MANAGING INSECTICIDES TO PREVENT GROUNDWATER CONTAMINATION

The potential for insecticide movement into groundwater exists wherever insecticides are used, but the extent varies with the chemical nature of the insecticide, the soil and other factors such as volatilization (with subsequent loss to the atmosphere), decomposition, soil retention and transport by water. Volatilization and decomposition reduce the total amounts of insecticides available for downward movement, soil retention decreases the availability of the insecticide for downward movement, and transport by water relates to the movement of insecticides with soil water. In addition, small quantities of insecticides are removed from the land in agricultural products.

The amount of insecticide applied affects the potential for groundwater contamination. The potential movement to groundwater of relatively mobile water-soluble insecticides may be much increased where large amounts have entered the soil, such as areas fused for fill stations, tank rinsing and equipment washing. In most areas, these practices should be carried out on concrete pads (or pads made from other impermeable material) and the liquid should be collected for disposal.

Organochlorines such as DDT and endrin were among the early synthetic organic insecticides. Low water solubility and a strong tendency to adsorb to soil have virtually precluded their appearance as groundwater contaminants resulting from agricultural applications.

Organophosphorus insecticides present a wide spectrum of both physiochemical properties and agricultural uses. They are generally less persistent than organochlorines and have been used to replace some organochlorine insecticides no longer registered in the United States. Breakdown in soil typically begins from reaction with water by natural and microbial hydrolysis. Examples of organophosphorus compounds include mevinphos, malathion and methyl parathion, listed in order of increasing persistence. This class of insecticides has not been detected in groundwater.

Three important members of the carbamate group are carbaryl, carbofuran and aldicarb. These are listed in order of increasing mobility, susceptibility to hydrolysis, and mammalian toxicity. Aldicarb (used also as a nematicide) is readily oxidized in soil. Aldicarb, aldicarb metabolites, and carbofuran have all been detected in groundwater. Pyrethroids include natural products and the newer family of synthetic derivatives, e.g., permethrin, cypermethrin, esfenvalerate and lambda cyhalothrin. They are usually degraded quickly in soil and are unlikely to leach. The following table gives the relative persistence and mobility of insecticides used in North Dakota. Bear in mind that the persistence and mobility classification assigned to each insecticide is approximate because environmental variation will influence persistence and mobility. Whenever several insecticide options exist for the pest/site to be treated, this data will help pesticide users and advisors select the insecticide that presents the least potential for movement to groundwater. This is particularly true when insecticide applications are anticipated in areas with a high risk for groundwater contamination.

Summary of Groundwater Contamination Potential As Influenced
by Water, Pesticide and Soil Characteristics

SUMMARY OF SUGGESTED PESTICIDE MANAGEMENT PRACTICES
TO PREVENT GROUNDWATER CONTAMINATION

Since site conditions, pest and crop patterns, and agricultural practices vary widely, specific recommendations for practices to reduce the risk of pesticide contamination must be specific and cannot be appropriate for all situation. However, measures to protect groundwater from pesticides generally involve the following objectives:

• Reduce the quantity of pesticide used (use the lowest effective rate).
• Use pesticides with less potential to leach.
• Use pesticides that are not persistent.
• Avoid pesticide application when conditions promote leaching.
• Prevent spills leading to a concentration of pesticide at a site which can leach to groundwater.
• Prevent back-siphoning or direct movement of pesticides down a well.

a   P = persistent; M = moderately persistent; N = nonpersistent;
^b   VM = very mobile; MM = moderately mobile; SM = slightly mobile; NI = nearly immobile; I = immobile

Protecting Your Groundwater Through Farmstead Assessment: There are numerous NDSU Extension circulars which address the issue of protecting groundwater from agricultural products. A listing and access to these circulars can be found on the internet at:

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