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Pesticides and Groundwater in Hawaii

Nature of Groundwater in Hawaii*

What is "Groundwater"?

Water occupying the voids in the rocks below the level of the soil is groundwater. Many people think that groundwater occurs in vast underground lakes, rivers, or streams. Usually, however, it is located in rock and soil. For at least 100 years, people in Hawaii have used groundwater for farming, drinking, cooking, washing, and other domestic uses. In 1975, just under 46 percent of all water used in Hawaii came from groundwater sources, and even more important, more than 90 percent of all water used for domestic purposes was groundwater. On Oahu, groundwater is more important than surface water sources such as ponds, lakes, streams, and rivers. In 1975, 85% of Oahu's water used for municipal, industrial, agricultural, and military purposes came from groundwater sources. During the same year, groundwater was less important on Kauaui (14%), Maui (46%), and Hawaii (18%) than surface water.

Aquifers and Recharge

An aquifer is a geologic body (such as a formation of rock or soil) which is porous and permeable enough to become saturated with water and yields water when wells are drilled into it. It is from aquifers that we get most of our groundwater. In some places, groundwater leaks to the surface as a "spring". By a complicated system of wells, pumps, pipes, valves, meters, and plumbing we are able to remove groundwater from aquifers and transport it to our homes, farms, and factories.

Recharge is water that soaks into the ground and adds to aquifers. Rainwater is the main source of recharge in Hawaii. Lakes, streams, rivers, and deep irrigation are other sources but their contribution to total recharge is small and highly variable. Fresh rainwater moves downward through small pores between particles of soil, through larger pores within and around rocks, and through cracks and channels within and between layers of old lava flows now deeply buried. (Water percolating through rock or soil can also move laterally.) When it reaches the zone of saturated rock or soil, it recharges the aquifer. The "dividing line" between the aquifer and the unsaturated rock or soil above it is called the water table.

BASAL AQUIFERS form in the base (at sea level) of some Hawaiian islands (those with caprock 1) such as Maui and Oahu. Compared to high-level aquifers described below, basal aquifers contain most of Hawaii's groundwater. Wells dug at lower elevations of Oahu and Maui reach to the tops of these basal aquifers and yield high quality drinking water. In agricultural areas of southern and central Oahu, the distance from the surface to the basal water table is 50 to 250 meters (164 - 820 feet).

In a younger island with little or no caprock such as Hawaii, recharge reaching sea level flows easily outward into the surrounding ocean. Thus the island of Hawaii has only a thin basal aquifer and groundwater is less important than surface water. In an older island with highly weathered surface soils such as Kauai, there is little recharge of the basal aquifer because water does not easily percolate into the soil. Instead, most of it runs off into surface rivers and streams. Here also, groundwater is less important than surface water.

HIGH-LEVEL AQUIFERS occur well above sea level. Two types of high-level quifers can be found in Hawaii: perched aquifers and dike-confined aquifers. They form underground where downward moving water is held up by impermeable layers of soil or dense older lava flows buried long ago by the newer soil and rock we see on the surface today. These high-level aquifers hold only a small portion of Hawaii's groundwater.

Perched aquifers form over impermeable horizontal layers such as beds of fine volcanic material (ash) or soil covered by newer volcanic material. Wells in central Oahu and Maui tap into perched aquifers. Dike-confined aquifers form in impermeable vertical (or nearly vertical) layers of dense lava aquifers called dikes2. Water tunnels dug into the windward side of Oahu's Koolau mountain range (at Waiahole and Waikane) tap into dike-confined aquifers.

Protecting Groundwater*

Pesticide Contamination of Groundwater

Leaching is the movement of a chemical (natural or synthetic) with water moving downward through soil or rock. When water that is moving downward from the surface contains chemicals - or comes into contact with them as it moves - the chemicals may be carried along with the water until they eventually reach the groundwater. Five major factors determine whether a pesticide will reach groundwater:

  • the practics followed by pesticide users,
  • the presence or absence of water on the surface of the site where the pesticides are released,
  • the chemical characteristics of the pesticides
  • the type of soil in the site where the pesticides are released
  • the location of the groundwater - its distance from the surface and the type of geological formations above it.
  • By being aware of these factors, you can handle pesticides in ways that will make the potential for groundwater contamination less likely.

