Soil pesticide migration

Migration and redistribution of pesticides in the soil profile is due to diffusion in the liquid and gas phases, capillary and gravitational movement of water, absorption and exudation by the root system of plants.

Migration of pesticides in the soil profile occurs mainly due to capillary-gravitational movement of water. The migration rate and the penetration depth of pesticides depend on many factors related to both soil and climatic characteristics (particle size distribution, colloid content and sorption capacity of soils, rainfall), and the properties and doses of preparations applied to the soil.

In the case of filtration under the influence of gravitational forces, the transfer of the drug through the soil profile is usually a combination of two processes: frontal flow and “preferential” mass transfer. Frontal flow considers the uniform movement of water and solutions through the soil. The “preferential flow”, expressed in a quick “slip” down the soil profile, is the result of various factors. The movement of solutions along specific “preferred” pathways is largely due to the presence of macropores in the structural soil (cracks, passages of soil fauna, canals along the passages of roots, etc.). The macropore flow results in a faster than expected leaching of chemicals used on the soil surface, so that the solutions in the fast flowing macropore flow do not have enough time to establish equilibrium with slowly moving or standing water in the soil matrix. Other types of preferential transport, such as “palmate” and “funnel” flows (Kung, 1991), can be observed in sandy soils. Studies of the “predominant flow” are described in a number of works: Van Genuchten et al., (1990), Kladivko et al., (1991), Steenhuis and Parlange (1991), Ghodrati and Jury (1992), Smetnik et al. (2005) . Frontal flow determines the movement of the bulk of the pesticide, while micro-quantities of the drug are transferred in transit flow. Taking into account strict restrictions on the content of pesticides in drinking water, it can be assumed that the uncontrolled micro-quantities of chemicals, having reached the level of groundwater, may exceed the accepted MPC.

In a temperate zone, the majority of toxicants are in most cases localized in a layer of 0-30 cm. Penetration of an insignificant amount of pesticides to a great depth of 50-80 cm and in some cases up to 100-120 cm was observed, as a rule, in soils characterized by light particle size distribution and low humus content, in conditions of excessive moisture, as well as with increasing doses or with repeated use of pesticides. The comparative penetration depth of individual pesticides within each of the classes of organic compounds largely depends on the degree of soil absorption and solubility (Ovchinnikova, 1987).

Of the above factors of migration of the pesticide along the soil profile (diffusion in the liquid and gas phases, capillary and gravitational movement of water, absorption and exudation by the root system of plants), the downward gravitational flow of water caused by precipitation or meltwater accounts for about 80 - 85% decrease in the initial concentration of the chemical in a given soil horizon (Bondarev, 1988).