SELENIUM ATTENUATION VIA REDUCTIVE PRECIPITATION IN DIVERSE SATURATED AND UNSATURATED SUBSURFACE CONDITIONS

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1 SELENIUM ATTENUATION VIA REDUCTIVE PRECIPITATION IN DIVERSE SATURATED AND UNSATURATED SUBSURFACE CONDITIONS David Silverman, Mike Hay, Gaston Leone, Bart Wilking 2 Arcadis, U.S, 2 Nu-West Inc. May 9, 208

2 Outline Idaho phosphate mining and selenium (Se) impacts Se release and attenuation Conceptual site model Case studies Conclusions and implications Arcadis May 208 2

3 Phosphate Mining in SE Idaho Western Phosphate Field Meade Peak Member of Phosphoria Formation source of phosphate ore Se content up to,040 mg/kg (World Shale average ~ mg/kg) Enriched in other metals (e.g., As, Cd, Cr, V) Strat column Mining-related surface water / groundwater impacts Local and regional Area-wide and site-specific investigations Arcadis 207 Boise Aquatic Sciences Lab 7 May 208 3

4 Se Release and Attenuation Oxidative dissolution of Se Sulfides/selenides, elemental Se, organic matter Groundwater, seeps > mg/l CO SeO HSeO - Se(0) NOMSe 2-3 Sulfide oxidation release of SO 4, divalent metals SeO 2-4 O 2 HSeO - 3 Se(0) Se 2- H 2 O 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) Ford et al., 2007, EPA/600/R-07/40 Arcadis May 208 4

5 Column Testing Data Saturated vs. unsaturated tests Leachable Selenium vs. Leachable Sulfate Unsaturated Range Correlation between leached Se and SO 4 Much lower Se:SO 4 ratio for saturated columns Less oxidative dissolution Se reduction Leachable Selenium (mg/l) Leachable Se:SO 4 ratios can be used to identify Se reductive precipitation Saturated Fit GW MCL Unsaturated columns Saturated columns Arcadis May 208 5

6 Conceptual Site Model ET RUNOFF PRECIP INFILTRATION Arcadis May 208 6

7 Overview of Sites Arcadis May 208 7

8 Se Reduction in Shallow Alluvium, Wetland Environment Site impacts Seeps: Se up to.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 SO 4 Se reduction in groundwater below wetlands Dissolved Selenium (mg/l) Dissolved Iron (mg/l) "oxic alluvium" CHMWA 04 CHMWA 05 Seeps Unsaturated Column Bounds Saturated Column Fit oxic alluvium CHMWA 05 CHMWA 04 Dissolved Manganese (mg/l) oxic alluvium CHMWA 05 CHMWA 04 Arcadis May 208 8

9 Se Reduction in Mine Pit Backfill Saturated pit backfill vs external waste dump Se, SO 4 release in external waste groundwater Dissolved Selenium (mg/l) BH 6 MWBR 9 BH Unsaturated Column Bounds Saturated Column Fit Selenium MCL (0.05 mg/l) Reducing conditions in pit backfill Low DO, elevated Mn and Fe Low Se, high SO 4 Se reduction Dissolved Iron (mg/l) Dissolved Manganese (mg/l) Waste rock configuration is key Arcadis 207 BH 6 BH MWBR 9 BH 6 BH MWBR 9 May 7, 208 9

10 Se Reduction in Shale Bedrock below Pit 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 Dissolved Selenium (mg/l) Dissolved Iron (mg/l) Grandeur Tongue groundwater Surface Water Saturated Column Fit Dissolved Manganese (mg/l) Meade Peak Groundwater Unsaturated Column Bounds Selenium MCL (0.05 mg/l) Grandeur Tongue GW SW Meade Peak GW Grandeur Tongue GW SW Meade Peak GW Arcadis May 208 0

11 Se Reduction in Unsaturated Waste Dump Impacted seeps, alluvial and bedrock GW on margins of dump Comparison between bedrock below, downgradient of dump Dissolved Selenium (mg/l) Downgradient of dump, diluted impacts Below dump, diluted attenuated impacts impacted alluvium MFSEP 03 MFMWD 2 MFMWD 4 MFMWR 20 MFMWR 22 Unsaturated Column Bounds Saturated Column Fit Selenium MCL (0.05 mg/l) Arcadis May 208

12 Conclusions and Implications 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 Arcadis May 208 2

13 Questions

14 Phosphate Mining in SE Idaho Arcadis May 208 4