Quality Water for Idaho Current Information Series No. 861

Groundwater supplies more than 90 percent of the drinking water consumed in Idaho. This resource is vital to homeowners and industry as well as Idaho's agricultural community. Best management practices (BMPs) for agricultural management are becoming more important. Groundwater is found in the pores and cracks of underground sand, gravel, and rocks. The formation through which it slowly flows is called an aquifer. The top of the water-saturated zone is the water table, and water percolating down to it is called recharge.

During recent surveys, Idaho water quality specialists have found trace amounts of nitrates and, in a few cases, pesticides in the drinking water. Nitrates have been detected in drinking water from natural, livestock, and human sources, but only people can cause pesticide contamination. Pesticides are found in groundwater through agricultural, industrial, and homeowner uses as well as from spills and improper disposal. Water is difficult and expensive to manage once it disperses underground.

Pesticides start breaking down naturally in the spray tank and as soon as they are applied. This normal pesticide breakdown happens in several ways; we need to know these ways to prevent the pesticides from entering the groundwater. The pesticides can undergo hydrolysis (broken down by water), be volatilized into the air, be broken down by sunlight, be absorbed (taken up by plants), be metabolized and transformed by microorganisms, undergo oxidation in the soil, or undergo adsorption (attached to the soil by charge) (fig. 1). Some pesticides are water soluble and can be transported through the soil by irrigation or rain water. Pesticide breakdown occurs most rapidly in well-aerated, moist topsoil, and slowest in groundwater.

Fig. 1. Pesticide movement and degradation pathways in a soil-bedrock system.

Cleaning a contaminated aquifer is almost impossible. Treatment is complicated, time consuming, and expensive. The best approach to groundwater problems is to prevent contamination in the first place. Careful pesticide handling practices will protect the groundwater and help insure personal and public safety. The following pesticide management and handling practices can help you reduce the potential for groundwater contamination.

Practice integrated pest management (IPM).
Carefully time and combine pesticide applications with other pest management practices. Accurately identify the pest and apply pesticides only when necessary, using the lowest label rate that gives adequate pest control. Minimizing pesticide use makes good economic sense and reduces the potential for pest resistance and environmental problems.

Select pesticides carefully.
Pesticides that are not adsorbed to soil particles can easily move in water, do not vaporize easily, are relatively stable, and thus have the greatest potential to leach into groundwater. Read the pesticide label carefully for information and restrictions on pesticide rate, timing, and placement. All these factors can influence a pesticide's potential for leaching. Also note any groundwater advisories or other water protection guidelines on the label. If you have several choices, choose a pesticide that is least likely to cause a problem.

Consider the vulnerability of the area.
Determine the relative susceptibility of the soil to leaching. Soil texture, organic matter content, and permeability affect pesticide movement. To the extent possible, determine the depth of the water table and the relative permeability of the geologic layers between the soil surface and the groundwater. Sinkholes (sinks) can be especially troublesome because they allow surface water to reach groundwater quickly with little natural soil filtering. Avoid spraying ditches and waterways along the field.

Consider the location and condition of wells.
Cap and seal wells properly to prevent groundwater contamination. Slope the surrounding ground away from the wellhead to keep runoff away from the well. Pesticides spilled near wells can move directly and rapidly into groundwater. Therefore, do not mix, store, or dispose of pesticides within 100 feet of a well. Close all abandoned wells properly and never dispose of wastes in unused wells.

Measure accurately.
Carefully calculate how much pesticide concentrate you need to treat the specific site with the equipment you plan to use. Careful calculations will save money by reducing the amount of pesticide you use and will help eliminate disposal problems associated with excess spray mix. Prepare only enough pesticide for immediate use. If measuring equipment needs rinsing, put rinsate in the spray tank.

Calibrate accurately.
Calibrate equipment carefully and often to be certain it will apply the proper amount of pesticide. To minimize the potential for pesticide accidents or spills, check the equipment for leaks and malfunctions with only water in the tank. Repair or replace faulty equipment.

Mix and load carefully.
Handle pesticides carefully to avoid spills. Mix and load pesticides on a concrete surface to avoid saturating the soil with pesticide. In some areas of the country, applicators loading and cleaning their equipment in the same spot year after year have caused pesticide-contaminated aquifers.

Fill the spray tank as far from the water source as possible. You can increase the length of the water hose or fill the tank in the field using an alternative water source. Never leave a spray unit unattended during filling.

Prevent back siphoning.
To prevent pesticides from siphoning back into the water supply, keep the end of the fill hose above the water level in the spray tank. Use an anti-backflow device (check valve) on the fill hose, especially when siphoning water directly from a pond or stream. Be sure wells are constructed with check valves to prevent back siphoning. Add check valves to existing systems if they are used for filling pesticide tanks or for chemigation.

Do not overlap.
Turn off the sprayer or granular applicator device on turns, especially at the bottom of an irrigated field.

Check the weather forecast.
Delay pesticide application if you expect heavy or sustained rain, or plan a heavy irrigation. Rainfall soon after application increases pesticide runoff and leaching. When you apply chemicals through irrigation systems (chemigation), carefully control the quantity of irrigation water to minimize the potential for pesticide leaching and runoff.

