U.S. Department of the Interior - U.S. Geological Survey
NUMBER 3, March 1995
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The Effects of Mining and Related Activities
on the Trace Element Geochemistry of Sediments
in Lake Coeur d'Alene, Idaho
by Arthur J. Horowitz¹, Kent A. Elrick¹, John A. Robbins², and Robert B. Cook³
¹ U.S. Geological Survey Water Resources Division, Suite 130, 3039 Amwiler Road, Atlanta GA 30360
² National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory, 2205
Commonwealth Blvd., Ann Arbor MI 48103
³ Dept. of Geology, 210 Petrie Hall, Auburn University, Auburn, AL 36849
Abstract
During 1989 and 1990, 12 gravity cores and 150 surface grab samples were collected
in Lake Coeur d'Alene, Idaho to determine trace element concentrations, partitioning
and surface and subsurface distribution patterns in the bed sediments of the lake.
Substantial portions of the surface and near-surface sediments in the lake are
markedly enriched in Ag, As, Cd, Hg, Pb, Sb and Zn, and somewhat enriched in Cu,
Fe and Mn. Surface and subsurface distribution patterns indicate that the source
of much of this enriched material is the Coeur d'Alene River. An estimated 75
million metric tons of trace-element-rich sediments have been deposited on or
in the lakebed. An ash layer from the 1980 Mt. St. Helens eruption, ages estimated
from 137Cs activity, and the presence of 80 discernible and presumably annual layers
in a core collected near the Coeur d'Alene River delta indicate that the deposition
of trace- element-rich sediments began, in the Coeur d'Alene River delta some
time between 1895 and 1910, dates consistent with the onset of mining and ore-processing
activities that began in the area in the 188O's.
Introduction
Lake Coeur d'Alene (CDA) is a natural lake in the
northern panhandle of Idaho. The main body of the lake is
about 3.2 km wide by 40 km long; however, the southern
part is composed of four smaller interconnected lakes that
were formed in 1906 when the Post Falls Dam (10 miles
downstream on the Spokane River) was completed, (Meckel
Engineering et al., 1983). The St. Joe and CDA Rivers
annually account for 94% of the inflow to Lake CDA; major
outflow from the lake occurs at the northern end through
the Spokane River (Meckel Engineering et al., 1983;
Javorka,1991).
The South Fork of the CDA River, which flows into
the CDA River, and thence into Lake CDA, drains a major
part of the CDA mining district. Until 1968, most of
the mining and ore-processing wastes were discharged directly
into the South Fork of the CDA River. These materials
were highly enriched in Ag, As, Cd, Cu, Fe, Mn, Pb, Sb and
Zn (Rabe and Bauer, 1977; Bender, 1991). In 1983
the U.S. Environmental Protection Agency (EPA)
established the Bunker Hill Superfund Site that encompasses
54 km2 around Kellogg and Smelterville, some 50 km
upstream of Lake CDA. (e.g., Bender, 1991).
Sample Collection, Treatment, and Analytical Methods
Sample Collection
Undisturbed (stratified) surface samples were collected
using a stainless steel Ekmann grab sampler in 1989
(Fig. 2). Subsamples were removed from the upper 2 cm of
sediment in each grab. Twelve gravity cores were
collected in 1990 using a 5 cm diameter, stainless steel
2.44 m gravity core with a clear polycarbonate liner and a
non-metallic core catcher (Fig. 1).
 Figure
1.
Sample Analyses
All the samples were analyzed for Al, As, Cd, Cu, Fe, Mn, Pb, Sb, Ti, and
Zn by inductively coupled plasma (ICP) methods following an HF/HC104/HN03
digestion. Hg was determined by AA cold vapor, and TOC by IR spectroscopy. Selected
samples were subjected to heavy-mineral separations using bromoform (2.96 g/cm³)
followed by total chemical analyses of the light and heavy fractions. Selected
heavy fractions also were examined and chemically analyzed using Scanning Electron
Microscopy/Energy Dispersive Analysis (SEM/EDA). Additional data on sediment-trace
element partitioning were obtained using partial chemical extractions (Horowitz
et al., 1993; 1995).
137Cs activity was determined using a high resolution solid-state HpGe detector
coupled to a multichannel analyzer. Approximately 3 days of counting time per
sample were required to achieve precisions between ±7 - 20% in the vicinity
of maximum activities because of the relatively small sample sizes and/or relatively
low activities (Horowitz et al., 1995).
Results and Discussion
Bulk Sediment Chemistry and Trace Element Distribution Patterns
The chemical data for all the samples clearly show
that the upper sediment column in Lake CDA is enriched in Ag,
As, Cd, Hg, Pb, Sb and Zn. The samples are less enriched
in Cu, Fe and Mn relative to both unenriched fluvial
sediments throughout the U.S.A. and to unenriched surface
and subsurface (e.g., core 146, Fig. 1) sediments from
within the lake (e.g., Horowitz 1991; Horowitz et al.,
1993, 1995; Table 1).
