Chapter 6-Impacts of historical mining in the Coeur d'Alene River Basin

By Laurie S. Balistrieri1, Stephen E. Box1, Arthur A. Bookstrom1, Robert L. Hooper2, and J. Brian Mahoney2

BACKGROUND

Mining began in the late 1880s in the
Coeur d'Alene mining district in northern
Idaho (fig. 1). Although only two
mines, the Galena and Lucky Friday,
currently are operating, more than 90
historical mines exist in this region
(Bennett and others, 1989). Most of the
mines are along the South Fork of the
Coeur d'Alene River and its major
tributaries (Bennett and others, 1989).
Total production records indicate that
this district ranks among supergiants
(top 1 percent of world producers) for
silver (34,300 metric tons Ag) and lead
(7,290,000 metric tons Pb) and among
giants (top 10 percent of world producers)
for zinc (2,870,000 metric tons Zn)
(Long, 1998a, 1998b).
Ore deposits in the district are
steeply dipping quartz veins and siderite
(FeCO3) veins containing
stratigraphically controlled Pb-Zn-Ag
ore shoots in Precambrian rocks of the
Belt Supergroup (Fryklund, 1964;
Hobbs and others, 1965; Zartman and
Stacey, 1971; Bennett and
Venkatakrishnan, 1982; Leach and
others, 1988; Criss and Fleck, 1990).
The veins are separated into two major
types by ore mineralogy: (1) lead- and
zinc-rich veins having argentiferous
galena (PbS) and sphalerite (ZnS) and
(2) silver-rich veins having argentiferous
tetrahedrite [(Cu, Ag)10(Fe,
Zn)2(As, Sb)4S13] and minor galena and
sphalerite. The vein types are spatially
separated, may represent one or two
ages of mineralization, and were deposited
from fluids of metamorphic origin
(Leach and others, 1998; Long, 1998a).
Pyrite (FeS2) is ubiquitous but variable
in abundance in the veins. Most veins
contain small amounts of chalcopyrite
(CuFeS2). Minor minerals include
arsenopyrite (FeAsS) and pyrrhotite
(Fe1-xS). Host rocks are primarily
quartzite and argillite, which contain
some interbedded carbonate-bearing
rocks. Studies at the Lucky Friday
Mine indicate that wall rocks around
veins are altered and typically contain
10 to 15 percent carbonate minerals.
Concentric zonation with respect to
three carbonate minerals [siderite
(FeCO3), ankerite [CaFe(CO3)2], and
calcite (CaCO3)] is common in the
altered wall rocks (Gitlin, 1986). The
predominant gangue minerals are
siderite and quartz. The absolute and
relative abundances of sulfide and
gangue minerals vary significantly
between different vein systems.
Milling and ore concentration practices
varied over time in the district
because of changing technology and
economics. Early ore separation methods,
which included coarse crushing
and gravity ("jig") mineral separation
methods, were not very efficient. Jig
tailings produced before 1915 ranged
from coarse gravel to fine powder and
were still very rich in metals. Development
of more efficient flotation methods
between 1915 and 1925 resulted in
tailings with finer grain size (fine sand
and finer) and lower metal concentrations.
Most tailings were deposited
directly into the Coeur d'Alene River
and its tributaries before environmental
regulations required the installation of
tailings ponds in 1968. A preliminary
accounting by Long (1998b) has estimated
that 56 million metric tons of
tailings containing 2,200 metric tons of
Ag, 800,000 metric tons of Pb, and at
least 650,000 metric tons of Zn were
dumped into the river system. Stream
transport, especially during major flood
events, has redistributed and continues
to redistribute metal-enriched sediment
from its sources for distances of more
than 240 km downstream throughout
the channel of the South Fork and main
stem of the Coeur d'Alene River and
their floodplains, into Lake Coeur
d'Alene, and into the Spokane River
(Horowitz and others, 1993; Horowitz
and others, 1995; Bookstrom and
others, 2001; Box and others, 2001).
Because the river gradient is steeper
and the flow faster in the upper Coeur
d'Alene River system, the major repositories
of the discharged mine tailings
were the channel and floodplain of the
lower Coeur d'Alene River (that is,
between Cataldo and Harrison) and
Lake Coeur d'Alene.
The Bunker Hill lead smelter in
Kellogg (see fig. 1) operated between
1917 and 1982 (Bennett and others,
1989). This smelter released more than
3,300 metric tons of Pb to the atmosphere
between 1965 and 1981. An
area of 54 km2 surrounding the smelter
was listed by the U.S. Environmental
Protection Agency (EPA) as one of the
nation's largest Superfund sites in
1983 (U.S. Environmental Protection
Agency, 1994). The listing was
prompted, in part, by very high levels
of Pb in the blood of children living in
Kellogg. A Natural Resource Damages
(NRD) lawsuit filed by the Federal
government and two Native American
tribes is awaiting a decision after 2
months of court testimony in 2001.
The EPA completed a 3-year Remedial
Investigation/Feasibility Study (RI/FS)
for the entire Coeur d'Alene Basin in
2001. The proposed plan, in its public
comment period as of May 2002,
recommends a 30-year, $300 million
remedial program as the first increment
of a longer remediation schedule.
