15-Apr-13

Trace element mobility and the role of subsurface heating

Presented by: Jon Fennell, MSc, PhD, PGeol VP, Geosciences & Water Security

April 20, 2013

§  Highest concentrations restricted to deeper aquifers (>50m)

§  Determined to be a natural phenomenon for the area (Komex, AENV, AHW)

§  Linked to weathering of local sediments

§  Regional monitoring conducted in 1996 identified elevated arsenic levels in east-central Alberta aquifers (up to 90 µg/L ⇒ drinking water guideline is 10 µg/L)

0

25

50

75

100

125

150

175

200

0 5 10 15 20 25 30

Arsenic (ug/L)

Depth(m)

current GCDWQ

Background

§  ERCB hearing for thermal in-situ project expansion in east-central Alberta questioned effects of aquifer heating on element mobility (in particular arsenic)

§  EUB approval condition: “[the proponent] must design and implement monitoring programs to specifically address the potential that its operations may have on liberating or introducing arsenic into the groundwater”

Background

§  Design and conduct controlled laboratory experiments and field-based program to assess element mobilization potential

§  Conduct geochemical and transport/fate modelling to assess attenuation potential for mobilized constituents

§  Resolve source(s) and potential mechanism(s) responsible for general water quality changes

Research Objectives

Hypothesis testing

Hypothesis 1 §  Heating of a freshwater aquifer

will cause certain constituents and elements to mobilize from the local sediments

Hypothesis 2 §  Constituents and elements

mobilized by localized heating will attenuate through various mechanisms (e.g., mineral precipitation, hydrodynamic dispersion, sorption, degradation)

Laboratory investigations

§  Mineralogical assessments (XRD, XRF, XES, EDS, Synchrotron Light, ICPM-MS)

§  Batch heating tests (sediment and aquifer water at escalating temperatures to from 5ºC to 200ºC)

§  Sediment sorption & re-mobilization tests

§  Soil extractions (and element speciation)

Field investigation

§  Well installations & aquifer testing

§  Passive tracer test to determine flow direction & velocity (dueterated water and δ2H measurements )

§  In-situ monitoring (pH, EC, temperature, redox)

§  Temperature profiling

§  Water quality sampling (main ions, trace elements & speciation, dissolved organics, gases, stable isotopes; microbial testing)

D55 OB-11

D55 OB-10

D55 OB-2

D55 OB-1

D55 OB-8

D55 OB-9

D55 OB-3

D55 OB-4

D55 OB-5D55 OB-6

D55 OB-7

D57 OB-2

D57 OB-3

D57 OB-4

D57 OB-1 D57 OB-5

D57 OB-6 D57 OB-7

D55 Pad

D57 Pad

General GWflow direction

D55 OB-12

D55 OB-13

D55 OB-14

Test site monitoring

network array

1 km

Groundwater flow direction

(rate = 50 to 60 m/yr)

Active pad (ca. 1990)

New pad

§  Alteration to GW flow field due to change in hydraulic conductivity

Effects of temperature on GW flow

K = kρg/µ

y = 6E-06x + 8E-05R2 = 0.99

5.0E-05

2.5E-04

4.5E-04

6.5E-04

8.5E-04

1.1E-03

1.3E-03

1.5E-03

0 25 50 75 100 125 150 175 200

Temperature

K (m

/sec

)

Elements released during heating

§  Numerous constituents released from sediments due to heating:

Lab: Ca > Na > Zn > K > Mo > As > Al > Sb

Field: Na/Ca > Si > K > Mg > B/Sr > Ba > As > Mo

§  Release of As evident between 30ºC and 50ºC o  Similar results expected for other elements o  Slower rate occurs at lower temperatures

Activation energy

y = 5101.2x - 8.3628R2 = 0.97

0

2

4

6

8

10

12

0.002 0.0022 0.0024 0.0026 0.0028 0.003 0.0032 0.0034 0.0036 0.0038

1/T (deg K)

-ln (r

ate)

Ea = 12 kcal/mol §  Activation energy for As release consistent with desorption, dissolution, & biologically-mediated reactions

§  Range of values from field consistent with lab

Ea = 6.3 to 11.0 kcal/mol

§  Similar results for other constituents

Ea = 1.2 to 10.3 kcal/mol

Reactions

Physical adsorption 2 to 6Aqueous diffusion <5Mineral dissolution or ppt 8 to 36Mineral dissolution via surface rxn control 10 to 20Ion exchange >20Isotopic exchange in solution 18 to 48Solid-state diffusion in minerals at low T 20 to 120Cellular and life-related reactions 5 to 20

