One drop does not stay in one place: How do pesticides move through the environment?

01

What happens after a pesticide is applied?

Scientists describe a pesticide’s journey as its “environmental fate and transport.” A substance may remain on foliage, reach the soil, break down in sunlight, be transformed by microorganisms, or move with air and water. Several of these processes can occur at the same time.

The product name alone does not predict the outcome. Solubility, volatility, binding to soil organic matter and half-life are considered alongside formulation, droplet size, equipment, application rate, wind, temperature, rainfall and slope.

Saying a pesticide was applied and then “disappeared” is therefore incomplete. It may fully degrade, temporarily bind to soil, move into another environmental compartment or form transformation products. Some of those products can be more mobile than the parent compound and may also require monitoring.

02

Movement through air: Are spray drift and volatilization the same?

The U.S. Environmental Protection Agency defines spray drift as pesticide droplets or dust moving through the air to a site other than the intended target during or soon after application. Fine droplets, unsuitable nozzle or pressure choices, excessive release height and wind can increase drift potential.

Post-application volatilization is different. Some substances can change to a vapor after reaching foliage or soil and then move through air. Temperature, the chemical’s volatility, formulation and surface conditions influence this pathway.

The two pathways create different exposure scenarios for nearby homes, workers, neighboring crops, wild plants and water bodies. Following label requirements for wind, buffers, equipment and timing—and identifying sensitive areas before work begins—can reduce off-target movement.

03

Does soil hold a pesticide or break it down?

Soil is not a uniform filter. Clay and organic matter, pH, moisture, temperature, microbial activity and pore structure affect how strongly a pesticide binds and how quickly it changes. A strongly sorbed pesticide may leach less, yet still reach surface water attached to eroded soil particles.

Breakdown may occur through sunlight, reaction with water and microbial activity. A “half-life” is an estimate of the time required for half of the starting amount to transform under stated conditions; it is not the time required for complete disappearance. Laboratory and field conditions can produce different results.

Persistence does not by itself prove harm, but it helps indicate how long exposure may continue. Environmental assessments therefore examine the parent active substance together with relevant transformation products, their mobility and their potential to accumulate.

04

How do pesticides reach surface water and groundwater?

Rainfall or irrigation can move dissolved pesticide and pesticide-bound soil particles downslope. This runoff may enter drainage channels, streams, lakes and wetlands. Heavy rain soon after application, steep or poorly covered soil and fields near water can increase the likelihood of transport.

Leaching is downward movement with water through the soil profile. High solubility, weak soil binding, longer persistence, permeable soil and shallow groundwater can increase leaching potential for some substances. It is not correct, however, to say that every pesticide will reach groundwater; the answer is chemical- and site-specific.

The U.S. Geological Survey identifies water as a major route by which pesticides move away from application areas and notes the importance of diffuse agricultural and urban sources in streams and groundwater. Monitoring therefore needs to look beyond harvested food and include the wider watershed.

05

How can exposure extend to food webs and non-target life?

Pesticides can contact beneficial insects, soil invertebrates, aquatic organisms, wild plants, birds and mammals as well as the target pest. Exposure may occur through direct spray, contaminated water or soil, treated plant material and predator–prey relationships.

The phrase “enters the food chain” does not mean every pesticide accumulates in organisms in the same way. Bioaccumulation depends on properties such as fat solubility and the rates of breakdown and elimination. Some substances transform quickly, while some persistent compounds can remain longer in tissue or sediment.

EFSA’s environmental risk approach combines how a pesticide behaves in the environment with ecotoxicology. The question is not only whether a chemical can be detected, but whether expected exposure could cause undesirable effects in fish, insects, birds, mammals, plants or ecosystem functions.

06

How can off-target movement and environmental risk be reduced?

The most effective starting point is preventing unnecessary applications. FAO’s integrated pest management approach combines monitoring, correct diagnosis, resistant varieties, crop rotation, biological control and physical measures, using pesticides in a targeted way only when technically and economically justified.

When an application is necessary, label requirements, correct dose, suitable equipment, drift-reducing nozzles, buffer zones, weather and rainfall, water bodies and sensitive habitats must be considered together. Storage, mixing, equipment cleaning and container disposal are also part of responsible lifecycle management.