EPA is expected to issue a Record of
Decision laying out its remediation
plan in mid-2002. The State of Idaho
has completed some remediation
projects and is currently doing several
more within the district but outside of
the Superfund site. Although our data encompass a
wide range of elements, the following
discussion of our work focuses on the
behavior of Pb and Zn in the nearsurface
environment because of their
importance to environmental and
health issues in the Coeur d'Alene
River basin. As discussed in the introductory
chapter (ch. 1), we examine
the results of our work in light of four
questions:
1. What is known about the distributions,
concentrations, and speciation
of Pb and Zn in this river
basin?
2. What is known about the processes
that release Pb and Zn from their
sources and then act to physically
and biogeochemically redistribute
them?
3. What is known about the impacts
of Pb and Zn on biota and can we
identify specific processes that
influence bioavailability?
4. What additional work needs to be
done to provide a more thorough
understanding of Pb and Zn cycling
in this basin?
Our work was done as part of the
Coeur d'Alene Project that was funded
primarily by the Mineral Resources
Program of the U.S. Geological Survey.
The EPA and U.S. Fish and Wildlife
Service provided supplemental funding.
DISCUSSION OF MINING IMPACTS

What is known about the distributions, concentrations, and speciation of Pb and Zn in the Coeur d'Alene River basin?

Particulate Lead

Pb exists primarily in the particulate or
solid phase rather than the dissolved
phase. This is because of the low solubility
of Pb minerals and the high affinity of
dissolved Pb for metal oxide particles at
neutral pH. Health problems in the basin
for humans and wildlife are linked to
high concentrations of particulate Pb in
surface soils and sediment and to ingestion
of those particles. Elevated concentrations
of particulate Pb are associated
with soils that formed over mineralized
rocks in the area (Gott and Cathrall,
1980), tailings from mills that processed
the mineralized rock (Long, 1998b), and
atmospheric fallout from smelters that
operated in the mining district (U.S.
Environmental Protection Agency, 1994).
There are no new sources of particulate
Pb from smelters or tailings today because
of closure of the smelters and
environmental regulations that prohibit
the dumping of tailings into rivers.
However, historically produced particulate
Pb from smelter fallout and mill
tailings is constantly being redistributed
by wind and water. Because Superfund
work focused on cleaning up smelter
fallout in houses and yards, the biggest
remaining challenge is grappling with the
environmental and health impacts of
fluvially distributed tailings.
The origin of the fluvially deposited
tailings is the historical milling and
processing of ore within the district and
subsequent disposal of tailings into the
river system. The concentration of
metals in tailings from mills in the
Coeur d'Alene mining district decreased
irregularly through time as ore concentration
methods improved (fig. 2).
Gravity separation or jigging was the ore
concentration method used between
1885 and 1925. Data from the historical
Morning Mill, the remnants of which are
just west of Mullan, indicates that
tailings contained between 4 and 9
percent Pb and Zn during this time (fig.
2). Flotation methods, used between
1925 and 1968, produced tailings containing
lower metal concentrations (<1.5
percent Pb and Zn) (fig. 2). The distribution of Pb as a function of
depth in the sediments of the Coeur
d'Alene River valley reflects the onset of
mining and history of milling practices
within the district (Bookstrom and
others, 2001; Box and others, 2001).
Before mining began in the late 1880s,
the concentration of Pb was <30 ppm in
the silty clay and fine sand of the basin
(Bookstrom and others, 2001). Similar
concentrations are observed today in the
deepest portion of cores collected from
transects across the river channel at
Killarney (between Rose Lake and
Harrison) and from the adjacent floodplain
(fig. 3). Changes in milling practices
over time also are reflected in the
varying concentrations of Pb with depth
across this transect. Sediments mixed
with early jig tailings that were deposited
before about 1925 have the highest
concentrations of Pb (up to 36,000 ppm).
A sharp drop in Pb concentration to less
than 10,000 ppm, followed by a gradual
upward decrease in Pb contents to 3,000-
5,000 ppm, mark sediments from the era
of flotation methods. A gradual upward
coarsening trend from silt at the base to
medium sand at the top is typical of the
river channel and bank deposits. Deposition
rates are still high on the river banks
(average of 0.8 cm/yr) and little, if any,
diminution of sedimentation rate or
metal content in river bank or Lake
Coeur d'Alene sediments has been noted
since the dumping of tailings dumping
into the river was stopped in 1968
(Horowitz and others, 1993; Box and
others, 2001).
In sediments finer than 175 mm in the
bed of the Spokane River, the Pb content
ranges from background concentrations
(<30 ppm) to about 2,500 ppm, whereas
Zn concentrations vary from about 50 to
5,000 ppm (fig. 4). A mixing model
suggests these sediments are composed
of three types of material that have
different sources: (1) background material
with low Pb (20 ppm) and low Zn
(50 ppm) concentrations, (2) material,
possibly an authigenic flocculent, that is
poor in Pb (50 ppm) and rich in Zn
(5,000 ppm), and (3) material derived
from the Coeur d'Alene River that has
moderately high concentrations of both
Pb (2,500 ppm) and Zn (4,000 ppm)
(Box and Wallis, 2002).
