4.1 Pesticide Movement (Runoff, Leaching, Volatilization) & Drift Management

Pesticide Movement and Drift Management

Quick Answer: Pesticides move off-target through runoff, leaching, volatilization, and spray drift. Drift management relies on choosing larger nozzles, reducing operating pressure, maintaining a low boom height, and avoiding temperature inversions.

Pesticides do not always remain where they are applied. Once a pesticide is released into the environment, it is subject to several physical processes that can carry it off-target. Understanding these pathways is essential for any Massachusetts pesticide applicator, both to protect public health and the state's diverse natural resources, and to avoid regulatory violations under 333 CMR 10.00. The primary mechanisms of pesticide movement include runoff, leaching, volatilization, and drift.

The Physical Pathways of Pesticide Movement

Runoff is the movement of water and suspended or dissolved materials across the soil surface. This occurs when rainfall or irrigation rate exceeds the soil's infiltration capacity, or when applications are made to sloped or compacted surfaces. Pesticides can run off dissolved in water or bound to eroding soil particles. The risk of runoff is determined by the pesticide's adsorption coefficient (Koc), its water solubility, the slope of the land, soil compaction, and the intensity and timing of precipitation. Runoff can carry pesticides directly into wetlands, streams, rivers, and coastal estuaries, which are highly sensitive habitats in Massachusetts.

Leaching is the downward movement of pesticides through the soil profile rather than across the surface. As water percolates down through the soil, it dissolves soluble chemicals and carries them toward the water table. Leaching can lead to groundwater contamination, which is a major concern in Massachusetts since a significant portion of the population relies on groundwater for public and private drinking water. Leaching risk is elevated when applying highly soluble pesticides with low soil adsorption properties to highly permeable, sandy soils.

Volatilization occurs when a solid or liquid pesticide transforms into a gas or vapor. Once volatilized, the pesticide can be carried long distances by wind currents, leading to off-target deposition and inhalation hazards. Factors that increase volatilization include high temperatures, low relative humidity, wind, dry soil surfaces, and the chemical's vapor pressure. Highly volatile formulations, such as ester-based herbicides, pose a significant risk of vapor drift, particularly during warm summer months.

Drift is the physical movement of pesticide droplets or dust particles through the air to non-target areas during or immediately after application. This is distinct from vapor drift (volatilization) and is referred to as spray drift. Managing spray drift is one of the most critical responsibilities of a certified applicator, as it is the most common cause of off-target damage complaints and pesticide exposure incidents.

Factors Influencing Spray Drift

Several environmental and equipment factors interact to determine the severity of spray drift. Applicators must evaluate these factors before and during every application.

• Wind Speed and Direction: Higher wind speeds carry droplets farther off-target. Wind is the primary driver of spray drift. Applying when wind is light and steady, typically between 3 and 10 miles per hour, blowing away from sensitive sites, is ideal. Dead calm or extremely light winds (less than 3 miles per hour) can indicate a temperature inversion, which is highly dangerous.
• Nozzle Orifice Size: Smaller orifices produce fine droplets that remain suspended in the air and drift easily. Select nozzles with larger orifices to produce medium to coarse droplets.
• Spray Pressure: High pressure forces liquid through the nozzle faster, shearing it into smaller, drift-prone droplets. Operate equipment at lower pressures within the manufacturer's recommended range.
• Boom Height: Higher spray booms increase the distance and time droplets travel through the air, giving wind more opportunity to blow them off-target. Keep the boom as low as possible while still maintaining uniform spray pattern overlap.
• Temperature and Humidity: High temperatures and low humidity accelerate droplet evaporation, shrinking their size and making them drift-prone. Avoid applications during the hottest, driest parts of the day.

Understanding Temperature Inversions

A temperature inversion is a meteorological condition that presents a severe risk for pesticide drift. Under normal conditions, air near the earth's surface is warmer than the air above it. As this warm air rises, it mixes vertically with cooler air, dispersing spray droplets that remain suspended. During a temperature inversion, this pattern is reversed: a layer of warm air traps a layer of cooler, denser air close to the ground.

Because there is no vertical air mixing, any fine pesticide droplets or vapors released during an inversion do not disperse. Instead, they form a highly concentrated cloud that floats horizontally just above the ground or crop canopy. This cloud can travel long distances with a light, barely noticeable breeze and deposit pesticides onto sensitive non-target sites, such as adjacent residential areas or organic crops.

Applicators must be able to recognize the indicators of a temperature inversion:

• Time of day: Inversions typically begin forming in the late afternoon or early evening as the soil cools, and they can persist until shortly after sunrise when the sun warms the ground.
• Wind conditions: Dead calm or extremely light winds (less than 3 miles per hour) are classic indicators.
• Visibility: Clear, cloudless nights allow heat to radiate rapidly from the soil, prompting inversions. The presence of low-lying fog, dew, or frost also suggests an inversion.
• Smoke behavior: A reliable field test is to observe smoke from a small fire or smoke generator. If the smoke rises vertically and disperses, an inversion is unlikely. If the smoke rises a short distance, flattens out, and drifts horizontally like a column or sheet, a temperature inversion is present.

Drift Management and Buffer Zones

To minimize drift, Massachusetts applicators should employ drift reduction technologies (DRTs), such as air-induction nozzles, which incorporate air into the spray droplets, making them larger and heavier. Drift-control adjuvants (deposition aids) can also be added to the spray tank to increase liquid viscosity and reduce the percentage of fine droplets.

Establishing buffer zones is another key practice. A buffer zone is an unsprayed area downwind of the application site that acts as a physical barrier to protect sensitive areas, such as surface water bodies, organic crops, residential lawns, and apiaries. Applicators must check the pesticide label for mandatory buffer zone distances, which are legally binding. When no buffer zone is specified on the label, professional judgment must be used to establish a voluntary buffer downwind to prevent off-target deposition. Under 333 CMR 10.03, applicators are legally liable for any off-target pesticide movement that causes injury or contamination.