Page 803 - The Toxicology of Fishes

Mining Impacts on Fish in the Clark Fork River, Montana: A Field Ecotoxicology Case Study 783

The question is of particular interest with regard to sediments, which are the repository of greatest mass
of contamination. Culpability for clean-up often depends on defining which activity was responsible for
the sustained contamination of sediments. The primary effect of mining and smelting is to disperse the
contamination that occurs naturally in the Earth. The size of the ore body and the concentration of a
particular metal in the soils covering the ore body are important determinants of the degree of dispersal.
The area of metal-enriched soil around an undisturbed deposit generally ranges from 0.4 to 2.4 times
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the exposed area of the ore body; for example, an ore body that has a surface expression of 1 km might
show a surface anomaly of 0.4 to 2.4 km . Mining greatly expands the fingerprint of the ore body on
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the surface of the Earth, as metal-rich material is dug up and moved around. Differences in the area of
the surface anomaly contribute to major differences in the distance that uncontained metals are transported
away from mined compared to unmined ore bodies (termed the dispersion train). Comparisons of a
variety of mines show that mining exaggerates the size of the natural ore body anomaly from about 100
to 500 times the natural anomaly (Helgen and Moore, 1996).
In the Butte mines, the major ore body has a surface area of approximately 4 km . After 150 years
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of mining activity, primary wastes adjacent to the open pit mine cover an area of about 50 km . In total,
about 1400 km of land is contaminated in the Clark Fork basin by the wastes generated from mining,
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processing, and smelting at Butte and Anaconda. Comparing after-mining metal concentrations with the
before-mining estimate of dispersion shows that approximately 2200 times more metal was released into
the upper Clark Fork basin from mining than would be expected from the original mineral deposit.
Before mining in the basin, sediment metal concentrations would have reached background concentra-
tions in about 20 to 30 km, based on cumulative basin area modeling (Helgen and Moore, 1996). Now,
after mining, contaminated sediments are dispersed into Lake Pend de Oreille, approximately 600 km
downriver from the mine (Axtmann and Luoma, 1991). Atmospheric deposition has also contributed to
metal contamination across the watershed. Between 1900 and 1980, the Anaconda smelter processed
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much of the locally mined Butte ore, with a dispersal range of up to 300 km (Moore and Luoma, 1990).
Arsenic, cadmium, copper, lead, and zinc associated with the smelting process caused adverse impacts
to soils, cropland, and farm animals.

Silverbow Creek
It is estimated that 2 million m of tailings was dumped directly into Silverbow Creek. The massive

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addition of sediment clogged the original stream and increased the potential for flooding (Weed, 1912).
Floods, accentuated by the aggraded channels, deposited that material onto floodplains all along
Silverbow Creek and the Upper Clark Fork (Brooks and Moore, 1989). As a result, unvegetated, low-
pH tailings deposits (termed slickens) lined Silverbow Creek over its entire length, with occasional
patches of metal-tolerant grasses and willows. The slickens contained contaminated soil up to 2 m
thick; some of the deposits were over 200 m wide. In 2002, typical metal concentrations in fine-grained
sediments (within 5 km of the inlet to the ponds) were cadmium, 21 µg/g dw; copper, 1660 µg/g dw;
zinc, 5690 µg/g dw (Dodge et al., 2003). Premining concentrations, determined from soil pits, were
cadmium, <1 µg/g dw; copper, 16 µg/g dw; zinc, 49 µg/g dw (Ramsey et al., 2005). The lack of vegetation
in the slickens also increased the erosion potential of the banks and adjacent floodplain, resulting in
continual recontamination of the stream and aiding downstream transport of the wastes. Everman (1892)
found the waters of Silverbow Creek to be “the consistency of thick soup, made so by the tailings it
receives from the mills at Butte. No fish could live in such a mixture.” Invertebrates were absent over
the entire 40 km of Silverbow Creek before 1975, after which the earliest treatments of mine wastes
began to take effect and metal-tolerant species began to appear (Chadwick et al., 1986). No fish were
found in the stream in published surveys through 2000 (see later discussion).
It is increasingly recognized that environmental remediation is a necessity as ore bodies are depleted
and mines or smelters are closed, but remediation can be a challenge where the extent of the problem
is large or the waste problem is complex (Moore and Luoma, 1990). Remediation efforts were initiated
on Silverbow Creek in 1988 as part of the USEPA’s Superfund cleanup process. Early efforts removed
contaminated soils or covered areas of the most severe contamination around the smelter, but the first
10 years were mostly dominated by litigation-driven conflict and proprietary data collection linked to