Calculations of the volume of sediment
and associated Pb in various areas of the
river basin were recently done using digital
maps of the South Fork and lower Coeur
d'Alene River and chemical analyses of
many sediment samples (Bookstrom and
others, 1999; Bookstrom and others, 2001;
Box and others, 2001). These calculations
indicate that there are 200,000±100,000
metric tons of Pb in the South Fork drainage
basin, 250,000±62,000 metric tons in
the lower Coeur d'Alene River valley
between Cataldo and Harrison, and
292,000±145,000 metric tons in Lake
Coeur d'Alene. Estimates have not yet
been made for the North Fork drainage
basin or the Spokane River. About 99
percent of the Pb contained in these
sediments was added as a result of miningrelated
activities. Of the Pb in the lower
Coeur d'Alene River valley, approximately
59 percent is in the floodplain, 37 percent
in the river channel, and 4 percent in the
riverbanks. The estimated sum of Pb added
to sediments of the South Fork River
valley, Coeur d'Alene River valley, and
Lake Coeur d'Alene (739,000 + 305,000
metric tons) amounts to about 87 percent
of the estimated Pb (850,000 + 10,000
metric tons) that was dumped into tributary
streams by the mills in the mining
district.
Dissolved Zinc
Although large amounts of particulate
Zn reside in the Coeur d'Alene River
basin, it is the dissolved form of Zn that
governs water quality in this region
because of its impact on the health of biota, particularly fish. Because of this
impact, we compare concentrations of
dissolved Zn observed in the basin to the
Criterion Continuous Concentration
(CCC) for Zn, which is the highest
concentration of total recoverable Zn to
which aquatic life can be exposed for an
extended period of time (4 days) without
deleterious effects (U.S. General Services
Administration, 1999). Within the
basin, total recoverable Zn in water
samples typically is equal to dissolved
Zn (Brennan and others, 2000; Brennan
and others, 2001). The CCC for Zn in
Idaho is a function of the hardness of the
water (U.S. General Services Administration,
1999). For example, for a range
in hardness of 15 to 65 mg/L, as observed
in the Coeur d'Alene River, the
corresponding CCC for Zn ranges from
21 to 73 mg/L (U.S. General Services
Administration, 1999). A water quality
criterion for pH has also been set and
ranges from 6.5 to 9 for freshwater (U.S.
Environmental Protection Agency,
1999).
Dissolved Zn concentrations are
plotted as a function of pH in figure 5
for different types of water (adit drainage,
ground water from deep wells,
porewater in the upper 30 cm of watersaturated
sediments, water from tailings
seeps, and river water) affected by ore
deposits or mining wastes within the
Coeur d'Alene River basin. Both pH and
dissolved concentrations of Zn show
large variations, and many samples,
particularly of ground water, do not meet
water quality criteria (Mink and others,
1971; U.S. Geological Survey, 1973;
McCulley Frick and Gilman, 1994;
Balistrieri and others, 1998; Golder
Associates, 1998; TerraGraphics Environmental
Engineering, 1998; Balistrieri
and others, 2000; McCulley Frick and
Gilman, 2000). Values of pH range from
a low of 2.72 at the Kellogg Tunnel (adit
drainage) to a high of 9.1 in the Coeur
d'Alene River. Dissolved Zn concentrations
range from 1.2 mg/L to 759 mg/L,
and vary significantly at near-neutral pH
values. Table 1 summarizes the range
and median values for pH and dissolved
Zn concentrations in the various waters.
Except for water from the Kellogg
Tunnel, ground water under tailingsbearing
floodplain sediments and tailings
piles has the lowest pH values and
highest dissolved Zn concentrations.
Porewater in metal-contaminated sediment
mostly tends to be slightly acidic
(median pH = 6.57) and can have moderately
high dissolved Zn concentrations
(up to 70 mg/L). Most adit drainage,
except from the Kellogg Tunnel, has
slightly alkaline pH values (median pH
= 7.34) and moderately high dissolved
Zn concentrations (up to 58 mg/L).
Water from rivers is mostly near neutral
and tends to have the lowest dissolved
Zn concentrations, although the median
value is still above the CCC.
Solid-phase speciation
We have conducted two studies on the
speciation of metal-enriched particles in
the Coeur d'Alene River valley. The
intent was to look at changes in mineralogy
and geochemical availability of
particulate Pb and Zn as they are dispersed
from their primary sources as
sulfide minerals (galena and sphalerite)
in the ore deposits and to determine how
the redox characteristics of the depositional
environment influence the mineralogy
and geoavailability of Pb and Zn.
Our conclusions are based on mineralogical
and geochemical leach studies of
the particles.
The first study looked at the speciation
of Pb in particles from paired sites
at three locations within the mining
district. At each location, the paired sites
are the floodplain, which contains
tailings from the jig era, and the
nearby river channel. Both types of site are oxidizing environments.
Mineralogical (R.L. Hooper and C.
Rowe, unpublished data, 2000) and
leach data using the methods of
Gasser (Gasser and others, 1996) for
these particles are shown in table 2.
The Pb, Fe, and Mn contents of these
samples vary substantially, considerably
higher concentrations generally
occurring in sediment from the floodplain.
