Application of geochemical modeling, including reactive transport simulation, to understand mobilization of metals in groundwater in response to acidification.

1. Heavy Metals in Groundwater:
Understanding the Big Picture
Walt W. McNab, Jr., Ph.D., P.G.
Roux Associates, Inc., Oakland, CA

2. Why do we Care About Heavy Metals?
2
 Common
groundwater
contaminants
 Cleanup liability
 Forensics
questions
 Different types of
sources
 Complex
behavior
Element MCL (mg/L)
Antimony 0.006
Arsenic 0.01
Barium 2
Beryllium 0.004
Cadmium 0.005
Chromium 0.1
Copper 1.3*
Lead 0.015*
Mercury 0.002
Selenium 0.05
Thallium 0.002

3. Outline
3
 Sources
 Environmental
behavior
 Tools to help
understand
 Applying the tools

4. Sources
4
 Industrial sources
 Plating shops, smelters, etc.
 Ambient (e.g., atmospheric
deposition)
 Natural sources
 Sulfide ore tailings (e.g., acid
mine drainage)
 Certain soil conditions

5. What Makes the Environmental Fate of Heavy
Metals in Groundwater so Complicated?
5
 Complexation
 Metals coexist with major ions (e.g.,
calcium, sulfate, carbonate) and H2O
 Metal ions can form complexes with
other ions, depending on what is
present in what concentrations
 Tendency to bind to mineral
surfaces
 Iron oxide surfaces
 Clay minerals
The environmental fate of metals in groundwater is fundamentally
linked with the geochemistry of the aquifer

6. Broader Sets of Questions Create Daunting
Uncertainty
 Source
 Release history – what, where, when, and how?
 Geochemistry
 Background concentrations?
 Distribution of mineral phases?
 Important reactions?
 Hydrology
 Groundwater flow – rate, direction, and
variability
 Combined geochemistry & hydrology
 Rates of chemical reactions versus rates of
groundwater flow

7. What to do? Models can Help
7
“Everything should be made as
simple as possible, but no
simpler.” – A. Einstein
 Modeling tools available to provide a
technical basis for informed decision
making
 Geochemical process models
 Reactive transport models (groundwater
flow + chemical transport + reactions)
 Data intensive
 Computationally intensive (possibly)
 Need to manage level of detail
 What are the major processes?
 What can be defensibly assumed?
 Must balance trying to match everything
against fine-tuning with too many knobs

8. Getting the Big Picture Through an
Example Problem
 Former industrial site
(anonymous)
 Sulfuric acid used in
processing activities
 Spilled acid appears to
have impacted
underlying groundwater
chemistry
 Elevated metals
detected in site
groundwater
0
5
10
15
20
25
30
35
40
45
DissolvedConc(mg/L)
Maximum Observed Concentrations
Site provides an unusual, unencumbered example of the interplay between
aquifer chemistry and the behavior of dissolved metals

9. Strategy for Modeling the Site
9
 Replicate current site conditions
 Construct a geochemical model for the site
 Assume near-neutral pH with low metals
concentrations as background
 Assume initial equilibrium between
groundwater and aquifer mineralogy
 Refine model to match observations (e.g.,
metals concentrations versus pH as sulfuric
acid is added)
 Test response of site geochemical model to
remedial option (e.g. addition of lime)
 Construct a hydrologic model for the site
 Hydraulic conductivity and gradient
 Distribution of key aquifer minerals
 Time span
 Combined reactive transport model – compare
modeled and predicted concentrations
 Assess remediation efficacy on field scale
(one scenario)

10. O
Replicating Site Groundwater Chemistry
10
 Effects of acidification:
1. Changes in surface charge and complexation
2. Dissolution of some of the iron oxyhydroxide
Cu
+
H
+
Cu
++
O
H
H
+
O
H
O
H
H
+
O
H
H
+
O
H
H
+
Added H+ ions displace
metals and make surface
more positively charged
Some iron oxyhydroxide
is dissolved, so additional
metals are released

11. Geochemical Modeling is an Iterative Process
11
 What is assumed?
 What is adjusted?
 What is not
adjusted?
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
0 1 2 3 4 5 6 7 8
pH
Al
Data
Model
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
0 1 2 3 4 5 6 7 8
pH
Cr
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
0 1 2 3 4 5 6 7 8
pH
Ni
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
0 1 2 3 4 5 6 7 8
pH
SO4SO4

12. Remediation Approach
12
Ca(OH)2 + 2H+  Ca2+ + 2H2O
Lime
Acid neutralization
 Concept is to neutralize pH
 Will the effect simply be to reverse the pH decline, or is it different?
 Extra calcium is added
 Elevated sulfate remains
 What could go wrong?

