미국지질조사소 발췌자료 (U.S Geological Survey)
ENVIRONMENTAL GEOCHEMISTRY OF MERCURY MINES IN ALASKA
The U.S. Geological Survey (USGS), in cooperation with the U.S. Bureau of Mines, U.S. Fish and Wildlife Service, and Calista Corporation (an Alaska native corporation), is investigating potential environmental contamination around naturally occurring, mercury-rich mineral deposits in Alaska. Mercury deposits are scattered over a wide region in southwestern Alaska covering several thousand square kilometers. Cinnabar (mercury sulfide) is the most common mercury-rich ore mineral in the deposits, but liquid mercury is found at some localities. Several of the mercury deposits have been mined, but most of the deposits are small and have not been developed. None of the mercury mines in Alaska are currently operating due to economic factors. Mercury is used in the manufacturing of electrical instruments, fungicides, pharmaceuticals, munitions, paper production, and in the extraction of gold in mining.
Mercury is a heavy metal that has no known metabolic purpose and is toxic to living organisms. In humans, mercury adversely affects the central nervous system. Mercury can be converted from inorganic compounds such as cinnabar to organic forms such as methylmercury that is easily absorbed by organisms. Thus, mercury deposits and mines in Alaska are a potential hazard to residents and wildlife because drainage from these deposits enters streams and rivers that are part of local ecosystems.
The primary sources of mercury are naturally occurring mineral deposits, rocks, and volcanic eruptions, as well as man-made industrial sources, such as factory effluents and incinerations. Southwestern Alaska has no major sources of industrial mercury; however, mineral deposits in the area are of concern because mercury is highly concentrated around the deposits.
The USGS has measured the concentration of mercury in sediment, water, and fish collected from streams near some of these mercury mines and deposits and has compared these data with those from streams in unmineralized (background) areas to evaluate environmental effects of these deposits.
TESTING MERCURY LEVELS IN STREAMS AND SEDIMENTS
Stream-sediment samples collected near some mines contain mercury concentrations in excess of 5,000 ppm (parts per million). Stream-sediment samples collected from streams in unmineralized areas typically contain less than 1 ppm mercury. It is reasonable to expect high mercury concentrations in stream sediments near the mines because the mercury-rich mineral cinnabar, which is often observed in the streams, is resistant to weathering.
Stream waters below the mines generally have pH measurements that are neutral to slightly alkaline. The formation of acid drainage below some mines containing sulfide minerals can be a problem; however, cinnabar is highly insoluble in water and does not easily form acid drainage during weathering. Thus, acid formation in streams below the mercury mines in southwestern Alaska is probably insignificant.
Samples of stream water collected below mercury mines contain as much as 0.75 ppb (part per billion) mercury, whereas background stream waters in the region typically contain less than 0.10 ppb mercury. Mercury concentrations in these stream waters are below the 2.0 ppb drinking water standard recommended by the State of Alaska and below the 2.4 ppb instream maximum concentration recommended by the U.S. Environmental Protection Agency (EPA).
Some stream water samples collected below mines, however, exceed the 0.012 ppb instream concentration that the EPA indicates may result in chronic effects to aquatic life. When stream water exceeds 0.012 ppb, edible portions of fish are analyzed to determine if mercury concentrations exceed the 1.0 ppm (wet weight) action level established by the Food and Drug Administration (FDA). At this concentration, sale of fish is restricted by the FDA. Because stream water samples collected near some of the southwestern Alaska mercury mines exceed the 0.012-ppb concentration, the USGS has determined mercury contents in fish throughout the region.
MERCURY LEVELS IN FISH DOWNSTREAM FROM MINES
In the USGS studies, maximum mercury concentrations in muscle samples (edible fillets) of native freshwater fish (Arctic grayling and Dolly varden) collected downstream from mercury mines are about 0.6 ppm (wet weight). Mercury concentrations for these fish are considered high because similar fish in southwestern Alaska collected from background streams contain only about 0.2 ppm mercury. The mercury levels in these fish, however, are still below the 1.0 ppm FDA action level.
Mercury concentrations were also measured in salmon collected from large rivers in the region because these fish are often consumed by local residents and sportsmen. Mercury concentrations for these salmon are low, less than 0.1 ppm, and are also below the FDA action level. These low mercury concentrations are understandable because salmon spend much of their lives in the ocean and are not significantly affected by mercury from the mines and deposits.
Cinnabar in the upstream mines is the dominant source of mercury in the stream environment. The elevated mercury concentrations in fish near the mines indicate that mercury is being converted from inorganic mercury in the sediment to organic mercury that is more biologically available. Most of the organic mercury resides in the stream sediments and is then transferred to stream water and food sources for the fish, such as larvae, insects, and other small fish that live in and around the mercury-rich sediment. When mercury enters the food chain, it can be hazardous because mercury tends to concentrate in the highest predators through a process called biomagnification. Mercury concentrations in fish are useful for studying the levels of mercury in the food chain that can eventually affect humans.
The results of this environmental study may be used for other similar mineral-deposit studies.
For additional details and data, please refer to:
Gray, J.E., Theodorakos, P.M., Budahn, J.R., and O'Leary, R.M., 1994,
Mercury in the environment and its implications, Kuskokwim River Region,
southwestern Alaska, in Till, Alison, and Moore, Tom (eds.),
Geologic Studies in Alaska by the U.S. Geological Survey, 1993:
U.S. Geological Survey Bulletin 2107, p. 3-13.
Gray, J.E., Meier, A.L., O'Leary, R.M., Outwater, C., and:
Theodorakos, P.M., 1996, Environmental Geochemistry of:
Mercury Deposits in Southwestern Alaska: Mercury:
Contents in Fish, Stream-sediment, and Stream-water Samples,:
in Moore, T.E., and Dumoulin, J.A., eds., Geologic Studies:
in Alaska by the U.S. Geological Survey, 1994::
U.S. Geological Survey Bulletin 2152, p. 17-29.