The mineralogical data indicate
the importance of Fe and Mn oxides
as host phases for Pb (also see fig. 6)
and, in some cases, the formation of
substantial amounts of Pb carbonates
and sulfates. Trace amounts of
sphalerite are present in all samples.
The sequential extraction data are in
accord with the mineralogical data in
that they indicate large fractions of Pb
associated with Fe and Mn oxides
(that is, in the EDTA or HNO3 leach)
or, in the case of sample 205, anglesite
(that is, in the MgCl2 leach).
Results from the leach simulating
gastric conditions suggest that a
major fraction of Pb in most of these
samples would be mobilized in the
human stomach. This work indicates
that the speciation of particulate Pb
changes from a sulfide mineral to Pb
carbonates and sulfates or Pb associated
with Fe and Mn oxides within a
short distance (<10 km) from the ore
deposits. These results are consistent
with observations of mineralogical
changes in Pb downstream from leadzinc-
fluorite-barite deposits in northeast
England (Hudson-Edwards and
others, 1996).
The second study looked at the
speciation of Pb and Zn in particles as a
function of depth from four different
environments (river bed, levee banks,
wetlands, and lateral lakes) in the lower
Coeur d'Alene River valley using a
combination of mineralogical and
chemical techniques (SEM, TEM, and
sequential leaches) (Hooper and
Mahoney, 2000, 2001). Care was taken
to maintain the redox characteristics of
the samples during collection, storage,
and analyses. A sequential extraction
method (Tessier and others, 1979) was
calibrated by determining the mineralogy
after each extraction step (fig. 7).
The individual steps in the sequential
extraction scheme generally are not
specific for given mineral phases.
However, patterns of metal release,
considering all of the extraction steps
along with the calibration results,
provide information about the dominant
phases in the samples.
Periodic flooding transports primary
ore minerals (sulfides) and
secondary mineral phases (oxides and
carbonates) from upstream tailings
deposits into the lower river valley.
Preliminary results of our analyses of
particles in the lower river valley
suggest that their mineralogy is very
complex and that they contain considerable
amounts of amorphous and
nanocrystalline material. The fate of
particles, which may involve mineral
dissolution, metal mobilization and
migration, and mineral precipitation,
depends on the redox characteristics,
permeability, organic content, and
microbial activity of the depositional
environment. Particles in the bed of
the river below the sediment-water
interface are in a reducing environment,
which acts as a sink for detrital
sulfides and carbonates (fig. 8).
Authigenic Fe, Pb, and Zn sulfides
appear to form near the sedimentwater
interface. Because of waterlevel
changes throughout the year,
particles in the levee environment,
near the edge of the river, cycle
between dry and wet conditions. This
results in oxidizing zones containing
oxide-coated grains interspersed with
reducing zones dominated by detrital and authigenic carbonate and sulfide
phases. Unexpectedly, detrital sphalerite
is present in oxidized levee samples,
indicating that some fraction of the
total amount of the mineral is resistant
to oxidation. Sediment at the top of
levees resides in oxidizing environments,
and samples from these environments,
contain small amounts of detrital
galena (PbS) and greater amounts of
cerrusite (PbCO3) and Pb-Fe-Mn oxides
(fig. 8). In some samples Pb is associated
with Mn oxides, and in others it is
associated with Fe oxides. Zn exists
primarily with Fe-Mn oxides. The
wetland environment is anoxic, and
most of the particulate Pb and Zn exist
as microcrystalline and nanocrystalline
to amorphous authigenic sulfides
associated with biofilms, rootlets, and
bacteria. Anoxic conditions also exist in
the sediments of the lateral lakes.
Particles in the lateral lakes contain
nanocrystalline inorganic and biogenic
sulfides in the upper third of the metalenriched
sediment and increasing
amounts of silt-size detrital sulfides
(especially sphalerite) closer to the
premining surface. In both the wetland
and lateral lake environments, microbial
activity is extremely effective in
removing metals from the water and
producing a nanocrystalline biofilm on
particles that is characterized by ZnS
and nonstoichiometric PbS, FeS, and
mixed metal sulfides.
What is known about the processes that mobilize Pb and Zn from their sources and then act to physically and biogeochemically redistribute them?

Physical processes

Several physical processes play important
roles in determining the distribution
and concentration of Pb and Zn in the
Coeur d'Alene River basin, including
those involved in historical mining
practices; hydrologic transport and
seasonal fluctuations in water levels;
sorting of particles by size fractions
during flood events; mixing of different
source waters; and transport of dissolved
metals across the sediment-water interface
by molecular diffusion. Each of
these is discussed here.