13. How Will Metals Concentrations
Respond to Reversing pH Decline?
13
1.E-12
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0 2 4 6 8 10 12 14 16
ModeledConcentration(mol/kgw)
pH
Fe
Co
Cu
Cr
Ni
Zn
Pb
V
CdAdded lime
Remediation approach less
effective at high pH

14. Modeled Zinc Complexation
(Example for Other Metals, too)
14
Zn+2 ZnHCO3+ ZnCl+
ZnSO4 ZnX2 Hfo_sOZn+
Hfo_wOZn+
ZnSO4 Zn+2 ZnCl+ ZnX2
Zn(OH)4-2 Zn(OH)3-
Initial Conditions (background)
Acidified
Alkaline (too much lime)
Mobile
Mobile
Immobile
Immobile
Mobile

15. Forecasting Mineralogical Impacts
from Aquifer Chemical Alteration
15
0.0
0.5
1.0
1.5
2.0
2.5
Initial Acidified Post-remediation
(neutral)
Post-remediation
(alkaline)
Mass(mol/kgw)
Ca & Mg-carbonate(s) Gypsum Fe(OH)3 Al(OH)3
Gypsum (~calcium sulfate)
• Aquifer clogging
• Armoring of lime
suspension
Mineral precipitation  porosity
reduction  permeability reduction
Water
Grain

16. 16
Groundwater Flow and Chemical Transport
Fixed-headboundary(surfacewaterbody)
Fixed-headboundary
Fine sand
Initial impacted zone
Treatmentzone
Coarse sand
• 90 x 100 grid (single layer)
• 10 m x 10 m cells
• Separate geochemical model for every cell
• 50-year simulated site history
(pre-remediation)

17. Addressing Spatial Heterogeneity in
the Transport Model
17
Hydraulic Conductivity Initial Iron Oxyhydroxide Distribution
Log (m/day)
Hydraulic conductivity field based on building distribution about measured site data.
Distributions were developed with a correlated random field generator.

18. 18
Addressing Spatial Heterogeneity in
the Transport Model: Groundwater Flow
Fixed-headboundary(surfacewaterbody)
Fixed-headboundary
Coarse sand
Fine sand
Initial impacted zone
Treatmentzone

19. pH Distribution
19
?
?
?
Field data (depth-integrated) Simulated (one realization)

20. Simulated Water Chemistry
20
SO4 Fe
Cr Cu

21. 0.01
0.1
1
0.01 0.1 1
Iron(mol/kgw)
Sulfate (mol/kgw)
Model
Data
Model vs. Data: Pre-remediation
21
Modeled iron-versus-
sulfate is key to supporting
conceptual model

22. 1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03
Vanadium(mol/kgw)
Copper (mol/kgw)
Model Data
Model vs. Data: Pre-remediation
22
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
0.001 0.01 0.1 1
Copper(mol/kgw)
Sulfate (mol/kgw)
Model Data
1.E-07
2.E-07
3.E-07
4.E-07
5.E-07
6.E-07
7.E-07
0.01 0.1 1
Lead(mol/kgw)
Sulfate (mol/kgw)
Model Data
 Other modeled relationships stem
from Fe-oxyhydroxide dissolution
hypothesis
 Model results consistent with data
(order-of-magnitude changes)
~1,000X
~1,000X
<10X>100X

23. 23
0.001
0.01
0.1
1
10
100
1000
10000
100000
0.001 0.01 0.1 1 10 100 1000 10000 100000
ModeledMaximumConcentration(mg/L)
Measured Maximum Concentration (mg/L)
SO4
Fe
Cd
Pb
V
Cr
Co
Zn
Cu
Ni
Majority of metals roughly
follow model results; Ni, Zn,
and Co are under-predicted
Model vs. Data: Pre-remediation

24. Remediation by Aquifer Chemistry
Alteration
24
Narrow zone option
• Less expensive
• Localized high concentrations
Broad zone option
• Greater capital and O&M costs
• Injectate impacts reduced
CaO + H2O CaO + H2O

25. Remediation by Aquifer Chemistry
Alteration: pH
25
Narrow injection zone: Simpler and
cheaper implementation, but produces
concentrated high pH zone
Narrow lime dispersal (t = 0) Broad lime dispersal (t = 0)
Broad injection zone: pH increase over
wider area, with a smaller region of
alkaline conditions

26. Remediation by Aquifer Chemistry
Alteration: pH
26
Narrow lime dispersal (+5 years) Broad lime dispersal (+5 years)

27. Remediation by Aquifer Chemistry
Alteration: Chromium
27
Narrow lime dispersal (+5 years) Broad lime dispersal (+5 years)
0.0002
-100 0 100 200 300 400 500 600 700 800 900
x (m)
-100
0
100
200
300
400
500
600
700
800
y()
-100 0 100 200 300 400 500 600 700 800 900
x (m)
-100
0
100
200
300
400
500
600
700
800
y()
Broad injection zone much more
effectively removes downgradient
elevated concentrations of chromium
than narrow-zone option

28. Remediation by Aquifer Chemistry Alteration:
Gypsum (Calcium Sulfate) Precipitation
28
Narrow lime dispersal (+5 years) Broad lime dispersal (+5 years)
Narrow injection zone leads to ~50%
more gypsum precipitation (localized)
 greater chance for armoring and/or
pore clogging

29. Summary
 Achieved:
 Built an internally-consistent conceptual/numerical model for metals
mobility under specific site conditions
 Predictions for remediation
 Broad application of lime more effective than narrow dispersal
 Low-conductivity heterogeneities and acidity locked in aquifer minerals
may limit efficacy of applied alkalinity, at least for a time
 Armoring via mineral precipitation could be problematic
 Need to monitor groundwater to assess spreading of pH impact
 May take years for engineered remedy to play out