    Practices for Pesticide Users

    The best way to keep from contaminating groundwater is to follow labeling directions exactly. Be sure to note whether the labeling requires you to take any special steps to protect groundwater. In addition, remember the following:

  • Use IPM (integrated pest management) strategies to reduce your need for pestcides.
  • Avoid the temptation to use more pesticide than the labeling directs. Overdosing will increase both the cost of the pest control and the odds that the pesticide will reach groundwater. Overdosing is also illegal. Keeping the use of pesticides to a minimum greatly reduces the risk of groundwater contamination.
  • Consider whether your application method presents any special risks. For example, soil injection of some pesticides may not be wise when groundwater is close to the surface.
  • Take precautions to keep pesticides from back-siphoning into your water source.
  • Locate pesticide storage facilities at least 100 feet from wells, springs, sinkholes, and other sites that directly link to groundwater to prevent their contamination from runoff or firefighting water.
  • Whenever possible, locate mixing and loading sites and equipment cleaning sites at least 100 feet from surface water or from direct links to groundwater. This will help prevent back-siphoning, runoff, and spills from contaminating the water sources. If you must locate one of these work sites near a water source, use structures such as dikes, sump pits, and containment pads to keep pesticides from reaching the water.
  • Do not contaminate groundwater through improper disposal of unused pesticides, pesticide containers, or equipment and container rinse water. Dispose of all pesticide wastes in accordance with local, State, and Federal laws.

  • Water on the Treated Surface

    If there is more water on the soil than the soil can hold, the water (along with any pesticides it contains) is likely to move downward to the groundwater. Prolonged heavy rain or excessive irrigation will leave excess water on the soli surface.


    If weather forecasts or your own knowledge of local weather signs cause you to expect heavy rain, delay outdoor handling operations - including mixing and loading, application, and disposal - to prevent wash-off, surface runof, or leaching.


    Pesticide movement into groundwater is affected by both the amount of water used in irrigation and how soon before or after a pesticide application the irrigation is done. If Irrigation water contains pesticides, be careful to prevent it from flowing into water sources.

    Pesticide Factors

    Some chemicals are more likely than others to move to groundwater. Such movement depends mainly on:

  • solubility - Some pesticides dissolve easily in water and are more likely to move into water systems.
  • adsorbtion - Some pesticides become tightly attached (strongly adsorbed) to all soil particles and are not likely to move out of the soil and into water systems.
  • persistence - Some pesticides evaporate or break down slowly and remain (persist) in the environment for a long time. These are more likely to move into water systems.
  • These factors area all related to one another. Pesticides that are most likely to move into groundwater are highly soluble, moderately to highly persistent, and are not strongly adsorbed to soil. A nonpersisitent pesticide would be less likely to move to groundwater, even if it is highly soluble or not strongly adsorbed to soil. A pesticide that is strongly adsorbed to soil would be less likely to move to groundwater even if it is persistent.

    Pesticide labeling usually does not tell you about these properties of the pesticide product. The Soil Conservation Service, Cooperative Extension Service, your trade association, or your pesticide dealer may have specific information about the characterisitics of the pesticides you are using.

    Soil Factors

    Soil is also an important factor in the breakdown and movement of pesticides. Your local Soil Conservation Service can help you determine the types for soil in your area and how they affect breakdown and movement. The three major soil characteristics that affect pesticides are texture, permeability, and organic matter.

    Soil texture is an indication of the relative proportions of sand, silt, and clay in the soil. Coarse, sandy soils generally allow water to carry the pesticides rapidly downward. Finer textured soils generally allow water to move at much slower rates. They contain more clay, and sometimes organic matter, to which pesticides may cling.

    Soil permeability is a general measure of how fast water can move downward in a particular soil. The more permeable soils must be managed carefully to keep pesticides from reaching groundwater.

    Soil organic matter influences how much water the soil can hold before it begins to move downward. Soil containing organic matter has greater ability to stop the movement of pesticides. Soils in which plants are growing are more likely to prevent pesticide movement than bare soils.


    The distance from the soil surface to the water table is the measure of how deep the groundwater is in a given location. If the groundwater is within a few feet of the soil surface, pesticides are more likely to reach it than if it is deeper down. The depth to the water table does not stay the same over the course of the year. It varies according to:

  • the amount of rain and irrigation water added to the soil surface,
  • the amount of evaporation and plant uptake,
  • how much groundwater is being withdrawn by pumping
  • The Soil Conservaation Service can provide you with valuble information on the geology of an area and on the potential for groundwater contamination on your property.

