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SELENIUM ATTENUATION VIA REDUCTIVE PRECIPITATION IN DIVERSE SATURATED AND UNSATURATED SUBSURFACE CONDITIONSDavid Silverman1, Mike Hay1, Gaston Leone1, Bart Wilking2
1Arcadis, U.S, 2Nu-West Inc.
May 9, 2018
© Arcadis 2017
• Idaho phosphate mining and selenium (Se) impacts
• Se release and attenuation
• Conceptual site model
• Case studies
• Conclusions and implications
Outline
17 May 2018 2
© Arcadis 2017
Phosphate Mining in SE Idaho• Western Phosphate Field
• Meade Peak Member of Phosphoria Formation source of phosphate ore– Se content up to 1,040 mg/kg (World Shale average ~
1 mg/kg)– Enriched in other metals (e.g., As, Cd, Cr, V)
• Mining-related surface water / groundwater impacts– Local and regional
• Area-wide and site-specific investigations17 May 2018 3
Strat column
Boise Aquatic Sciences Lab
© Arcadis 2017
• Oxidative dissolution of Se– Sulfides/selenides, elemental Se, organic matter– Groundwater, seeps > 1 mg/L
• Sulfide oxidation release of SO4, divalent metals– Abundant buffering capacity, neutral pH
• Reduction to elemental Se, selenides– Biotic and abiotic (slow) processes – Requires actively reducing conditions – Immobilization of Se (re-oxidation slow)
Se Release and Attenuation
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SeO42-SeO42- HSeO3
-HSeO3- Se(0)Se(0) Se2-Se2-
O2O2 H2OH2O
SeO42- HSeO3
- Se(0) Se2-
O2 H2O
Ford et al., 2007, EPA/600/R-07/140
SeO42-SeO42-
CO2CO2 NOMNOM
HSeO3-HSeO3- Se(0)Se(0) Se2-Se2-SeO4
2-
CO2 NOM
HSeO3- Se(0) Se2-
© Arcadis 2017
• Saturated vs. unsaturated tests
• Correlation between leached Se and SO4
• Much lower Se:SO4 ratio for saturated columns– Less oxidative dissolution– Se reduction
Column Testing Data
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Se:SO4 ratios can be used to identify Se reductive precipitation
0.000
0.001
0.010
0.100
1.000
10.000
1 10 100 1000 10000
Leacha
ble Selenium
(mg/L)
Leachable Sulfate (mg/L)
Leachable Selenium vs. Leachable Sulfate
UnsaturatedRange
SaturatedFit
GW MCL
Unsaturatedcolumns
Saturatedcolumns
© Arcadis 2017 17 May 2018 6
Conceptual Site ModelPRECIP
ET
RUNOFF
INFILTRATION
1
2
3
4
© Arcadis 2017 17 May 2018 7
Overview of Sites
© Arcadis 2017
• Site impacts– Seeps: Se up to 1.46 mg/L – Oxic alluvial groundwater: Se up to 0.7 mg/L
• Isolated natural reducing conditions– Wetlands along Goodheart Creek– DO, Fe, Mn, seasonally variable – Low Se, high SO4
• Se reduction in groundwater below wetlands
Se Reduction in Shallow Alluvium, Wetland Environment
0.00001
0.0001
0.001
0.01
0.1
1
10
1 10 100 1000Dissolved Mangane
se (m
g/L)
Sulfate (mg/L)oxic alluvium CHMWA‐04CHMWA‐05
0.0001
0.001
0.01
0.1
1
10
1 10 100 1000
Dissolved Iro
n (m
g/L)
Sulfate (mg/L)oxic alluvium CHMWA‐04CHMWA‐05
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0.0001
0.001
0.01
0.1
1
1 10 100 1000
Dissolved Selen
ium (m
g/L)
Sulfate (mg/L)"oxic alluvium" CHMWA‐04CHMWA‐05 SeepsUnsaturated Column Bounds Saturated Column Fit
© Arcadis 2017
Se Reduction in Mine Pit Backfill• Saturated pit backfill vs external
waste dump
• Se, SO4 release in external waste groundwater
• Reducing conditions in pit backfill– Low DO, elevated Mn and Fe– Low Se, high SO4
– Se reduction
• Waste rock configuration is key
0.001
0.01
0.1
1
10
100 1000 10000
Dissolved Iro
n (m
g/L)
Sulfate (mg/L)
BH‐6 BH‐1 MWBR‐9
0.001
0.01
0.1
1
10
100
100 1000 10000
Dissolved Mangane
se (m
g/L)
Sulfate (mg/L)
BH‐6 BH‐1 MWBR‐9
0.0001
0.001
0.01
0.1
1
1 10 100 1000Dissolved Selen
ium (m
g/L)
Sulfate (mg/L)BH‐6 MWBR‐9BH‐1 Unsaturated Column BoundsSaturated Column Fit Selenium MCL (0.05 mg/L)
May 17, 2018 9
© Arcadis 2017
Se Reduction in Shale Bedrock below Pit
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• Impacted SW, infiltration– Se up to 0.74 mg/L
• Infiltration to Bedrock groundwater– Phosphatic shale member – Dolomite (Grandeur Tongue Member)
• Se reduction in shale member, where conditions suboxic
0.0001
0.001
0.01
0.1
1
1 10 100 1000Dissolved Selen
ium (m
g/L)
Sulfate (mg/L)Grandeur Tongue groundwater Meade Peak GroundwaterSurface Water Unsaturated Column BoundsSaturated Column Fit Selenium MCL (0.05 mg/L)
0.0001
0.001
0.01
0.1
1
10
1 10 100 1000
Dissolved Iro
n (m
g/L)
Sulfate (mg/L)
Grandeur Tongue GW SW Meade Peak GW
0.0001
0.001
0.01
0.1
1
10
1 10 100 1000Dissolved Mangane
se (m
g/L)
Sulfate (mg/L)
Grandeur Tongue GW SW Meade Peak GW
© Arcadis 2017 17 May 2018 11
• Impacted seeps, alluvial and bedrock GW on margins of dump
• Comparison between bedrock below, downgradient of dump
• Downgradient of dump, diluted impacts
• Below dump, diluted attenuated impacts
0.0001
0.001
0.01
0.1
1
1 10 100 1000
Dissolved Selen
ium (m
g/L)
Sulfate (mg/L)
impacted alluvium MFSEP‐03MFMWD‐12 MFMWD‐14MFMWR‐20 MFMWR‐22Unsaturated Column Bounds Saturated Column FitSelenium MCL (0.05 mg/L)
Se Reduction in Unsaturated Waste Dump
© Arcadis 2017
• Se reduction to below levels of concern observed in diverse subsurface conditions in the Phosphate Patch
• Improves conceptual understanding of Se dynamics in environment
• Opportunities to harness natural process
– Mine planning /reclamation
– Mine remediation
Conclusions and Implications
17 May 2018 12
Questions
© Arcadis 2017
Phosphate Mining in SE Idaho
17 May 2018 14
