Chapter 1-Introduction to Pathways of Metal Transfer from Mineralized Sources to Bioreceptors

By Roger P. Ashley, Elizabeth A. Bailey, Laurie S. Balistrieri, Andrea L. Foster, Larry P. Gough, Floyd Gray, Randolph A. Koski, James J. Rytuba, Robert R. Seal II, Kathleen S. Smith, Lisa L. Stillings, and Richard B. Wanty

Since 1995, the Mineral Resources Program of the U.S. Geological Survey has funded a number of studies in the western United States that address environmental issues associated with mineral deposits. Projects with environmental components in the western United States include Geoenvironmental Impacts of Hg and As; Humboldt River Basin Assessment; Coeur d'Alene Life-Cycle Models; Southwest Mineral and Environmental Investigations; and Geologic and Geoenvironmental Studies of the Western Phosphate Field. Toxicity to humans and other biota is a primary issue in these studies. Such toxicity is related to high concentrations of environmentally significant elements
released during natural weathering prior to mining, during mining of mineral
deposits, and during modern weathering
of historic mine wastes. Environmentally
significant elements, including but not
limited to Al, As, Cd, Cu, Fe, Hg, Mo,
Pb, Sb, Se, and Zn, are defined to be
elements that are essential to or adversely
impact the health of organisms.
Allowable concentrations of many environmentally
significant elements are
regulated by state and Federal agencies.
Toxicity is also related to the speciation
and location of elements in the
ecosystem. The term "speciation" in
this context means the exact chemical
forms in which an element occurs and
the quantitative distribution of different
coexisting forms.
Results of these studies provide a
scientific basis for local, state, and Federal
agencies charged with minimizing
the impacts of trace and potentially
toxic elements on the environment and
health of biota to make decisions, develop
strategy, and assess mitigation
alternatives. As some of these projects
draw to a close, we want to summarize,
in a consistent manner, the results of
these studies, identify gaps in our knowledge,
and propose topics for future research.
This collection of papers aims to
meet those goals.
The common themes among the various
environmental studies are (1) determining
the distributions and concentrations of
environmentally significant elements
throughout the study areas, (2) understanding
how elements move through the
environmental systems, and (3) identifying
the relationships between geochemical
behavior of these elements and
impacts on biota. These themes provide
a framework for examining and comparing
the various studies.
This framework or conceptual model,
presented in figure 1, has the advantage
of being transferable among ecosystems
of widely varying scales. It is composed
of three linked components: source, processes,
and bioreceptors. The component includes natural and anthropogenic
entities such as mineralized terranes,
mineral deposits and mines, mine
tailings, and waste rock. Characterization
of the source component involves
determining the concentrations of elements
in the source, identifying the
phases where trace elements reside, assessing
the speciation of the trace elements
(for example, oxidation state and
type of bonding to the mineral structure),
and determining the rates and
mechanism of release from the source.
The processes component of the
model includes all of the physical and
biogeochemical processes that act either
to redistribute or chemically transform
trace elements among sources and secondary
phases. For example, particulate
phases such as mine tailings can be
physically moved away from the source
by hydrologic transport, taken up by
bioreceptors (for instance, ingestion of
particulate Pb by humans or birds), or
transformed into dissolved species by
oxidation, reduction, dissolution, or desorption
and exchange reactions. Dissolved
species can be physically
transported by advection or diffusion
away from the source, taken up by
bioreceptors (for instance, assimilation
of methyl mercury by algae), or incorporate into particulates through precipitation and adsorption reactions. The cycling of elements between particulate
and dissolved phases is a recurring process
in a given ecosystem.
The third component of the model is
bioreceptors (including humans, livestock,
wildlife, plants, and microorganisms).
Information in this component
addresses the biogeochemical factors (for
example, molecular speciation of elements
in host phases and mechanisms of
uptake) that control the accessibility and
availability of trace elements to biota.
Using this three-component conceptual
framework, table 1 presents an overview
of the individual study areas,
summarizing their locations; characteristics
(that is, deposit types, ecoregions
and climates, elements of interest, and
relative ranges of pH); documented processes
operating within the systems;
bioreceptors and probable mechanisms
of uptake; and topics that need further
research. These studies encompass a
range of deposit types, climate regimes,
and elements of interest. Many of the
critical processes affecting element distributions
and behavior are similar in
these diverse environments and include
those related to physical movement (for
example, aeolian transport, erosion, hydrologic transport and mixing, and
fluxes across interfaces) and to transformations
between dissolved and particulate
phases (for example, adsorption and
desorption, mineral precipitation and
dissolution, and organic matter production
and oxidation). Microorganisms
mediate many of the geochemical reactions.
Bioreceptors that have been identified
in these systems include humans,
livestock, game animals, fish, amphibians,
birds, aquatic and benthic invertebrates,
and plants.
The chapters that follow discuss the
geologic setting of each study area and
answer the following questions, most of
which are directly related to the conceptual
framework:
1. What is known about the distributions,
concentrations, and speciation
of environmentally significant
elements in the system?
2. What is known about the processes
that mobilize elements from their
sources and then act to physically and
biogeochemically redistribute them?
3. What is known about impacts to
biota, and can specific processes be
identified that influence the accessibility
or availability of trace elements,
acid, and other chemical
species to biota? 4. What are the major gaps in knowledge
(that is, new directions for
research) remaining from the
study?
The ability to answer the above questions
varies among study areas because
of the scope of ongoing work, the level
of involvement of collaborative partners,
and the length of time that research has
been done. One study is just starting,
whereas others are in their fifth or sixth
year of study.
From examination of this collective
work, we propose to focus our future
research on the processes component of
the conceptual model. The following
research themes have been identified:
1. Speciation of dissolved and solid
phases and the influence of speciation
upon geoavailability (that is,
biogeochemical mobility and accessibility)
and bioavailability (that is,
uptake by humans and wildlife) of
trace elements.
2. Microbial activities and transformations.
3. Processes at physical, geochemical,
and biological interfaces.
4. Precipitation and transformations of
metal oxyhydroxides and metal
oxyhydroxysulfates and associated
interactions with trace elements. 5. Physical transport and dispersion of
trace elements.
6. Conceptual and quantitative modeling
of element behavior in mineralized
ecosystems and understanding
how elements and ecosystems respond
to natural and anthropogenic
perturbations.
Research to address these themes is
now being done in the Pathways Project,
which is funded by the Mineral Resources
Program. Specific tasks of the
Pathways project (and the related themes
from above) include:
1. Impact of Hg from placer gold
mine tailings on bioreceptors
(themes 1 and 2).
2. Biogeochemical and biochemical pathway
investigations of Cd in subarctic
ecosystems using a hyperaccumulator
species (willow) (theme 1).
3. Effect of microbially mediated Fe,
Mn, and SO4 reducing conditions
on Hg and As transformation in a
historic mining environment
(themes 1, 2, 3, and 4).
4. Mobility and bioavailability of As
and Se in the Great Basin as a function
of redox conditions (themes 1,
2, 3, and 4).
5. Modeling the underlying processes
that determine the concentrations and distributions of elements in
environmental systems as a function
of time (themes 1 through 6).
Results from these tasks will advance
our understanding of specific
processes that link geoavailability of
environmentally significant elements
to bioavailability in large-scale systems
where mineralized areas are a long-term
source of metals and other elements to the
systems. By determining the dominance
and influence of particular physical,
chemical, and biological processes that
control metal mobility and accessibility to
biota, we will be able to identify which
specific processes should be targeted
during remediation or management of
these systems to best minimize the impacts
of trace and toxic elements on the
environment and the health of biota.