Chapter 8: Pesticides in the Environment

The five properties that drive movement

Whether a pesticide moves off target depends on site conditions, your practices, and five properties of the pesticide itself (all detailed in Chapter 3). Two of them pull in opposite directions, which is a favorite exam theme.

Solubility is how easily a chemical dissolves in a liquid, usually water. A highly soluble pesticide, once dissolved, tends to move with water — leaving quickly in surface runoff and leaching downward into groundwater. Sorption is the opposite story: it's the process of one substance attaching to or mixing with another (the term covers both absorption and adsorption). A pesticide strongly adsorbed (bound) to soil particles resists being carried off by runoff or leaching — but don't mistake "bound to soil" for "immobile." In dry, windy conditions, that pesticide can still go off site if the soil it's attached to blows away, carrying it to sensitive organisms. So solubility moves a pesticide with water, while adsorption holds it in place — until the soil moves.

Volatility is the tendency to turn into a vapor or gas; volatile pesticides can move off site and injure susceptible organisms, sometimes far away. Persistence measures how long a pesticide stays in an active form — the same idea as residual activity. Persistence cuts both ways: it's desirable when you want long-term control, but undesirable past the time it's needed, because the longer a pesticide persists, the more chance it has to move off target, harm nontarget species, contaminate soil or water, or damage later crops. Finally, degradation is the breakdown of a pesticide into simpler, usually less toxic compounds. It happens through chemical, physical, and biological processes — biological breakdown coming from fungi, bacteria, and other microorganisms, and sunlight (radiation) breaking certain chemical bonds.

How pesticides reach water

Protecting both groundwater and surface water is a central concern. Precipitation during or soon after application (or a spill) can greatly increase movement into surface water through runoff or into groundwater through leaching — which is why some labels list a "rain fast" time (for example, two hours), after which the product is less likely to wash off.

It helps to be precise about the two kinds of water. Groundwater is water contained in the cracks and pores of rocks underground; when it's contaminated, so is the drinking water for people and livestock that depends on it. Surface water contamination, by contrast, can injure or kill wildlife, including fish and waterfowl. Groundwater carries a seasonal risk: spring and fall are often the times of greatest recharge (rain and snowmelt) and therefore the highest water tables, and that combination of water moving down and tables sitting high raises the contamination risk (heavy rains add risk whenever they fall). The hard truth is that once groundwater is contaminated, it's difficult if not impossible to fix — about all you can do is stop the source and wait for levels to fall. That's why prevention — following the label and best practices — is the best solution.

Point vs. nonpoint sources

Water contamination comes from one of two source types, and telling them apart is a common test item. Point source pollution comes from a specific, identifiable place or incident — a spill directly entering a water body or storm sewer, back-siphoning from the spray tank into a well (the unwanted reverse flow of water), repeated spills at mixing and loading sites, careless wash-water spills, mishandled spills during transport and storage, and improper disposal of containers, rinsate, and excess pesticide. Nonpoint source pollution comes from a widespread area with no easily identifiable source — the diffuse runoff, leaching, and drift that carry pesticide off many acres over time. The quick test: a single, traceable event is point source; a broad, untraceable pattern is nonpoint.

Reading the label and using vegetative strips

Labels carry water-protection guidance, usually in the Environmental Hazards section — warnings about toxicity to fish and aquatic life, warnings about leaching potential and shallow water tables, buffer areas, and vegetative buffer strips. A key distinction from Chapter 2 reappears here: some label statements are advisory and some are mandatory. "Use caution when applying near water" is advisory; a statement that begins "Construct and maintain..." is mandatory. The example label requires a maintained vegetative filter strip of at least 25 feet of grass or permanent vegetation between the field and down-gradient aquatic habitat.

Site conditions: soil and terrain

Even by-the-label applications can harm the environment, so you assess soil type, terrain, and drainage. A soil that's deep, high in organic matter, medium-to-fine in texture (silt or clay), and well structured and drained is relatively good at holding a pesticide until it can degrade. In general, the greater the depth to groundwater, the more the soil filters and the greater the chance the pesticide degrades before reaching it. Table 8.1 sorts the key traits: coarse soils (sand, rock, gravel) have fast flow, high permeability, and little adsorption, while fine soils (clay, silt) flow slowly; and high organic content means more retention, less flow, and greater adsorption.

Terrain is the lay of the land in elevation and slope, and it tells you where a pesticide might run off. Hills and slopes increase the chance that runoff or erosion carries pesticide off site, so you take extra care where water would drain toward a storm sewer, surface water, or other sensitive area — and you consider adding a buffer even if the label doesn't require one. A buffer is an area along the edge of a site where you don't apply pesticide; it keeps the application away from sensitive areas and helps intercept runoff.

