solid-phase speciation and post-depositional mobility...
TRANSCRIPT
SOLID-PHASE SPECIATION AND POST-DEPOSITIONAL
MOBILITY OF ARSENIC IN LAKE SEDIMENTS IMPACTED
BY ORE ROASTING AT LEGACY GOLD MINES NEAR
YELLOWKNIFE, NT, CANADA
Christopher E. Schuh1, Heather E. Jamieson1, Michael J. Palmer2, & Alan J. Martin3
1Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, ON2Department of Geography and Environmental Studies, Carleton University, Ottawa, ON
3Lorax Environmental Services Ltd., Vancouver, BC
Ore Roasting in the Yellowknife Area
• Giant Mine: greenstone-hosted gold
• In operation from 1948-1999
• Refractory gold ore hosted in
arsenopyrite (FeAsS)
• Ore roasting
• 2FeAsS + 5O2 Fe2O3 + As2O3 + 2SO2
• Over 20,000 tonnes of As2O3 dust
released as stack emissions over the
course of operating life
GNWT, 1993
85%
St-Onge, 2007
2,500 tonnes
• Anticipate a combination of
geogenic and
anthropogenic inputs
• Focus of recent studies:
establishing background As
concentrations
• Anomalously high
concentrations in lake
waters and sediments to
the west and northwest of
Giant Mine; decrease with
distance
Palmer et al., 2015
Canadian As Guidelines
Drinking water = 10 µg L-1
Sediment quality = 5.9 mg kg-1
Site-specific guideline for
sediments at Yellowknife boat
launch = 150 mg kg-1
Arsenic in Surface Waters and Sediments
Modified from Plumlee & Morman, 2011Lowest
Highest Arsenic trioxide As2O3
Calcium iron arsenate
Yukonite Ca7Fe12(AsO4)10(OH)20·15H2O
Pharmacosiderite KFe4(AsO4)3(OH)4·6-7H2O
Amorphous iron arsenate (HFA) Fe/As = 1 to 3
Arsenic-bearing iron oxyhydroxide (HFO) Fe/As >3
Arsenic-bearing sulfides
Arsenic-rich pyrite FeS2, Realgar As4S4
Arsenopyrite FeAsS
Scorodite FeAsO4·2H2O
In Vitro Arsenic Bioaccessibility
Authigenic
Anthropogenic
Geogenic
CBC, 2014
• 5 km downwind of Giant Mine roaster
• Fred Henne Territorial Park (Long Lake beach,
boat launch, campground)
• Surface water As = ~40 µg L-1
• Surface area = 115 ha
• Max basin depth = ~7 m
• Bedrock-bound (mostly granite)
• “Terminal” lake hydrology
Study Site: Long Lake
1. To characterize As-hosting solid phases in sediments
• Are sediment As concentrations elevated from the aerial deposition of roaster-
generated As2O3 or from the weathering of mineralized bedrock?
• Is As2O3 stable and able to persist in lake sediments? Does its dissolution result in
the formation of less bioaccessibleAs-hosting phases?
• What is the relative contribution of each As-hosting phase to total sediment As
concentrations?
• Can vertical variations in sediment As concentrations and solid-phase speciation be
related to the timeline of ore roasting in the Yellowknife area?
• How do the concentrations and distributions of As-hosting solid phases differ in
shallow- and deep-water environments?
2. To determine whether sediments are source or sink of As to surface waters
• What is the rate and direction of diffusive transport of As across the SWI?
• How much As is diffusion across the SWI contributing to surface-water As
concentrations?
Research Objectives
Analyses:
• ICP-OES and ICP-MS
• 210Pb and 137Cs dating
• SEM-based automated mineralogy
(MLA)
• EMPA
• Synchrotron-based microanalyses
(µ-XRF and µ-XRD)
Sample Collection and Analysis
Lorax Environmental, 2016
Field Methods:
• Sediment cores collected from
shallow-water (0.7 m water depth) and
deep-water sites (5.8 m water depth)
• Installation of dialysis arrays (peepers)
at the shallow-water site
Shallow-Water Site (LLPC)
• Arsenic maximum (90 mg kg-1 As) occurs at SWI
• Low relative to Yellowknife site-specific guideline for
sediments of 150 mg kg-1 As
• Concentrations decrease to levels at or below
detection (1 mg kg-1) below ~3 cm depth
• Arsenic maxima at 3.5 cm depth (1000 mg kg-1 As)
and 17.5 cm depth (1500 mg kg-1)
• Lower peak is coincident with the period of maximum
emissions from the Giant roaster (1949-1951)
• Upper peak occurs in sediments deposited after
operations had ceased at Giant; redox boundary?
