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The river made the National Priorities List (NPL) on October 30th, 1990 as the Carson River Mercury Superfund site (CRMS) due to investigations that showed trace amounts of mercury in the wildlife and watershed sediments.

Superfund Site:
Mercury is the primary concern in regards to the Carson River Mercury Superfund site. There are several forms of mercury that are prevalent in the open water source including inorganic mercury (Hg2+) and organic methylmercury (MeHg). MeHg is produced from bacteria in river systems and can be ingested by the wildlife in the area. The methylmercury, due to its specific properties that include it being fat soluble, can lead to bioaccumulation and biomagnification of mercury in Carson River Aquatic wildlife.

Signs of Mercury poisoning due to oral ingestion are most common found in effect to the kidneys, central nervous system, and development - especially pertaining organic mercury.

History:
The Carson River watershed was a shared territory within these two tribes. The Northern Paiute of wester9i Nevada was grouped up in to several bands relating to the area that they inhabited. The group that established itself along the Carson River were known in the Shoshone language as "Toituked". The Washoe peoples that had residence in the Carson Valley were also divided in to a seperate group and called "Pau wa lu". Ancient people and indigenous tribes used the Carson River as a resource for livelihood, depending on the river for fish, waterfowl, plant materials, and especially water in the aired Nevada desert climate. Displacement of these tribes and changes of lifestyle did not take place until the development of European settlements.

Mercury and Gold Mining Process
The Mercury presence in the Carson river is primarily introduced in to the river through sediment deposition resulting from mine tailings during the Comstock Lode. These tailings were attributed to the mining process that settlers used in the 1850's to recover silver and gold. This process is known as mercury amalgamation. Mercury amalgamation has been used in the process of mining precious metal since the Middle Ages. Mercury amalgamation is accomplished by pulverizing the ore and covering it with mercury using drums or tables. Once the mercury binds to the gold or silver it is separated from the barren soil to a retorting vessel where the mercury is vaporized and condensed off of the amalgam. During the Comstock Lode a new, Washoe "hot-pan" amalgamation, process was created specifically to treat Nevada ore in 1861. Washoe "hot pan" mining used a cast iron pan with the soil mixture, mercury, sulfuric acid, and other materials to remove the precious metals.

Losses of mercury that attributed to the contamination of the Carson River, and eventually to the CRMS, occurred during these processes. Mercury loss in the mining operations ranged from transportation failures, crude equipment, and poor handling. The mercury was transferred about using leather bags, pails, and barrels that leaked and spilled. Equipment was not up to standards and not practical for containing tailings. However, most of the mercury loss happened throughout the amalgam process. Recovery amount of the vaporized mercury is completely dependant on the technology used and the efficiency of mercury binding depends on the ability of the precious metal. Both of the actions were not completely refined themselves, and attributed to as much as 0.7-1 kg of mercury waste/ per tonne of ore processed in Nevada mines.

Hydraulic mining was also used in the Carson River region. Hydraulic mining uses mercury to bond to the gold and silver in the mining slurry. The gold/silver amalgam drops to the bottom of the sluice while gravel and other tailings pass over. As time passes the mercury is ground in to a fine layer that coats the bottom of the sluice and can contaminate the surrounding area through leaks and the effluent of water containing the mercury. It is estimated that this practice in California attributed to mercury losses up to 10-30%.