Use of mercury in mining 125 years ago continues to impact waterfowl populations: Implications for current artisanal...

Legacy and active gold mining using elemental mercury (Hg⁰) as an amalgam have influenced mercury (Hg) concentrations in terrestrial and aquatic organisms. The use of Hg⁰ in gold and silver mining over 125 years ago continues to impact wood ducks in the lower Carson River, Nevada, USA. Here, we leveraged a large archive of feather samples to model how environmental, temporal, and biological factors influence Hg accumulation in wood duck feathers. We evaluated the effects of hydrologic variables (annual streamflow, drought), age class, and site location on feather Hg concentrations, testing the hypothesis that lower flows, when conditions may favor methylmercury production, would be associated with higher feather Hg concentrations in hens and offspring. Contrary to our hypothesis, feather Hg concentrations were highest during high water-flow years, suggesting that increased flows may mobilize legacy Hg from sediments, increasing exposure. We also quantified water quality parameters, total Hg and methylmercury in water and sediment, and Hg in dietary sources such as duckweed and macroinvertebrates to identify routes of exposure for wood ducks. Our results indicate that food web pathways, including aquatic vegetation and invertebrates, are key sources of Hg exposure. Although water management could theoretically mitigate Hg exposure to wildlife, the extent of legacy contamination in the lower Carson River makes substantial reductions unlikely. These findings have important implications for regions currently affected by artisanal gold mining, where ongoing Hg use will result in persistent ecological and human health risks for years to come.