The role of biofilm on the long-term performance of a shallow water cover to limit reactive tailings oxidation and

metal mobilisation.

Bernard Vigneault*, Y.T. John Kwong*

and Lesley Warren╪

*Environment Group, CANMET-MMSL╪McMaster University

For 15th Annual BC MEND Workshop, Vancouver, December 3 and 4 2008

Current study

Louvicourt Mine Experimental Cells• Constructed in 1996 to study the performance of shallow water coverto limit oxidation of reactive tailings

• 21 m x 21 m

• 3 m of mine tailings

• 0.3 m water cover

• First geochemical field studyconducted from 1996 to 1999 (MEND project 2.12.1, report published in 2002)

Louvicourt Mine Experimental Cells

• Current study, initiated 7 years after the last measurements

• Cells decommissioned in 2005

July 2005 August 2005

Biofilm

Project Objectives•To assess the long-term performance of the shallow water cover by determining porewaterchemistry and submerged tailings geochemistry nine years after disposal.

•To provide additional information on the role and impacts of the fully established periphyton layer at the tailings–water interface and on observable changes in geochemistry and mineralogy of the submerged tailings.

Methodological Approach

Sampling and porewater analysis• Electrochemical micro-profiles• Interstitial water profiles

Biofilm/Tailings Analysis• Core sampling and extrusion• Sequential extraction analysis• Mineralogical analysis (SEM-EDX)• Molecular biological analysis

Micro-Profiles – O2

Dissolved oxygen (µM)

0 100 200 300 400 500 600 700

Dep

th (m

m)

-20

-15

-10

-5

0

5

10

15

5700 Lux 18500 Lux >20000 Lux

Micro-Profiles - pH

pH

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

Dep

th (m

m)

-30

-20

-10

0

10

20

>20000 Lux5700 Lux 18500 Lux

Porewater

-6

-4

-2

0

2

4

[ ] (µg · L-1)

0.0 0.1 0.2 0.3 0.4-6

-4

-2

0

2

4

0 10 20 30 40

Dep

th (c

m)

-6

-4

-2

0

2

4

Copper

Zinc

Cadmium

Cu Zn Cd

Diff

usiv

e flu

x to

the

wat

er c

over

(m

ol ·

cm-2

· s-1

)

10-19

10-18

10-17

10-16

10-15

10-14

1998 2005

No flux

Porewater - Metal Fluxes

Core Extrusion

biofilm

4.5 – 5.0 cm of tailings

0 - 0.5 cm of tailings

Total Concentrations (2005)

MetalCu Cd Zn

Tota

l con

cent

ratio

ns

(mg

· kg-

1 )

1

10

100

1000

10000

Biofilm 0 - 0.5 cm (tailings) 4.5 - 5.0 cm (tailings)

Seq. Ext. – First 0.5 cm of TailingsCu - 1996 Cu - 2005

Cd - 1996 Cd - 2005

Zn - 1996 Zn - 2005

MgCl2CH3COOH (pH 5)NH2OH-HClNH2OH-HCl at 96 °CH2O2HF, HNO3 and HClO4

Cu

Cd

Zn

Most labile

Least labile

Mineralogical ObservationsIncreasing abundance of sulfides with depth but well crystalline pyrite persists at tailings-water interface

(C2: 0-0.5 cm)

Molecular Biological Analysis

26%

33%

26%

4%

4%7%

unknown function

methanogenic consortium;lithotrophs, anaerobic NH4oxidationPhototrophs

chemoorganoheterotrophs

grazer

fungus-decomposer

Does the biofilm affect metal toxicity of discharged water?

Assessing the Biological Impacts of Submerged Tailings

• Update of the MEND literature review conducted in 1993

• Methods reviewed included the Biotic Ligand Model for the prediction of metal toxicity in water

• The BLM approach allows prediction of acute toxicity of several metals within a factor of 2 to 3.

Biotic ligands

Ca2+

Na+Mg2+

H+

interactions at toxic binding sites

reactions with dissolved ligands

cationic competitioncomplexation

Lorg. = DOC (FA and HA)

Linorg. = Cl-, SO42-, HCO3

-…

Mez+

Ligands

The Biotic Ligand Model approachProtective effect of DOC derived from the biofilmin the Louvicourt water cover?

[ ] (mg · L-1)

10 15 20 25 30 35 40

Dep

th (c

m)

-6

-4

-2

0

2

4DOC

Water Cover: DOC Profiles

Effect Ratio Rainbow trout = Toxicity with DOCToxicity without DOC

(also for Daphnia magna or Ceriodaphnia dubia)

• Hydroqual BLM used to predict median lethal concentrations (LC50)•The final effect ratio is the geometric mean of the effect ratios (CCME 2005).

Water Cover: DOC Protective Effect

Water Cover: DOC Protective Effect

Copper Zinc Cadmium

DO

C E

ffect

Rat

io

0

5

10

15

20

25

30

Water Cover: DOC Protective Effect

Cu Zn Cd

Con

cent

ratio

ns (µ

g · L

-1)

0.001

0.01

0.1

1

10

100

1000

EstimatedWQG (CCME 2006)Site Specific WQG

Conclusions• The system had not reached equilibrium after nine years of operation.

• Mobilisation of trace metals occurred at the tailings surface but the overlying biofilm effectively trapped the released metals.

• Given the water volume in the actual tailings pond the observed Cu, Cd and Zn fluxes would not lead to exceedance of water-quality guidelines.

• Biofilm development on submerged tailings isgenerally beneficial for the application of water covers.

Future WorkBiofilms has a key component of shallow water cover tailings disposal:

• At Louvicourt, the conditions in the actual tailings impoundment might differ from the experimental cells

• Biofilms have likely developed at other sites with shallow water covers

• Can biofilm be used as tool ?

Acknowledgements• John Chaulk (CANMET-MMSL)

• Teck (formerly Aur Resources)

• INRS-ETE

• Mine Environment Neutral Drainage (MEND) Program

• Mining Association of Canada

Summary of Observations• O2 is limited to the first -0.5 mm to -1.4 mm of the biofilm/tailings with concentration affected by photosynthesis.

• pH is locally elevated near the tailings/water interface due tophotosynthesis.

• Fluxes for Zn and Cd to the overlying water are 10x lower than those obtained 7 years ago but the tailings have becomea source of Cu to the overlying water.

• Between 1996 and 2005, there is a notable reduction in H2O2-oxidizable Cu, Cd and Zn (presumably as sulfides) in the surface tailings and the amount of Zn in the more labile fractions has significantly decreased while that for Cd has slightly increased.

Summary Observations (cont.)• Sulfide abundance increases with depth but well crystalline pyrite persists at tailings-water interface.

• There is evidence of galvanic interaction among the prevailing sulfides which could explain the efflux of Cu observed in 2005.

• The biofilm and associated tailings & Fe-oxyhydroxidehave elevated metal concentrations.

• The biofilm was a highly dynamic system in terms of vertical distribution and temporal variation.

Evidence of Galvanic Interaction

pysp

ank

(C2: 4.5-5.5 cm)

- Note that Cu is also widespread among the sulfide phases- This could explain the efflux of Cu undetected previously