Dispersal of mercury-contaminated sediments by geomorphic processes,sixmile canyon,Nevada, USA: Implications to site characterization and remediation of fluvial environments

The discovery of the Comstock Lode near Virginia City, Nevada, in 1859 resulted in the construction of more than 30 stamp-mills along Sixmile Canyon and its tributaries to process the gold and silver ore. Extraction of the precious metals from the ore was accomplished using a crude mercury (Hg) amalgamation process. As a result, a substantial quantity of Hg was released along with tailings materials to this tributary of the Carson River. During the past 134 years, Hg-contaminated sediments have been eroded from the mill sites and transported downstream by fluvial processes, thereby expanding the influence of Hg pollution. Geomorphic and geochemical data have been combined in this study to document the distribution, quantity, and physical dispersal of Hg-contaminated materials from Sixmile Canyon to the Carson River. These data show that the influx of Hg to the Carson River has varied through time as a function of the erosional and depositional processes operating on the Sixmile Canyon Alluvial Fan located between the canyon and the Carson River channel; relatively high influx rates to the river occurred immediately after mining began and from approximately 1933 to 1948. Hg-polluted sediments are located within discrete areas of the fan and comprise about 21 % of the total active and relict fan surface. Mass balance calculations estimate that about 31,500 kg of Hg, 18,200 oz of An, and 1,205,800 oz of Ag are contained within 710,700 m³ of contaminated materials. If site remediation is conducted, extraction of An and Ag, which is worth about $12 million at current market prices, would greatly defray the costs of clean-up activities. The study also illustrates that the effects of Hg may be temporally and spatially displaced from the period and location of milling activities. Thus, we conclude that to accurately assess the site for remediation, an understanding of the spatial and temporal variations in geomorphic dispersal processes is required.