There is considerable public concern about water quality, and agriculture is coming under increasing scrutiny regarding practices that can affect water quality. Many pesticides and fertilizers are soluble in water and can leach through the soil to contaminate underlying groundwater. Several factors affect the movement of chemicals in the soil and their likelihood of reaching groundwater. Consideration of these factors can minimize the threat to groundwater.
Solubility is the ability of a chemical to dissolve in a solvent. Pesticides that are highly soluble in water are often desirable from an applicator's perspective because they are less likely to clump and/or separate. However, they also pose potential water quality issues as they can easily leach into the groundwater supply. If practical, to reduce leaching potential select the least soluble material and use the lowest effective rate.
Adsorption is the binding of a chemical to the surfaces of soil particles and organic matter. Some chemicals are tightly adsorbed and do not easily leach from soils; however, they have a higher risk of moving with soil when there is surface erosion. Others have low adsorption, and will leach more easily.
Persistence refers to the amount of time a chemical will stay in the environment before being broken down into nontoxic substances. The rate of breakdown is affected by sunlight, temperature, soil pH, moisture and microbial activity. Pesticide persistence is measured in terms of half-life, which is the length of time needed for one half of the amount applied to break down. Persistent chemicals break down slowly, increasing the chance for them to leach into the soil. Conversely, short-lived materials may be degraded before significant leaching occurs. Many pesticides are broken down by sunlight (photodegradation) and/or microbial action. Incorporation of pesticides into the soil reduces or eliminates photodegradation. As depth in the soil increases, there is less microbial degradation. Any practice that slows degradation increases persistence and the likelihood of leaching. Generally, foliar applied materials are more likely to break down before significant leaching occurs than those that are applied to the soil.
Soil Characteristics and Topography. Soil texture and organic matter greatly influence the movement of pesticides and fertilizers. Fine textured soils and those with high amounts of organic matter are highly adsorptive, whereas sandy soils low in organic matter are not. Highly permeable soils with permeable underlying layers allow for rapid downward movement of water and dissolved chemicals. Steep grades and long slopes allow water to build up speed and carry soil and contaminants off-site. Know your soils and apply chemicals accordingly.
Water Table. High water tables are especially vulnerable to contamination because little time is required for chemicals to reach groundwater. Tile drainage allows contaminants to leave the field faster and enter surface waters directly through outlet pipes. Excessive precipitation or irrigation, or poorly timed irrgation, immediately after pesticide or fertilizer applications, can also move contaminants off-site and into surface and ground water quickly.
Contact Cooperative Extension and the Natural Resource Conservation Service about questions you may have regarding the use of certain pesticides on your soils.
Leachable pesticides. The following are some pesticides used on vegetables that have been identified as having high leaching potential:
- alachlor (Lasso)
- atrazine (Aatrex)
- bentazon (Basagran)
- chlorothalonil (Bravo)
- DCPA (Dacthal)
- methomyl (Lannate)
- metolachlor (Dual)
- metribuzin (Sencor/Lexone)
- simazine (Princep)