Ea values (kcal/mol)Typical range of

98

4.3.8 HEATING AND SEDIMENT REPLACEMENT TESTS

Results of the initial phase of heating and sediment replacement experiments are provided

in Figure 27. As indicated in Section 4.2.4, the main goal of these experiments was to

determine the amount of As that could be released at elevated temperature conditions,

whether a limit exists for the amount released from the test sediment, and the ability of

fresh test sediment to attenuate As from aqueous solution at lowered temperature

conditions. Concentrations of As in the aqueous phase have been shown at each step of

the process, displayed as µg/L at the top of each reaction vessel schematic.

Figure 27: Results of heating and sediment replacement experiments. (Testing entailed reacting F01-05 sediment with test water from D57OB2 - further explanation of sequencing is provided in Figure 10; values displayed at the top of the reaction vessel schematics represent As concentrations in µg/L; SR indicates the sediment replacement sequence)

21

51

Homogenized sediment Aquifer

water

344 144 41

34 15

23

27

19 10 7

Heat 1

Heat 2

Heat 3

SR 1 SR 2 SR 3

9

Laboratory heating experiments

§  Attenuation with successive sediment replacements

§  Finite release potential

§  Slight reduction in attenuation with repeated sediment heating (speciation control)

-85%

-94%

-71%

-10% -52% -66%

-47% -33%

-58% -88% -93%

Laboratory heating experiments

0%

20%

40%

60%

80%

100%

0 1 2 3[As]

aq re

mov

ed fr

om te

st w

ater

Run 1Run 2Run 3

SR at 5ºC

0%

20%

40%

60%

80%

100%

0 1 2 3

[As]

aq re

mov

ed fr

om te

st w

ater

Run 1Run 2Run 3

SR at 50ºC

§  Less effective attenuation at elevated temperatures (reduction in sorption sites)

100

from the original concentration recorded. Sediment replacements reduced values to 19,

10, and 7 µg/L, respectively, and essentially back to the ambient value of the test water

(9 µg/L). Similar results were obtained for duplicate experiments run on D57OB3

sediments (Appendix II), as well as tests conducted using a lower sediment-water ratio

(i.e., Tests 1 versus 3 – including duplicate analyses). Similar findings were made by

Kakandes and Grandstaff (1988) for the various major and trace element constituents

released or attenuated.

Figure 29: Results of heating and sediment loading experiments. (Testing entailed reacting F01-05 with test water from D57OB2 - for further explanation of sequencing is provided in Figure 11; values displayed at the top of the reaction vessel schematics represent As concentrations in µg/L; SR indicates the sediment replacement sequence; the first value shown above reaction vessel schematics designated as SR1, 2 and 3 represents results from sediment replacement experiments conducted at 5ºC; the second value represents results from sediments replacement experiments conducted at 50ºC)

191

37/41 17/22

86/102

176/217

147/133

Homogenized sediment Aquifer

water

230

261

60/127

42/116 26/83

41/118

Run 1

Run 2

Run 3

SR 1 SR 2 SR 3

7

§  Potential sources of trace elements identified Bulk sample: 1 - 1.5 mg/kg As Clay fraction: 8 mg/kg As

§  Most likely associated with Fe-rich smectite

framboidal

euhedral

Pyrite (<<1%)

am-Goethite coatings (<<1%)

Potential sources (mineral identification)

Clay-sized fraction (approx. 10%)

Clay sample

Standards

Potential sources (sequential soil extraction)

§  SSE conducted to identify the likely phases responsible for Arsenic release

§  Applicable to other elements of concern (Mo, Sb, etc.)

64

Table 8: Sequential Soil Extraction steps applied to test sediments.

Step Extractant Extraction Time Type of Arsenic Extracted

1 0.05M (NH4)2SO4 4 hours F1 - non-specifically bound

2 0.05M (NH4)H2PO4 16 hours F2 - specifically bound

3 0.2M NH4-oxalate buffer, (pH 3.25)

4 hours (in dark) F3 - amorphous hydrous oxide (of Fe and/or Al), bound

4a 0.2M NH4-oxalate buffer + 0.1M ascorbic acid

30 minutes (at 96oC) F4 - crystalline hydrous oxide (of Fe and/or Al), bound

4b 0.68M Na-citrate + Na-dithionite 12 hours F4 - crystalline hydrous oxide (of Fe and/or Al), bound

5 Concentrated HF + HNO3 + HCl 12 hours (at 100oC) F5 – residual

The sequential extraction procedures for F1 and F2 represent As that is adsorbed to

sediment surfaces by loose electrostatic bonds (outer-sphere complexes) and by more

specific covalent bonds (inner-sphere complexes), respectively. Binding might be to the

surface of metal oxides, such as Fe- or Al-hydrous oxides, clays, or carbonate minerals.

