell, United Kingdom.
authors.forename: Robin G
authors.initials: RG
authors.lastname: Oliver
authors.affiliation: Agrible, Champaign, Illinois, USA.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Harbourt
authors.affiliation: Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, Illinois, USA.
authors.forename: Essam M
authors.initials: EM
authors.lastname: El-Naggar
mesh_terms: D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000032:analysis*; D002250:Carbon Radioisotopes / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D006540:Herbicides / Q000032:analysis; D007753:Laboratories; D008027:Light; D010782:Photolysis / Q000528:radiation effects; D011753:Pyrones / Q000032:analysis* / Q000737:chemistry; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis* / Q000737:chemistry; D014872:Water Movements
chemical_list: D019086:Bridged Bicyclo Compounds, Heterocyclic; D002250:Carbon Radioisotopes; D006540:Herbicides; D011753:Pyrones; D012987:Soil; D012989:Soil Pollutants; C000607401:bicyclopyrone
keywords: Abiotic transformation; Environmental fate; Pesticide; Photolysis; Soil