6.1 Groundwater/surface-water protection, drift prevention & buffer zones

Keeping Pesticides Out of Water

Applicators have a legal and ethical duty to keep pesticides where they are applied and out of the water people and wildlife depend on. Contamination reaches water by two main routes: leaching downward into groundwater and runoff across the surface into ponds, streams, and lakes. Once water is contaminated, cleanup is slow and costly, so prevention is the only practical strategy.

Groundwater and Leaching

Groundwater is the water stored in the saturated zone beneath the soil; its upper surface is the water table, and wells draw from it. Leaching is the downward movement of a dissolved pesticide through the soil with water. A pesticide is most likely to leach and reach groundwater when it is highly water soluble (dissolves and travels with water), persistent (a long half-life, so it does not break down before it moves), and weakly adsorbed to soil particles (low binding).

Site conditions multiply the risk:

• Sandy, coarse, porous soils with low organic matter, where water and pesticide move quickly with little binding.
• A shallow water table, a short distance for the pesticide to travel.
• Heavy rain or over-irrigation soon after application, which pushes the chemical down.
• Sinkholes, gravel, or karst geology, which offer direct pathways to the aquifer.

To protect groundwater, follow the label rate exactly, calibrate equipment, avoid applying before heavy rain, keep required setbacks from wells, and on vulnerable sites choose products that are less soluble, less persistent, or more tightly bound to soil.

Surface Water and Runoff

Surface water includes ponds, lakes, streams, rivers, and wetlands. The main threat is runoff, pesticide moving over the ground with rain or irrigation, either dissolved in the water or attached to eroding soil particles. Runoff risk rises with heavy rain soon after treatment, sloping or compacted ground, and products that bind to soil that then erodes. Reduce runoff with vegetative buffer strips, by not treating before storms, and by observing setback distances from water. Never rinse equipment or dump excess where it can reach a ditch, storm drain, or waterway.

Point-Source and Non-Point-Source Contamination

Point-source contamination comes from a single, identifiable spot, such as a spill at the mixing-and-loading pad, a leaking tank, back-siphoning into a well through the fill hose, or improper rinsate disposal. Non-point-source contamination is spread out, like runoff from an entire treated field or drift over a large area. Point sources cause a large share of well contamination and are the most preventable. Guard against them by mixing and loading away from wells and surface water on an impervious pad, keeping an air gap or anti-back-siphon device between the fill hose and the tank so tank water cannot be drawn back into the well, and cleaning up spills immediately.

Spray Drift

Drift is the movement of pesticide through the air away from the target during or shortly after application. It wastes product, injures non-target plants and animals, and can violate the label. There are two kinds: particle drift, the physical movement of spray droplets (worst with fine droplets), and vapor drift, the movement of pesticide fumes after the product volatilizes (worst with volatile products in hot weather).

Factors That Control Drift

The single most important controllable factor is droplet size. Larger droplets fall faster and drift less; fine droplets stay airborne and travel far. Applicators make droplets larger by using lower spray pressure, larger-orifice or drift-reduction nozzles, and drift-control adjuvants, and by keeping the boom low over the target.

Temperature inversions deserve special caution. In an inversion, cool air is trapped near the ground beneath a warmer layer; fine droplets hang suspended in this stable air and can drift far downwind as a concentrated cloud. Inversions form on calm, clear evenings and nights and are signaled by ground fog, dew, or smoke and dust that hang and drift sideways. Do not spray during an inversion. Counterintuitively, a light, steady breeze blowing away from sensitive areas is safer than dead calm.

Before every application, assess the weather at the site, not just the forecast: check wind speed and direction at the target, watch for signs of an inversion, and record the conditions. If the wind blows toward a sensitive area such as homes, water, an apiary, or a school, stop and wait or reposition. Applicators are legally responsible for drift damage, so documenting weather and following the label's wind and buffer limits protects both the environment and the applicator.

Buffer Zones and Setbacks

A buffer zone, or setback, is an unsprayed strip between the treated area and something you must protect, such as a stream, well, home, school, beehive, or endangered-species habitat. Many labels state a mandatory downwind buffer, for example "do not apply within 25 feet of an aquatic habitat." Required buffers may grow with the application method (aerial versus ground) and the equipment used. These statements are enforceable label directives, not suggestions. Buffers combined with drift- and runoff-reduction practices keep pesticide on target and out of water and sensitive sites.