Herbicides are used to keep utility and pipeline rights-of-way, work areas, and storage sites clear of weeds in order to maintain safe and reliable service. They also play an important role in keeping railroad tracks and roadsides weed free to allow safe, unobstructed traffic flow. By choosing the right herbicides and applying them correctly, herbicides can achieve the intended goals without harming the environment.

Environmental fate refers to what happens to the herbicide after it is applied to the right-of- way. When a herbicide leaves the application equipment, it is subject to some well-defined environmental processes. These processes are influenced by the chemical and physical properties of the herbicide and the spray solution. These processes move herbicides around in the environment or restrict their movement, and regulate the breakdown of the herbicides.

Once a herbicide is released into the air, currents can move herbicide off target. Movement in the air can be reduced, but not completely eliminated. Movement of herbicides off target is an important concern to applicators. Uncontrolled movement in air can be from particle drift or volatilization. Both have the potential to cause damage to areas outside the treated site and to non-target organisms. Minimizing drift and volatilization are the applicator's prime responsibility.

Particle drift occurs when wind physically moves the herbicide and carrier from the target site during the spraying operation. Spray drift can damage susceptible plants adjacent to the right-of- way or unintentionally expose animals or people to a chemical. An application should cease if the spray cannot be kept on the target area.

• As the distance from the spray nozzle to the ground increases,
• As droplet or particle size decreases, and
• As wind speed increases.

• Spray at lower pressure,
• Use spray tips with narrow discharge angles,
• Use the largest practical nozzle openings to obtain large droplets,
• Reduce the distance between the nozzle and the weed,
• Reduce application speed,
• Use drift control additives that increase solution viscosity,
• Spray during the calmer times of the day, and
• Use attachments or modifications to spray equipment, such as shielded booms and drift-reducing nozzles.

The applicator should evaluate the wind speed and direction prior to spraying to determine the potential damage that could occur. If there are sensitive areas downwind (such as homes, water, and crops), try to make spray applications when the wind is calmest -- usually early morning or early evening, or when the wind is not blowing toward the sensitive area. If in doubt, the area should not be sprayed.

Volatilization can occur either during spraying or after application has occurred. Volatilization occurs when there is a physical change of the herbicide into gas. The resulting gas or vapor is readily moved by any wind. Herbicide vapors are created by the evaporation of the herbicide as the droplets falls from the sprayer or as the herbicide residue rests on surfaces, such as leaves, soil, asphalt, ballast, and gravel.

• As air temperature increases,
• As surface temperatures increase, and
• As relative humidity decreases.

Volatility can occur hours after an area is treated, especially during a hot summer afternoon. Vapor losses are reduced or stopped when the herbicide becomes bound or adsorbed to plant and soil surfaces, or penetrates the foliage or soil. Volatilization is not as common as particle drift, but it has the potential for moving the herbicide a greater distance from the target.

Volatility varies greatly among herbicide products and is usually associated with emulsifiable concentrate formulations. Amine formulations of most herbicides are less volatile than their ester forms. Therefore, choosing nonvolatile herbicide formulations and avoiding application during periods of excessively high temperatures can reduce volatility. Some labels note temperature limits.

Flowing water (creeks, streams, ditches) is a means for herbicides to move in the environment. Herbicides can reach water as a result of direct application, drift, spills, incorrect filling, cleanup, disposal methods, erosion, runoff, and leaching. Water from treated ditches flowing into fields can cause crop damage and may also have the potential for contaminating water used for drinking, fishing, recreation, and irrigation. Turning off the sprayer when crossing water and leaving buffer strips are effective ways of keeping herbicides out of water.

Some or all of most herbicide applications eventually reach the soil. Herbicides deposited on foliage, for example, may be washed off onto the soil by rainfall or reach the soil with dead plant parts. High-volume liquid applications may drip onto the soil. Some herbicides are applied directly onto the soil. The impact a herbicide has on the environment depends greatly upon its fate in the soil.

Herbicides move in soil in two major ways: leaching, downward movement through the soil, and lateral movement, movement of the herbicide across the soil surface or within the soil profile, usually in connection with flowing water. The amount of herbicide lost through leaching depends on

• adsorption (the attraction between soil particles and herbicide molecules),
• soil texture (amount of sand, clay and silt particles),
• herbicide solubility,
• amount and intensity of rainfall, and
• degree of soil compaction

• greatest through sandy soils and
• least through compacted soil or soils containing clays or organic matter.

Desirable trees can be injured if their roots absorb the herbicide. This can be a problem when soil active herbicides are used around roadside plantings or when they are used where the right-of-way passes through forests, along the edge of woodlots, near shade trees, or near orchards. Lateral movement becomes a concern when intense rainfall comes soon after application, or soil active herbicides have been applied to saturated soil and the rainfall cannot soak into the soil. Light showers are more likely to move the herbicide into the soil and reduce lateral movement. Use caution when applying herbicides on moderate to steep slopes, on saturated soils, on frozen soils, or on areas that may be subject to water erosion since all soil active herbicides can be move laterally. Movement down slope can be a problem after guide rail applications because the guide rails are often placed on fill slopes. The fingers of dead vegetation or scalloped edges will be visible throughout the growing season and perhaps visible into the next year. Compaction caused by construction, or hard dry soil during drought periods, encourages surface water flow. As water flows over the treated site, it picks up some of the herbicide and moves it laterally as runoff, which can be deposited where it is not wanted. Treated soil can injure plants if carried off the application site by water erosion. In sloping areas, use bare-ground treatments with caution. Downward leaching can move herbicides from inches to a couple of feet, whereas runoff or lateral movement can move herbicides much greater distances (Figure 1 a and 1b).