Changes
in the Benthic Community of Lake Creek, Mt,
Resulting from Mine Tailings Contamination
Barry
S. Hansen
Confederated Salish and Kootenai Tribes
Pablo, MT, 59855
Abstract:
The benthos of Lake Creek, Lincoln Co., MT were sampled in 1984-85 to determine
the effects of a 2.5 years old tailings impoundment located adjacent to the
stream. Sampling results were compared spatially in t he 1984-85 data set, and
temporally with a baseline data set collected three years prior to installation
of the impoundment. Lack of replication in the baseline sampling design
precluded the use of two-way ANOVA for analysis of temporal changes. Therefore
temporal comparisons were made on the basis of changes downstream of the
impoundment relative to changes upstream, as measured over time. Spatial
comparisons upstream and downstream were made by one-way ANOVA.
Determination of the effects of the tailings impoundment was
confounded by a tailings spill that directly contaminated the downstream
stations. The spill was assumed to have had a much larger biological effect than
did the tailings impoundment itself. Simuliidae and Rhithrogena spp. increased
in abundance after the spill the former responded as an opportunist, the later
was a coincidental benefactor since it was in the adult stage during the spill.
The before and after data sets were collected with different
sampling gear and had unequal replicate samples within plots. The gradient
effect of Bull Lake on Lake Creek was also a concern. These concerns were
minimized by utilizing a methodology that analyzed the change in the
relationship between control and impact areas over time.
Of 16 taxa analyzed, 10 decreased, two increased, and four
showed no significant change in abundance in the impact area relative to the
control from 1977-78 to 1984-85. Spatial comparisons above and below the
impoundment in 1984-85 showed eight taxa significantly more abundant in the
upstream area five with comparable abundances, and three with greater abundance
downstream.
Editor's Note: This paper investigates a creek outside the
Clark Fork drainage but addresses a problem of much concern in this drainage.
Hence the paper is included in this symposium because it provides useful
insights on the likely reactions of similar creeks lo similar disturbances
within the Clark Fork basin.
This study was undertaken to measure potential aquatic impacts resulting from a
new mining operation. It was inspired by the importance of biological monitoring
as a tool to judge the accuracy of pre-development impact predictions and by the
need to provide alarm systems when cumulative impacts approach unacceptable
levels.
I collected benthic samples in a northwestern Montana stream
three years after the installation of a tailings impoundment adjacent to the
stream. An Environmental Impact Statement was prepared five years prior to my
sampling. The tailings are a fine waste product from milling of copper/silver
ore.
My objectives were to document any differences in the
macroinvertebrate community between the before and during mining periods, and
between the areas upstream and downstream of the impoundment. Significant
problems experienced during this field study were: 1) 'pseudo-replication' (Hurlbert
1984) in the baseline sample design, 2) the presence of a lake upstream of the
study site that may have confounded results, and 3) the over-whelming influence
of a tailings spill between the control and impact stations in June 1984.
Lake Creek originates as the outflow from Bull Lake, near Troy Montana, and
flows for 28 km over a gradient of 3.2%, before joining the Kootenai River.
Metasedimentary Belt rocks such as quartzite, siltite, and argillite are common
in the basin, which was once entirely covered by the Cordilleran ice sheet.
Land use in the basin is predominately
for timber production, although about one fifth of the drainage is in wilderness
condition. Lake Creek water is calcium- bicarbonate type typically high in
oxygen, and low in turbidity and organic matter. Alkalinity measurements range
from 10 to 50 mg/l as CaCO and mean annual discharge is 510 cfs.
The tailings impoundment covers about 400 acres along 2000 m
of stream channel, and is located 5 km downstream of 1,200 acre Bull Lake.
Mining began in 1981, is expected to last until 1997, and results in the annual
deposit in the impoundment of about three million tons of tailings.
