STATEMENT BY HERBERT E. ALLEN BEFORE THE
HOUSE RESOURCES COMMITTEE
SUBCOMMITTEE ON ENERGY AND MINERAL
RESOURCES
SEPTEMBER 25, 2003
Good afternoon, Madam Chairman, Mr. Kind, and members of the
Committee: Mining waste materials often contain metals, such as
copper and lead, and metalloids, such as arsenic, that are commonly
regarded as having the potential to be toxic to humans and to
organisms in the environment. Whether toxicity occurs depends
critically on the physical and chemical composition of the waste
material and on the chemical conditions in the environment receiving
the waste material.
The total concentration of a contaminant is not predictive of the
toxicity observed for either humans or for other animal or plant
species. Only a fraction  the bioavailable fraction  causes a toxic
response. Both the chemical form of a substance and interactions of
the contaminant and the organism with other substances affect the
bioavailability of the contaminant. In addition there must be exposure
of the organism to the contaminant before any toxic response can
occur. I would like to discuss exposure and chemical effects on
bioavailability as two important factors that must be considered in the
evaluation of toxicity.
Toxicity data are commonly developed in the laboratory using soluble
salts. This generally supplies the contaminant in its most bioavailable
form. Some contaminants in soil, sediment, water and wastes are
often less available than would be concluded based on the total
amount present in the test. A recent panel of the National Academies
recommended greater use of bioavailability in risk assessments.
To exert a toxic response, a contaminant must be able to interact with
a receptor in the exposed organism. Generally, this requires that the
contaminant be dissolved. This can occur in the environment or
within the body of the organism. Extractions of lead and arsenic from
contaminated soils have been shown to correlate well with uptake by
animal surrogates for humans. Often, less than one-half of the lead
in a sample is bioavailable. Contaminants from other solid materials
are likewise poorly soluble in the acidic stomach of a mammal. A
soil-dwelling organism can access only a small fraction of the metal,
that which is soluble and in soil solution or is dissolved in its gut
following ingestion of soil particles.
Aquatic organisms are exposed to both inorganic particles and food
particles, and to dissolved forms of contaminants. Understanding
bioavailability of contaminants in food is an area of active research
that is being pursued by the Environmental Protection Agency (EPA)
and others. Present water quality criteria address dissolved
contaminants. However, the toxicity of a metal can vary over 200-fold
depending on the chemistry of the water. The principal factors
controlling the bioavailability are water hardness, acidity and the
concentration of dissolved natural organic matter. A new generation
of water quality criteria for metals that incorporate these factors for
the protection of aquatic life is being readied for release by EPA.
Toxicity is accurately predicted by a model called the Biotic Ligand
Model that accounts for the interaction of metals with substances that
modify their toxicity and by substances in the water that modify the
response of organisms to metals.
Currently EPA is developing a Framework for Assessing Hazards and
Risks of Metals and Metal Compounds. The goal is to develop a
cross-Agency framework describing the basic principles that need to
be considered in assessing the hazards and risks posed by metals. It
will present a consistent approach for making these assessments.
The EPA Science Advisory Board panelists who reviewed the
Agency’s plans for development of the Framework stressed the
importance of environmental chemistry and its relevance to the
assessment of both inorganic and organometallic compounds. They
also pointed out the importance of bioavailability in assessment of
risks and hazards posed by metals. The Agency should be
encouraged to apply the best science available as they complete the
Framework. This would emphasize the key roles of incorporating
exposure and bioavailability in the assessment of risk of metals.
Thank you again for this opportunity to address the Committee. I
would be pleased to answer your questions.