Selenium Release During Saturated Periods from Soils of the Sudbury Region, Ontario

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1 Selenium Release During Saturated Periods from Soils of the Sudbury Region, Ontario Lindsay Robertson, MSc Laurentian University, MIRARCO CEM Klohn Crippen Berger Ltd

2 Objectives of Study I. Determine the quantity of trace elements being released into the soil during saturated periods; II. Determine the effects of flooding on ph and reduction oxidation potential; and, III. Determine the controls on the release of selenium using the statistical relationships between Se and other metals.

3 The Big Picture

4

5 Sudbury's Heritage Logging Discovery of Nickel Mining Early Ore Roasting

6 Sudbury Smelters 1900s 1950s Current Smelter Fallout

7 Selenium Chemistry Selenium exists in four oxidation states Selenide (-II) Elemental Se (0) Selenite (IV) Selenate (VI) Selenite and selenate are the two most soluble of species Se (IV) and Se (VI) form metal complexes depending on redox potential

8 Water Decomposition Reduction oxidation sequence in soils of ph 7 (from McBride 1994)

9 Oxidation-reduction Potentials of Se Oxidation-reduction potentials of Se couples and redox couples affecting Se oxidation between ph 4 and 9. (Geering et al 1968)

10 Eh/pH Selenium Soluble Species Eh (Volts) HSeO - 4 H 2 SeO 3 HSeO - 3 H 2 Se HSe - SeO 2-4 SeO ph

11 Selenium Distribution 0 to 5 cm

12 Selenium Distribution 0 to 5 cm

13 Ore Data Sample ID Ni Cu Co Zn As Se LL (ppm) UL (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) Main Feed ICP-MS, Aqua Regia Main Tailings ICP-MS, Aqua Regia Bulk Concentrate ICP-MS, Aqua Regia Sample ID Al P S Mn Fe Co Ni Cu Zn LL (ppm) UL (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) Main Feed ICP-AES, Aqua Regia Main Tailings ICP-AES, Aqua Regia Bulk Concentrate ICP-AES, Aqua Regia From Hechler 2006

14 Sample Site Locations Control Sample Site Smelter 30 km

15 Vegetation Site 2 Site 6 Site 4

16 Bisequal Soil Podzol Soil Types Dystric Brunisol

17 Total Metal Content Site Horizon Se As Fe % Mn Ni Cu Pb Zn 2 LFH Ae B B Control LFH Ae B B

18 Redox and ph Trends Over Time ph Site 2 LFH ph Eh Time (Days) Eh (mv) Control LFH ph ph Eh Time (Days) Eh (mv)

19 Selenium and Redox Potential Se (g/m 3 ) Se Eh Eh (mv) Control LFH Site 2 LFH Time (Days) Se (g/m 3 ) Se Eh Eh (mv) Time (Days)

20 Fe (g/m 3 ) Selenium Correlation with Iron Se (g/m 3 ) y = x R 2 = Fe (g/m 3 ) Control LFH R 2 = 0.84 Site 2 LFH Se (g/m 3 )

21 Selenium Release by Site Site Se ppb (ug/kg) Se ppm XRF % in Solution 1 LFH LFH LFH LFH LFH LFH Control LFH Ae Ae Ae Ae Ae Ae 25.3 ND ND Control Ae

22 Iron Concentration in Spatial Relation to INCO Smelter Fe (g/m 3 ) Hour 1 Day 10 Days 20 Days 50 Days Distance from INCO Smelter (km)

23 Selenium Concentration in Spatial Relation to INCO Smelter Se (g/m 3 ) Hour 1 Day 10 Days 20 Days 50 Days Distance from INCO Smelter (km)

24 Summary Total metal concentrations were highest closest to the smelter sources. A reduction in redox potential was observed over time across all seven sites. The more acidic soil samples had a higher percentage of metals released into solution.

25 Conclusions I. Determine the quantity of trace elements being released into the soil during saturated periods; On average, 5% of metals are being released into the soil solution. Se release ranges from 1% to 52%, approximately 5% being released at the more contaminated sites.

26 Conclusions II. Determine the effects of flooding on ph and reduction oxidation potential; ph fluctuated over the first 10 days of incubation, general increasing trend over time. The E H decreased over time from oxic to suboxic, this shift was more evident in the LFH horizon of soils.

27 Conclusions III. Determine the controls on the release of selenium using the statistical relationships between Se and other metals; Selenium release was strongly correlated with the reduction oxidation potential. There is a relationship between the release of Se and Fe into soil solution.

28 Conclusions Metal release into the soil is regulated by both ph and redox potential. The initial concentration of a metal regulates the concentration of metals released. Metal interactions play an important role in selenium solubility and potential bioavailability. Flooded soils can potentially release metals into the pore water and become plant available.

29 Future Work The total concentration of Se does not accurately predict its mobility and potential bioavailability. The determination of Se species should be done to better understand the dynamics of Se behaviour in the soil solution. Current Se status in the Sudbury soils has not yet reached the toxicity threshold, but could in the near future.

30 Acknowledgements Dr. Joinal Abedin Post Doctoral Fellow Laurentian University ENVIRONMENTAL MONITORING Dr. Les Evans Dept. Land Resource Science University of Guelph Troy Maki MSc Candidate (Chemistry) Laurentian University