HUMAN EXPOSURE TO MERCURY MAY DECREASE AS ACIDIC DEPOSITION INCREASES

It has been hypothesized that human mercury (Hg) exposure via fish con sumption will increase with increasing acidic deposition. Specifically , acidic deposition leads to reduced lake pH and alkalinity, and incre ased sulphate ion concentration ([SO42-]), which in turn should cause increased Hg levels in fish, ultimately resulting in increased human H g exposure via fish consumption. Our empirical test of this hypothesis found it to be false. We specifically examined Hg levels in the hair of Ontario Amerindians, who are known consumers of fish from lakes acr oss the province, and observed a weak negative association with increa sing sulphate deposition. An examination of Hg levels in lake trout, n orthern pike and walleye, three freshwater fish species commonly consu med by Ontario Amerindians, found a similar weak negative association with increasing sulphate deposition. Further analysis of these fish da ta found that fish [Hg] was most significantly (positively) associated with lake water concentrations of dissolved organic carbon (DOC), not pH, alkalinity or [SO42-]. Lake DOC levels are lower in regions of gr eater acidic deposition. We propose an alternate hypothesis whereby hu man Hg exposure declines with increasing acidic deposition. In particu lar, we propose that increasing sulphate deposition lends to reduced l ake DOC levels, which in turn leads to lower Hg in fish, ultimately re ducing human Hg exposure via fish consumption.