The area that includes Beimen and the immediate surrounding villages of southwestern Taiwan is recognized as the home of the world's only well-documented Blackfoot Disease (BFD) occurrence - where extremely high concentrations of arsenic (As) have been found in the artesian wells from the Chianan Plain that were previously used as a drinking water supply. In the past decade, very high levels of arsenic have also been discovered in the potable water from the Lanyang Plain of northeastern Taiwan, but BFD does not occur there. From regional geological/geophysical field investigations in these two areas and a literature survey of arsenic problems in the drinking water of other countries, we have identified five potential primary geologic sources of the As for the BFD area and six sources for the Lanyang Plain. We also report measurements of arsenic in two "new" areas: (1) the sea water west of the BFD area (three samples with As levels of 2.32, 7.46, and 9.71 μg/L; and (2) stream bed deposits east of the BFD area (two samples with As concentrations of 0.00485 wt% and 0.23000 wt%). With the current information available, it seems most plausible that the primary source of the arsenic in the BFD area is from fluid escape from the accretionary prism of Taiwan or from in situ formation of arsenic-laden sulfide minerals in the water-rich coastal sediments, whereas the most likely sources of the high concentrations of arsenic in the Lanyang Plain are hydrothermal venting in the Okinawa Basin and/or weathering of sulfide minerals associated with gold and copper ores along the Lishan fault to the south. Although oxidation is known to play a role in the formation of some arsenic compounds, most of these 11 primary sources and their associated rocks and minerals are originally related to reducing environments; however continued research is necessary to fully explain each source's expulsion mechanism (mobility), transport (if any), and final concentration or enrichment of arsenic in the groundwater of Taiwan.
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