solid-phase speciation and post-depositional mobility...

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