Evidence for Iron-Mediated Anaerobic Methane Oxidation in a Crude
Oil Contaminated Aquifer.
* Richard T. Amos
[email protected]
Dep. of Earth and Environmental Sciences, Univ. Waterloo, Waterloo, ON, N2L 3G1
Barbara A. Bekins
[email protected]
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
Isabelle M. Cozzarelli
[email protected]
U.S. Geological Survey, 431 National Center, Reston, VA 20192
Mary A. Voytek
[email protected]
U.S. Geological Survey, 430 National Center, Reston, VA 20192
Julie D. Kirshtein
[email protected]
U.S. Geological Survey, 430 National Center, Reston, VA 20192
Elizabeth J. P. Jones
[email protected]
U.S. Geological Survey, 430 National Center, Reston, VA 20192
David W. Blowes
[email protected]
Dep. of Earth and Environmental Sciences, Univ. Waterloo, Waterloo, ON, N2L 3G1
Supplemental Information: Detailed Calculations
S1. Electron donors
S1.1 Electron donor transport calculations
The groundwater linear velocity at the Bemidji site was estimated at 22 m yr-1 and
the porosity was estimated at 0.38 (Essaid et al. 1995) giving a Darcy velocity of q = 2.7
x 10-7 m s-1.
For CH4, direct push profiles collected in 2007 were used to obtain estimates of
flux. Each vertical profile was divided into segments with one sample point within each
segment and the segment length for sample point zj defined by:
z j  z j 1  z j  / 2  z j  z j 1 / 2
(A1)
where zj is elevation of the current sample point and the subscripts j+1 and j-1 refer to the
sample points above and below this point, respectively. For the uppermost sample point
the upper bound of the segment was defined by the measured water table elevation. For
the lowest sample point the lower bound of the segment was assumed to be 0.25 m below
the sample point, generally resulting in a segment length of 0.5 m, consistent with
segment lengths at higher elevations. The mass transport of CH4 (mol s-1) across the 1-mwide plane defined by a vertical profile was determined by summing the mass transport
across each segment along the profile:
NP
FP,i  1000 qz j Ci , j
(A2)
j 1
where Ci,j is the concentration (mol L-1) of compound i (in this case CH4) measured at zj,
and NP is the number of segments in the profile.
1
BTEX and NVDOC concentrations used for flux calculations were measured in
samples collected from monitoring wells. Wells completed at various elevations were
grouped based on distance from the source zone, with each cluster containing one to five
wells. The mass transport (mol s-1) of individual BTEX components and NVDOC across
a well cluster is computed with a weighted average using the screen lengths:
Nw
Nw
j 1
j 1
FW ,i  1 0 0 
0Z q Lj Ci , j /  Lj
(A3)
where Nw is the number of wells within the cluster, Z is the estimated plume depth at the
cluster, Lj is the well screen length at well j, and Ci,j is the measured concentration (mol
L-1) of compound i at well j. The plume depths were estimated based on the depth of the
CH4 plume beneath the water table using 2007 direct push data, which were 2.75 m at 45
m, 3.85 m at 81 m, 3.81 m at 105 m and 2.6 m at 125 m.
Mass transport values were converted to electron equivalents (mol e- yr-1) assuming
all compounds degrade fully to CO2. The coefficients used were 8 for CH4, 30 for
benzene, 36 for toluene, and 42 for ethylbenzene, o-xylene, and m,p-xylene. The
coefficient used for NVDOC was 4.63 based on a model carbon compound C19H24O6
(Cozzarelli et al., 2010) that was determined through elemental analysis of NVDOC in
samples from Bemidji wells (Thorn and Aiken, 1998).
S1.2 Variability in electron donor transport
The estimates of electron donor transport are based on measurements of CH4,
BTEX and NVDOC at specific times. However, the variability in the in concentrations
over time will add some uncertainty to these estimates. Methane concentrations have
been relatively consistent over the time period considered (Amos et al., 2010) and the
2
contribution to electron donor transport from BTEX compounds is relatively small (Table
1), so that the greatest source of uncertainty is from the variability in NVDOC
concentrations (Figure 4).
