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Mercury Contamination of the Carson River

Winter 1992

Mining of the Comstock Lode in Virginia City, Nevada took place largely between 1860 and 1895. Gold and silver were extracted from the ores using the mercury amalgamation process. Amalgamation is the alloying and collection of fine gold-silver particles in puddles, droplets, or coatings of mercury. The mercury is then collected and heated (evaporated) away from the precious metals and recondensed in a retort for reuse. Some loss of mercury (and precious metals) is attendant in the many steps in this metallurgical process.

Amalgamation milling of Comstock gold-silver ores took place within the watershed of the Carson River. Approximately 15 million pounds of mercury (NBMG Bulletin 41) were lost to the Carson River drainage system in the milling of ore containing 8 million ounces of gold and 192 million ounces of silver (NBMG Bulletin 70). It is estimated that 3 million ounces of gold and 64 million ounces of silver were also lost. These tremendous amounts of mercury, gold, and silver now reside in mill tailings and the channel sediments and flood plain deposits of the Carson River, largely along the 70 mile stretch between Carson City and Fallon, Nevada. As many as 200 mills may have processed Comstock ore at one time or another within the Carson and Truckee River watersheds.

Although most milling was done in Six Mile Canyon and along the banks of the Carson River, some Comstock ore was shipped to mills in Washoe Valley. Preliminary data show some mercury contamination in Washoe Lake (now dry) and Little Washoe Lake sediments. These lakes drain into the Truckee River. Concentrations of 200-300 ppb total mercury are common in the upper 30-40 cm of lake sediments in the center of Washoe Lake. Closer to the edges of the playa 100-150 ppb total mercury is more common. At the north end of Little Washoe Lake 2-14 ppm mercury is found in the subsurface lake muds. Some mercury has been flushed out of Little Washoe Lake and into a pond located north of Washoe Hill in Pleasant Valley, and beyond.

Because of the substantial amount of silver sulfide (Ag2S, the mineral acanthite) which was present in the Comstock ores a variation of the conventional amalgamation process was used. This process, called the Mexican Patio Process, included the addition of sodium chloride (NaCl, common salt) to enhance the recovery of silver. Because of the water soluble nature of HgCl2 (mercuric chloride) this addition of sodium chloride may have converted some of the elemental mercury to the water soluble form, contributing to the dispersion of mercury to the river systems. In Washoe Lake samples a general correlation of higher water soluble mercury with high chloride content suggests conversion of a portion of the original native mercury to HgCl2. No relationship of total or leachable mercury to organic content of the samples has been observed.

One method of assessing which of the many mill tailings piles contain the highest levels of mercury is to measure the relative quantity of mercury vapor being emitted by each dump. A very simple method of collecting mercury vapor above the dumps is to suspend a piece of silver foil or wire inside an inverted plastic funnel partially buried in the dump material. Soil gas is allowed to flow past the foil for a specific period of time (24 hours is sufficient); mercury vapor is alloyed to the foil and fixed for later analysis. The foils are then placed inside a graphite furnace which is attached to an atomic absorption spectrometer, the tube is heated, and the evolved mercury is quantified. The analysis takes approximately 15 seconds and results are reported as nanograms of mercury per square meter of ground per day (ng/ml/day). This type of vapor sampling probably underestimates the actual concentrations.

Results of mercury vapor sampling indicate that the south end of Washoe Lake is emitting approximately twice as much mercury vapor from the playa surface as from more central areas farther to the north, possibly due to generally drier sediments to the south. Mercury vapor fluxes of 7-15 ng/ml/day have been recorded. Highest mercury vapor fluxes have been recorded from mill tailings in Six Mile Canyon (47-138 ng/ml/day) and at a contaminated site along the Carson River near Dayton (526 ng/ml/day).

The Carson River has been designated a Superfund site by the Environmental Protection Agency because of the huge amounts of contained mercury. Possible remediation scenarios include recovery of gold and silver to help defray the cost of the mercury cleanup. Preliminary data indicate that mercury, gold, and silver are not limited to the Carson River channel but were also deposited in the overbank sediments during flooding. Ore grade concentrations of gold and silver are present in some of the mill tailings along Six Mile Canyon and the Carson River.

Mercury/gold ratios are not constant in Carson River sediments; our data indicate variations from 8 to 313. Higher ratios tend to occur downstream, farther from the source mills. Mercury in samples with relatively high gold values (up to 2.7 ppm by weight) and low to moderate mercury/gold ratios is most likely contained in amalgam particles. Because various geochemical processes would be expected to preferentially dissolve mercury and silver from the amalgam particles, mercury in downstream samples with relatively high mercury/gold ratios is probably not contained primarily in amalgam particles; it may be adsorbed onto clays, iron hydroxides, or organic constituents in the sediments.

Sediments from Lahontan Reservoir have up to 100 ppm (by weight) of mercury, up to 300 ppb of gold, and up to 20 ppm of silver. Mercury vapor is being emitted from the surface of the reservoir, much of which is now dry. Concentrations in the range of 5 to 40 ng/ml/day have been detected at various localities.

This initial data collection, which is providing details about the mercury contamination in the Carson River Project area, has been partially funded by a grant from the National Institute of Environmental Health Science.

---Paul J. Lechler, Chief Chemist/Geochemist