Baseline samples were collected with a one square foot Surber sampler, using a
1000 micron mesh capture net, at single sites upstream and downstream of the
proposed site of the tailings impoundment Five samples were collected at each
site in April, July, October, and February 1977- 78. I used this baseline data
for determining temporal changes by 1984. I sampled on the same dates as the
baseline, but modified the study design. I used a Hess-type sampler (1000 micron
mesh), which seals more effectively to the substrate, increased upstream and
downstream stations to two each to eliminate pseudo-replication, and increased
replicate sample number at each station from five to eight for greater
precision. The changes did not invalidate a comparison between periods, and
ensured statistical validity.
Sampling stations
were located in riffles that were chosen primarily on the basis of their
proximity upstream or downstream of the tailings impoundment. Nearness to the
impoundment was of primary importance. Sample sites were further evaluated
subjectively for uniformity in stream width, depth, and velocity. The control
stations were 1.2 and 0.7 km upstream of the tailings impoundment, and the
impact stations were 0.1 and 1.0 km downstream of it.
Whole samples were systematically sorted in the lab. Most
organisms for which published keys are available were identified to species
level. Simuliids and chironomids were identified only to the family level.
Approximately 50 taxa were identified in the samples. Abundance data suggested
lognormal distributions as histograms of the data were generally skewed to the
right and plots on lognormal probability paper produced nearly straight lines.
Regressions were run on the 16 most common taxa to determine the dependence of
the variance on the mean. According to Anderson (1965) at least three
individuals are required per sample before a variance-stabilizing transformation
can be found. Log and fourth root transformations were chosen according to which
most successfully stabilized the variance.
According to Green (1979). two-way analysis of variance (ANOV
A), which determines the interaction of site and time, is the strongest test of
mining effects. Two-way ANOV A was inappropriate for this study, since the
baseline design was pseudo-replicated, meaning only single control and impact
stations were present in 1977- 78. Without replicated control and impact sample
sites there can be no estimate of among area variance, and only the null
hypothesis of "no difference between sites" rather than "no
difference between upstream and downstream areas" would be tested.
Therefore the spatial and temporal analyses were done separately.
Temporal changes were determined by a procedure described by
Stewart. Oaten et at. (1986), who resolve pseudo-replication by evaluating the
data as a time series and the sampling dates as replicates. The procedure is
based on a comparison of the differences between the control and impact areas
before and during mining, Impacts are deduced from a consistent change in the
comparison of control and impact areas over time between the baseline and
1984-85 period. A t-test with three degrees of freedom (four dates were sampled)
determines the level of significance.
Spatial differences (between the upstream and downstream
areas) after mining began were determined by one-way ANOVA. Significant
differences between control and impact areas support a rejection of the null
hypothesis that mean abundance at the control area equals mean abundance at the
impact area.
Six taxa were strongly influenced by tailings contamination as demonstrated by
greater abundances (a=0.05) at the spatial and temporal controls (Table I).
Heterlimnius corpulentus and Hydropsyche are the best examples.
Hydropsyche is a large caddis fly that relies on seston which
it fillers by means of an intricate silken net. Since all sampling stations were
located downstream of Bull Lake, a seston-gradient through the study area may
exist, and if so, Hydropsyche may not serve as a good indicator of water quality
in Lake Creek. Several researchers have shown hydropsychid abundance to decrease
with distance from a lake outlet (Valett and Stanford 1987, Sheldon and Oswood
1977). Carlson et al. (1977) observed a rapid reduction in seston in "a few
hundred meters" of a lake outlet, and Maciolek and Tunzi (1968) measured a
60% decrease within 400 m of a lake. Since the first control station was located
about 2000 m downstream of Bull Lake, I considered it beyond the bounds of the
lake-outlet community. Furthermore, in the baseline period, Hydropsyche was more
common at the impact station than the control station on three of four dates
sampled. In 1984-85 the situation was changed and Hydropsyche was more common in
the control area than the impact area on all dates sampled (p<O.003). The
consistently greater hydropsychid abundance in the downstream area before
mining, and the greater abundance upstream of the tailings impoundment during
mining, suggests a negative influence at the site of the impoundment.
Riffle beetles, Heterlimnius corpulentus, are clingers found
in cobble and gravel that were more common at the control site both before and
during mining. H. corpulentus presents unusual problems for impact detection,
since it was abundant in the control and rare in the impact area even before
mining began (Figure 1). This problem is compounded by the fact that H.
corpulentus increased in the impact area in 1984-85 relative to baseline levels.
