Pesticide drift from the target area is a common occurrence during spray applications. As discussed in a previous post (Pesticides Drift from Spray Applications) spray applications affect where pesticides end up in the environment and their potential impact. Here, we provide important considerations regarding the fate of sprayed pesticides. Pesticide applicators are generally responsible for where pesticides end up.
Where pesticides end up after spray applications depends on their properties and environmental conditions. Key pesticide properties to consider include:
Solubility is the ability of a pesticide to dissolve in a solvent (usually water) – expressed in milligram per liter (mg/L) or parts per million (ppm). Solubility determines how far a pesticide can move in a given liquid. Therefore, solubility assists in determining how far the pesticide will go from the spray applicate site to other potential places in the environment. High solubility (greater than 1000 ppm) means a pesticide can travel farther from the application site, while low solubility (less than 100 ppm) indicates limited movement. Solubility data are not typically found on the pesticide label. However, Labels may use terms like “miscible,” “dispersible,” “suspension,” “emulsifiable,” or “soluble in water” to help indicate solubility. For example, highly soluble pesticides in solution form can travel farther, while emulsifiable concentrates usually have lower solubility and limited movement. A label indicating that a pesticide is an emulsifiable concentrate generally suggests low water solubility, meaning the pesticide is less likely to travel far from the application site. However, this is not true for all emulsifiable concentrates. Environmental conditions also affect pesticide movement; for instance, pesticides sprayed in aquatic environments are more likely to travel farther compared to those applied in terrestrial environments.
Volatility refers to a pesticide’s tendency to evaporate or turn into gas, determined by its vapor pressure which is a feature that determines whether determines the likelihood of being in the liquid or gas states. High vapor pressure indicates a greater tendency to vaporize, leading to more significant movement. Organo-auxin ester-formulated pesticides, like the ester forms of triclopyr, 2,4-D, chlopyrolid, dicamba, and MCPA, have high vapor pressures and can move considerable distances from the application area. States like Florida, Texas, Arkansas, and North Carolina have placed restrictions on these pesticides. As can be imagined, environmental conditions affect volatility and movement of air borne pesticides. High temperatures increase volatility, and windy conditions enhance dispersion.
Adsorption is the tendency of pesticides to bind to soil particles, influenced by the soil type (clay, silt, sand, loam) and organic matter content. Soil types are classified by their size, texture, proportions and different forms of organic and mineral compositions. Pesticides with lower water solubility (oil-soluble pesticides) tend to bind strongly to clay and organic matter-rich soils. Positively charged pesticides, like paraquat, diquat, and chlormequat chloride, bind strongly to negatively charged soil particles (Clay and organic matter particles), resulting in pesticides remaining localized to the area of spray application. Therefore, pesticides that stick to soil particles move less in the environment and are generally localized to the area of their application.
Persistence affects the chemical activity (potency) in any environment. Persistence is the duration a pesticide remains active in the environment, measured by its half-life (time required to reduce the pesticide concentration by 50%). Pesticides with long half-lives stay active longer, increasing their potential to move from the application site. Persistence is influenced by environmental factors like soil type, water, pH, temperature, oxygen content, among other factors. The same pesticide can have very different half life under different environmental conditions. For example, the half life of 2,4-D in water containing oxygen is short, while the half life of 2,4-D in water absent of oxygen is significantly longer. Another example is 2,4-D ester, when in water at moderately acidic pH has a long half-life (~ 1 year) while at moderately alkaline pH has a half-life that only spans hours.
Understanding these properties helps applicators make informed decisions about pesticide use and their potential environmental impact. These properties may not always be present or obvious on pesticide labels, but can may be found in product information sheets, technical data sheets or safety data sheets (SDS).
For more detailed information on pesticides properties, refer to the following databases:
