conceptual model of selenium release from shale units within the meade peak member of the phosphoria...

Conceptual Model of Selenium Release from Shale Units Within the Meade Peak

Member of the Phosphoria FormationKathryn Johnson, Ph.D.

• Understanding release mechanisms is necessary – to design and interpret leaching experiments– Develop source terms for predictive groundwater

transport models– Evaluate waste management alternatives

• Conceptual model developed from reviews of general studies and site specific data

Column Leaching of CWS from Smoky Canyon Panels F&G, Environmental Geochemistry Report, Maxim 2005

Column Leaching from Blackfoot Bridge SPBF, Geochemical Characterization Study, Whetstone Associates, Inc., 2008

Phosphoria FormationGeological Historical of Marine Shale

• Apatite (phosphate) depositional detritus • Diagenesis – mineral alteration• Oil generation created pore space • Thrusting and faulting created fracture zones• Volcanism produced warm fluids carrying

selenium• Alteration by oxidizing fluids following fracture

zones

Mineralogical AnalysisData from USGS study, Hein 2004

• Organic carbon, calcite, dolomite, and carbonate fluorapatite appear to have formed primarily through diagenesis

• Pyrite associated with apatite pelloids formed through diagenesis

• Epigenetic pyrite along fractures, veins and bedding planes

Mineralogical Analysis of SeleniumElectron Microscopy and Microprobe Data from USGS study, Hein 2004

• Present mostly as elemental Se• Often associated with organic carbon• Se adsorbed on fracture surfaces, pyrite, iron

oxide, kaolinite, iron-rich phosphate minerals• Selenite adsorbed on oxide surfaces• Little enrichment of Se in apatite pellets• 10- to 100-fold enrichment of Se in pyrite

Leachable Se as Percentage of Total Se

• Column Leachates - < 2%• SPLP – 0.1 % - 5.4% [Tetra Tech, 2008]

• Sequential Extraction [Hein, 2004]

• Water soluble 0.2 – 2%• Elemental 0 – 8%• Organic 0.4 – 14%• Oxide/Carbonate 9 – 49%• Sulfide 37 – 88%

Column Leachates pe/pHdiagram from Masscheleyn & Patrick, 1993

Column Leaching from Blackfoot Bridge SPBF, Geochemical Characterization Study, Whetstone Associates, Inc., 2008

Column Leaching of CWS from Smoky Canyon Panels F&G, Environmental Geochemistry Report, Maxim 2005

Column Leaching from Blackfoot Bridge SPBF, Geochemical Characterization Study, Whetstone Associates, Inc., 2008

Observations from Column Leachates

• pe/pH predict predominance of selenite/selenate• Dissolved oxygen present in all columns• Oxygen consumption during first 3 - 4 PV (all

columns)• Increased dissolved Mn in saturated v unsaturated

columns suggesting slightly lower redox potentials• Slightly depressed pH in PV1 in unaltered v altered• Greater Se and SO4 in unaltered v altered• Greater Se and SO4 unsaturated v saturated

Increased Se from Unsaturated v Saturated• Hypothesis One– Se from unsaturated column represents desorption– Lower Se from saturated column due to reduction of

Se to Se(0) or Se(-2)– Inconsistent with pe/pH conditions and SO4

• Hypothesis Two– Increased desorption of Se from unsaturated column

by oxidation from selenite to selenate– Se from unsaturated column includes oxidation of

Se(0) or Se(-2) associated with pyrite– Accounts for increased SO4, depressed pH in early PV

from unsaturated columns and is consistent with redox conditions

Increased Se and SO4 from Unaltered v Altered

• Geologic alteration of shale – oxidation of sulfides, dissolution of carbonates, precipitation of oxides

• Unaltered v Altered Shale– Less geologic oxidation and leaching– More adsorbed Se available for desorption– Se and sulfur available for oxidation– Sulfide oxidation limited by armoring from oxyhydroxides,

carbonates and phosphates– Slow oxidation rates at circum-neutral pH