The milling practices and subsequent
disposal of tailings into the river system
before 1968 were important factors in
determining the present day geochemical
characteristics of the basin. Long (1998b)
estimated that 109 million metric tons of
metal-enriched mill tailings were produced
by jigging and flotation methods
within the district. About 51 percent of
those were disposed of in the Coeur
d'Alene River and its tributaries, 37
percent were placed in impoundments or
used as stope fill, and 12 percent were
stockpiled along the floodplain. Hydrologic
transport, especially during flood
events, resulted in sorting of this metalenriched
material and in the deposition of
fluvial tailings deposits not only within
the mining district, but also throughout
the lower Coeur d'Alene River valley, in
Lake Coeur d'Alene, and in the Spokane
River. There is a potential long-term
source of metal-enriched material to the
floodplain, Lake Coeur d'Alene, and the
Spokane River because a large fraction of
Pb (about 41 percent of Pb in the lower
Coeur d'Alene River valley) resides or is
stored in the river channel and riverbanks
(Box and others, 2001) and is subject to
movement during high flows.
Pb and Zn data for particles in the
river bed and on the levee banks (Box
and others, 2001) and for particles
suspended in the water of the Coeur
d'Alene River during two flood events
(Box and others, in press) are used to
illustrate the importance of sources of
sediment to the river (fig. 9).
Riverbanks and natural levees of the Coeur d'Alene River and alluvial
terraces of the South Fork have lower
Zn than Pb concentrations because Zn
is preferentially leached from sediments
stored in oxidizing environments.
In contrast, bed sediments in the
river channel are more enriched in Zn
than Pb. The major rain-on-snow flood
of 1996 began suddenly, when Lake
Coeur d'Alene was low and hydraulic
head between the river and the lake was
high. Floodwaters ran red with oxidized
suspended sediment, and Zn/Pb ratios of
suspended sediment were less than one,
indicating that most of the suspended
sediment was mobilized from oxidizing
environments. By contrast, the major
1997 spring runoff flood began gradually
and continued for about a month,
with relatively clear waters in the upper
basin, sluggish currents along the Coeur
d'Alene River, and floodwater backing
up from Lake Coeur d'Alene as it
overfilled. During this 1997 and previous
spring runoff floods, Zn/Pb ratios of
suspended sediment were greater than
one, indicating that fines, winnowed out
of riverbed sediment, predominated over
sediment from oxidizing environments.
In both events, however, sandy
riverbank deposits were derived from
mobilization of nearby bed sediments.
Another physical process that influences
the distribution of elements is
mixing of different source waters. Adit
drainage within the district that is enriched
in dissolved elements mixes with
rivers and creeks that have lower concentrations
of elements (Balistrieri and
others, 1998). The North Fork of the
Coeur d'Alene River supplies less metalenriched
water and sediment to the lower
Coeur d'Alene River than the South Fork
at their confluence above Cataldo. Also,
variations in metal loading along the
South Fork and its tributaries suggest
mixing of surface water with more metalenriched
ground water (Barton, 2002).
Dissolved Zn, other metals, and nutrients
can be supplied to Lake Coeur
d'Alene by inflowing rivers and by
transport across the sediment-water
interface (benthic flux). Benthic fluxes
were determined from concentrations in
the water at the bottom of the lake and in
porewater in the sediment using Fick's
First Law and time-series data from insitu
benthic flux chambers (Balistrieri,
1998; Kuwabara and others, 2000). The
results indicate that transport of dissolved
elements and species is controlled by
molecular diffusion and that the magnitudes
of the benthic fluxes of dissolved
Zn, Cd, phosphate, and nitrogen species
into the water column of the lake are
similar to fluxes supplied to the lake by
the St. Joe and Coeur d'Alene Rivers.
Biogeochemical processes
Water-rock interactions act to mobilize
and redistribute elements within the river
basin. Microorganisms mediate many of
these reactions. Oxidation reactions
involving primary sulfide minerals in the
ore deposits result in the release of
metals, sulfate, and in some cases (for
example, pyrite oxidation) acid to the
water. This acidity can be neutralized by
the dissolution of buffering minerals,
primarily carbonates such as calcite or
ankerite. The relative molar amounts of
pyrite and carbonate minerals in the
deposits and host rocks that react or the
reacting pyrite-to-calcite ratio can be
predicted from the composition of drainage
from adits (Balistrieri and others,
1999; Balistrieri and others, 2002). The
predicted reacting pyrite-to-calcite ratios
(assuming precipitation of ferrihydrite)
for deposits in the Coeur d'Alene mining
district range from 0.02 to 0.5 for nearneutral
drainage to about 1 for the acidic
drainage from the Kellogg Tunnel at the
Bunker Hill mine site (fig. 10). Data for
drainage from other polymetallic vein deposits in the Colorado Mineral Belt and
the Humboldt River Basin, Nevada, are
also shown in figure 10 for comparison
(Plumlee and others, 1993; Plumlee and
others, 1999; Nash, 2000). Theoretically,
neutralization of acid generated during
oxidation of one mole of pyrite requires
between 2 and 4 moles of calcite (that is,
reacting pyrite-to-calcite ratios between
0.25 and 0.5) (Cravotta and others, 1990).
Data for water draining polymetallic vein
deposits in Idaho, Colorado, and Nevada
are consistent with the theoretical predictions,
showing drainage pH values that
range from near neutral to alkaline when
reacting pyrite-to-calcite ratios for the
deposits are <0.3.
The precipitation of secondary minerals
such as carbonates (cerrusite and
smithsonite), sulfates (Zn sulfate and
anglesite), and oxides (Fe and Mn
oxyhydroxides and Fe
oxyhydroxysulfates) and adsorption of
dissolved elements onto the secondary
oxide phases occur after mobilization of
species from the primary sulfide minerals.
