Pesticides and Wyoming Homeowners

Wyoming Agriculture

Factors that increase pesticide movement toward water

Pesticide Movement in Soils

When a pesticide reaches the soil, it may be absorbed by...

Photos: Amy Jerup Teton County Weed & Pest

  • the plant,
  • destroyed by degradation processes,
  • attached (adsorbed) to soil particles, or
  • leached down through the soil.

Where the pesticide ends up depends on

  • the cumulative effects of pesticide and soil properties,
  • application methods, and
  • site conditions.

Several factors that affect pesticide movement are discussed below. The importance of these factors to leaching varies with each situation. A single factor may be more important than another in one situation and of very little consequence in the next. The diagram below shows how different factors can effect pesticide absorption into water sources.

Factors that cause chemical contamination of Water

Pesticide immigration through soils is controlled by several functions...

  • Adsorptivitiy
  • Solubility
  • Volatility
  • Degradation

Depending on the type of chemical(s) present within a pesticide, the degree of the above variables varies from one pesticide to another.

Adsorptivity is the electrical attraction between a chemical compound and soil. Pesticides strongly adsorbed to soil particles usually remain in the root zone, where they can be absorbed by plants or degrade. Weakly adsorbed pesticides move down through the soil profile with rain or irrigation water. The amount of adsorption depends upon a pesticides chemical properties, its concentration in the soil water, and on the soil type and its holding potential.

Solubility is the tendency to dissolve in water. Percolating water can carry dissolved pesticides to groundwater. In general, a highly soluble pesticide is more likely to leach than a less soluble one.

Volatility measures how quickly a pesticide evaporates in air. Highly volatile pesticides vaporize into the atmosphere, thus reducing the leaching potential. However, placing or sealing a pesticide in the soil reduces volatilization and increases leaching potential.

Degradation rate is the time required to break down a pesticide into other chemicals. Pesticides degrade by hydrolysis (reaction with water), photolysis (breakdown by sunlight), and by soil bacteria and fungi. Most degradation occurs due to sunlight and soil microorganisms. Most degradation occurs at the soil surface or within the root zone. Rapid degradation reduces leaching potential. Long-lived pesticides are more apt to leach. When a pesticide leaches below the root zone, conditions reduce degradation and increase groundwater contamination potential.

Site Conditions

Photos: Amy Jerup Teton County Weed & Pest

Geologic layers between the soil and groundwater influence pesticide movement. They vary from highly permeable gravel and fractured rock that permit rapid drainage to clay, which is virtually impermeable.

Weather, climate, and irrigation practices affect leaching and degradation. Heavy rainfall or irrigation can saturate soil. Water moves faster through saturated soil. Saturation reduces degradation potential and increases contamination risk. Generally, warmer soil temperatures speed degradation while cool temperatures retard breakdown.

Soil Properties

Soil texture and organic matter largely control pesticide movement in soil. Because sandy soils are coarse and porous, they permit relatively rapid water movement. Finer-textured clay soils have much more surface area, more adsorptivity, and limit pesticide and water movement better than sandy soils do. Organic matter has the most pesticide adsorption and water holding potential. Higher percentages of organic matter greatly lower the risk of contamination. Depth of soil to groundwater affects the amount of infiltration and degradation that can occur.