The sources of mercury contamination (USGS 1970) are summarized in Table 8.3.7. Mercury vaporizes from ore deposits and is washed back to earth by rain. The concentration of mercury in rainwater averages about 0.2 ppb. Mercury is held tightly by the upper 2 in or so of soil. Surface waters generally contain less than 0.1 ppb of mercury. Underground waters are higher due to longer contact with minerals. Hot springs and exposed ore bodies contribute higher concentrations in solution and by erosion. Man-created sources add to the natural sources. Suspended matter and sediments may contain 5-25 times as much mercury as the surrounding water bodies.
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