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
Return to the Ask-An-Earth-Scientist © page