Geology Reference
In-Depth Information
ponds contain approximately 9000 t arsenic, 200 t cadmium, 90,000 t
copper, 20,000 t lead, 200 t silver, and 50,000 t zinc. These metals enter
streams and rivers as solutes and particulates and contaminate sedi-
ments in the river and reservoirs far downstream from the primary
sources (Figure 4).
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Downstream concentrations follow an exponential
decline when viewed over several hundred kilometres. The sediment of
Milltown Reservoir, more than 200 km from the mines and smelters at
Butte and Anaconda, is highly contaminated with various metals and
arsenic. It is believed that oxidation-reduction processes release arsenic
from the reservoir sediments and cause contamination of an aquifer
from which water drawn through wells, now closed, contains arsenic
higher than the EPA drinking water standards.
61
Davis and Atkins,
63
however, have pointed out that, in the bottom sediments of the Clark
Fork River channels, the fraction of fine-grained (
o
63 mm) clay/silt
material, which contains high metal concentrations and to which benthic
organisms are likely to be directly exposed, is small relative to sands and
gravels. Furthermore, the metals occur predominantly in sulfides fre-
quently armoured with an oxide rim and other sparingly soluble phases.
Figure 4 Types of contamination resulting from large-scale metal extraction
(Reprinted with permission from ref 61,
r
American Chemical Society, 1990)
