THE ROLE OF GEOMORPHIC PROCESSES IN THE TRANSPORT AND FATE OF MERCURYIN THE CARSON RIVER BASIN, WEST-CENTRAL NEVADA

The historic processing of precious metal ores mined from the Comstock Lode of west-central Nevada resulted in the release of substantial, b ut unquantified amounts of mercury-contaminated mill tailings to the C arson River basin. Geomorphic and stratigraphic studies indicate that the introduction of these waste materials led to a period of valley-fl oor aggradation that was accompanied by lateral channel instability. T he combined result of these geomorphic responses was the storage of la rge 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 a re characterized by variations in mercury levels that exceed three ord ers of magnitude. Continued lateral instability, coupled with an episo de of channel-bed incision, followed the decline of Comstock mining, a nd has reexposed contaminated debris within the banks of the river. Er osion of bank sediments reintroduces mercury-enriched particles to the modern channel bed. It is suggested on the basis of geochemical and s edimentological data that during the bank erosion process, much of the mercury associated with fine (< 63 mu) valley-fill deposits are carri ed downstream without being incorporated to any appreciable extent wit hin the channel-bed sediments. In contrast, mercury associated with la rger and denser particles, particularly mercury-gold-silver amalgam gr ains, are accumulated in the channel-bed sediments as the river traver ses polluted reaches of the Carson River valley. Concentration pattern s developed along the modern channel indicate that the valley fill is the primary source of mercury to the river today. Thus, these data imp ly that efforts to reduce the influx of mercury to the aquatic environ ment should examine methods for reducing bank erosion rates.