Carson River Mercury Site — Overview
HISTORIC MINING (cont.)
The bottom of the lode was abruptly reached in 1877 at a depth of about 1,650 feet, and 1878 was the first year of dramatically reduced production. Between 1877 and 1878, ore production dropped from 562,519 tons to 272,909 tons and the total value decreased from $36,301,536 to $19,661,394. In 1879, production and value dropped even further. In 1901, the first cyanide-leaching operation began in Sixmile Canyon. Cyanide leaching was capable of recovering more gold and silver from lower-grade material than was possible by amalgamation methods, and during the early 1900s mining operations consisted of mining lower-grade material and reworking former ore dumps and tailings piles. Between approximately 1920 and 1950, large tonnages of low-grade ores were mined. Since approximately 1950, mining operations have been extremely limited in scope.
The most widely used ore-processing method during the Comstock era was the "Washoe Process". With this process, the raw ore is wet crushed with stamps, the crushed ore is separated from the slurry in a settling tank and then the crushed ore is charged with mercury (approximately 10 percent of the weight of the ore) in the amalgamation pan. The amalgam is separated from the slurry and the silver and gold is separated from the amalgam with a retort. It is thought that the majority of the mercury released to the environment was associated with tailings which were separated from the amalgam slurry and discharged into the drainage. Other possible release mechanisms would have included air emissions from the retort, fugitive air emissions throughout the process, and spilling throughout the process where mercury was handled. It is estimated that the loss of mercury exceeded 1 pound for each ton of ore milled which translates to approximately 14,000,000 pounds of mercury. MERCURY AS A CONCERN
Elevated mercury levels in the Carson River drainage basin were discovered in the early 1970s when sampling conducted by the U.S. Geological Survey (USGS) revealed elevated levels in river sediment and unfiltered surface water from the Carson River downstream from pre-1900 ore milling sites. Subsequent studies by a number of investigators have further delineated the extent of mercury in river and lake sediment. Based largely on the information presented in these studies, the Carson River basin below New Empire in Carson City was added to the National Priorities List (NPL) in August, 1990 due to the widespread presence of mercury. Sources of mercury in the Carson drainage basin and Washoe Valley include mercury imported from the Almaden area of San Jose, California during the Comstock era and, possibly, naturally occurring mercury. There is insufficient information to characterize the full extent and significance of naturally occurring mercury in the Carson drainage basin and Washoe Valley. However, according to reports which characterize the geology of the Carson River drainage basin, naturally occurring deposits of mercury of economic importance do not exist in the basin.
Less significant natural occurrences of mercury can be associated with mineralized zones and hot springs deposits. Although it is possible that there are such natural occurrences of mercury in the region, such sources are not considered important relative to the large amount of mercury imported to the region during the Comstock era.
Fluvial transport is considered the most important mechanism for distributing mercury throughout the Carson Drainage and Washoe Valley. This is because mill tailings are considered the most significant release mechanism and this material is easily transported by fluvial processes. Aeolian (airborne) transport mechanisms may also account for the widespread dispersion of mercury in the region. The fate and transport of gaseous mercury emissions to the atmosphere is not well defined, however, it is believed that gaseous mercury was released to the environment from mills while operating and that mercury evasion is presently occurring. Also included as a transport mechanism is percolation which refers to the vertical movement of mercury through the subsurface. This transport mechanism would account for the vertical movement of elemental mercury or amalgam that was released to the environment.
Several historic millsites were located in and around Dayton (see maps of milling sites; 5mb). Because Dayton is located at the mouth of Gold Canyon and on the flood plain of the Carson River, tailings may have been deposited in and around Dayton from other upgradient source areas. Samples were collected from soil, groundwater, air, and domestic produce; and exposure point concentrations were derived. In addition to the Dayton area, soil samples were also collected from Sixmile Canyon, Gold Canyon, the alluvial fan below Sixmile Canyon, the Carson River flood plain, the beach areas of Lahontan Reservoir, Washoe Lake, and Indian Lakes; and exposure point concentrations were derived to represent the level of contamination in these areas. Exposure point concentrations were also derived from muscle tissue from fish and waterfowl using data from Nevada Department of Wildlife, Nevada Division of Environmental Protection, and United States Fish and Wildlife Service.