Pesticides

EPA defines a pesticide as a substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest or intended for use as a plant regulator, defoliant, or desiccant. Pesticides may target many organisms, among them insects, fungi, certain weeds, mites, or nematode worms. In recent years, USGS and the NYSDEC have monitored selected tributaries and Cayuga Lake for pesticides using analytical testing methods that can detect these compounds at very low concentrations. Results indicate that measurable concentrations of pesticides and their breakdown products (metabolites) have been detected in both the streams and the lake. The concentration of no individual chemical in the lake water exceeds its associated water quality standard designed to protect human health and the environment. However, toxicological data on the effects of pesticide metabolites and mixtures of chemicals are limited. The chemicals detected in Cayuga Lake and its tributaries in highest concentrations are herbicides used to control weeds in corn and soybean production. Residential land uses may also be a source.

Pesticides and their metabolites may enter ground and surface water in solution, in emulsion, or bound to soil colloids and may impair water for its designated uses. Some types of pesticides are resistant to degradation and may persist and accumulate in aquatic ecosystems. Pesticides may harm the environment by eliminating or reducing populations of desirable organisms, including endangered species. Sublethal effects include the behavioral and structural changes of an organism that jeopardize its survival. For example, certain pesticides have been found to inhibit bone development in young fish or to affect reproductive success.

Herbicides in the aquatic environment can destroy the food source for higher organisms, or reduce the amount of vegetation available for habitat and stabilization of soft sediments. Also, the decay of plant matter exposed to herbicide-containing water can cause reductions in dissolved oxygen concentration (North Carolina State University, 1984).

A major source of contamination from pesticide use is a result of their normal application. Other sources of pesticide contamination are atmospheric deposition, spray drift during the application process, misuse, and spills, leaks, and discharges that may be associated with pesticide storage, handling, and waste disposal.

The primary routes of pesticide transport to aquatic systems are (Maas et al., 1984):

  1. Direct application;
  2. In runoff;
  3. Aerial drift;
  4. Volatilization and subsequent atmospheric deposition; and
  5. Uptake by biota and subsequent movement in the food web.

The amount of field-applied pesticide that leaves a field in the runoff and enters a stream primarily depends on:

  1. The intensity and duration of rainfall or irrigation;
  2. The length of time between pesticide application and rainfall occurrence;
  3. The amount of pesticide applied and its soil/water partition coefficient;
  4. The length and degree of slope and soil composition;
  5. The extent of exposure to bare (vs. residue or crop-covered) soil;
  6. Proximity to streams;
  7. The method of application; and
  8. The extent to which runoff and erosion are controlled with agronomic and structural practices.

This variability in the actual loss of field-applied material presents challenges for a monitoring program designed to estimate the magnitude and significance of pesticide loss from agricultural fields in the Cayuga Lake watershed. An intensive monitoring of three subwatersheds in the summer of 1998 for two pesticides indicated highly variable concentrations both temporally (over the course of the storm) and spatially (between the subwatersheds with different soils, geology, and agricultural practices). (Eckhardt et al 1999).

Pesticide losses are generally greatest when rainfall is intense and occurs shortly after pesticide application, a condition for which water runoff and erosion losses are also greatest. Pesticides can be transported to receiving waters either in dissolved form or attached to sediment. Dissolved pesticides may be leached to ground-water supplies. The rate of pesticide movement through the soil profile to ground water is inversely proportional to a chemical-specific adsorption partition coefficient Kd (a measure of the degree to which a pesticide is partitioned between the soil and water phase). The higher the value of Kd, the less tendency the chemical will have to move with water. Both the degradation and adsorption characteristics of pesticides are highly variable.

Methods to Control Pesticide Loss

The most effective approach to reducing pesticide pollution of waters is, first, to release fewer pesticides and/or less toxic pesticides into the environment and, second, to use practices that minimize the movement of pesticides to surface water and ground water (EPA 1993).

The pesticide management measures identify a series of steps or thought processes that producers should use in managing pesticides. A careful field-specific review of pest problems, previous pest control measures, and cropping history must be conducted. Each area targeted for application should review soil and hydrologic conditions to estimate the potential for off-site migration to groundwater or surface water. Integrated pest management (IPM) strategies should be used to minimize the amount of pesticides applied. Pesticides should be applied efficiently and at times when precipitation or high winds are unlikely. Storage, mixing and disposal of pesticides and containers must consider the potential for losses to groundwater and surface waters. Equipment should be tested and calibrated prior to use.

EPA has compiled a list of BMPs for pesticides, illustrating the types of practices that can be applied successfully to minimize this important aspect of agricultural nonpoint source pollution (EPA 1993). The EPA list is summarized below.

(1) Inventory current and historical pest problems, cropping patterns, and use of pesticides for each field.

This can be accomplished by using a farm and field map, and by compiling the following information for each field:

(2) Consider the soil and physical characteristics of the site including mixing, loading and storage areas for potential for the leaching and/or runoff of pesticides.

In situations where the potential for loss is high, emphasis should be given to practices and/or management practices that will minimize these potential losses. The physical characteristics to be considered should include limitations based on environmental hazards or concerns such as:

(3) Use IPM strategies to minimize the amount of pesticides applied.

Following is a list of IPM strategies:

(4) When pesticide applications are necessary and a choice of materials exists, consider the persistence, toxicity, and runoff and leaching potential of products along with other factors, including current label requirements, in making a selection.

Users must apply pesticides in accordance with the instructions on the label of each pesticide product and must be trained and certified in the proper use of the pesticide. Labels include a number of requirements including allowable use rates; classification of pesticides as "restricted use" for application only by certified applicators; safe handling, storage, and disposal requirements; and any restrictions needed to protect ground water; and other requirements.

(5) Maintain records of application of restricted use pesticides (product name, amount, approximate date of application, and location of application of each such pesticide used) for a 2-year period after such use, pursuant to the requirements of the 1996 Farm Bill.

(6) Use lower pesticide application rates than those called for by the label when the pest problem can be adequately controlled using such lower rates.

(7) Consider the use of organic farming techniques that do not rely on the use of synthetically compounded pesticides.

(8) Recalibrate spray equipment each spray season and use anti-backflow devices on hoses used for filling tank mixtures.

Purchase new, more precise application equipment and other related farm equipment (including improved nozzles, computer sensing to control flow rates, radar speed determination, electrostatic applicators, and precision equipment for banding and cultivating), as replacement equipment is needed.

(9) Integrated crop management system: A total crop management system that promotes the efficient use of pesticide and nutrients in an environmentally sound and economically efficient manner.

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