Article pubs.acs.org/est
Stable Hg Isotope Signatures in Creek Sediments Impacted by a Former Hg Mine Robin S. Smith,†,‡ Jan G. Wiederhold,*,†,‡ Adam D. Jew,§ Gordon E. Brown, Jr.,§ Bernard Bourdon,∥ and Ruben Kretzschmar† †
Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CH-8092 Switzerland Isotope Geochemistry Group, Institute of Geochemistry and Petrology, ETH Zurich, CH-8092 Switzerland § Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305, United States ∥ Laboratoire de Géologie de Lyon, ENS Lyon, UCBL and CNRS, F-6936 Lyons, France ‡
S Supporting Information *
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 δ202Hg range (−0.09 to 0.16‰), while roasted calcine materials exhibited a very large variability in δ202Hg (−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−1) and displayed relatively narrow mass-dependent fractionation (MDF, δ202Hg; ± 0.08‰, 2SD) ranges (−0.58 to 0.24‰) and little to no massindependent fractionation (MIF, Δ199Hg; ± 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 δ202Hg 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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INTRODUCTION
Mercury derived from legacy mining sites is a result of widespread contamination from waste materials disposed of onsite and near streams or rivers.19−23 One such mine in the San Francisco Bay watershed is the former New Idria Hg mine, located 225 km southeast of San Francisco in the Diablo Mountains of the California Coast Range. Site runoff discharges via the natural drainage San Carlos Creek and contributes water to the San Joaquin River, a major tributary to the northern reach of the Bay estuary.24,25 The mine was in operation from 1853 to 1972 and produced over 20 000 tons of elemental Hg, utilizing both retort furnace and rotary furnace roasting techniques.26 It has been estimated that over 1 million m3 of waste, the majority of which is the roasted ore waste (known as calcine), were deposited in tailings piles adjacent to San Carlos Creek.26 A second potential source of Hg to the creek
Mercury (Hg) is a highly pervasive global pollutant that is considered by the World Health Organization as one of the top ten chemicals of major public health concern.1,2 The highly complex global biogeochemical cycle of Hg and the variability of emission sources prompted the use of stable Hg isotopes to further elucidate the fate of Hg in the environment.3 In the past decade, advances in mass spectrometry techniques have revealed that Hg exhibits both mass-dependent fractionation (MDF; δ202Hg see definitions below) and mass-independent fractionation (MIF; Δ199Hg).4 Recent reviews detail this rapidly expanding field and the processes which fractionate stable Hg isotopes in nature.5−7 Analyzing both δ202Hg and Δ199Hg provides a two-dimensional stable Hg isotope signature, which can be employed as source8−14 and biogeochemical process tracer.15−18 For source tracing, it is imperative that all potential Hg sources to the system of interest are isotopically distinct and in situ fractionation by secondary processes that might alter the original signature are identified. © XXXX American Chemical Society
Received: July 16, 2014 Revised: December 5, 2014 Accepted: December 9, 2014
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DOI: 10.1021/es503442p Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
als;14,33,36,37 however, more soluble secondary minerals23,28,38 and elemental Hg33 were also detected. In line with these results, sequential extractions indicated a higher abundance of more soluble Hg species in calcines than in unroasted ore waste materials.31,39 Column experiments examining the leaching of Hg from calcine materials suggested colloidal transport of HgS as the primary mode of Hg transport, rather than leaching of dissolved Hg.35 Furthermore, elevated Hg concentrations have been detected in the surface water of San Carlos Creek immediately downstream of New Idria, in both the unfiltered and filtered (20‰) within individual calcine pieces collected from different waste piles.30 This suggested that a very large number of calcine samples would have to be analyzed in order to derive a reliable “source signature” representative for the mining site, which would be required for source tracing of Hg at larger spatial scales, such as the San Francisco Bay watershed. Studies at other Hg mines have also reported Hg isotope variations in bulk calcine, with a δ202Hg range of about 3‰.14,27,37 While much work has been conducted on Hg concentrations and speciation in San Carlos Creek near New Idria,26,32,40,41 the stable Hg isotope variation in the creek sediments, which may be relevant for source tracing of Hg from the mine to the downstream environment at larger spatial scales, has not been studied to date. Therefore, the aim of this study was to examine the Hg isotopic composition of sediments in San Carlos Creek between the mine and 28 km downstream. Sediment samples were collected at two depths (0−10 and 10−20 cm) in the river bed and were analyzed for Hg concentrations and Hg stable isotope signatures. Size-fractionated samples were analyzed to reveal possible concentration or isotopic differences between particle size fractions, which typically exhibit different transport behavior in river environments and may be derived from different source materials. Additionally, a column experiment was carried out with two calcine materials to obtain an isotopic signature of mobilizable colloids, which has been proposed as a transport pathway of Hg into San Carlos Creek.35,42
sediments is unroasted ore waste that was not considered high enough in Hg to be economically viable. A pile of this material is situated close to the only open mine adit from which acid mine drainage flows in a small rivulet into San Carlos Creek. The calcine piles cover a larger areal extent than the unroasted ore waste (Figure 1) and both contain Hg concentrations of environmental concern (up to 1000 μg g−1).14,22,27−31 Numerous studies have suggested that Hg-rich mining wastes may release Hg to the surrounding environment,22−26 including previous studies at New Idria.32−35 Cinnabar (α-HgS) and metacinnabar (β-HgS) were found to be the primary Hg species in unroasted Hg ore waste and calcine materi-
Figure 1. Map of the sediment sampling locations and New Idria with respect to San Francisco, CA, USA. Lower box in upper panel displays the mine area shown in the lower panel. Red shaded area displays the approximate extent of calcine piles, orange shaded area displays the approximate extent of acid mine drainage, and black shaded area displays the approximate extent of unroasted ore waste pile, after EPA (2010). Red and gray hexagons represent sampling locations of calcine and unroasted ore waste samples previously collected.31 B
DOI: 10.1021/es503442p Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
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MATERIALS AND METHODS Sample Collection and Preparation. The New Idria Hg ore is hosted in a silica carbonate type Hg deposit intruded into Franciscan Complex and Great Valley Sequence sedimentary rock formations.43 More information on the regional geology of the site can be found in the Supporting Information (SI). Sediment samples were collected in April 2011 from 10 locations (S1−S10) along San Carlos Creek downstream of the mine (Figure 1). Downstream of S9, the creek merges with Larious Creek and becomes Silver Creek. At the time of sampling, acid mine drainage (AMD) from the open adit still ran uncontrolled over the mining site and discharged into the creek. Sampling location S2 was located on this AMD runoff, before merging with the creek. All sediment samples were collected as grab samples using a trowel from two sediment depth ranges (approximately 0−10 and 10−20 cm), following EPA sediment sampling procedures (see SI). All sediment samples were first oven-dried at 30 °C and dry-sieved to