NVDOC concentrations from samples collected in 2010 were used for the
electron donor transport calculation. These values are higher than previous measurements
of NVDOC at the site, suggesting that the 2010 samples can be considered maximum
values based on the available data. Estimates of electron donor transport using 1992-1995
data indicate a change in NVDOC transport from 46 to 125 m of 158 mol e- yr-1 and a
total change in electron donors over the same interval of 277 mol e- yr-1, representing a
50% decrease in the total reducing capacity compared to the estimates using 2010 data.
Figure S1-1. Non-volatile dissolved organic carbon (NVDOC) concentrations (mg C L -1) from
individual sampling points along the contaminant plume during various time periods. Note: Only one
sampling point is available in 1999.
NVDOC concentrations are subject to season variations. Measurements collected in several wells
over a one year period from May 1987 to August 1988 (Baedecker and Cozzarelli, unpublished data)
indicate that concentrations fluctuate considerably throughout the year, although maximum
3
concentrations are observed in the summer months. The data presented in Figure S1, including the
2010 was collected in the summer, generally in late July or early August, again suggesting that the
estimates of NVDOC transport from the 2010 data represent maximum values.
S2. Transport of dissolved electron acceptors within the groundwater
Mass transport estimates of dissolved electron acceptors within the groundwater
were calculated in the same manner as demonstrated above for CH4 using equations A1
and A2. Calculated transport estimates were multiplied by a stoichiometric coefficient to
convert each to electron equivalents. Coefficients used were 4, 5 and 8 for O2, NO3- and
SO42-, respectively.
S3. Transport of dissolved electron acceptors across the water table.
S3.1 Diffusion
Diffusive flux of O2 across the water table was calculated assuming a
concentration gradient of 0.21 atm, 10 cm above the water table, to 0.0 atm, 10 cm below
the water table (Amos et al., 2011). The total diffusive mass transport across a given
length, X, of water table 1 m wide was determined by:


FD ,O2   D *CO2 / z X
(A4)
where C is concentration, z is vertical distance set to 20 cm, and D* is the effective
diffusion coefficient given by:
D *  n D O2
(A5)
where n is porosity (0.38), τ is tortuosity (0.39, Chaplin et al., 2002), and DO2 is the
diffusion coefficient of O2 in water (1.67 x 10-09 m2 s-1, CRC Press, 2009).
The mass transport estimates were converted to electron equivalents.
4
S3.2 Recharge
Recharge rates of 0.2 to 0.3 m yr-1 have been measured at the Bemidji site using
soil-moisture measurements, and between 0.1 and 0.2 m yr-1 using water level changes
observed in hydrographs (Herkelrath and Delin, 2001). Essaid et al. (2003) estimated
average annual recharge at 0.178 m yr-1 using inverse modeling and also determined that
average annual recharge amounted to 28 % of average precipitation. However, Bekins et
al. (2005) showed surface topography has a major control on recharge and varied across
the site. Recharge is focused in the topographic low above the floating oil, amounting to
65 % of precipitation in 2002, while downgradient from the oil body, where the surface
elevation is higher, annual recharge in 2002 was equivalent to 20 % of precipitation. This
suggests that average annual recharge in the CH4 oxidation zone would be about 0.13 m
yr-1.
Assuming an average annual recharge rate of 0.13 m yr-1 (Bekins et al., 2005;
Essaid et al., 2003) over a 1 m2 area gives a recharge volume, Vr, of 0.13 m3 yr-1. The
mass transport of each compound across a given length, X, of water table 1 m wide was
determined by:
FR ,i  Vr C i   X
(A6)
where Ci (mol m-3) is the concentration of compound i. The concentration of O2 was
assumed to be in equilibrium with the atmosphere at 9 °C. Sulfate and NO3concentrations in recharge water were assumed to be similar to average concentrations
measured in uncontaminated background water, 3.6 mg L-1 and 0.4 mg L-1, respectively.
Background SO42- and NO3- concentration were measured in well 310, 200 m upgradient
5
from the source zone, from 1988 to 1995. The mass transport estimates were converted to
electron equivalents.