The highest trace element concentrations
in Lake CDA sediments, typically by a factor of two or more,
are associated with subsurface rather than surface
material (Ag, Cu, Pb, Zn, Hg, As and Sb, the maximum column
in Table 1); however, this pattern reverses when the
surface and subsurface median concentrations are considered
(with the exception of Ag and Sb, the median column in
Table 1).
Table 1
The highest median concentrations for Cu, Pb, Zn, Cd,
Hg and As are found in the surface sediments, whereas the
highest median concentration for Ag is found in the
subsurface sediments. The median concentration for sediment-
associated Sb is about the same for both. The generally
higher median surface sediment concentrations may be the
result of post-depositional remobilization, upward diffusion,
and subsequent reprecipitation caused by reducing
conditions in the sediment column which may have stripped
some subsurface trace elements associated with Fe oxides
from the sediments.
All the enriched surface and subsurface samples were
collected from the main body of Lake CDA, and all the
major, and many of the minor bays north of Conkling Point
and Carey Bay (near the southern end of Lake CDA, but
north of the mouth of the St. Joe River, Fig. 1). Samples
collected from the lake south of Carey Bay and Conkling
Point and from the very back
of several of the northern bays (Wolf Lodge, Rockford
and Windy Bays) display trace element concentrations similar
to unimpacted fluvial sediments throughout the U.S.A.
(Table 1; e.g., Horowitz, 1991).
The chemical distribution patterns in Lake CDA surface
sediments appear consistent with the CDA River acting as
a major source for the enriched trace elements. Additionally,
the patterns also appear to reflect the velocity and
direction of water movement through the system (generally
from south to north). These observations are in agreement
with previous findings and are consistent with the enriched
trace elements being transported into the lake by the CDA
River. (e.g., Punk et al., 1973, 1975).
Estimated Masses of Enriched Sediments and Associated Trace Elements
Estimates of the mass of trace-element-rich sediments,
as well as the mass of each enriched trace element, currently
on or in the bed of Lake CDA were calculated on the basis
of the chemical data from the 12 cores, and an estimated
sediment bulk density of 2.0 g/cm33. These estimates indicate
that there are about 75 million metric tons of trace-
element-rich sediments currently blanketing about 85% of
the bed of Lake CDA (Table 2). The masses of excess trace
elements range from about 260 metric tons for Hg to more
than 468,000 metric tons for Pb (Table 2).
Table 2
Trace-Element Partitioning
The various techniques used to determine partitioning
indicate that the majority of the enriched Pb, Cd,
Zn, As, and Cu are associated with an operationally defined
Fe oxide phase (by extraction with .25 M hydroxylamine
hydrochloride in .25 M HCl heated at 50° C for 30 min.),
the Ag could be associated with either Fe oxides or sulfides,
whereas the Sb is predominantly associated with a refractory phase.
Trace element contributions from the heavy mineral fraction
in the surface sediments are limited. Although the
chemical concentrations from these fractions are elevated, their
contribution to the overall chemical levels in the
sediments are low because the concentration of heavy minerals
also are low. The highest concentration of heavy
minerals in the surface sediments (11%) is within the CDA River
delta; this decreases rapidly to <1% within 2 km of
the delta. Similar results were found for the majority of the
core samples where major heavy mineral concentrations
were limited to a few distinct bands.
The similarity in mineralogy, as well as the association
of the vast majority of the enriched trace elements with an
operationally defined Fe oxide phase, in both the surface
and subsurface samples implies that the sources and/or
concentrating mechanisms for the trace-element-rich sediments
probably were the same throughout the course of their
deposition in Lake CDA.
Sediment-Geochemical History of Lake CDA
Efforts to determine a geochemical history for the lake
were concentrated on core 123 because of: a) the overall
thickness of its trace-element-rich section (119 cm), b)
the thickness and number of its readily discernible individual
layers (80), c) the presence of a datable (1980) Mt. St.
Helens ash layer (20.5-21.5 cm) and d) the occurrence of
background trace element levels at its base (119-126 cm
depth). Three separate approaches were used to estimate the
age of the base of the trace-element-rich zone.
An initial age for the base of the trace-element-rich
zone was made by considering the 1990 date the core was
obtained, the position and age of a 1980 Mt. St. Helens
ash layer, and the 9 layers between them. These factors
indicate that each layer may represent annual deposition.
Because there are 80 layers between the top of the core and
the base of the trace-element-rich zone, deposition began around 1910.
1
3
7Cs
activity was determined for all the sampled layers
in core 123. The data show a strong maximum at about
45 cm, and a secondary peak at 55 cm. No
1
3
7Cs
was detected below 60 cm, nor in the ash layer between
20.5-21.5 cm. These features are consistent with the
assignment of dates of 1963-1964 for the maximum, and
1958-1959 for the secondary peak. The onset of measurable
fallout and sedimentary
1
3
7Cs
activity occurs around 1954.