The conclusion of significant differences between periods is based on the
assumption that in the absence tailings contamination, Populations in the
control and impact areas should have similar fluctuations in abundance.
Therefore downstream abundances should have increased to the same degree as they
did upstream, and the fact that they failed to have may be due to a stress in
the downstream area.
Three taxa increased in abundance downstream of the tailings
impoundment (Table 1). Rhithrogena, a heptagenl1d mayfly t was marginally more
abundant the control station in 1977- 78, and substantially more abundant at the
impact stations than the control in 1984-85 (Figure
2). The variability in
differences between data accounts for the lack of significance (t=I.84~3). In
1984-85 Rhithrogena was significantly more abundant at the Impact stations than
at die control stations on three of four dates. One possible explanation is the
coincidental emergence Of Rhithrogena prior to the tailings spill, which may
have enabled it to escape the peak of pollution stress and recolonize after
competitive pressure was greatly reduced Rhithrogena emerge in nearby Kootenai
River during the first two weeks in June (Perry 1983), suggesting they were in
the adult stage during the spill. Rhithrogena was present in nearly equal
abundance at the control and impact sites in April Prior to the spill, but much
more common in the impact area on all dates after the spill.
The genus Simulium was the most common black fly in Lake
Creek, and like Rhithrogena, it also increased in the impact area. 11 acts as a
filter feeder by utilizing its cephalic fans for removing particles from the
current. During the baseline period the control station averaged less than one
simuiliid per sample, supporting earlier contention that the control area is not
part of a lake-outlet community. During 1984-85 simuliids erupted into large
concentrations, in which one particular sample contained 476 larvae and pupae.
On all dates in 1984-85 except April, abundance at the control areas was greater
than in 1977- 78. Increased sample efficiency and sample size may largely
explain that difference, but not why there was a tremendous increase in the
impact area in October and February, and only a moderate increase in April and
July .The timing and the magnitude of the impact suggest that the tailing spill of
June 1984 was primarily responsible. The spill may have caused a large, short
term reduction in the population of benthos, in which case organisms, best
adapted for colonization, like simuliids, would be favored to fill the empty
habitat. Short generation times and multivoltine life cycles may have provided
simuliids the competitive advantage to exploit the disturbance caused by the
spill (Carlson 1977).
Four taxa, Cinygmula, Rhyacophila, Perlidae, and
Chloroperlidae demon. demonstrated no difference in abundance between areas
either spatially or temporally. Notably, all except Cinygmula are predacious.
The low population density d predators may render them less effective as
monitors of biological change.
In the 1984-85 period, taxa was greater at the control
stations than the impact stations on all dates sampled. The average number of
taxa identified in the control area was 39, white only 31 taxa were present in
the impact area.
CONCLUSIONS
Significant differences in abundances between areas during the 1984-85 period
were measured in 10 taxa. Seven taxa had higher abundances in the control and
three had higher abundances in the impact area. All of these differences appear
attributable to tailings contamination. It is very unlikely that the significant
temporal and spatial differences in so many taxa is a chance occurrence.
Furthermore, the changes are generally explainable in terms of the spill event
of the tailings impoundment from those of the tailings spill, although it is
assumed that the spill was of much greater influence. None of the biotic changes
measured in Lake Creek up to July 1985 appear to be extreme enough to risk local
extinction. The absence of extreme changes at this point does not preclude more
severe changes later. Lake Creek may be in what Bormann (1982) categorized as
Stage 1 or 2, in which an ecosystem serves as a sink for pollutants without
exhibiting significant biological changes.
Any discussion of significant impacts involves value
judgments concerning the components that are lost. The biotic changes measured
in this study do not clearly translate into reduced outputs (i.e. fish,
wildlife, water, etc.) to society from Lake Creek, although the link between
those changes and the tailings transport and storage facility makes the
continued ecological health of Lake Creek a valid concern. I propose that the
statistically significant changes measured in this study are also biologically
significant, until long term monitoring proves them otherwise.
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