• Elevated concentrations below 1949; downward
diffusion and precipitation?
Deep-Water Site (LLCD)
Sediment Geochemistry and 210Pb Dating
a) As2O3
• High solubility of this phase
precludes its precipitation in water-
saturated conditions, suggesting it is
of roaster origin
• Solubility is likely limited by Sb
content (average 0.13 wt.%),
therefore able to persist in lake
sediments for more than 60 years
b) As-sulfide
• Poorly crystalline (no diffraction)
• Atomic ratio of As to S is 1:1,
suggesting that it is realgar (As4S4)
• Forms from the partial dissolution of
As2O3 in sediment horizons where
reduced sulfur is available
*Sb-Lβ1 and Ca-Kα have similar energies
Arsenic-Hosting Solid Phases
c) As-bearing Fe-oxyhydroxide
• Predominant host of As in near-
surface sediment horizons
• Poorly crystalline (no diffraction)
• Average As content changes with
depth (4 wt.% in near-surface
sediments; 2 wt.% deeper in
sediment column)
d) As-bearing pyrite
• Framboidal; precipitates in sediment
horizons where reduced sulfur is
available
• Average As content of 0.2 wt.% in all
samples; no change with depth
• A negative correlation of As with S
implies that As is substituting for S
*Sb-Lβ1 and Ca-Kα have similar energies *Negligible arsenopyrite*
Arsenic-Hosting Solid Phases
Distributions of Arsenic-Hosting Solid Phases
Porewater Geochemistry (Shallow-Water Site)
Zone of Fe-oxyhydroxide
(re)precipitation
Zone of diffusion
1. Congruent porewater profiles of As and Fe indicate mobility of As governed by reductive
dissolution of As-bearing Fe-oxyhydroxide during burial (~90% of total sediment As)
• Complete dissolution and release of As between -10 cm and -20 cm
2. Linear portion of As profile indicative of upward diffusion toward SWI
3. Inflection at -3 cm indicative of resorption/reprecipitation
• Sufficient to prevent diffusion into overlying water column?
Complete dissolution of
As-bearing Fe-oxyhydroxide
Sediment
Porewater
Diffusive Input of Arsenic to the Water Column
• Diffusive input of As to the water column calculated using assumed linear concentration
gradients across the SWI
• In reality non-linear due to scavenging by Fe-oxyhydroxide; overestimation of
magnitude of concentration gradient
• Rate of diffusive efflux estimated using Fick’s first law:
𝐽𝑧 = −𝐷𝑜
𝐹𝑗𝜑
𝑑𝑐
𝑑𝑧
• Impact to water column calculated using lake residence time:
[𝐴𝑠]𝐽𝑧= 𝐽𝑧 ∗𝐴 ∗ 𝑡𝑟𝑉
• Diffusive efflux contributes ~90% of water column As concentration
• Likely higher as other transportation mechanisms ignored
Site Sampling
period
Dºj
(cm2 s-1)
Porosity Efflux
(µg cm-2
month-1)
Impact to
water column
(µg L-1)
Measured
water
column As
(µg L-1)
LLPC July 2015 7.91E-06 0.8 -0.133 35.6 39.7
• Arsenic trioxide from the Giant Mine roaster has persisted in Long
Lake sediments for more than 60 years
• Maximum As concentrations in deep-water sediment core are roughly
coincident with the period of maximum emissions from the Giant roaster
(1949-1951)
• Evidence that the dissolution of As2O3 results in the formation of less
bioaccessible As-hosting solid phases
• Fe-oxyhydroxide is the predominant host of As in near-surface
sediments from shallow-water sites; As2O3 and As-sulfides are
predominant hosts in deep-core sediments from deep-water sites
• Little evidence of geogenic As (no arsenopyrite)
• Sediments are an ongoing source of As to surface waters
Conclusions