Conceptual Model of Selenium Release
from Shale Units Within the Meade Peak
Member of the Phosphoria Formation
Kathryn 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
2.5
Se, mg/l
2.0
1.5
1.0
0.5
0.0
0
2
4
6
8
Pore Volume Smoky Canyon F&G Columns
Unsat alt Se
Unsat unalt Se
Sat alt Se
Sat unalt Se
10
12
Column Leaching from Blackfoot Bridge SPBF, Geochemical
Characterization Study, Whetstone Associates, Inc., 2008
3.0
2.5
Se, mg/l
2.0
1.5
1.0
0.5
0.0
0
2
4
6
8
Column Leach Cycle Blackfoot Bridge SPBF
Unsat Se dissolved
Sat Se dissolved
10
12
Phosphoria Formation
Geological 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 Analysis
Data 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 Selenium
Electron 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/pH
diagram from Masscheleyn & Patrick, 1993
Column Leaching from Blackfoot Bridge SPBF, Geochemical
Characterization Study, Whetstone Associates, Inc., 2008
9.0
8.0
7.0
O2, mg/l
6.0
5.0
4.0
3.0
2.0
1.0
0.0
0
2
4
6
8
Column Leach Cycle Blackfoot Bridge SPBF
Unsat O2 dissolved
Sat O2 dissolved
10
12
Column Leaching of CWS from Smoky Canyon Panels F&G,
Environmental Geochemistry Report, Maxim 2005
1,200
SO4, mg/l
1,000
800
600
400
200
0
0
2
4
6
8
10
Pore Volume Smoky Canyon F&G Columns
Unsat alt SO4
Unsat unalt SO4
Sat alt SO4
Sat unalt SO4
12
Column Leaching from Blackfoot Bridge SPBF, Geochemical
Characterization Study, Whetstone Associates, Inc., 2008
2.5
Mn, mg/l
2.0
1.5
1.0
0.5
0.0
0
2
4
6
8
Column Leach Cycle Blackfoot Bridge SPBF
Unsat Mn dissolved
Sat Mn dissolved
10
12
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