    The permeability of geological layers between the soil and groundwater is also important. If surface water can move down quickly, pesticides are more likely to reach groundwater. Gravel deposits are highly permeable. They allow water and any pesticides in it to move rapidly downward to groundwater. Regions with limestone deposits are particularly susceptible to groundwater contamination, because water may move rapidly to the groundwater through caverns or "rivers" with little filtration or chemical breakdown. On the other hand, layers of clay may be totally impermeable and may prevent most water and any pesticidse in it from reaching the groundwater.

    Sinkholes are especially troublesome. Surface water often flows into sinkholes and disappears quickly into the groundwater. If a pesticide is released into an area that drains to a sinkhole, even a moderate rain or irrigation may carry some of the pesticide directly to the groundwater.

    The Certified Applicator's Role

    Some pesticides or certain uses of some pesticides may be classified as restricted use because of groundwater concerns. As a certified applicator, you have a special responsibility to handle all pesticides safely in and near use sites where groundwater contamination is particularly likely. Take extra precautions when using techniques that are known to be likely to cause contamination of groundwater, such as chemigation and soil injection.

    When a pesticide product has been found in groundwater or has characteristics that may pose a threat of contamination of groundwater, the pesticide product labeling may contain statements to alert you to the concern. Typical pesticide labeling statements include:

    "This chemical has been identified in limited groundwater sampling and there is the possibility that it can leach through the soil to groundwater, especially where soils are coarse and groundwater is near the surface."
    "This product is readily decomposed into harmless residues under most use conditions. However, a combination of permeable and acidic soil conditions, moderate to heavy irrigation and/or rainfall, use of 20 or more pounds per acre, and soil temperature below 50 F (10 C) at application time tend to reduce degradation and promote movement of residues to groundwater. If the above describes your local conditions and groundwater in your area is used for drinking, do not use this product without first contacting (registrant's name and telephone number)."

    * * *

    At one time, it was thought that contamination of Hawaii's groundwater was highly unlikely. The great depth to the water tables and the filtering action of the percolation process provided a sense of insurance against chemical contamination. However, between 1980 and 1983, EDB, DBCP, TCP and atrazine were found in various central Oahu wells. hawaii's aquifers, on which we depend so heavily, were indeed vulnerable to pollution. And because they are so deep and large, once contaminated they are difficult to clean up.

    Since water treatment is expensive and since no treatment can remove 100% of a contaminant, prevention is the key to keeping Hawaii's drinking water safe. Pesticide applicators can contribute toward this goal by learning more about Hawaii's groundwater and avoiding both direct and indirect contamination.

    * This section of the leaflet is based mainly on Chapter 11 Groundwater in the second edition (1983) of a book, Volcanoes in the Sea: the Geology of Hawaii. Authors: Gordon A. MacDonald, A. Abbott, and F. Peterson. Publisher: University of Hawaii Press (Honolulu).
    1 Caprock forms like an apron on the island's coastal margins. It reduces the flow of fresh water from a basal aquifer outward to the surrounding ocean and allows a thicker zone of fresh water to accumulate. Caprock is built up over a few million years of water-deposited sediment eroded from the island and of marine sediment such as coral and sand. Maui and Oahu are old enough to have extensive caprock that makes basal aquifers bulge (like a lens) with fresh water upward and downward. The Pearl Harbor-Honolulu aquifer is the largest in Hawaii.
    2A dike is formed when magma (molten rock deep in the earth) is forced upward through rock or soil near the surface. The magma remaining underground cools and hardens as vertical layers of rock called dikes. Dikes are less permeable to water than surrounding rock in which they formed and can also slow the outward flow of water towards the coast. In areas where many dikes are closely spaced, water can accumulate in and saturate the more permeable rock between them and form an aquifer.

    This handout was prepared for Pesticide Applicator Training courses sponsered by the Cooperative Extension Service (part of the College of Tropical Agriculture & Human Resources, University of Hawaii). Direct comments to Charles Nagamine; Dept. of Environmental Biochemistry; 1800 East-West Rd., #328; Honolulu, HI 96822. Phone (808) 956-6007.