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.
Fertilizers: Nitrogen (N) in the nitrate form is highly soluble, persistent and not adsorbed to soil particles. Nitrate N is not only leachable but is recognized as a health threat at concentrations above 10 ppm in drinking water. Infants and the elderly are most susceptible to nitrate in drinking water. The ammonium form of N is adsorbed by soil particles and is less subject to leaching. However, ammonium N is converted to nitrate N in the soil, and this can occur quite rapidly. Note that sources of N, including organic materials and urea, are converted in the soil to ammonium and then to nitrate.
Appropriate management practices can reduce the likelihood of nitrate leaching. Any time large amounts of N are applied, significant leaching can occur if there is heavy rain. By applying some of the needed N at planting and the rest during one or more sidedressings, you can avoid having large amounts of nitrate present at any one time. Not only can this reduce leaching, it can improve production by providing N during periods of greatest crop uptake.
Nitrogen left over in the soil at the end of the season is highly subject to leaching. A cover crop should be planted to take up unused N. The N will again become available for future crops as the cover crop breaks down.
Phosphorus (P) is tightly bound by the soil, but if over-applied, can leave the field through soil erosion and concentrate in and contaminate surface waters. Excess application and accumulation of P in the soil can be associated with high application rates of mineral fertilizers, manure, manure-based compost, sludge or livestock grazing. Monitor P levels by conducting soil tests on a regular basis and avoiding excess applications.
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)