geochemical behaviour of adit drainage under saturated and...

Geochemical behaviour of adit drainage under saturated and unsaturated conditions at two

underground mines in Northern British Columbia

John Dockrey & Alison Shaw

22nd annual BC MEND ML/ARD workshopDecember 2-4, 2015

Overview

1. Two geologically distinct mine sites with similar mining histories2. Similarities in geochemical trends

o Sulphate concentrations increase when mine workings are unsaturated

o Metal cation (Zn and Cd) concentrations decrease when mine workings are unsaturated

3. Identify mechanisms that could result in decline in metal cation solubility between saturated and unsaturated time periods

4. Results show that metal cation behaviour is controlled by pH related solubility constraints in neutral mine drainage.

Site Naming Conventions

Site Background: GeologySite Ao Epithermal Au-Ag deposit hosted in stockwork

veining intruding a primarily volcanic host rock.o Existing excavations are primarily in hydrothermally-

altered PAG rock consisting of volcanic and volcaniclastic sedimentary rock units

Site B: aka the tunnel of loveo Ag-Pb-Zn carbonate replacement deposit hosted in a

limestone formationo Existing excavations are primarily in non-PAG

limestone with minor amounts of mudstone and ore excavated as part of bulk sampling program.

1984 1990 1995 2001 2006 2012 2017

SaturatedUnsaturated

Site Background: Mining History

Site B:Total Excavations: 2400 m

• Nearly all mine workings are below adit elevation in both sites• No backfilled waste rock or tailings are present at either site

Site A:Total Excavations: 7200 m

Site A Timeline

Site B Timeline

Conceptual Model of Geochemical Loading: Unsaturated

O2

GW

GW

GW

Wall RockMuck Rock

GW Seepage Shotcrete

Unsaturated Geochemical Load = Wall Rock Oxidation + Rinsing of Freshly Exposed Surfaces + Additives + Groundwater

Conceptual Model of Geochemical Loading: Saturated

GW

GW

Short-Term Geochemical Load = Flushing Load + Groundwater

Long-Term Geochemical Load = Groundwater

Flushing of Rock Surfaces

0

500

1,000

SO4

mg/

L

SaturatedUnsaturated

1984 1990 1995 2001 2006 2012 2017

0

200

400

SO4

mg/

L

Sulphate Results

Site A

Site B

o Sulphate concentrations reflect impact of oxygen ingress and sulphide mineral oxidation during unsaturated time periods at both minesites.

1984 1990 1995 2001 2006 2012 2017

0

200

400

SO4

mg/

L

Site A

0

500

1,000

SO4

mg/

L

SaturatedUnsaturated

Sulphate Results

1984 1990 1995 2001 2006 2012 2017

Site B

o Sulphate concentrations return to the range observed in background groundwater when mine workings are saturated.

1984 1990 1995 2001 2006 2012 2017Background Groundwater

0.00

0.10

0.20

0.30

D-Zn

mg/

L

0.0

4.0

8.0

12.0

D-Zn

mg/

L

SaturatedUnsaturated

Zinc Results

Site A

1984 1990 1995 2001 2006 2012 2017

o Zinc concentrations tend to be lower when mine workings are unsaturated.

Site B

1984 1990 1995 2001 2006 2012 2017

0.0

4.0

8.0

12.0

D-Zn

mg/

L

SaturatedUnsaturated

Zinc Results

Site A

1984 1990 1995 2001 2006 2012 2017

o Zinc concentrations decline to below background groundwater when mine workings are drained, indicating that zinc is being attenuated

0.00

0.10

0.20

0.30

D-Zn

mg/

L

Site B

1984 1990 1995 2001 2006 2012 2017Background Groundwater

Potential Attenuation Mechanisms

1. Secondary Mineral Precipitationo solution chemistryo equilibrium concentrations can be

calculatedo thermodynamic data only

available for end-memberso potential rate limitations

2. Sorption to Particle Surfaceso solution chemistryo availability of surfaceso mineralogy of surfaceso must be empirically determined

powellite precipitation on calcite

The pH Dependence of Metal Sorption on Fe-oxide (After Stumm, 1992)

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

6 8 10 12

D-Zn

mg/

L

pH

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

6 8 10 12D-

Zn m

g/L

pH

• Site A: sorption controlled• Site B: secondary mineral control

Potential Attenuation Mechanisms

Smithsonite (ZnCO3)

ZnCO3:H2O

Zn(OH)2

Site A Site B

Smithsonite (ZnCO3)

ZnCO3:H2O

Zn(OH)2

Factors influencing sorption and solubility during active mining

1. Elevated TSS: increased particle surfaces available for sorption

2. Cement in additives: dissolution of CaO

3. Cation exchange: increase in carbonate mineral equilibrium pH

4. Aeration of mine water: leading to degassing of carbon dioxide

Elevated TSS

• During active mining TSS is observed to increase at both minesites

• Blasting and ground disturbance releases and suspends silt and clay particles

Cement in Additives

CaO + H2O → 2OH- + Ca2+

Cement Dissolution:

• Dissolution of cement in grout or shotcrete can drive up pH

Cation Exchange

Ca2+ + Na2:Ex → 2Na+ + Ca:Ex

2CaCO3 + 2H2O →2HCO3- + 2OH- + Ca2+

Cation Exchange:

0

40

80

120

2009 2010 2012 2013 2014 2016

D-N

a

0

20

40

60

80

2009 2010 2012 2013 2014 2016

D-Ca

• Water quality trends provide evidence that Na:Ca exchange reactions are occurring and potentially driving up pH at site A.

• Similar trend observed in early humidity cell data from freshly crushed drill core samples

Site A

Aeration of Mine Drainage

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

6 7 8 9

log_

pCO

2(g

)

pH

Saturated Mine Drainage

Unsaturated Mine Drainage

Site A

atmospheric pCO2

(1) CO32- + H2O = HCO3

- + OH-

(2) HCO3- + H2O = H2CO3 + OH-

(3) H2CO3 = H2O + CO2(g)

Carbon Dioxide Degassing:

• A number of reactions are caused by mine water aeration

• Mine drainage at unsaturated time points is closer to atmospheric CO2 equilibrium, indicating that degassing has occurred.

7

7.5

8

8.5

-4

-3.5

-3

-2.5

-2

0 25 50

pH

Log_

pCO

2

Weeks

atmospheric pCO2

pCO2

pH

Aeration of Mine Drainage

Ageing Experiment of Saturated Portal Drainage (Site B)

Portal Drainage Stored in 20L

Jug at 2°C

• Aeration of saturated drainage and degassing of carbon dioxide leading to an increase in pH and attenuation of Cd and Zn.

0

0.01

0.02

0.03

0.04

0

1

2

3

4

0 25 50

Cd m

g/L

Zn m

g/L

Weeks

Zn

Cd

Conclusions

• Geology is not the dominant control of metal cation behaviour.

• Solubility controls dictate metal cation behaviour in neutral pH settings. Solubility decreases when mine workings are drained due to:– Elevated TSS– Dissolution of cement– Cation exchange reactions – Aeration

• Interpreting neutral mine drainage datasets and predicting future water quality must take into account geochemical reactions which are highly sensitive to pH in addition to the intrinsic metal leaching potential of mine rock.

Questions?