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

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© 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

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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

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Questions

© Arcadis 2017

Phosphate Mining in SE Idaho

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