ContentsPutah and Cache: McLaughlin Mine

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Downstream: How Mercury Travels

Bob Speirs

Gold mining in California is a well known and important aspect of American history. However, associated with this gold mining is a less well known but equally important activity, the mining of mercury that is used in the amalgamation process to separate the gold from sand and other impurities. Amalgamated gold and mercury is a mixture which can be separated by heating and vaporizing the mercury, leaving the gold behind. Old timers involved with placer mining had a number of different ways of accomplishing this. In rural areas the amalgam was often placed on a shovel and heated over a fire, leaving behind the collected gold in the form of nuggets. A smaller-scale procedure involved cutting a potato in half, scooping out the middle and filling it with amalgam, wiring the potato together and then placing it in coals of a fire. In a couple of hours the potato could be opened revealing a little nugget of gold.

In more sophisticated procedures such as hydraulic mining the gravel and soil were washed through long sluice boxes. In some cases heavy sand containing gold dust from the sluice boxes was dumped along with a few pounds of mercury at the bottom of a tub or barrel to form an amalgam which was later heated to separate out the gold. In other cases the mercury was placed directly in the sluice boxes, which could be over a thousand feet long, to amalgamate with the gold and then collected and heated. These sluice boxes would accommodate several hundred pounds of mercury at a time. Retorts and distillation procedures were eventually utilized to salvage the mercury so it could be reused.

Such inefficient procedures resulted in thousands of tons of mercury being released into the atmosphere in the Sierra Nevada and adjoining mining regions. Because of its high atomic weight and ready capacity for vaporization the mercury would spread in a grasshopper fashion. It would vaporize and be carried away by winds until, in the presence of moisture and sunlight, it would oxidize and return to earth as a component of rain drops. Soil bacteria converted some of it back to elemental mercury which would revaporize and recycle spreading a bit further. This process would be repeated over and over for as much as a year, gradually spreading the mercury further away from its original source.

The mercury used in the Sierra Nevada and other areas was mined in a mercuriferous zone along the Coastal Mountain Range extending 350 miles south from Lake County. Local residents used to collect cinnabar rocks and bring them home to pulverize. The crushed rock was often placed on a slab of sheet metal and heated. This converted the cinnabar or mercuric sulfide into liquid elemental mercury which was collected in a jug or flask. During the process much mercury was vaporized and released into the atmosphere or retained as elemental mercury within the waste rock. The retort and distillation processes which were eventually used greatly increased yield but its low efficiency resulted in a great deal more mercury being released into the atmosphere or dumped into the slag heaps adjacent to the mines.

There are roughly 30,000 abandoned mines (gold, mercury, silver and other metals) scattered throughout California. Discarded minerals and other waste materials have been scattered around these sites and exposed to weathering. Any elemental mercury remnants in these slag heaps slowly volatilizes or becomes eroded into nearby streams and waterways. The pulverized cinnabar was also eroded by rain into adjacent streams. One such waterway is Cache Creek and its tributaries.

In 1974 authorities in the Bay-Delta area began issuing fish advisories after tests indicated that many of the local fish contained methylmercury, sometimes in high, potentially toxic levels. They began to investigate the origin of this mercury and looked upstream at the rivers and tributaries feeding into the Sacramento Delta. Cache Creek appeared to carry most of the mercury incorporated in the sediment resulting from upstream erosion. Tributaries from the Sierra Nevada mountains and foothills, with a watershed area approximately 20 times larger than the Cache Creek watershed, also contributed mercury but not as much. In one winter alone one ton of mercury was washed down Cache Creek into the Yolo Bypass and from there into the Delta and Bay area. Some of this mercury eventually becomes converted by bacteria into a compound called methyl mercury which is one of the most hazardous materials known to man. Methyl mercury is readily absorbed by aquatic organisms and then becomes part of the food chain for higher animals. Thus mine sites which are no longer active and have been abandoned for many years are still contributing to the mercury pollution in the waterways leading into the Bay Delta area.

This situation raises many questions. Only a few of the mercury, gold and silver mines are located in Yolo County and there is little or no mercury eroding into the streams within Yolo County. Instead most of the mercury is coming from upstream areas in Lake County. Some mercury which is coming from the Sierra Nevada on the eastern side of the valley passes into the Yolo Bypass during periods of flooding. Therefore why should any of this be of concern to Yolo County Citizens? One reason is that mercury in either natural form (cinnabar) or in its elemental form is relatively heavy and settles deep into the gravel beds. As the eroded material from Lake County passes downstream much of it is deposited in the gravel beds along Cache Creek. In some marsh areas or wetlands the accumulated mercury sediment is converted to methylmercury which as it passes further downstream can be incorporated into the aquatic biota and eventually into fish and fish eating mammals including humans. Mining of gravel in the creek bed along Cache Creek stirs up these deep deposits and directly or indirectly results in an increase in availability of mercury for methylation. Thus decisions made about the mining of gold and the removal of gravel from Cache Creek can materially affect downstream wildlife and users of the water.

Mercury was an issue when Homestake Mining Company became operative in Lake and Napa Counties. Environmental activists pushed them into improving their mining practices. A similar situation applied to gravel mining within Cache Creek. Environmental groups are important in helping to keep our environment safe. They really don't win many battles per se, but they do at least persuade companies to improve their activities and lower the risks to the environment and to human health and welfare.

I hope there will always be a place for active environmentalists even if it seems a discouraging undertaking.

Further references to mercury pollution in Northern California can be found on the web:

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