Site Conditions Affecting the Mobility of Pesticides in Soil
The site conditions affecting the mobility of pesticides in the soil include groundwater table, hydrogeologic conditions, and climatic and weather conditions.
Generally, the groundwater table is shallower in humid regions than in regions that are more arid. A shallow depth to the groundwater offers fewer opportunities for pesticide sorption and degradation.
The travel time of the pesticide to the water table may range from days to a week if the depth to the water table is shallow, and soil is permeable, and the amount of rainfall exceeds the water-holding capacity of the soil.
In contrast, the travel time may be about decades in arid regions where the water table is tens of meters below the land surface.
Hydrogeologic conditions (underground plumbing) beneath the soil profile may dictate the direction and rate of chemical movement.
The presence of impermeable lenses or layers in the soil profile and underlying strata may limit the vertical movement of pesticides. Such impermeable layers may, however, contribute to the lateral flow of shallow groundwaters and to the eventual discharge of ground waters and its contaminants into surface waters.
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On the contrary, the presence of high-permeability earth materials such as sands and gravel may greatly accelerate the vertical and horizontal flows of contaminants. Of particular concern is the presence of karsts (limestone) and fractured geologic materials that generally transmit water and chemicals rapidly to the groundwater body.
Climatic and weather conditions other than rainfall may also influence the mobility of pesticides. Warmer temperatures tend to accelerate physical, chemical, and biological processes such as volatility, water solubility, and microbial degradation, respectively.
High winds and high evaporation rates may accelerate volatilization and other processes that contribute to gaseous losses of pesticides (National Academy of Sciences, 2018).
