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REACTIVE AND INERT GASES IN
GROUNDWATER CONTAMINATION
AND REMEDIATION STUDIES
K. Ulrich Mayer
Dept. Earth, Ocean and Atmospheric Sciences
University of British Columbia
Acknowledgements
 Randi Williams, M.A.Sc., UBC, 2005, now at
BGC Engineering Ltd.)
 Richard Amos (Ph.D, UBC, 2006, now at
Carleton Univ)
 Sergi Molins (Ph.D., UBC, 2007, now at LBNL)
 Katie Jones (M.Sc., UBC, 2009, now at BGC
Engineering Ltd.)
 Natasha Sihota (Ph.D. UBC, 2014, now at
Chevron Energy Technology Company )
 Andrea Chong (M.Sc., UBC, in progress)
 Barbara Bekins, USGS, Menlo Park, CA
 Jared Trost, USGS, Minneapolis, MN
Dissolved gases and soil gas and
the field of contaminant
hydrogeology
Components of talk
 Gas exsolution and ebullition in the saturated
zone
 Monitored natural attenuation of hydrocarbons
 GW remediation with permeable reactive barriers
 Gas transfer during GW remediation
 Reactive and non-reactive gases as tracers
in the vadose zone
 Monitored natural attenuation of hydrocarbons
 Surficial gas efflux measurements for source
zone delineation and quantification of
contaminant degradation rates
Reactive and non-reactive gases
Saturated zone processes
Modified from USGS Fact Sheet 084-98
Microbially mediated gas generation
Gas exsolution and ebullition
25
CH4 (mg L-1)
Saturated
Zone:
Evidence for
Gas Exsolution
B: 2002
20
15
R2 = 0.8889
10
5
R2 = 0.7386
0
-0.2
A: 2002
CH4 (mg L-1)
25
20
0.2
0.6
Ar Depletion
Downgradient Wells
Source Zone Wells
Background Wells
15
R2 = 0.9178
10
R2 = 0.8640
5
0
-0.2
0.2
0.6
N2 Depletion
1.0
From Amos et al, WRR, 2005
1.0
Henry’s Law constants and gas exsolution
Gas
Henry’s law constant
[mol L-1 atm-1] reported as log KH,cp
O2
-2.77
CH4
-2.86
CO2
-1.47
N2
-3.19
He
-3.40
Ne
-3.30
Ar
-2.72
Kr
-2.44
Noble gases as gas exsolution tracers
 Noble gas
ratios and
concentrations can be
related to
gas
generation
(reaction
progress)
 Strongest
response for
Ne/Kr ratio
From Jones et al, GCA, 2014
Noble gas signature for ebullition
Reversal of pNe/pXe
and pKr/Xe ratios
From Jones et al, GCA, 2014
 PHREEQC
modeling of gas
exsolution due
to CH4 and CO2
generation by
methanogenesis
 For high gas
production
volumes noble
gas ratios may
be useful to
prove ebullition
in the field
Gas exsolution in
permeable
reactive barriers
 Treatment of Fe
and SO4 in mine
drainage by
organic carbon
mixture
 Generation of CO2
and CH4
 N2 and Ar indicate
occurrence of gas
exsolution
From Williams et al, Appl. Geochem., 2007
Current project:
Enhanced ebullition due to
groundwater remediation?
 Focus on chlorinated solvents
 Consider various treatments including permanganate
and enhanced bioremediation
Reactive and non-reactive gases
Vadose zone processes
Modified from USGS Fact Sheet 084-98
Contaminant degradation in the
vadose zone
Vadose Zone
O2, CH4, CO2, and N2
From Jones et al, GCA, 2014
Vadose Zone – Interpretation
 Methanogenic Zone
2 O 2  CH
4
 CO
2
 Increases gas
pressure
 Induces an upward
advective gas flow
 Deplete non-reactive
gases
 2 H 2O
 Methanotrophic Zone
2 CH
O ( aq )  CO
2
2(g )
 CH
4(g )
 Decrease in gas
pressure
 Induces an inward
advective gas flow
 Enrichment nonreactive gases
Can we use depletion and enrichment of nonreactive gases to constrain the
dynamics between reactions and fluxes?
Reactive Transport Modeling of
Vadose Zone Processes
Mass Balance Equation

t
S
Ta
a
k



S
t
   S a D a Ta
k
Tg
g
k
    q
    S
k
Ta
a
D g  Tg
g
k
k
    q
 Q
k
a ,a
k
Tg
g