The assignment of these dates [plus the dated ash layer
(1980)], leads to an age-depth model which produces a
predicted age of 1895 for the base of the
banded (trace-element-rich) zone.
By combining (a) the date of collection for the core
(1990), (b) with the date for the ash layer (1980), and (c) with
the three major dates estimated from the
1
3
7Cs
activity, it is possible to estimate average sedimentation rates.
Assuming constant deposition between the dated points, the
average rates are: 2.1 cm yr1
(1980-1990), 1.7 cm yr1
(1965-1980),
1.3 cm yr1
(1959-1965) and 1.4 cm yr1
(1954-1959).
The decline in sedimentation rates with increasing depth could
be the result of compaction. The thickness of the undated
portion of the trace-element-rich zone is 58 cm; if
deposition was 1.35 cm yr1,
then the undated section
represents a period of 43 years and the age for the start of trace-
element-rich sediment deposition is 1911.
The relative consistency of the three estimated dates
(1910, 1895, and 1911) for the base of the trace-element-rich
banded zone in core 123, using three somewhat different
approaches, is encouraging. Certainly, the three estimated
dates are consistent with the 1880-1890 period assigned for
the onset of mining and ore-processing activities in the
CDA mining district (e.g., Bender, 1991).
Conclusions
Substantial portions of the surface and subsurface
sediments blanketing 85% of the bed of Lake CDA are enriched
in Ag, As, Cd, Cu, Hg, Pb, Sb and Zn. The CDA River seems
to be the source for the majority of the trace-element-
rich sediments in the lake. The similarity in the locations
of the trace-element-rich surface and subsurface sediments,
the trace-element concentrations, and the trace-element
partitioning, all suggest that the concentrating mechanisms
and/or sources causing the trace-element enrichment in
the lake probably have been much the same
throughout the last 100-110 years. An estimated 75 million
metric tons of trace-element-rich bed sediments have been
deposited in Lake CDA. Most of the enriched trace elements
are associated with an operationally defined Fe oxide
phase. The age of the onset of trace element enrichment
in the lake probably falls between 1895 and 1910. These
dates are consistent with the onset of mining and ore
processing in the CDA mining district, and these activities
probably represent the major source for the extreme trace
element enrichments in the sediments of Lake CDA.
References
Bender, S., 1991. Investigation of the chemical
composition and distribution of mining wastes in Killarney Lake,
Coeur d'Alene Area, northern Idaho: Unpublished
M.S. Thesis, University of Idaho, Moscow: Idaho, 98 p.
Funk, W., Rabe, F. and Filby, R., 1973, The biological
impact of combined metallic and organic pollution in the
Coeur d' Alene-Spokane River drainage system, Project
Completion Report OWRR B-044 WASH and B-015 IDA,
Idaho Water and Energy Resources Institute, Moscow, Idaho, 202 p.
Funk, W., Rabe, F., Filby, R., Bailey G., Bennett,
P., Kishor, S., Sheppard, J., Savage, N., Bauer, S., Bourg, A.,
Bannon, G., Edwards, G., Anderson, D., Syms, P.,
Rothert, J. and Seamster A., 1975, An integrated study on the
impact of metallic trace element pollution in the
Coeur d' Alene-Spokane Rivers and Lake drainage system, Office
of Water Research and Technology Project Completion
Report, Title II, Project C4145, Washington State
University-University of Idaho, 332 p.
Horowitz, A. 1991, A Primer on Sediment-Trace Element
Chemistryi, 2nd Ed., Lewis Publishers, Inc., Chelsea,
Michigan, 136 p.
Horowitz, A., Elrick, K. and Cook, R., 1993, Effect of
mining and related activities on the sediment trace element
geochemistry of Lake Coeur d'Alene, Idaho, USA. Part I:
surface sediments: Hydrological Processes, v. 9,
p. 403-423.
Horowitz, A., Elrick, K. Robbins, J. and Cook, R.,
1995, Effect of mining and related activities on the sediment
trace element geochemistry of Lake Coeur d'Alene, Idaho,
USA. Part II: subsurface sediments: Hydrological
Processes, v. 9, p. 35-54.
Javorka, E., 1991, Lake water quality assessment Coeur
d'Alene Lake, Benewah and Kootenai Counties, Idaho.
Coeur d'Alene Basin Interagency Group, Coeur d'Alene
Tribe of Idaho: Coeur d'Alene Subagency, Plummer,
Idaho, 42 p.
Meckel Engineering and Brown and Caldwell Consulting
Engineers, 1983, Kootenai County lakes master plan:
Meckel Engineering/Brown and Caldwell, Coeur d'Alene,
Idaho, pp. 1-30-1-35, pp. 2-24-2-32.
Rabe, F and Bauer, S., 1977, Heavy metals in lakes of
the Coeur d'Alene River valley: Northwest Science, v. 51,
p. 183-197.
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