S3.3 Water table fluctuations
To calculate the potential mass transport of gases, including O2 and CH4, as a
result of the entrapment and release of gas bubbles during water table fluctuations the
following calculations were used. A 2 m column of aquifer centered at the water table
was divided into 10-cm-high control volumes, and the volume of gas entrapped and
released during water table fluctuations was determined for each control volume. The
volume of trapped gas was calculated for each year from 1992 and 2007 based on the
amplitude of the water table fluctuation taken from measured water levels, which ranged
from 4 cm to 59 cm. The effective trapped gas saturation, Segt, for each volume was
determined by (Kaluarachchi and Parker, 1992):

1  Sem a i n
1  Sa a
Se g 
i n
t
m
1  RL (1  Se a) 1  RL (1  Sa

,
)a
(A7)
where RL is the empirically derived Land’s parameter given by:
RL 
1
m ax
S egt
 1,
(A8)
and Segtmax is the maximum effective trapped gas saturation assumed to be 0.1. Seamin is the
minimum effective aqueous saturation, defined as:
S ema i n
Sa  Sr a
,
1  Sr a
(A9)
where Sa is the aqueous phase saturation calculated at the lowest water table position
(Wosten and van Genuchten, 1988):
6
Sa  Sr a
1 Sr a
1 
n m
a
,
(A10)
Sra is the residual saturation, ψa is the aqueous phase pressure head, and α = 4, n = 3.5,
and m = 1-1/n, are soil hydraulic function parameters (Dillard et al., 1997). Saa is the
apparent effective aqueous phase saturation, defined by equation A12 with Sa calculated
at the higher water table position. The trapped gas phase saturation Sgt was determined
using the relation:
S egt 
S gt
1  S ra
,
(A11)
and the volume of trapped gas was determined by:
Vg nVcSg t
(A12)
where n is porosity and Vc is the volume of the control volumes. The oxygen transport
into the groundwater was calculated assuming that the trapped gas volume contains
atmospheric O2 levels, 0.21 atm.
To calculate the release of CH4 during a subsequent drop in the water table, the
following assumptions were made; 1) that after a water table rise, all O2 in the entrapped
gas was consumed through aerobic oxidation processes, 2) that the remaining CH4 partial
pressure in the groundwater was equivalent to 0.38 atm from 45 to 57 m then varied
linearly from 0.38 atm at 57 m to 0.0 atm at 125 m (Amos et al., 2011), and 3) the only
other gas in the groundwater and the entrapped gas phases was N2, initially at
atmospheric concentrations. The partial pressure of CH4 re-equilibrated with the
entrapped gas phase and subsequently released to the atmosphere when the water table
elevation declines was calculated using the relations (Cirpka and Kitanidis, 2001):
7
PT 
Ng
p
(A13)
i
i 1
and
pi 
Ti
Ki Sa  Sg t/ RT
(A14)
where PT is the total gas pressure, equal to 1 atm near the water table, pi (atm) is the
partial pressure of each gas, i, Ki is the Henry’s Law constant (mol L-1 atm-1), Sa and Sgt
are the aqueous and gas phase saturations, respectively, R is the gas constant (0.08206
atm L mol-1 K-1), T (K) is the temperature, and Ti (mol L-1) is the cumulative gas
concentration of each gas in the water and gas phases calculated by:
Ti  Ci ( a ) S a  Ci ( g ) S gt
(A15)
where Ci(a) (mol L-1) is the concentration of the gas in the aqueous phase, and Ci(g) (mol
L-1) is the concentration of the gas in the gas phase. Substituting in Equation A14,
Equation A13 was solved for Sgt and subsequently pi was determined using equation A14.
Given Sgt, the volume of the gas bubble can be calculated (Equation A12) and the amount
of CH4 released was calculated using the partial pressure pi.
S4. Fe(III) mass balance calculations
The overall oxidation capacity for sediment Fe(III) (mol e- yr-1) is given by:
FFe 
CFe( s)Va b
(A16)
Ny
where ∆CFe(s) is the change in solid phase sediment Fe(III) concentrations from 1993 to
2007 (13.4 μmol g-1; Figure 8), Va is the volume of the aquifer section considered (80 m
8
length by 4.3 m in elevation by 1 m wide), ρb is the bulk density (1.65 g cm-3) and Ny is
the number of years (1993 to 2007). It is assumed that Fe(III) is converted to Fe2+(aq).
S5. Free Energy Calculation
Free energy calculations for individual sampling points were based on a ∆Gr0 for
Equation 1 of -664 kJ mol-1 using ∆Gf0 ( kJ mol-; from Wagman et al., 1982, unless
otherwise noted) values of -34.39 for CH4(aq) , -687 for Fe(OH)3(am) (Nordstrom et al.,
1990), -78.87 for Fe2+, -586.8 for HCO3- , 0 for H+, and -237.1 for H2O.
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