 Q a , s  Q a , ext  Q g , ext  0
k
k
k
k  1, N
c
Momentum Balance Equation
k
q
g
 
k rg k

 p
g
Tg
  g g z 
g
N
pg 
g

i
pg
p g  RTc
i
i
g
i 1
Molins and Mayer, Water Resour. Res., 2007
Molins et al., JCH, 2010
Bemidji Vadose Zone Simulation
 = 0.38
Kh=10-11 m2
Kv=3·10-12 m2
lens:
 = 0.30
Kh=10-13 m2
Kv=3·10-14 m2
Molins et al., JCH, 2010
Simulated O2,
CH4, and N2
Concentrations
 Good qualitative
agreement was
obtained with the field
observations
 … Model also allows
to visualize fluxes and
rates
Noble gases in the vadose zone at
the Bemidji site
 CH4
production:
preferential
depletion of
heavy noble
gases
 CH4 oxidation:
preferential
enrichment of
heavy noble
gases
Jones et al., GCA, 2014
MIN3P-Dusty simulation of gas
generation and fate
 Use of noble gases as tracers for advection/pressure gradients
 Heavy noble gases are the most sensitive tracers in the vadose
zone
Jones et al., GCA, 2014
Implications for Natural Attenuation
Research
 Simulations and field
observations suggest that
 CH4 generation is focused on
smear zone
 More than 95% of carbon will leave
via CO2 gas efflux
 Gas migration in soil is very
sensitive to soil moisture and
soil structure
 Uncertainties for C-balance
remain
 Need to measure rates (or
fluxes)
?
?
?
CH4
CO2
?
?
?
Model results indicate that most
CO2 reports to the ground surface
C flux/rate
[moles d-1 m-1]
Biodegradation
22.3
Calcite dissolution 0.033
Total source
22.33
Recharge to
0.25
saturated zone
C flux/rate (%)
Change in storage 0.13
Gas efflux to
21.96
atmosphere
Total sink
22.33
0.6
98.3
99.8
0.2
1.1
From Molins et al., 2010 and Sihota et al., 2011
How can we measure contaminant
degradation rates?
 Measure CO2 efflux above, upgradient, and
downgradient of source zone
 Real-time infrared gas analysis
 Need to distinguish between background soil
respiration and contaminant degradation
24
CO2 Efflux Measurements
Contaminant respiration and soil respiration
530
603
44
604
66
9016
9015
301
9014
601
534
9013
533
9101
532
518
531
9017
88
310
(umolCO2/m2/sec)
Measured flux
A) Well locations associated with surficial carbon dioxide flux
22
00
434
432
432
430
430
428
428
432
430
428
426
426
426
Water table
424
424
424
Groundwater flow
422
15%
5% 1%
1100
00
55 00
22 55
00
-2- 2
55
-5- 5
00
5
-7
0
0
-1
-1
2
5
5
-7
Distance from the center of the oil body (m)
7755
422
-205 0
0
422
-1
Elevation (masl)
B) Carbon dioxide in the vadose zone
4434
34
 Use background
correction
 Flux attributable
to SZNA is
2.6 μmol m-2 s-1
 Corresponds to 4
depth-integrated3
4
rate of
biodegradation
4
3
 Method effective
4
2
3
for source zone
4
0
and rate 248
delineation24and
Sihota et al.,
6
2
4
4
2
ES&T,
2
2011
How do we know that enhanced CO2 efflux
is due to contaminant degradation?

14C
in CO2 provides
direct measurement of
TPH-derived CO2
 Half-life of 14C is 5,730 years
 TPH will have 14C signature
of 0 percent modern carbon (pmc)
 Analyze 14C contents of CO2 in soil gas
https://seaborg.llnl.gov/facilities.php
 Calculate rates based on 14C contents
 Compare results to rate measured using CO2 efflux method
26
Radiocarbon and
stable carbon isotopes
 Radiocarbon allows to
distinguish between soil
respiration and contaminant
degradation
 Model highly constrained
Sihota and Mayer, VZJ, 2012
Conclusions and Outlook
 Reactive gases provide important information
on contaminant fate in the vadose and GW
zones
 Inert gases can serve as powerful indicators
for transport and reaction processes, but are
often underutilized in GW contamination
studies
 Gas efflux measurements show promise for
delineating contaminant degradation rates
Thank You – Questions?
 Amos, R. T., K. U. Mayer, B. A. Bekins, G. N. Delin, and R. L. Williams. 2005. Use of
dissolved and vapor phase gases to investigate methanogenic degradation of petroleum
hydrocarbon contamination in the subsurface, Water Resour. Res., 41, W02001,
doi:10.1029/2004WR003433
 Williams, R. L., K. U. Mayer, R. T. Amos, D.W. Blowes, C. J. Ptacek, and J. Bain, 2007.
Using dissolved gas analysis to investigate the performance of an organic carbon
permeable reactive barrier for the treatment of mine drainage, Appl. Geochem., 22:90-108
 Amos, R. T., and K. U. Mayer, 2006. Investigating the role of gas bubble formation and
entrapment in contaminated aquifers: Reactive transport modeling, J. Contam. Hydrol.,
87:123-154
 Molins, S., K. U. Mayer, R. T. Amos, and B. A. Bekins, 2010. Vadose zone attenuation of
organic compounds at a crude oil spill site - Interactions between biogeochemical
reactions and multicomponent gas transport, J. Contam. Hydrol, 112:15-29
 Sihota, N.J., O. Singurindy, and K. U. Mayer, 2011. CO2 efflux measurements for
evaluating source zone natural attenuation rates in a petroleum hydrocarbon
contaminated aquifer, Environ. Sci. Technol., 45:482-488
 Sihota, N.J., and K.U. Mayer, 2012. Characterizing vadose zone hydrocarbon
biodegradation using CO2-effluxes, isotopes, and reactive transport modeling, Vadose
Zone J., 11, doi:10.2136/vzj2011.0204.
 Jones, K.L., M.B.J. Lindsay, R. Kipfer, and K.U. Mayer, 2014. Atmospheric noble gases
as tracers of biogenic gas dynamics in a shallow unconfined aquifer, Geochimica
Cosmochimica Acta, 128:144-157
1/--pages
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