Surface runoff, pesticide transport

Disulfoton is also transported through soils or from soil to surface water (streams or rivers) via runoff. Pesticides with water solubilities >10 mg/L move mainly in solution phase in runoff water (Racke 1992). Disulfoton, with a water solubility of 25 mg/L (Sanborn et al. 1977), is expected to be found mainly in runoff water. In a runoff event from agricultural soil in Nebraska, low levels of disulfoton were detected both in the dissolved state and in eroding soil particles in the sorbed state (Spalding and Snow 1989). 

The environmental factors that influence occurrence and concentrations of pesticides include amount and timing of rainfall after pesticide application, and dilution by water bodies. Another factor that appears to influence pesticide concentrations in streams is soil permeability. Well-drained soils allow water to percolate into the groundwater. As water percolates through soil, some pesticides are filtered by the soil and broken down to degradates by bacteria. In areas with impermeable soils, more water enters streams as surface runoff. Such areas also require tile drains to make the land arable. Because tile drains lessen the underground filtration of the soils, they can transport elevated concentrations of pesticides [95,96]. 

Boyd, P.M., J.L. Baker, S.K. Mickelson, and S.I. Ahmed (2003). Pesticide transport with surface runoff and subsurface drainage through a vegetative filter strip. Tram. Am. Soc. Agric. Eng., 46(3) 675-684. 

The documented occurrence of pesticides in surface water is indicative that runoff is an important pathway for transport of pesticide away from the site of appHcation. An estimated 160 t of atrazine, 71 t of simazine, 56 t of metolachlor, and 181 of alachlor enter the Gulf of Mexico from the Mississippi River annually as the result of mnoff (47). Field appHcation of pesticides inevitably leads to pesticide contamination of surface runoff water unless runoff does not occur while pesticide residues remain on the surface of the soil. The amount of pesticides transported in a field in runoff varies from site to site. It is controlled by the timing of mnoff events, pesticide formulation, physical—chemical properties of the pesticide, and properties of the soil surface (48). Under worst-case conditions, 10% or more of the appHed pesticide can leave the edge of the field where it was appHed. 

Pesticide transport by surface runoff and soil erosion is a function of time lag between rainfall and application the chemical nature and persistence of the pesticide the hydrological, soil, and vegetative characteristics of the field and the method and target of application (43). Wauchope (44) found that unless severe rainfall occurred shortly after pesticide application, total losses for the majority of pesticides due to runoff were less than 0.5% of the amount applied in most cases, although single-event losses from small plots or watersheds can be much greater. 

Transport processes describe movement of the pesticide from one location to another or from one phase to another. Transport processes include both downward leaching, surface runoff, volatilization from the soil to the atmosphere, as well as upward movement by capillary water to the soil surface. Transport processes do not affect the total amount of pesticide in the environment however, they can move the pesticide to sites that have different potentials for degradation. Transport processes also redistribute the pesticide in the environment, possibly contaminating sites away from the site of application such as surface and groundwater and the atmosphere. Transport of pesticides is a function of both retention and transport processes. 

Organophosphorus insecticides are applied to plants and soils using a variety of methods and formulations. Because formulation and initial placement affect exposure of these compounds to transformation processes and their availability for transport in surface runoff, the influence of these factors must be understood. Formulation in particular may exert an important influence on organophosphorus insecticide loads in surface runoff. Organophosphorus insecticides are rarely applied alone, but are mixed with other substances to enhance their performance and safety. These formulation ingredients can make up to 99.5% of the applied pesticide product and include organic solvents, surfactants and polymers. 

Sorption to soil solids and plant cuticular material represents an important process influencing the chemodynamic behavior of insecticides, including their transport in surface runoff Sorption phenomena affect the volatilization, hydrolysis, photolysis and microbial transformation of organophosphorus insecticides. Furthermore, species sorbed to soil particles are transported by erosion processes rather than as solutes in the water phase. Sorption to foliar surfaces reduces the amount of pesticide mobilized by washoff. 

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) ... 

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). 

Nonpoint sources of pollution are more difficult to measure because they often cover large areas or are a composite of numerous point sources. Examples of nonpoint sources include pesticide and fertilizer runoff from agricultural fields, and urban runoff contaminated with pollutants from automobile emissions. Nonpoint sources may not be directly located next to a surface water body pollutants may be transported to surface waters by runoff from the land, by groundwater inflow, or by atmospheric transport. 

Mass balance equations of pesticide fate and transport are developed for the surface and subsurface zones in PRZM. In the surface zone, avenues of loss include soluble loss in runoff, percolation to the next zone, sortoed loss in erosion, and decay in both phases. In the subsurface zones, losses include plant uptake and percolation in the soluble phase, and decay in both phases. A backward difference, Implicit numerical scheme is used to solve the partial differential solute transport equations, with a time step of one day and a spatial increment specified by the user. 

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