Page 801 - The Toxicology of Fishes

Mining Impacts on Fish in the Clark Fork River, Montana: A Field Ecotoxicology Case Study 781

The Mining/Smelting Sites

The specific contamination problems resulting from mineral extraction are ultimately determined by the
nature of the ore and what is done to extract its metals. The ore body in the Clark Fork basin consists
of high-grade metal sulfide veins enclosed in lower grade rock. The mineral forms of several metals
were mined over the history of the deposit. Silver mining dominated for a brief period after the gold
was exhausted; zinc was extracted at times, and modern operations remove molybdenum. But, most of
the history of the Butte mine involves the extraction of copper. The richest veins of ore (depleted long
ago) contained up to 80% copper minerals; the lower grade altered rock around the central ore deposit

contains approximately 0.2% copper. The mining activity in Butte removed approximately 400 million

3
m of rock from the subsurface, 90% of which was discarded as waste rock and tailings. Trace element
contaminants in the various wastes included arsenic, antimony, cadmium, lead, and zinc. Copper con-
centrations in discarded mine tailings average 6730 µg/g; in smelter flue residues, they can be 37,100
µg/g (Moore and Luoma, 1990). As a comparison, copper concentrations are 20 µg/g soil dry weight
2
(dw) in unmineralized soils in Western Montana. As much as 35 km of land has been buried under piles
or ponds of contaminated materials in the direct vicinity of the mine and smelter. The sulfur content of
the Butte ore exceeded 30%, and sulfur accounts for 0.5 to 4.0% of the rock enclosing the ore. Sulfide
ores become unstable when they are mined and moved to the oxygen-rich surface environment. Sulfide
minerals in the mining waste oxidize, forming acids that leach metals to surfacewater and groundwater
systems. Although low pH is common within the pore waters of tailings deposits (Benner et al., 1995),
overland acid mine drainage is now rare in the Clark Fork basin. Most of the large-scale contamination
is from dispersal of the particulate mine wastes.

The Clark Fork River System
Physical characteristics of a river system influence its exposure to and effects of contamination. Physical
processes are a primary determinant of biological characteristics. The size of the watershed and the
nature of the tributaries determine the basic geochemical characteristics of the water body and down-
stream dilution of wastes. Flows, floods, and impoundments affect the distribution of wastes. Other
stressors in the watershed can influence ecological impacts. The waste complex in the Clark Fork basin
begins in the headwaters of Silverbow Creek at the city of Butte (Figure 19.1). The creek extends
approximately 40 km to Anaconda, where the smelter was located. The Warm Springs tailings ponds,
adjacent to Anaconda, capture the entire flow of Silverbow Creek at its downstream boundary. The creek
is treated with lime, and sediments are settled out in the ponds. Below the release point of the ponds,
two other creeks join Silverbow, formally defining the headwaters of the Clark Fork River.
The watershed of the Clark Fork is large, encompassing 57,400 km . It is sparsely populated and
2
mountainous, and the major human development is agriculture. Small nutrient inputs from local cities
and summer dewatering of the river for irrigation are potential stressors (typical of most Montana
streams). In general, the river is a steep-gradient, cobble- and gravel-bottom stream. The Upper Clark
Fork River extends 150 km, from below Warm Springs Ponds to Milltown Reservoir (Figure 19.1). It
has four major tributaries, each of which approximately doubles its discharge. It has wide floodplains
through the uppermost 60 km (the Deer Lodge Valley, extending past Deer Lodge). Downstream from
there it is often confined by canyons, with occasional areas of relatively extensive floodplain. Three
major river systems join the Clark Fork through this reach. The Lower Clark Fork lies below the
confluence of the Blackfoot River and extends to Lake Pend de Oreille. Three additional impoundments
and one large pulp and paper mill lie between Milltown Reservoir and the mouth of the Clark Fork. The
impoundments have greatly changed the physical nature of the river, and the mill adds waste. These
additional stressors add difficulty to evaluating the causes of reduced fisheries in this reach. This chapter
considers only the Upper Clark Fork.