The identity and composition of these
precipitates were determined using
mineralogical, chemical, and modeling
techniques. We conducted several sets of
experiments that mixed surface water
with ground water, adit drainage, or
porewater and assessed the ability of
thermodynamic models to predict the
precipitation of mineral phases and
adsorption of metals onto metal oxide
phases (Paulson and Balistrieri, 1999;
Tonkin and others, 2002) using
PHREEQC (Parkhurst and Appelo, 1999)
and a database containing sorption
parameters for elements onto hydrous
ferric oxide (Dzombak and Morel, 1990).
Many important conclusions resulted
from this work. First, thermodynamic
modeling successfully predicted observed
pH changes, precipitation of ferrihydrite,
and removal of Cd, Cu, Pb, Zn, and, in
some cases, As in the mixing experiments
(fig. 11). Second, the relative efficiency
of removal of metals by adsorption onto
Fe oxide is Pb>Cu>> Zn~Cd. Third, Fe
oxide is a stronger adsorbent than Al
oxide, and adsorbed organic matter is an
important adsorbent for Zn and Cd.
Fourth, the adsorption characteristics of
ferrihydrite and schwertmannite can be
described by the same set of adsorption
parameters (that is, site density, surface
area, and adsorption complexation
constants). Fifth, more metal is removed
by adsorption if Fe oxides that
precipitate during the mixing of acidic
ground water and near-neutral surface
water remain suspended in the water
(transported scenario) rather than being
separated from the water and remaining
in the aquifer (retained scenario) along
a flow path. Because pH increases
along the flow path, greater metal
removal occurs in the transported than
in the retained case because the zone of
precipitation of Fe oxide, which occurs
at a lower pH, is not separated from the
zone of adsorption of metals onto Fe
oxide, which occurs at a higher pH.
The influence of organic matter
diagenesis on redox state has been
demonstrated in porewater studies in the
sediments of Lake Coeur d'Alene and in
sediments along the river's edge and in
marshes of the lower Coeur d'Alene
River valley (Balistrieri, 1998; Balistrieri
and others, 2000). Organic matter
decomposition occurs using a thermodynamically
predictable sequence of
oxidants (oxygen, nitrate, Mn oxide, Fe
oxide, and sulfate) and results in redox
conditions that range from oxic to
suboxic, or to anoxic and sulfidic as
depth increases in the sediment (Froelich
and others, 1979). Redox state, in turn,
influences the mobility of elements,
particularly Zn, because of the release of
associated elements during reduction of
oxide phases under suboxic conditions
and the insolubility of authigenic metalsulfide phases under anoxic and sulfidic
conditions. Porewater data from sediments
in the lower Coeur d'Alene River
valley suggest that Zn is more mobile
(that is, higher concentrations in
porewater) under oxidizing than under
reducing conditions, most likely because
of formation of authigenic Zn sulfides
under anoxic and sulfidic (reducing)
conditions.
Modeling of coupled physical and
biogeochemical processes
Metal distributions and concentrations
in the environment are a result
of the interaction of many physical
transport processes and biogeochemical
reactions. Understanding the dominant
processes is critical for successful design
of remediation activities and for longterm
management of ecosystems affected
by human activities. Development
of mathematical models that describe
metal cycling in such systems synthesizes
our understanding of how the
system works by identifying and describing
dominant processes and potentially
allows for sensitivity tests to assess
how a system might respond to perturbations.
Models can also assess our understanding
of system behavior by comparing
observations and predictions during
and after remediation activities. One
such model describing the behavior of
dissolved Zn in Lake Coeur d'Alene is
discussed here.
We used a mass balance approach to
examine various processes that control
the concentration of dissolved Zn in
Lake Coeur d'Alene (L.S. Balistrieri and
P.F. Woods, unpublished data, 2002).
Lake Coeur d'Alene is treated as a
single, completely mixed system (that is,
a one-box model) (fig. 12). The following
equation describes changes in dissolved
Zn concentrations in the lake as a
function of time (dCZn/dt):
dCZn/dt = IZn + PZn - OZn - RZn (1)
where IZn accounts for all external inputs
of dissolved Zn to the lake, PZn accounts
for all internal sources of dissolved Zn
to the lake, OZn accounts for fluxes of
dissolved Zn out of the lake, and RZn
accounts for internal removal of dissolved
Zn from the water column. All
terms have units of mg/L per day.
Dissolved Zn enters the lake through
river inlets (Coeur d'Alene, St. Joe, and
St. Maries Rivers) and leaves through a
single outlet (Spokane River). These
processes are depicted by the IZn and OZn
terms in equation 1. Benthic fluxes of
dissolved Zn, represented by the PZn term
in equation 1, act as an additional source
of dissolved Zn to the lake, whereas
scavenging of dissolved Zn to particulate
Zn and sedimentation, depicted by RZn,
removes dissolved Zn from the water
column of the lake.
