Transport in surface runoff

Although the downward transport of triazines by water is the most important route in evaluating the potential for presence in groundwater, other modes of transport away from the site of application should also be taken into consideration. These include plant uptake, upward transport to the soil surface by water, transport in surface runoff water and sediment, volatilization from the soil surface, spray drift during application, and movement on wind-eroded particles. This chapter will cover triazine transport across the soil surface and through the soil profile. 

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. 

Formulation and initial placement influence the susceptibility of organophosphorus insecticides to transport in surface runoff, as well as their degradation by abiotic and microbial processes. Formulation affects the kinetics of insecticide release into soil water and overland flow, as well as sorption to soil solids and plant surfaces. Spray adjuvants affect initial placement by influencing the amount of insecticide depositing on foliar and soil surfaces. Initial placement determines the relative importance of such processes as volatilization, photolysis, biodegradation, and leaching out of the zone of interaction with overland flow. 

After organophosphorus insecticides are deposited on or incorporated into the soil matrix, they distribute themselves between the air, soil air, soil water and soil solid phases. Insecticides applied to foliar surfaces partition between the plant cuticle, air and any water present on the leaf surface (e.g., dew). Phase distribution affects the amount of organophosphorus insecticide available for transfer into overland flow, as well as the mode of transport in surface runoff. 

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. 

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). 

Bowman, B.T., G.J. Wall, and D.J. King (1994). Transport of herbicides and nutrients in surface runoff from com cropland in southern Ontario. Can. J. Soil Sci., 74 59-66. 

Rohde, W.A., L.E. Asmussen, E.W. Hauser, M.L. Hester, and H.D. Allison (1981). Atrazine persistence in soil and transport in surface and subsurface runoff from plots in the coastal plain of the southern United States. Agro-Ecosystems, 1 225-238. 

Total phosphorus in surface runoff can be estimated from total suspended sediments (Bolton et al., 1991). In an agricultural watershed in Indiana, about 90% of the total phosphorus transported was bound to sediment (Monke et al., 1981). 

Misra, A.K. (1994). Effectiveness of vegetative buffer strips in reducing herbicide transport with surface runoff under simulated rainfall. Ph.D. Dissertation. Iowa State University, Ames, IA. 

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. 

Kloppel, H., Kordel, W., and Stein, B., Herbicide transport by surface runoff and herbicide retention in a filter strip — rainfall and mnoff simulation studies, Chemosphere, 35, 129-141, 1997. 

Transport and Transformation Processes Affecting Organophosphorus Insecticide Loads in Surface Runoff... 

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. 

Transport in water is an important mechanism for transfer of biogeochemical elements between the atmosphere, land, and oceans. In particular, rain is the primary means of removal from the atmosphere for many substances, and rivers (and to some extent groundwater) convey weathering products and runoff from the land surface to the oceans. 

RICEWQ was the first model developed for agrochemical runoff from paddy fields, incorporating aircraft application, dissipation by drift, adhesion on leaf surfaces, and dissipation from the leaf surface in addition to the processes affecting degradation and transport in sediment and paddy water. An important parameter, desorption from sediment to paddy water, is not considered, although this is not as important as other parameters in paddy fields such as sedimentation rate, behavior of SS, etc. 

Transport rates for dissolved material are based on the internal and external fluxes (flows) computed in the hydrology section of the module. Soluble chemicals are transported down through the soil profile and are washed out into streams with surface runoff, interflow and groundwater flow. Sediment... 

In a general way, the overall movement of phosphorus on the continents can be considered as the constant water erosion of rock and transport of P in both particulate and dissolved forms with surface runoff to river channels and further to the oceans. The intermediate transformations are connected with uptake of P as a nutrient by... 

Endrin is extremely persistent when released to the soil. It adsorbs strongly to soil particles and tends to be immobile, based on an estimated Koc of 34,000 (Kenaga 1980 Swann et al. 1983). Endrin on soil may be transported to surface water via runoff from rain or irrigation. Since endrin in solid form is... 

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 extent of transport of dissolved contaminants in overland runoff is controlled by the topography and morphology of the land (also affected by anthropogeiuc activity), the depth of chemical incorporation into soil, and the time between rainfall initiation and surface runoff commencement. In addition to these factors, transport of adsorbed contaminants on suspended particles is affected by rainfall intensity, which favors soil erosion. 

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