Clear Lake always contaminated with Mercury, which is why it's clear? Not exactly.
anthro.ucdavis.edu
There are few historical records to tell us when the serious pollution of Clear Lake began, or what the most important agents in its degradation were. But researchers at UC, Davis have found that the mud from the bottom of Clear Lake contains a chronicle of its contamination.
Pete Richerson of Environmental Studies and a large team of other scientists have been reconstructing the history of human impact on the lake by analyzing sediment cores. They obtain the cores by sinking a long, narrow tube into the lake bed and extracting a plug of mud. A lead-based dating technique reveals the ages of the cores, each of which are about 8 feet long and span 200-300 years. The scientists then examine the composition of the mud at 2- inch intervals to which they can attach specific dates. They analyze the mercury content as well as concentrations of other chemicals that provide clues about the environment at the time the sediment was being deposited. For example, this method can detect influxes of earth and rock to the lake, indicating erosion of the nearby hills. The cores tell a compelling tale.
Around 1927, a dramatic jump occurred in mercury contamination of Clear Lake, even though mining had been well-developed long before this date. By 1873, for example, the Sulfur Bank Mercury Mine was in full swing and had extracted 72% of all the mercury it would obtain before the century was up. Nevertheless, after 1927, mercury levels in the lake increased ten-fold over the previous 75 years. At the same time, sediment influx to the waters also rose markedly. What happened in 1927 to cause these massive changes?
Richerson and his colleagues suggest that the key was the upsurge around this time in the use of large, powered earth-moving machinery. This made it possible to operate Sulfur Bank Mercury Mine as an open pit mine, a method that brought earth and rock to the surface, producing 30-foot high tailings and exposing mercury deposits to erosion and leaching. Mechanized equipment also greatly increased gravel mining, road-building, and other erosive activities. The result was an influx of mercury and sediment to what was rapidly becoming an unclear lake.
Researchers are struck by the absence of any notable human impact during the first half of European settlement at Clear Lake. It seems that intensive human labor and use of draft animals during the 19th and early 20th centuries were relatively unpolluting, yet effective, methods of mining.
Do the cores have any good news? There is some evidence that water quality of Clear Lake has improved modestly over the past 20 years, probably due to repair work on the mercury mine, as well as restrictions on gravel mining. However, researchers from the UCD team warn that far more needs to be done before contamination of Clear Lake is merely a part of its history.
Mercury is a widespread problem in Northern California. Abandoned mercury mines dot the Coast Range and in the Sierras the use of mercury in gold mining led to extensive contamination of the drainage. The research at Clear Lake will have relevance and impact throughout the state.
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