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by Mika Pringle Tolson |
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| Finding the source of pollution is often the first step in successful remediation. In the case of Clear Lake, it is well known that the mercury originates in the remains of the Sulphur Bank Mercury Mine, whose tailings are piled on the shore of the lake. What is not known is exactly how the mercury gets into the lake. William Shipp, a master's student in Geology at UC Davis, has received a TSR&TP fellowship to identify the source of the acid-sulfate waters in Clear Lake. Says Shipp, "The overall goal of my project is to determine the sources and flow rates of the subsurface fluids discharging into Clear Lake, including acid rock drainage, and to provide baseline monitoring data for the site before cleanup strategies are implemented." The Sulphur Bank Mercury Mine initially began as a sulphur mine in the 1860’s, but when it became "contaminated" with cinnabar, the ore of mercury, the sulphur lost its value and the mine was closed. It was reopened in the 1870’s as a mercury mine in a subset of the existing tunnels and shafts. After several decades, mining underground became impossible due to the risks of hydrogen sulfide gases and scorching temperatures. |
 Tailings of the Sulphur Bank Mercury Mine are piled on the shore of Clear Lake. Photo by William Shipp. |
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 Herman Pit, on the right, is thought to push contaminated fluids into Clear Lake by subsurface flow through the sediments. Photo by William Shipp. |
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| With the advent of diesel power in the 1920’s, the site was converted to an open pit mine, and the remains of this process are most of what is visible today. There are several pits at the site, but the largest and most significant is Herman Pit, located about 750 feet from the lake. The surface water of this pit is about ten feet higher than the lake, making a pressure gradient, which is thought to push fluids into the lake by subsurface flow through the sediments. | |||||||||||||||
| Water in Herman Pit is very acidic, making the mercury more soluble and therefore enhancing its transport. The lake water is slightly alkaline, and when the two meet, they form a flocculent precipitate, a white, gooey aluminum silicate material high in mercury. This precipitate appears to form just above the sediment layer in the lake, and sulfate-reducing bacteria in the sediments are known to convert the inorganic mercury to the organic and bioavailable form, methylmercury. | |||||||||||||||
Shipp explains, "The flocculent forms near the mine site at the face of the waste rock dam and has a near neutral buoyancy, which facilitates its transport. If we can understand the source, migration pathways, and flux of acid-sulfate fluids, then we can potentially understand methods for reducing the environmental impact of the Sulphur Bank Mercury Mine on the Clear Lake ecosystem." There are other potential sources of fluids that may turn out to be more significant than Herman Pit, such as direct subsurface flow from waste rock and active hot springs or leaking wells, but no one has measured and compared them. Says Shipp, "We don’t have a good handle on the hydrogeology between the mine site and the lake." Shipp’s research will develop methodology to fingerprint the source of fluids and establish local flow rates to determine which source is the most significant for mercury contamination. The results of his research will also provide baseline monitoring data before any cleanup strategies are implemented. Shipp feels the methods he’s developing can become "a quick economical way of evaluating the fluid discharge that can be applied to other sites in California, not just for mercury, but where mines near lakes have groundwater discharging out into the lake." |
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 William Shipp at Clear Lake. Courtesy photo. |
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| William Shipp can be reached via email at shipp@geology.ucdavis.edu | |||||||||||||||
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