Environmental characteristics of a pesticide can often be determined by the environmental hazards statement found on pesticide product labels. The environmental hazards statement (referred to as “Environmental Hazards” on the label and found under the general heading “Precautionary Statements”) provides the precautionary language advising the user of product specific concerns. Potential environmental impacts include contamination of surface water or groundwater and toxicity to non-target organisms.
The key to preventing pesticide impacts on the environment is an understanding of the physical and chemical characteristics that determine a pesticide’s interaction with the environment: solubility, adsorption, persistence, and volatilization. These characteristics influence the potential for pesticide runoff, leaching, or drift. Once applied, pesticides can move off-site in several ways: in water, in air, attached to soil particles, and on or in objects, plants, or animals.
To prevent the off-site movement of pesticides, site-specific characteristics and prevailing conditions should be evaluated, in addition to the selection of the appropriate pesticide. Site-specific characteristics, such as soil type, depth to the water table, geology, and proximity to surface water should all be considered before selecting and applying pesticides. For example, highly permeable materials such as gravel deposits or the sandy soils of Maryland’s eastern shore allow water and dissolved compounds to freely percolate down to groundwater. In western Maryland, karsts (limestone formations with sinks or separations in the rock) underlie the soil. These sinks can act as direct entryways to groundwater for dissolved pesticides.
In addition to site characteristics, prevailing weather conditions, such as chance of precipitation, prevailing wind, humidity, etc., should be evaluated with respect to the timing of pesticide applications. For example, if rainfall is high and soils are permeable, water that carries dissolved pesticides may take only a few days to percolate downward to the groundwater.
Leaching and Runoff
Most pesticide movement in water is either by surface movement off the treated site (runoff) or by downward movement through the soil (leaching). Runoff and leaching may occur when:
- Too much pesticide is applied or spilled onto a surface.
- Too much rainwater or irrigation water moves pesticide through the soil off-site or into groundwater.
- Highly water-soluble or persistent pesticides are used.
Weakly sorbed pesticides (compounds with small Koc values) are more likely to leach through the soil and reach groundwater. Conversely, strongly sorbed pesticides (compounds with large Koc values) are likely to remain near the soil surface, reducing the likelihood of leaching but increasing the chances of being carried to surface water via runoff or soil erosion.
Drift
Pesticide movement away from the application site by wind or air currents is called drift. Pesticides may be carried off-site in the air as spray droplets, vapors, or solid particles, even on blowing soil particles.
Air drift: Air drift is a function of droplet size. Small, fine drops with diameters of 100 microns or less tend to drift away from targeted areas.
Vapor drift: Volatile pesticides can change readily from a solid or liquid form into a gas under the right conditions (i.e., high temperatures) and cause vapor drift.
Particle drift: Particle drift is the movement of solid particles from the target area by air during or just after an application. These solid particles may include pesticides formulated as dust or soil particles to which pesticides are attached.
Application techniques and the equipment used greatly influence the amount of drift that may occur. The pesticide label should be reviewed for specific information on drift reduction techniques or requirements. The applicator must also check weather conditions such as air stability, temperature, and wind direction and speed at the time and place of the application and follow all weather-related restrictions on the label.
Environmental fate- and transport-related topics are covered in detail in the following publications:
- Chapter 7 “Pesticides in the Environment” of the Maryland Pesticide Applicator Core Manual.
- Pesticide Information Leaflet No. 8: Factors Affecting Groundwater Contamination. 2012. University of Maryland Extension.
- Pesticide Information Leaflet No. 9: Protecting Groundwater from Pesticides. 2012. University of Maryland Extension.
Best Management Practices
- Use drift retardants when needed to reduce spray drift by hindering formation of small, drift-prone droplets.
- Consider pesticide sorption principles in selecting pesticides.
- Understand site characteristics that are prone to leaching losses (e.g., sand-based putting greens, coarse-textured soils, and shallow water tables) and select pesticides for these areas that have a low leaching potential.
- Understand site characteristics that are prone to runoff losses (such as steep slopes) and select pesticides for these areas with a reduced runoff potential.
- Select pesticides with reduced impact on pollinators.
- Select pesticides that, when applied according to the label, have no known effect on listed species or species of concern present on the facility.
- Time product applications for favorable prevailing conditions.
- Do not make applications during windy conditions or during temperature inversions.
- Avoid using volatile pesticides. If used, follow precautionary statements on the label and do not apply during high temperatures.
- Apply pesticides at the appropriate rate and prevent unintended releases.
- Exercise caution when using spray adjuvants that may facilitate off-target movement.
- Schedule the timing and amount of irrigation needed to water in products (unless otherwise indicated on label) without over-irrigating.
