the Penn Mine in Calaveras County, Calif.
that operated intermittently from the early 1860s to the late 1950s. During that
time, the refinery operators drained slag off the molten metal and poured it
into bathtub-shaped molds. When it cooled and hardened, they dumped the waste
material along the banks of the nearby Mokelumne River. The Penn Mine is
currently the site of an environmental restoration project. The project
sponsors, however, have assumed that the slag dump does not pose an
environmental hazard and so have not included it in their clean-up plan.
Parsons has measured relatively high proportions by weight of a number of toxic
elements in the Penn Mine slag: zinc levels as high as 28 percent, copper levels
up to 6 percent, lead levels to 11 percent, cadmium levels to 1.4 percent, and
arsenic levels approaching 1 percent.
In 1963, a dam was built downstream of the mine. So today the reservoir it
created, Camanche Lake, which is used for drinking water and irrigation, floods
the slag dump for about six months a year. Through a combination of field and
laboratory studies Parsons has determined that the slag dump, which contains
about 250,000 cubic yards of material, is a significant source of cadmium,
copper and zinc levels in the reservoir that exceed the EPA chronic toxicity
guidelines for the protection of aquatic life. These levels are restricted to
the immediate vicinity of the dump, and peak when the reservoir is drawn down in
the spring. During this period the slag is exposed to groundwater than can be as
acidic as vinegar. But the elements leached from the slag rapidly combine with
the lake sediments so they don't remain in the water for long, he has
determined.
Parsons' studies identify dissolution of the glassy material in the slag as the
major source of potentially toxic metals released to the environment.
Ironically, many modern smelters use water jets to quench their slag to form a
fine-grained, glassy material that is convenient for disp
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Contact: David Salisbury
salisbury@stanford.edu
650-725-1944
Stanford University
9-Dec-1998
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