Endosulfan transport: II. Modeling airborne dispersal and deposition by spray and vapor.

Endosulfan (C9H6O3Cl6S; 6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin 3-oxide) and other agricultural chemicals can be transported from farms to rivers by several airborne pathways including spray drift and vapor transport. This paper describes a modeling framework for quantifying both of these airborne pathways, consisting of components describing the source, dispersion, and deposition phases of each pathway. Throughout, the framework uses economical descriptions consistent with the need to capture the major physical processes. The dispersion of spray and vapor is described by similarity and mass-conservation principles approximated by Gaussian solutions. Deposition of particles to vegetation is described by a single-layer model incorporating contributions from settling, impaction, and Brownian diffusion. Vapor deposition to water surfaces is described by a simple kinetic formulation dependent on an exchange velocity. All model components are tested against available field and laboratory data. The models, and the measurements used for comparisons, both demonstrate that spray drift and vapor transport are significant pathways. The broader context, described in another paper, is an integrative assessment of all transport pathways (both airborne and waterborne) contributing to endosulfan transport from farms to rivers.

Endosulfan transport: I. Integrative assessment of airborne and waterborne pathways.

To reduce endosulfan (C9H6O3Cl6S; 6,7,8,9,10,10-hexachloro-1,5, 5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin 3-oxide) contamination in rivers and waterways, it is important to know the relative significances of airborne transport pathways (including spray drift, vapor transport, and dust transport) and waterborne transport pathways (including overland and stream runoff). This work uses an integrated modeling approach to assess the absolute and relative contributions of these pathways to riverine endosulfan concentrations. The modeling framework involves two parts: a set of simple models for each transport pathway, and a model for the physical and chemical processes acting on endosulfan in river water. An averaging process is used to calculate the effects of transport pathways at the regional scale. The results show that spray drift, vapor transport, and runoff are all significant pathways. Dust transport is found to be insignificant. Spray drift and vapor transport both contribute low-level but nearly continuous inputs to the riverine endosulfan load during spraying season in a large cotton (Gossypium hirsutum L.)-growing area, whereas runoff provides occasional but higher inputs. These findings are supported by broad agreement between model predictions and observed typical riverine endosulfan concentrations in two rivers.