Calculations were done using the
computer program AQUASIM (Reichert,
1994), which allows for simulations of
aquatic systems. Discharge and dissolved
concentrations in the rivers for
three water years (WY1999-2001; note
that water year 2000 runs from October
1, 1999 to September 30, 2000) were
used (Brennan and others, 1999;
Brennan and others, 2000; Brennan and
others, 2001) (fig. 13). Discharge during
the 3 water years was variable; WY2000
had 90 percent of the discharge of
WY1999, whereas WY2001 had only 37
percent of the discharge of WY1999.
Several assumptions were made in the
modeling:
1. The inflow is equal to the outflow;
in other words, the volume of the
lake does not change over time.
This assumption is a good approximation
because the lake volume
changes by less than 9 percent
throughout the year (Woods and
Beckwith, 1997). 2. The volume of the lake is 2.8 x 1012
L and the surface area of the lakebottom
sediment is 1.08 x 1012 cm2
(Woods and Beckwith, 1997).
3. The exchange of dissolved Zn
across the sediment-water interface
is by molecular diffusion; therefore,
a molecular diffusion coefficient is
used to describe this process. This
assumption is supported by the
work of Kuwabara and others
(2000). The average value of the
molecular diffusion coefficient of
dissolved Zn at bottom water
temperatures in Lake Coeur
d'Alene is 0.279 cm2 per day (Li
and Gregory, 1974; Balistrieri,
1998).
4. A range in the concentration of
dissolved Zn in the porewater just
below the sediment-water interface
is used to calculate the input of
dissolved Zn to the lake by benthic
fluxes. Dissolved Zn concentrations
of 1,070 (25th percentile), 1,210
(median), and 1,650 (75th percentile)
mg/L are used, based on 10
published values (Balistrieri, 1998;
Kuwabara and others, 2000); these
concentrations do not vary seasonally
in the model calculations.
5. Scavenging of dissolved Zn to
particulate Zn and subsequent
sedimentation is described by a
first-order rate coefficient (kscav),
and scavenging of dissolved Zn
derived from the riverine input is
the same as scavenging of dissolved
Zn derived from the benthic flux
source. In other words, there is no
difference in the chemical behavior
or speciation of dissolved Zn from
the two sources.
Model results depend on the formulation
of the model (for example, the
processes considered and their parameterization)
and the measured data that
are used. Assuming that all of the important
processes are incorporated into the
model, then the most reliable data are
the physical characteristics of the system
such as lake volume and surface area of
sediment, the measured discharge and
dissolved Zn concentrations for the
inflows and outflow, and the molecular
diffusion coefficient for dissolved Zn.
The least well known data are the
porewater concentrations of dissolved
Zn, which determine the benthic flux,
and the parameterization of the removal
of dissolved Zn from the water column.
Little information is available on spatial
values and no information on seasonal
values of dissolved Zn in the porewater
just below the sediment-water interface
in Lake Coeur d'Alene. The removal of
dissolved Zn from the water column is
assumed to be by transformations to the
particulate phase and settling of particles.
No sediment trap data are available
to support this assumption. In
addition, this removal is assumed to
follow first-order kinetics. Limited
information from in situ microcosm
studies in lakes suggests that removal of
dissolved metals from the water column
follows a first-order rate equation
(Santschi and others, 1986; Anderson
and others, 1987).
Model results indicate that the relative
importance of inputs of dissolved Zn to
Lake Coeur d'Alene from rivers and
benthic flux varies through the year and
from year to year (fig. 14A,B). The
benthic flux contribution to dissolved Zn
concentrations in the lake was larger
than the river contributions during
October of the first two water years
(WY99-WY00); otherwise, contributions
from the inflowing river dominated.
During WY01, which was a lowdischarge
year, the supply of dissolved
Zn to the lake by benthic flux dominated
during the first 6 months (October
through March), whereas the supply
from inflowing rivers was dominant during the subsequent 4 months (April
through July). Benthic flux was the
primary input of dissolved Zn to the
lake for the remainder of the water
year. Low discharge results in less
input of dissolved Zn to the lake from
rivers and a higher proportion of input
from benthic flux. In addition, the
relative importance of sources of
dissolved Zn to the lake will change
as cleanup efforts within the mining
district and lower Coeur d'Alene
River valley proceed.
Values of the first-order scavenging
rate coefficient that are needed to fit
the measured dissolved Zn concentrations
in the outflow (Spokane River)
range by a factor of 46, from 0.0005
to 0.023 per day (fig. 14C). The
highest values of this coefficient were
determined in the third or fourth
quarter of the water year and slightly
precede the minimum in dissolved Zn
concentration. For comparison, values
of scavenging coefficients for dissolved
Zn from whole-lake experiments
and microcosm studies using
radiotracers range from 0.03 to 0.06
per day (Santschi and others, 1986;
Anderson and others, 1987). These
values are at the high end of values
determined by the modeling of dissolved
Zn in Lake Coeur d'Alene.
What processes are responsible for
variations in the scavenging rate coefficient?