F3 and F4 extractions describe As that is occluded in amorphous and crystalline Fe- and

Al-hydrous oxides, respectively. Two differing methods of extraction were used for the

F4 step to test extraction efficiency (c.f., Table 8). Finally, the F5 fraction represents all

remaining source minerals not affected by the previous extractions. This represents the

most recalcitrant phase in the sequential soil extraction technique, and requires the use of

hot, concentrated hydrofluoric, nitric and hydrochloric acids. There are a number of

potential As-bearing phases associated with this fraction, including:

• pyrite and other sulfide minerals;

• metal arsenates (i.e., X3(AsO4)2 where X = Zn2+, Ba2+, Pb2+ or Ca2+);

• As associated with silica, silicate phases, or incorporated into the framework

lattice of clay minerals; and,

• As associated with phosphate minerals (e.g., apatite).

87

0

10

20

30

40

50

60

0 1 2 3 4 5 6

Fraction

% T

ota

l A

rsen

ic i

n F

racti

on Pretest average

Heated average

SR average

0

20

40

60

80

0 1 2 3 4 5 6

Fraction

% R

an

ge

of

Va

lue

s

Pretest range

Heated range

SR range

Figure 19: Relative proportions of arsenic in sequential extraction fractions and range of values. (Results of testing indicates the F3 fraction (amorphous phases of Fe- and Al-(hydr)oxides) as the dominant controlling fraction; errors bars represent the percent range of total arsenic recovered)

The greatest percentage of the As in the averaged sediment aliquots resides in the F5

fraction (38%), or residual fraction, followed by the F3 (31%), F4 (21%), F2 (10%) and

finally F1 (<1%) fractions. For context, the average concentration of As in the pre-test

Results

§  SSE indicates amorphous phases of Fe and Al as dominant players (clay minerals suspected; consistent with ligand-promoted release)

Surface-controlled reactions

§  Release of 10B due to heating supportive of clay minerals as the source

§  Association of As and 10B supports release of As from clay minerals as well

y = -169.49x + 15.013R2 = 0.32

-10

-5

0

5

10

15

20

0.00 0.02 0.04 0.06 0.08 0.10

Arsenic (mg/L)

del 11

B (p

er m

il)

y = -0.6011x + 18.311R2 = 0.99

y = -1.1398x + 14.791R2 = 0.97

-10

-5

0

5

10

15

20

0 5 10 15 20 25 30

Temperature (deg C)

del 11

B (p

er m

il)

D57OB1

D57OB5

y = 0.0594x - 14.827R2 = 0.4933

-16

-15

-14

-13

-12

-11

0 10 20 30 40

Temperature (deg C)

del

13C

(HC

O3)

per

mil

Other influencing factors

§  Dissolved gases, microbial tests and stable isotopes support reductive processes enhancing Arsenic release

0

2

4

6

8

10

12

0 10 20 30 40Temperature (deg C)

Resp

onse

tim

e (d

ays)

IRBSRB

0

4

8

12

16

20

Jan-02 Jan-03 Jan-04 Jan-05

del

34S(

SO4)

per m

il

0

10

20

30

40

50

Tem

pera

ture

(deg

C)SO4

Temp.

0

2

4

6

8

0 10 20 30 40Temperature (deg C)

CH

4 (m

g/L)

0

20

40

60

80

CO

2 (m

g/L)

Methane

Carbon dioxide

Poly. (Methane)

Poly. (Carbondioxide)

Enhanced microbial activity

CO2-reduction

Gas production

SO4-reduction

Results §  Re-mobilization tests

at room temperature indicate SRBs as effective mobilizing agents

§  Reductive processes implicated

§  Release at lower temperature supports a change in bonding configuration (lower energy after sorption)

0.00

0.02

0.04

0.06

0.08

0.10

0 4 8 12 16 20 24

Weeks

As

(mg/

L)

F 01-­‐05  lo aded  s ediment

F 01-­‐05  natural  s ediment

F 01-­‐05  natural  s ediment  (dup.)

As-loaded sediment

Natural sediment

Natural sediment (dup.)

104

0.00

0.01

0.02

0.03

0.04

0 4 8 12 16 20 24

Weeks

As

(m

g/L

)F01-05 loaded sediment

F01-05 natural sediment

F01-05 natural sediment (dup.)

D57OB3 natural sediment

Figure 31: Arsenic re-mobilization Test 1. (During this experiment pH conditions were not controlled and redox values were maintained between -25 to -150 mV; loaded sediment = soil exposed to As-laden test water during the sediment replacement phases of the heating experiments; natural sediment = fresh test sediment; dup. = duplicate run).

Re-mobilization of the As from the “loaded sediment” at the comparatively low

temperature of the experiment (20ºC), as opposed to the natural sediments, is an

indication of a weaker bond configuration compared to the natural sediments and suggests

formation of surface-complexes with the associated mineral surfaces. Because arsenate

and arsenite tend to form inner-sphere binuclear or mononuclear complexes with reactive

mineral surfaces under natural conditions (Foster, 2003), the lack of release from the

natural sediments at lower temperature is consistent with a stronger bond configuration

and possible occlusion of the As into the surface layers of the mineral or minerals

involved.

As-loaded sediment

Natural sediment Natural sediment (dup.) Natural sediment

§  Log SI values indicate potential for dissolution of: o  Feldspars o  Some carbonates o  Selected micas o  Hornblendes o  Most clays o  Fe(hydr)oxides

§  Saturated conditions for: o  Calcite & dolomite o  Quartz o  Some micas o  Major sulphides

Well7°C 29°C 7°C 20°C

Mineral Type/Class 1-Dec 4-Dec 1-Dec 5-DecAlbite Na-feldspar -2.3 -2.1 -2.5 -1.9Anorthite Ca-feldspar -10.0 -8.0 -10.1 -7.8Orthoclase K-feldspar -0.1 -0.3 -0.3 0.3Calcite Carbonate 0.0 0.0 0.0 0.0Siderite Carbonate 0.2 -0.6 -0.1 0.2Dolomite Carbonate 0.1 0.4 0.1 0.3Magnesite Carbonate -0.7 -0.2 -0.7 -0.5Brucite Mg-hydroxide -6.4 -5.2 -6.4 -5.9Quartz Silicate 0.5 0.3 0.5 0.3Cristobalite Silicate 0.0 -0.2 0.0 0.0Sepiolite Mg-silicate (hydrated) -5.6 -4.2 -5.7 -4.8Pargasite Hornblende -25.4 -23.5 -25.2 -25Muscovite Mica (K) 11.9 10.6 11.9 11.4Paragonite Mica (Na) 11.4 10.1 11.3 10.8Phlogopite Mica (K/Mg/Fe) -3.3 -2.7 -3.1 -3.3Illite Clay -3.3 -2.7 -3.5 -2.4Kaolinite Clay 0.0 0.0 0.0 0.0Gibbsite Clay -1.0 -0.5 -1.0 -0.2Chlorite (7A) Clay -10.7 -9.6 -10.6 -10.7Chlorite (10A) Clay -7.2 -6.2 -7.0 -7.3Na-smectite Clay -2.5 -2.2 -2.5 -2.2Goethite Fe(hydr)oxide 1.4 -1.4 2.0 -1.1Fe(OH)3 (am) Fe(hydr)oxide -3.8 -4.5 -3.2 -2.8Troilite (FeS) Sulphide 0.8 0.5 0.5 0.0Pyrite (FeS2) Sulphide 8.8 5.6 8.1 7.1

D57OB1 D57OB6Mineral saturation

indices

Data analysis (Hierarchical Cluster Analysis)

§  Data groupings from HCA support:

o  Occurrence of reductive processes

o  Release of certain constituents from clays

o  Expulsion of saline porewater from rafted marine shale

Cluster Tree

0 1 2 3 4Distances

TEMP

PH

EH

K

NA

HCO3

CLSO4

DOC

HARD_D

AS

BA

B

FEMN

MO

P

SI

SR

§  Results from PCA support:

o  Dissolution of aluminosilicates

o  Reductive processes o  Release of saline

pore water o  Surface reactions

Data analysis (Principal Component Analysis) Variable(

1 2 3 4Temperature 0.95 -0.08 -0.01 -0.08Na 0.91 0.01 -0.02 0.03K 0.91 -0.10 0.08 0.32Ba 0.88 0.23 0.03 -0.08Cl 0.87 0.24 -0.06 -0.10SO4 0.86 0.21 -0.09 0.02Mn -0.83 0.02 -0.18 0.25Si 0.81 -0.21 0.00 0.47Sr 0.76 0.55 -0.06 -0.03Fe -0.67 0.38 0.53 0.13As 0.62 -0.36 0.28 0.37Hardness 0.08 0.83 -0.28 -0.23pH 0.02 0.67 0.22 0.32Mo 0.09 -0.30 0.82 0.12P -0.03 0.25 0.82 -0.27DOC -0.07 0.08 -0.26 0.72B 0.08 0.00 0.16 0.71HCO3 0.07 0.47 0.05 -0.52Eh -0.38 0.11 -0.25 -0.07%JofJvarianceJexplained 40.8 12.2 10.7 11.0

FactorVariable(1 2 3 4

Temperature 0.95 -0.08 -0.01 -0.08Na 0.91 0.01 -0.02 0.03K 0.91 -0.10 0.08 0.32Ba 0.88 0.23 0.03 -0.08Cl 0.87 0.24 -0.06 -0.10SO4 0.86 0.21 -0.09 0.02Mn -0.83 0.02 -0.18 0.25Si 0.81 -0.21 0.00 0.47Sr 0.76 0.55 -0.06 -0.03Fe -0.67 0.38 0.53 0.13As 0.62 -0.36 0.28 0.37Hardness 0.08 0.83 -0.28 -0.23pH 0.02 0.67 0.22 0.32Mo 0.09 -0.30 0.82 0.12P -0.03 0.25 0.82 -0.27DOC -0.07 0.08 -0.26 0.72B 0.08 0.00 0.16 0.71HCO3 0.07 0.47 0.05 -0.52Eh -0.38 0.11 -0.25 -0.07%JofJvarianceJexplained 40.8 12.2 10.7 11.0

FactorVariable(1 2 3 4

Temperature 0.95 -0.08 -0.01 -0.08Na 0.91 0.01 -0.02 0.03K 0.91 -0.10 0.08 0.32Ba 0.88 0.23 0.03 -0.08Cl 0.87 0.24 -0.06 -0.10SO4 0.86 0.21 -0.09 0.02Mn -0.83 0.02 -0.18 0.25Si 0.81 -0.21 0.00 0.47Sr 0.76 0.55 -0.06 -0.03Fe -0.67 0.38 0.53 0.13As 0.62 -0.36 0.28 0.37Hardness 0.08 0.83 -0.28 -0.23pH 0.02 0.67 0.22 0.32Mo 0.09 -0.30 0.82 0.12P -0.03 0.25 0.82 -0.27DOC -0.07 0.08 -0.26 0.72B 0.08 0.00 0.16 0.71HCO3 0.07 0.47 0.05 -0.52Eh -0.38 0.11 -0.25 -0.07%JofJvarianceJexplained 40.8 12.2 10.7 11.0

FactorVariable(1 2 3 4

Temperature 0.95 -0.08 -0.01 -0.08Na 0.91 0.01 -0.02 0.03K 0.91 -0.10 0.08 0.32Ba 0.88 0.23 0.03 -0.08Cl 0.87 0.24 -0.06 -0.10SO4 0.86 0.21 -0.09 0.02Mn -0.83 0.02 -0.18 0.25Si 0.81 -0.21 0.00 0.47Sr 0.76 0.55 -0.06 -0.03Fe -0.67 0.38 0.53 0.13As 0.62 -0.36 0.28 0.37Hardness 0.08 0.83 -0.28 -0.23pH 0.02 0.67 0.22 0.32Mo 0.09 -0.30 0.82 0.12P -0.03 0.25 0.82 -0.27DOC -0.07 0.08 -0.26 0.72B 0.08 0.00 0.16 0.71HCO3 0.07 0.47 0.05 -0.52Eh -0.38 0.11 -0.25 -0.07%JofJvarianceJexplained 40.8 12.2 10.7 11.0

FactorVariable(1 2 3 4

Temperature 0.95 -0.08 -0.01 -0.08Na 0.91 0.01 -0.02 0.03K 0.91 -0.10 0.08 0.32Ba 0.88 0.23 0.03 -0.08Cl 0.87 0.24 -0.06 -0.10SO4 0.86 0.21 -0.09 0.02Mn -0.83 0.02 -0.18 0.25Si 0.81 -0.21 0.00 0.47Sr 0.76 0.55 -0.06 -0.03Fe -0.67 0.38 0.53 0.13As 0.62 -0.36 0.28 0.37Hardness 0.08 0.83 -0.28 -0.23pH 0.02 0.67 0.22 0.32Mo 0.09 -0.30 0.82 0.12P -0.03 0.25 0.82 -0.27DOC -0.07 0.08 -0.26 0.72B 0.08 0.00 0.16 0.71HCO3 0.07 0.47 0.05 -0.52Eh -0.38 0.11 -0.25 -0.07%JofJvarianceJexplained 40.8 12.2 10.7 11.0

