Current vs. Pre-Industrial Mercury Deposition: Where Do High Fish Levels of Mercury Come From?

摘要

Current measured wet deposition levels of divalent mercury in precipitation within the continental United States range from about 5 to 25 micrograms per square meter per year in a typical year. The lowest range of values, from samplers of the Mercury Deposition Network, tend to occur in the intermountain west. Global data from a variety of surrogate deposition records, including glacial ice cores, peat hummocks, lake bottom sediment cores, etc., show consistently that mercury deposition globally in the pre-industrial era (prior to 1750 or so) were in the range of about 1 to 10 micrograms per square meter per year. Nonetheless, most states west of the Mississippi have significant mercury fish advisories posted, and data showing widespread occurrence of fish with tissue above the 0.3 ppm level mercury health criterion established by the U.S. EPA. This occurrence of fish with "excess" (greater than regulatory threshold) mercury levels in regions of modeled and measured moderate deposition raises the more basic question: where did the mercury originate, both currently and prior to 1750, to correspond to high-tissue levels? In some cases, crustal mercury is being leached into or discharged to waterways, while in other cases, atmospheric wet and dry deposition play a dominant role. The ubiquity of pre-industrial-equivalent deposition levels in present-day North America coinciding with fish mercury advisories leaves unresolved the issue of whether pre-European America experienced widespread mercury intoxication from fish diet.