Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.h51j..05s&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #H51J-05
Other
1815 Erosion, 1821 Floods, 1823 Frozen Ground, 1825 Geomorphology: Fluvial (1625)
Scientific paper
Nineteenth century hydraulic mining in the Sierra Nevada foothills of California displaced ~ 1.0 x 109 m3 of sediment, much of which constructed large tailings fans that linked up into valley-scale fans (e.g. Yuba fan) and graded into the Central Valley more than 50 km downstream. Additionally, ~4.0 x 106 kg of mercury used in gold separation was lost, leading to widespread contamination of mined sediments and the potential for tracing the evolution of the tailings fans over the last century. We conducted total mercury analysis on sediments from a range of geomorphic units spanning the region from the gold mining districts to the Central Valley to investigate aspects of the basin-scale adjustment to this extreme sediment loading. Samples were primarily extracted from bank exposures along vertical sections and from floodplain sediment cores, enabling determination of various phases of deposition and erosion. They were subsequently sieved to 63 microns to normalize samples from different geomorphic units containing a wide range of grain sizes and analyzed for total mercury on a Tekran cold vapor atomic fluorescence spectrophotometry detector. The resulting data show good discrimination in total mercury between the various sedimentary units ranging over two orders of magnitude, wherein prehistoric soils have values ~50 ppb and primary mining tailings contain concentrations up to ~7000 ppb. The data from the Yuba fan, which are corroborated by radionuclides and other geochemistry, support a conceptual model of transport and storage that resembles glacial outwash analogues, wherein successive phases of fan development and dissection can be identified. More importantly, the data suggest ongoing, episodic transport of hydraulic mining sediment from the Sierra piedmont to the lowland valley areas and delta, where conditions are favorable for mercury methylation (conversion of mercury from inorganic to organic forms). This sediment remobilization occurs largely during major flooding episodes, where the Yuba River migrates laterally into high (e.g. up to 20 m) terraces of highly contaminated mining sediments. These fan terraces pose a sobering risk to downstream ecosystems.
Aalto R.
James L.
Singer Michael Bliss
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