Stable Hg Isotope Signatures in Creek Sediments Impacted by a Former Hg Mine

Abstract : The goal of this study was to investigate the Hg stable isotope signatures of sediments in San Carlos Creek downstream of the former Hg mine New Idria, CA, USA and to relate the results to previously studied Hg isotope signatures of unroasted ore waste and calcine materials in the mining area. New Idria unroasted ore waste was reported to have a narrow delta Hg-202 range (-0.09 to 0.16 parts per thousand), while roasted calcine materials exhibited a very large variability in delta Hg-202 (-5.96 to 14.5 parts per thousand). In this study, creek sediment samples were collected in the stream bed from two depths (0-10 and 10-20 cm) at 10 locations between the mine adit and 28 km downstream. The sediment samples were size-fractionated into sand, silt, and (if possible) clay fractions as well as hand-picked calcine pebbles. The sediment samples contained highly elevated Hg concentrations (8.2 to 647 mu g g-1) and displayed relatively narrow mass-dependent fractionation (MDF, delta Hg-202; +/- 0.08 parts per thousand, 2SD) ranges (-0.58 to 0.24 parts per thousand) and little to no mass-independent fractionation (MIF, Delta Hg-199; +/- 0.04 parts per thousand, 2SD) (0.00 to 0.10 parts per thousand), similar to what was observed previously for the unroasted ore waste. However, due to the highly variable and overlapping delta Hg-202 signatures of the calcines, they could not be ruled out as source of Hg to the creek sediments. Overall, our results suggest that analyzing creek sediments downstream of former Hg mines can provide a more reliable Hg isotope source signature for tracing studies at larger spatial scales, than analyzing the isotopically highly heterogeneous tailing piles typically found at former mining sites. Creek sediments carry an integrated isotope signature of Hg transported away from the mine with runoff into the creek, eventually affecting ecosystems downstream.
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Submitted on : Thursday, October 31, 2019 - 10:08:35 AM
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Robin S. Smith, Jan G. Wiederhold, Adam D. Jew, Gordon E. Brown, Bernard Bourdon, et al.. Stable Hg Isotope Signatures in Creek Sediments Impacted by a Former Hg Mine. ENVIRONMENTAL SCIENCE & TECHNOLOGY, 2015, 49 (2), pp.767-776. ⟨10.1021/es503442p⟩. ⟨hal-02340923⟩

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