Subject: Groundwater Contaminants
Is there a way of determining how much of the pesticides will migrate down into the groundwater in Hawaii? Once contaminants are in the groundwater, why is there a deviation in the amount of contaminants detected at the well site? Do the contaminants float on the surface of the water in the aquifers or does it thoroughly mix with the water? Land here has been used extensively for agriculture. Can one estimate how much contaminants will eventually migrate down into the groundwater or is that a wild guess?
Is there a way of determining how much of the pesticides will migrate
down into the groundwater in Hawaii?
Groundwater contamination by agriculture chemicals is of concern here in Hawaii and in many parts of the country. The problem is site specific though and a combination of field, lab, and office work is needed to make educated guesses or estimates that can answer your questions. The "office" work is formally called "modeling" which concerns the use of calculation schemes, known as simulation models. The models consist of mathematical equations that describe water and chemical movements in the ground. The problem is complicated due to many factors, including chemical and biological reactions, field changes due to geological factors, and the uneven distribution of water saturation. In general, it is very difficult to describe the rock or soil properties that can allow us to make accurate predictions. To use models, we need to know all pertinent information about the chemical in question, rock properties, rainfall and irrigation dates and amounts, chemical application dates and amounts, etc. Some of these can be measured in the lab, but most should be measured in the field. Model use requires expertise that may not be readily available.
Once contaminants are in the groundwater, why is there a deviation in
the amount of contaminants detected at the well site?
Chemicals are applied on the soil surface. Physical and chemical processes transport the chemicals down what is known as the unsaturated zone or zone of partial saturation. The amount that go through the unsaturated zone is the balance between what the farmer applies minus what the plant uses. The thickness of such a zone varies in Hawaii between a few feet near the coast to more than a 1000 ft. in central Oahu. The major physical processes are convection and dispersion. They are responsible respectively for moving the chemical down and mixing it with water. (Most chemicals are soluble in water). The chemical concentration of water reaching the aquifer is likely to be smaller than the starting value near the soil surface, depending on the amount of mixing, and on chemical and biological transformations. The aquifer is the source of water use where wells are installed and water is pumped for various uses. Once the "contaminated" water reaches the aquifer, which differs from the unsaturated zone by being fully saturated, additional mixing, motion, and transformations occur. Therefore a well that is closer to the source of chemicals should have, in general, higher concentrations. In some cases, however, chemicals can move in a different path than what is expected, due to certain geological features, such as rock cracks. The movement of water in the aquifer depends on the rate of well pumping, rates of water recharge from above, conditions near the aquifer, including the distance to the coast, etc.
Do the contaminants float on the surface of the water in the aquifers
or does it thoroughly mix with the water?
As stated above, most agricultural chemicals dissolve in water. Other chemicals, such as gasoline, float on the surface in a bulk but also dissolves in water. Thus you expect to see a floating body of gasoline, and a dissolved body of gasoline constituents.
Land here has been used extensively for agriculture. Can one estimate
how much contaminants will eventually migrate down into the groundwater
or is that a wild guess?
See the first paragraph of my answer. I am working currently on a project to assess agricultural contamination by nitrate (from fertilizers). The Department of Health and the Board of Water Supply are responsible for monitoring water for chemicals contamination. Records show that nitrate has not exceeded the maximum concentration limit. My study will help in identifying the future concentrations based on the current or future use of fertilizers. The best guess for time of travel through the unsaturated zone is about 15 years.
Dr. Aly El-Kadi, Associate Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822