Pesticides and Groundwater in Hawaiʻi
Nature of Groundwater in Hawaiʻi*
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 Hawai'i have used groundwater for farming, drinking, cooking, washing and other domestic uses. In 1975, just under 46 percent of all water used in Hawai'i came from groundwater sources, and even more important, more than 90 percent of all water used for domestic purposes was groundwater. On O'ahu, groundwater is more important than surface water sources such as ponds, lakes, streams and rivers. In 1975, 85% of O'ahu's water used for municipal , industrial, agricultural, and military purposes came from groundwater sources. During the same year, groundwater was less important on Kaua'i (14%), Maui (46%), and Hawai'i (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 Hawai'i. 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 caprock1) such as Maui and O'ahu. Compared to high level aquifers described below, basal aquifers contain most of Hawai'i's groundwater. Wells dug at lower elevations of O'ahu and Maui reach to the tops of these basal aquifers and yield high quality drinking water. In agricultural areas of southern and central O'ahu, 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 Hawai'i, recharge reaching sea level flows easily outward into the surrounding ocean. Thus the island of Hawai'i has only a thin basal aquifer and groundwater is less important than surface water. In an older island with highly weathered surface soils such a Kaua'i, 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 aquifers can be found in Hawai'i: 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 Hawai'i'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 O'ahu and Maui tap into perched aquifers. Dike-confined aquifers form in impermeable vertical (or nearly vertical) layers of dense lava called dikes2. Water tunnels dug into the windward side of O'ahu's Ko'olau mountain range (at Waiahole and Waikane) tap into dike-confined aquifers.
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 practices 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 pesticides.
- Avoid the temptation to use more pesticide than the labeling directs. Overdosing will increase both the cost of 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 soil 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 runoff, 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.
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.
- adsorption - Some pesticides become tightly attached (strongly adsorbed) to 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 are 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 nonpersistent 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 characteristics of the pesticides you are using.
Soil is also an important factor in the breakdown and movement of pesticides. Your local Soil Conservation Service can help you determine the types of 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 Conservation Service can provide you with valuable 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 pesticides in it from reaching the
Sinkholes are especially troublesome. Surface water often flows into sinkholes and disappears quickly into the groundwater. Ifa 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 use conditions and groundwater in
your area is used for drinking, do not use this
product without first contacting (registrant's name and telephone
At one time, it was thought that contamination of Hawai'i' 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 O'ahu wells. Hawai'i'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 Hawai'i's drinking water safe. Pesticide applicators can contribute toward this goal by learning more about Hawai'i's groundwater and avoiding both direct and indirect contamination.
* This section of the webpage 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).
1Caprock 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 O'ahu are old enough
to have extensive caprock that makes basal aquifers bulge Oike a lens) with fresh
water upward and downward. The Pearl
Harbor-Honolulu aquifer is the largest in Hawaii.
2 A 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 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.