Scavenging rate coefficients
encompass the partitioning of metal
between dissolved and particulate
phases and the settling velocity or flux
of particles (Sigg, 1985). Partitioning
between dissolved and particulate
phases can be described by a distribution
coefficient and is a function of the
particle type, particle concentration,
pH, ionic strength, and temperature of
the system. Scavenging rate coefficients
are largest when dissolved metals
have a strong affinity for particulate
phases and the flux of particles in the
system is large. Some insight into
variations of scavenging rate coefficients
for Zn is given by work done in a
lake in Switzerland (Sigg and others,
1996). Although they examined a
relatively pristine lake relative to Lake
Coeur d'Alene (Lake Greifen, Switzerland,
where Zn concentrations are 0.65-
2.6 mg/L), the chemical behavior of Zn
should be similar in the two lakes.
Using sediment trap data, they found
that the sedimentation of Zn (or transformations
from dissolved to particulate
phases and subsequent settling) was
related to sedimentation of algae and
manganese dioxide. Because primary
productivity is seasonal, they observed
corresponding seasonal variations in
sedimentation of Zn. About 87 percent
of the dissolved Zn that enters Lake
Greifen is trapped within the lake, compared
to 56 percent in Lake Coeur
d'Alene. Sigg and others (1996) concluded
that biological mechanisms of uptake and
binding are significant for the removal of
dissolved Zn from the water column in
Lake Greifen. The largest scavenging rate
coefficients for dissolved Zn determined
for Lake Coeur d'Alene occur during
periods when the maximum in biological
productivity would be expected. Further
work is needed to understand the mechanisms
and the role of biology in controlling
transformations of dissolved to
particulate Zn in Lake Coeur d'Alene.

What is known about impacts of Pb and Zn to biota, and can specific processes that influence bioavailability be identified?

The health impacts of mining in the
Coeur d'Alene River basin on humans
and other biota are highly dependent on
the phase of the metal. The critical
phase for Pb is the particulate form, whereas for Zn it is the dissolved form.
Some of the highest levels of Pb in
the blood of children in the United
States were measured in the Coeur
d'Alene mining district around the
Bunker Hill smelter and the eventual
Superfund site (fig. 15). The primary
pathway of Pb into human blood is
ingestion of Pb-enriched particles
that are soluble in the stomach.
Solubility of the particles depends on
the speciation of Pb in the solid
phase (Ruby and others, 1992; Gasser
and others, 1996). In 1974, one
year after a fire destroyed the air
emission controls on the Bunker Hill
smelter stack, 98 percent of children
1 to 9 years old in the area around
the smelter had blood Pb levels >40
mg/dL (U.S. Environmental Protection
Agency, 1994). Emergency
response actions (for example, education
about hygiene, smelter closure,
and yard remediation) resulted
in lower blood Pb levels, most of
which are now below the maximum
recommended level of 10 mg/dL.
However, recent studies of blood Pb
levels in children living outside of
the Superfund site, but within the
Coeur d'Alene River basin, indicate
that about 15 percent of children
between the ages of 9 months and 9
years have higher than recommended
levels of Pb in their blood
(TerraGraphics Environmental Engineering
and others, 2001). Warnings
of potential human exposure to high
concentrations of metals in sediment
and fish and related health impacts
are posted at recreational beaches
along the lower Coeur d'Alene River,
Coeur d'Alene Lake, and Spokane
River.
High concentrations of Pb in surface
sediments within the basin and dissolved
Zn in rivers also have proved
harmful to terrestrial and aquatic
wildlife. The metal-enriched marshes
within the lower Coeur d'Alene River
valley are prime resting and feeding
areas for migratory birds, and deaths
related to Pb poisoning during feeding
in this area have been reported for
waterfowl (Beyer and others, 1998).
High dissolved Zn concentrations in
the South Fork of the Coeur d'Alene
River and its tributaries have prevented
the re-establishment of a viable fishery,
despite attempts to improve fish
habitat in streams (T. Maret, unpublished
data, 1999).
Factors that affect the
geoavailability and, ultimately,
bioavailability of Pb are linked to the
speciation of Pb in the solid phase,
which depends on the redox conditions
of the depositional environment. The
impact of Zn on biota depends on the
mobilization of this element from
metal-enriched tailings and subsequent
transport and reaction between ground
water and surface water.
What are the major gaps in knowledge remaining from this research?

1. Quantitative and predictive dynamic
models are needed that provide an
understanding of the underlying
mechanisms controlling the transport,
reaction, and fate of Pb, Zn,
and other elements in the Coeur
d'Alene River basin and other largescale
systems affected by mining
activities. Sensitivity tests with such
models would provide information
on the response of large-scale
systems to natural and humaninduced
perturbations, would identify
data gaps, and could be used to
direct remediation and management
of the systems.
2. The interrelationships among
processes and factors responsible for chemical speciation, mobilization,
and transport of Pb, Zn, and
other elements in the environment
(geoavailability) and the uptake of
Pb, Zn, and other elements by biota
(bioavailability) need to be further
defined.
3. The role of microorganisms in
determining chemical speciation,
geoavailability, and bioavailability
for Pb, Zn, and other elements
needs to be further evaluated.
ACKNOWLEDGEMENTS
Robert Seal, Michael Zientek, Tom
Frost, and Peter Vikre of the U.S. Geological
Survey provided comments on an
earlier draft of this paper. Peter Stauffer's
editing of the manuscript helped improve
its clarity.
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