MERCURY CONTAMINATION IN THE CARSON RIVER, NEVADA - A PRELIMINARY-STUDY OF THE IMPACT OF MINING WASTES

From 1860 to 1890, approximately 7 100 metric tons of metallic mercury (Hg) were released into the Carson River-Lahontan Reservoir watershed as a by-product of Comstock Lode silver (Ag) and gold (Au) ore refini ng. Present-day Hg contamination is most severe in mine tailings, wher e total Hg concentrations can exceed several hundred mu g/g. Hg-laden tailings were also dumped directly into the Carson River, and were sub sequently transported downstream into the Lahontan Reservoir and the S tillwater Wildlife Management Area. The Hg, Ag and Au contents of sedi ments from the Carson River and the Lahontan Reservoir are well above local background levels, and both Ag and Au contents are positively co rrelated to Hg. Thus, tailings-derived Hg has been redistributed throu ghout the entire Carson-Lahontan watershed over the last century. Tota l Hg concentrations in water samples from the Carson River at seven lo calities show that: 1) elevated (e.g., >20 ng/L) Hg levels in Carson R iver waters first appear downstream from accumulations of mill tailing s, 2) total Hg concentrations in unfiltered and filtered water from th e Carson River increase downstream (i.e. away from the tailings piles) , and 3) Hg concentrations in both the Carson River (downstream from t he tailings piles) and the Lahontan Reservoir are among the highest kn own worldwide (100 to 1000 ng/L). Filtered water samples from the Cars on-Lahontan system also have high Hg contents (up to 113 ng/L), and su ggest that the >0.4 mu m particle fraction constitutes over 60% of the total water-borne Hg.