Prevent chemigation and back flow.
Idaho's chemigation law requires antipollution devices to prevent back flow into the water source when adding chemicals through the irrigation system. A chemical injection line check valve and an interlock are needed to shut off the chemical injection when the water supply fails. A sprinkler system will need an irrigation line check valve, an automatic low pressure drain, and an inspection port. Idaho law also requires using a combination air and vacuum relief valve. A goose-neck pipe loop is an alternative. Read Idaho's chemigation laws and regulations and consult with the Idaho Department of Agriculture for possible variances.

Store pesticides safely.
Minimize your pesticide inventory by buying only what you need for the current season or specific application. Construct the pesticide storage area on a concrete floor sealed with an impervious material, located away from all water sources. This will make pesticide cleanup easier in the event of a spill or leak. Inspect containers regularly for leaks and corrosion.

Use the pesticides that have been stored longest before using those just purchased. Inspect bulk pesticide storage tanks frequently. Place tanks on concrete pads surrounded by dikes to contain pesticides if a leak develops or a spill occurs. Keep storage locked so unauthorized people do not have access to pesticides.

Dispose of wastes carefully.
Follow Idaho regulations for disposing of pesticides and pesticide containers; label directions for disposal may be illegal in Idaho. Triple rinse or pressure rinse pesticide containers as soon as they are emptied and pour the rinsates into the spray tank. Excess spray mix and rinsates from equipment cleaning can be sprayed on another site or crop listed on the label. A water source at the application site makes it easier to rinse equipment and spray rinsates in the field. When practical, store excess spray mix or rinsates in the tank to use in a later spray mix.

Never dispose of pesticides or pesticide containers near a water source, over a shallow water table, in sinkholes or in abandoned wells. Such disposal violates both federal and state laws. You may give away general use pesticides. Restricted use pesticides must be disposed of at legally approved sites or turned in on approved pick-up days. You can safely store excess pesticide concentrates until a hazardous waste collection day. You also can dispose of them through a hazardous waste transporter, but this is very expensive. Triple-rinsed containers are not considered hazardous and may be disposed of at a landfill that accepts them. You must dispose of unrinsed containers and excess pesticides at a disposal site approved by the Environmenal Protection Agency.

Prevent spills.
Anyone hauling pesticides is responsible for their safe transport. Anchor the containers so they do not tip or spill. If a spill occurs, contain and clean it up immediately. If spills occur on roads or other public areas, notify the appropriate authorities. Repeated pesticide spills in the same area can exceed the capacity of the soil to adsorb or degrade the chemical, and increases the likelihood of groundwater contamination.

Leave buffer zones around a sensitive (easily contaminated) area.
When mixing, applying, or storing pesticides, consider sensitive ground and surface waters locations. These include springs, streams, ponds, wetlands, and other surface waters. Also consider wells and groundwater recharge areas. Establish a buffer zone between a pesticide use or handling site and a sensitive area by planting vegetation or leaving an untreated border in the field.

The fate of a pesticide and the likelihood of its movement into groundwater are affected by its chemical and physical properties and those of the soil. Geology and climate, as well as the applicator's pesticide handling practices, are also important factors. Consider each factor when determining the susceptibility of groundwater to pesticide contamination.

Be sure to understand how your activities, including pesticide handling and usage, can affect groundwater. Seek assistance from your local Extension agricultural agent with questions or problems.

Checklist for Protecting Water from Pesticides

___	Does your storage facility have a concrete floor?
___	Do you clean your pesticide application equipment so that you can easily collect rinsates?
___	Does your water hose have a check valve to prevent back siphoning?
___	Have you sealed the wellhead?
___	Have you sloped the area around your well to divert surface runoff away from the well?
___	Have you properly closed all abandoned wells near a pesticide handling or application site?
___	Are there dikes around your bulk tanks to prevent off-site pesticide movement?
___	Do you know if the pesticides you use have a potential for soil leaching?
___	Do you delay the pesticide application if rain is forecast or if a major irrigation is scheduled?
___	Do you always read pesticide labels for information about irrigation practices, rates, and application methods?
___	Do you leave a border of untreated vegetation between treated and sensitive areas?
___	Do you have information about the geologic features in your area and about the depth of the groundwater?
___	Do you use pesticides only when necessry and then at the lowest label rate needed to control a pest?
___	Do you load your sprayer at least 100 feet away from a well?

Need More Information?
Obtain pesticide recommendations for various crops and pests from your local Extension agricultural agent. Contact your county Extension office for information.

The Authors -- Hugh W. Homan, Extension entomologist; Robert L. Mahler, soil scientist and Extension water quality coordinator; Gene P. Carpenter, entomologist and Extension pesticide coordinator; all in the University of Idaho Department of Plant, Soil and Entomological Sciences, Moscow.

This publication is one of a series on water quality issues produced by the University of Idaho Cooperative Extension System for the people of Idaho. The material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 90-EWQUI-1-9216.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, gender, age, disability, or status as a Vietnam-era veteran, as required by state and federal laws.

10,000 1-94 (revised) Printed with special grant funds from USDA

Comments to author: karenl@uidaho.edu

All contents copyright © 1997-2002.
College of Agricultural and Life Sciences, University of Idaho.
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Revised: January 3, 2002

URL: http://www.uidaho.edu/wq/wqpubs/cis861.html