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 δ<sup>202</sup>Hg range (−0.09 to 0.16‰),
while roasted calcine materials exhibited a very large variability
in δ<sup>202</sup>Hg (−5.96 to 14.5‰). 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 μg g<sup>–1</sup>) and displayed
relatively narrow mass-dependent fractionation (MDF, δ<sup>202</sup>Hg; ± 0.08‰, 2SD) ranges (−0.58 to 0.24‰)
and little to no mass-independent fractionation (MIF, Δ<sup>199</sup>Hg; ± 0.04‰, 2SD) (0.00 to 0.10‰), similar
to what was observed previously for the unroasted ore waste. However,
due to the highly variable and overlapping δ<sup>202</sup>Hg
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