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Preliminary report on mercury geochemistry of placer gold dredge tailings, sediments, bedrock, and waters in the Clear Creek restoration area, Shasta County, California

Open-File Report 2002-401
By: , , , , and 
https://doi.org/10.3133/ofr02401

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Abstract

Clear Creek, one of the major tributaries of the upper Sacramento River, drains the eastern Trinity Mountains. Alluvial plain and terrace gravels of lower Clear Creek, at the northwest edge of the Sacramento Valley, contain placer gold that has been mined since the Gold Rush by various methods including dredging. In addition, from the 1950s to the 1980s aggregate-mining operations removed gravel from the lower Clear Creek flood plain. Since Clear Creek is an important stream for salmon production, a habitat restoration program is underway to repair damage from mining and improve conditions for spawning. This program includes using dredge tailings to fill in gravel pits in the flood plain, raising the concern that mercury lost to these tailings in the gold recovery process may be released and become available to biota. The purposes of our study are to determine concentrations and speciation of mercury in sediments, tailings, and water in the lower Clear Creek area, and to determine its mobility. Mercury concentrations in bedrock and unmined gravels both within and above the mined area are low, and are taken to represent background concentrations. Bulk mercury values in flood-plain sediments and dry tailings are elevated to several times these background concentrations. Mercury in sediments and tailings is associated with fine size fractions. Although methylmercury levels are generally low in sediments, shallow ponds in the flood plain may have above-normal methylation potential. Stream waters in the area show low mercury and methylmercury levels. Ponds with elevated methylmercury in sediments have more methylmercury in their waters as well. One seep in the area is highly saline, and enriched in mercury, lithium, and boron, similar to connate waters that are expelled along thrust faults to the south on the west side of the Sacramento Valley. This occurrence suggests that mercury in waters may at least in part be from sources other than placer mining.

Suggested Citation

Ashley, R.P., Rytuba, J.J., Rogers, R., Kotlyar, B.B., and Lawler, D., 2002, Preliminary report on mercury geochemistry of placer gold dredge tailings, sediments, bedrock, and waters in the Clear Creek restoration area, Shasta County, California: U.S. Geological Survey Open-File Report 2002-401, 47 p., https://doi.org/10.3133/ofr02401.

ISSN: 2331-1258 (online)

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Preliminary report on mercury geochemistry of placer gold dredge tailings, sediments, bedrock, and waters in the Clear Creek restoration area, Shasta County, California
Series title Open-File Report
Series number 2002-401
DOI 10.3133/ofr02401
Year Published 2002
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description 47 p.
Country United States
State California
County Shasta County
Other Geospatial Clear Creek restoration area
Additional Online Files (Y/N) N
Additional publication details