Preliminary Report on Mercury Geochemistry of Placer Gold Dredge Tailings, Sediments, Bedrock, and Waters in the Clear Creek Restoration Area, Shasta County, California
Citation
Roger P. Ashley, James J. Rytuba, Ronald Rogers, Boris B. Kotlyar, and David Lawler. 2002. 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 02–401. U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY
Summary
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 [...]
Summary
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.
