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. Schuh 1, Heather E. Jamieson 1, Michael J. Palmer 2, & Alan J. Martin 3 1 Department of Geological Sciences and Geological Engineering, Queen s University, Kingston, ON 2 Department of Geography and Environmental Studies, Carleton University, Ottawa, ON 3 Lorax 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 + 5O 2 Fe 2 O 3 + As 2 O 3 + 2SO 2 Over 20,000 tonnes of As 2 O 3 dust released as stack emissions over the course of operating life 2,500 tonnes 85% GNWT, 1993 St-Onge, 2007
Arsenic in Surface Waters and Sediments 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 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 Palmer et al., 2015
In Vitro Arsenic Bioaccessibility Highest Arsenic trioxide As 2 O 3 Anthropogenic Calcium iron arsenate Yukonite Ca 7 Fe 12 (AsO 4 ) 10 (OH) 20 15H 2 O Pharmacosiderite KFe 4 (AsO 4 ) 3 (OH) 4 6-7H 2 O Amorphous iron arsenate (HFA) Fe/As = 1 to 3 Arsenic-bearing iron oxyhydroxide (HFO) Fe/As >3 Arsenic-bearing sulfides Arsenic-rich pyrite FeS 2, Realgar As 4 S 4 Authigenic Arsenopyrite FeAsS Geogenic Lowest Scorodite FeAsO 4 2H 2 O Modified from Plumlee & Morman, 2011
Study Site: Long Lake 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 CBC, 2014
Research Objectives 1. To characterize As-hosting solid phases in sediments Are sediment As concentrations elevated from the aerial deposition of roastergenerated As 2 O 3 or from the weathering of mineralized bedrock? Is As 2 O 3 stable and able to persist in lake sediments? Does its dissolution result in the formation of less bioaccessible As-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?
Sample Collection and Analysis 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 Analyses: ICP-OES and ICP-MS 210 Pb and 137 Cs dating SEM-based automated mineralogy (MLA) EMPA Synchrotron-based microanalyses (µ-xrf and µ-xrd) Lorax Environmental, 2016
Sediment Geochemistry and 210 Pb Dating 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 Deep-Water Site (LLCD) 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?
Arsenic-Hosting Solid Phases a) As 2 O 3 High solubility of this phase precludes its precipitation in watersaturated 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 (As 4 S 4 ) Forms from the partial dissolution of As 2 O 3 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 nearsurface 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*
Distributions of Arsenic-Hosting Solid Phases
Porewater Geochemistry (Shallow-Water Site) Zone of Fe-oxyhydroxide (re)precipitation Zone of diffusion Complete dissolution of As-bearing Fe-oxyhydroxide Sediment Porewater 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?
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: J z = Do F jφ dc dz Impact to water column calculated using lake residence time: [As] Jz = J z A t r V Diffusive efflux contributes ~90% of water column As concentration Likely higher as other transportation mechanisms ignored Site Sampling period Dº j (cm 2 s -1 ) Porosity Efflux (µg cm -2 month -1 ) Impact to water column (µg L -1 ) LLPC July 2015 7.91E-06 0.8-0.133 35.6 39.7 Measured water column As (µg L -1 )
Conclusions 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 As 2 O 3 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; As 2 O 3 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