The role of geomorphic processes in the transport and fate of mercury in the Carson River basin, west-central Nevada |
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Authors: | J R Miller P J Lechler M Desilets |
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Institution: | (1) Department of Geology, Indiana University-Purdue University at Indianapolis, 723 W. Michigan Street, Indianapolis, IN 46202, USA, IN;(2) Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557, USA, US |
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Abstract: | The historic processing of precious metal ores mined from the Comstock Lode of west-central Nevada resulted in the release
of substantial, but unquantified amounts of mercury-contaminated mill tailings to the Carson River basin. Geomorphic and stratigraphic
studies indicate that the introduction of these waste materials led to a period of valley-floor aggradation that was accompanied
by lateral channel instability. The combined result of these geomorphic responses was the storage of large volumes of mercury-enriched
sediment within a complexly structured alluvial sequence located along the Carson River valley. Much of the contaminated sediment
is associated with filled paleochannels produced by the cutoff and abandonment of meander loops, and their subsequent infilling
with contaminated particles. Geochemically, these deposits are characterized by variations in mercury levels that exceed three
orders of magnitude. Continued lateral instability, coupled with an episode of channel-bed incision, followed the decline
of Comstock mining, and has reexposed contaminated debris within the banks of the river. Erosion of bank sediments reintroduces
mercury-enriched particles to the modern channel bed. It is suggested on the basis of geochemical and sedimentological data
that during the bank erosion process, much of the mercury associated with fine (<63 μ) valley-fill deposits are carried downstream
without being incorporated to any appreciable extent within the channel-bed sediments. In contrast, mercury associated with
larger and denser particles, particularly mercury-gold-silver amalgam grains, are accumulated in the channel-bed sediments
as the river traverses polluted reaches of the Carson River valley. Concentration patterns developed along the modern channel
indicate that the valley fill is the primary source of mercury to the river today. Thus, these data imply that efforts to
reduce the influx of mercury to the aquatic environment should examine methods for reducing bank erosion rates.
Received: 13 December 1996 · Accepted: 15 April 1997 |
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Keywords: | Mercury pollution Mining Fluvial geomorphology Sedimentation |
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