Abstract
The hydraulic gold-mining process used during the Californhttia Gold Rush and in many developing countries today contributes enormous amounts of sediment to rivers and streams. Commonly, accompanying this sediment are contaminants such as elemental mercury and cyanide used in the gold extraction process. We show that some of the mercury-contaminated sediment created by hydraulic gold mining in the Sierra Nevada, between 1852 and 1884, ended up over 250 kilometers (km) away in San Francisco Bay; an example of the far-reaching extent of contamination from such activities. A combination of radionuclide dating, bathymetric reconstruction, and geochemical tracers were used to distinguish the hydraulic mining sediment from sediment deposited in the bay before hydraulic mining started (pre-Gold Rush sediment) and sediment deposited after hydraulic mining stopped (modern sediment). Three San Francisco Bay cores were studied as well as source material from the abandoned hydraulic gold mines and river sediment between the mines and bay. Isotopic and geochemical compositions of the core sediments show a geochemical shift in sediment deposited during the time of hydraulic mining. The geochemical shift is characterized by a decrease in eNd, total organic carbon (TOC), Sr and Ca concentrations, Ca/Sr, and Ni/Zr; and, an increase in 87 Sr/ 86 Sr, Al/Ca, Hg concentrations, and quartz/ plagioclase. This shift is in the direction of the geochemical signature of sediments from rivers and gold mines in hydraulic mining areas. Mixing calculations using Nd isotopes and concentrations estimate that the hydraulic mining debris comprises up to 56% of the sediment in core sediments deposited during the time of hydraulic mining. The surface sediment of cores taken in 1990 were found to contain up to 43% hydraulic mining debris, reflecting a continuing remobilization and redistribution of the debris within the bay and transport from the watershed. Mercury concentrations in pre-Gold Rush sediment range between 0.03 and 0.08 µg g -1 . In core sediments that have characteristics of the gold deposits and were deposited during the time of hydraulic mining, mercury concentrations can be up to 0.45 µg g -1 . Modern sediment (post-1952 deposition) contains mercury concentrations up to 0.79 µg g -1 and is likely a mix of hydraulic mining mercury and mercury introduced from other sources.
Highlights
When human activities add inorganic mercury to the environment, the most important environmental outcome is generation of methylmercury
The analyses show that the chemistry of the hydraulic mine gravels is relatively similar among the mines (Table 1)
Sediment deposited in San Pablo Bay during the time of large-scale hydraulic gold mining in the Sierra Nevada (1852–1884) is geochemically different from earlier deposited sediment
Summary
When human activities add inorganic mercury to the environment, the most important environmental outcome is generation of methylmercury. About 25% to 50% the elemental mercury used to extract the gold remains with the sediments or is otherwise lost to the environment when large volumes of tailings are discharged to a watershed (Nriagu 1994). In oxidizing conditions this elemental mercury (Hg) can be converted to Hg+2 and some fraction will be methylated and biomagnify through the aquatic food web. Tracking the fate of the sediments originating from hydraulic mining, as well as the mercury that accompanies those sediments is a problem of global interest
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
