Distribution, speciation, and transport of mercury in stream-sediment, stream-water, and fish collected near abandoned mercury mines in southwestern Alaska, USA

Concentrations of total Hg, Hg (II), and methylmercury were measured in str eam-sediment, stream-water, and fish collected downstream from abandoned me rcury mines in southwestern Alaska to evaluate environmental effects to sur rounding ecosystems. These mines are found in a broad belt covering several tens of thousands of square kilometers, primarily in the Kuskokwim River b asin. Mercury ore is dominantly cinnabar (HgS), but elemental mercury (Hg d egrees) is present in ore at one mine and near retorts and in streams at se veral mine sites. Approximately 1400 t of mercury have been produced from t he region, which is approximately 99% of all mercury produced from Alaska. These mines are not presently operating because of low prices and low deman d for mercury. Stream-sediment samples collected downstream from the mines contain as much as 5500 mu g/g Hg. Such high Hg concentrations are related to the abundance of cinnabar, which is highly resistant to physical and che mical weathering, and is visible in streams below mine sites. Although tota l Hg concentrations in the stream-sediment samples collected near mines are high, Hg speciation data indicate that concentrations of Hg (II) are gener ally less than 5%, and methylmercury concentrations are less than 1% of the total Hg. Stream waters below the mines are neutral to slightly alkaline ( pH 6.8-8.4), which is a result of the insolubility of cinnabar and the lack of acid-generating minerals such as pyrite in the deposits. Unfiltered str eam-water samples collected below the mines generally contain 500-2500 ng/l Hg; whereas, corresponding stream-water samples filtered through a 0.45-mu m membrane contain less than 50 ng/l Hg. These stream-water results indica te that most of the Hg transported downstream from the mines is as finely-s uspended material rather than dissolved Hg. Mercury speciation data show th at concentrations of Hg (11) and methylmercury in stream-water samples are typically less than 22 ng/l, and generally less than 5% of the total Hg. Mu scle samples of fish collected downstream from mines contain as much as 620 ng/g Hg (wet wt.), of which 90-100% is methylmercury. Although these Hg co ncentrations are several times higher than that in fish collected from regi onal baseline sites, the concentration of Hg in fish is below the 1000 ng/g action level for edible fish established by the US Food and Drug Administr ation (FDA). Salmon contain less than 100 ng/g Hg, which are among the lowe st Hg contents observed for fish in the study, and well below the FDA actio n level. (C) 2000 Elsevier Science B.V. All rights reserved.