Dissolved insecticide transport

Once dissolved pesticides have been extracted from the soil matrix into overland flow or reach the surface through shallow interflow, they are transported toward the field outlet with surface runoff For one-dimensional overland flow, dissolved insecticide transport can be expressed as 76) ... 

When rainfall intensity exceeds infiltration rate and surface-storage capacity has been reached, overland flow begins. The transfer of dissolved pesticides from the soil matrix to overland flow consists of several mechanisms desorption from soil organic matter, mineral surfaces and plant residues dissolution of insecticide crystals or granules and diffusive and turbulent transport of dissolved insecticide from soil water into overland flow (2, 62). The relative importance of each process depends on the physico-chemical properties of the chemical, formulation, initial placement, soil properties, recent hydraulic history and vegetation (62). 

Dissolved insecticides are transferred from soil solution to surface runoff through the concurrent mechanisms of molecular diffusion, raindrop impact induced turbulent diffusion, and shear stress induced mass transfer (63, 64). In addition, shallow interflow may contribute dissolved chemicals to surface runoff as it returns to the surface downslope or seeps into rills and furrows 65). Most studies of dissolved chemical transport into overland flow have employed inorganic tracers such as bromide, gypsum (CaS04 2H20) and 66, 67). The behavior of organophosphorus insecticides, however, is considerably more complex due to association with particulate and colloidal natural organic matter. 

Finally, lipid-soluble chemicals like the insecticide DDT, polychlorinated biphenyls, and methylmercury are excreted readily as dissolved chemicals into milk fat droplets. Lead is secreted into milk using the same transport system as calcium. Nursing mothers may therefore transfer environmental contaminants to their infants - not to the point, however, of negating the well-established benefits of breastfeeding. 

A DDT is a toxin still found in the fatty tissues of some animals. DDT was transported into our lakes and streams as runoff from agricultural operations where it was originally used several years ago as an insecticide. In the lakes and streams it did not dissolve to any great extent it collected in the lake and stream bottoms. It entered the bodies of animals via fatty tissues in their diet microorganisms collected the DDT, the fish ate the microorganisms, and... 

Irrigation and precipitation events result in the entry of organophosphorus insecticides into the hydrologic cycle. Organophosphorus compounds deposited on foliar surfaces, dissolved in soil water and sorbed to particles are mobilized and transported with surface runoff The main mobilization processes are discussed below. 

During transport with surface runoff, organophosphorus insecticides redistribute themselves between the dissolved, colloidal and suspended particle phases. Such phase redistribution during overland transport has not been investigated and the common assumption of phase equilibrium at the field outlet has not been tested. The validation of physically based numerical models of pesticide transport in surface runoff will require careful laboratory and field experimentation that includes the effects of infiltration and sorption (77). 

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