Drift and overspray — not the same thing

Drift is pesticide movement away from the site by wind or air currents, and it comes in three forms. Particle drift is movement as a granule, pellet, or dust. Vapor drift happens when a volatile pesticide turns into a gas and moves off site — and this one is sneaky, because it can occur hours or even days after the application, and since gasses are so much smaller than the smallest spray droplets, they travel the farthest (certain herbicide vapors have damaged plants miles away). Spray drift is off-site movement during a liquid application — the key word is "during." And don't assume indoors is safe: indoor applications are not immune to drift, because ventilation systems, fans, and forced-air heating and cooling can move pesticide where you didn't plan.

Overspray is a different failure. It's the direct application of pesticide beyond the target boundary — and unlike drift, which weather can drive, overspray is always applicator error: forgetting to shut off the spray while turning at the end of a field, spraying across a property line, or not turning off the equipment while crossing a stream. Keeping drift and overspray straight — air movement versus direct misapplication — is worth doing before the exam.

Reducing drift: the weather you can't control

The factors that cause spray drift split into weather-related and application-related groups. On the weather side, wind direction matters as much as speed — you need to know what's downwind at all times so you can wait until the wind moves away from sensitive sites. Counterintuitively, very light winds of 0 to 3 mph are not ideal: they can shift in variable directions near the surface and can signal a temperature inversion. Labels often specify an acceptable range such as 3 to 10 mph, and you shouldn't spray above or below it — remembering that the distance droplets travel, and the chance they leave the target, rise as wind speed rises.

A temperature inversion is when cooler air lies below warm air. The stable, dense, cool air near the ground keeps small droplets concentrated and can carry them far horizontally — miles, in enough concentration to cause damage. Inversions are strongest when a hot day is followed by a cool night, peaking in the early morning and breaking up as the sun warms the ground. This ties directly to air stability: stable air (dense, cool, near the ground, with little mixing) keeps droplets concentrated and drift-prone, while mixing air (warmed by the sun and rising) spreads droplets apart and dissipates them — think of dust on a gravel road lingering in still air but lifting and scattering when the air mixes. Humidity and temperature matter too: low humidity and/or high temperature increase evaporation, which shrinks droplets and increases drift (a small droplet can fully evaporate into a tiny drop of pure pesticide), and a pesticide's volatility rises as temperature increases and humidity decreases.

Reducing drift: the application you can control

Even in good weather, technique decides drift — and droplet size is the single most important factor. The rule is simple and worth memorizing: drift increases as droplet size decreases. So your goal is to produce the coarsest spray that still gives an effective treatment, while minimizing the volume of small droplets. For scale, a micron is 1/1,000 of a millimeter, and droplets 100 microns or smaller are the most prone to drift — barely visible and long-lived in the air.

Several equipment factors change droplet size. Increasing nozzle pressure makes droplets smaller (more drift), with aerial application the exception, where you may raise pressure to adjust for speed. Nozzle type matters a lot: flood nozzles run coarser than flat-fan, which run coarser than hollow-cone, and larger orifices with greater flow rates make coarser droplets; drift-reduction nozzles produce fewer small droplets. Low-drift nozzles make coarser sprays with fewer drift-prone droplets, but they cannot eliminate drift, and because bigger droplets often mean reduced coverage, they aren't right for every job. Nozzle height is the last lever: keep booms, spray guns, and backpack wands as low as possible for good coverage — but never below the nozzle's recommended height — because shorter tip-to-target distances mean less drift. When you need the specifics, the label's Directions for Use typically list nozzle recommendations, droplet-size categories, release heights, minimum and maximum wind speeds, buffer zones, and drift-reduction adjuvants.

How to study this chapter

1. Start with the movement pathways (drift, volatilize, runoff/erosion, rinse-in, leach, secondary poisoning) — they frame the whole chapter.
2. Learn the five properties and, above all, the solubility vs. adsorption contrast: soluble moves with water; adsorbed stays put until the soil blows or washes away.
3. Fix the degradation facts: chemical/physical/biological, microorganisms for biological, sunlight breaks bonds.
4. Nail the water facts: groundwater = cracks and pores of rock, spring/fall highest risk, contamination is nearly irreversible, prevention is best; rain-fast ≈ 2 hours.
5. Separate point vs. nonpoint by example — a spill or back-siphoning is point; runoff/leaching/drift over a wide area is nonpoint.
6. Memorize advisory vs. mandatory ("Construct and maintain..." = mandatory) and the 25-ft filter strip.
7. Distinguish the three drift types (dust = particle, gas = vapor and travels farthest, "during" = spray) from overspray (direct, always applicator error).
8. For weather, lock in 0–3 mph is risky, 3–10 mph typical range, inversion = cool air below warm (early morning, after a hot day), stable air concentrates / mixing air disperses, and hot + dry = more evaporation and volatility.
9. For equipment, the headline is droplet size is #1 and drift rises as size falls: higher pressure → smaller, flood > flat-fan > hollow-cone, low as possible (not below recommended), and ≤100 microns drifts most.