Factor

Geochemical modelling

§  PHREEQC modelling supports release of Arsenic from clays as mechanism

0.00

0.04

0.08

0.12

0.16

0.20

0 25 50 75 100 125 150 175 200 225 250

Temperature (deg C)

Con

cent

ratio

n (m

g/L)

H3AsO3

H2AsO4-

HAsO42-

As (total)

Series1

Poly. (As(total))

4 → 1

2 → 4

4 → 15

9 → 43

7 → 2

5 → 3

5 → 4

§  As(III) released as opposed to As(V) (weaker bond configuration)

§  Possible reduction of As(V) to As(III)

§  Sequestering of As(V) in precipitated minerals (i.e., FeS)

As(III) to As(V) ratios

Retardation factors

§  Variable rates of movement observed in the field o  evidence of attenuation and

chromatographic dispersion

§  Lab Kd values for As (1.3 to 5.3) much greater than determined from field data (0.11) - not surprising

Cl B Si AsD57OB1 1.0 1.1 1.5 1.6D57OB5 1.0 1.1 1.4 1.6D57OB6 1.0 1.1 1.4 1.6D57OB7 1.0 1.1 1.5 1.6

Retardation factor (Rf)Well differencem/y %

D57OB1Cl 56.6 1.0B 51.1 0.9Si 39.0 0.7K 33.2 0.6As 35.3 0.6D57OB5Cl 56.7 1.0B 53.3 0.9Si 40.7 0.7K 36.9 0.6As 36.5 0.6D57OB6Cl 59.5 1.0B 56.0 0.9Si 42.2 0.7K 33.6 0.6As 36.4 0.6

rate

Transport & fate modelling

§  Transport & fate modelling indicates active transport of mobilized constituents

§  Retardation factor of 1.6 required to match simulated breakthrough curves for As o  DL = 10 o  DT = 1 o  Flow = 60 m/y

D55 OB-11

D55 OB-10

D55 OB-2

D55 OB-1

D55 OB-8

D55 OB-9

D55 OB-3

D55 OB-4

D55 OB-5D55 OB-6

D55 OB-7

D57 OB-2

D57 OB-3

D57 OB-4

D57 OB-1 D57 OB-5

D57 OB-6 D57 OB-7

D55 Pad

D57 Pad

General GWflow direction

D55 OB-12

D55 OB-13

D55 OB-14

0

10

20

30

40

50

60

70

80

Jan-95 Jan-00 Jan-05 Jan-10 Jan-15 Jan-20 [As]

in e

xces

s of

bac

kgro

und

(ug/

L)

Simulated

Measured

0

10

20

30

40

50

60

70

80

Jan-95 Jan-00 Jan-05 Jan-10 Jan-15 Jan-20

[As]

in e

xces

s of

bac

kgro

und

(ug/

L)

Simulated

Measured

Conclusions

1.  Increased temperature near active thermal in situ wells causes the release of various elements to the groundwater

o  this will be sediment-dependent

2.  Groundwater flow field is influenced by changes in hydraulic conductivity

o  important to know when positioning monitoring wells

3.  Mineral dissolution and surface release reactions are the cause

o  clay minerals, in particular Fe-rich smectite, the dominant source of released elements

4.  Increased microbial activity implicated in release of Arsenic and other constituents

o  reductive dissolution o  ligand-promoted dissolution

5.  Variable transport rates occur for different elements

o  different retardation factors

6.  Attenuation of elements evident via dispersion, sorption and mineral precipitation

So what does this all mean?

1. Results of this study indicate a concern regarding element mobilization due to subsurface heating

Some elements more mobile than others

2. Large areas of Alberta will experience subsurface heating due to in situ development of oil sands deposits

However, a concern does not necessarily equal an issue

3. Risk is relative, and requires the presence of a source, pathway and receptor

Source need to be mobile (not subject to attenuation)

Pathways need to be open and active (flow velocity important)

Receptors need to be sensitive (not all are)

Source

Pathway Receptor

Risk

Thank You jon.fennell@integratedsustainability.ca (587) 891-5831 www.integratedsustainability.ca