Even in the early days, panning for gold wasn't an environmentally clean venture. Gold hunters often used mercury to make it easier to separate out the precious metal. Since then, river flow has entombed the toxic mercury under silt and stone.
Enter the modern suction dredge, which vacuums sand, gravel and stones from the river bottom, then sends it all through a floating sluice where the gold, heavier than the rest of the material, sinks and is trapped, while the remainder is returned to the river. In the process, the dredge unearths the mercury and releases it into the water.
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