&EPA
United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/S-97/001 April 1997
ENVIRONMENTAL
RESEARCH BRIEF
Characterization of Organic Matter in
Soil and Aquifer Solids
M.J. Barcelona,3 M.E. Caughey,b R.V. Krishnamurthy,c D.M. Shaw,cand K. Maasc
Abstract
The focus of this work was the evaluation of analytical methods
to determine and characterize fractions of subsurface organic
matter. Major fractions of total organic carbon (TOC) include:
particulate organic carbon (POC) in aquifer material, dissolved
organic carbon (DOC) and both volatile (VOC) and non-volatile
(NVOC) organic carbon sub-fractions.
POC makes up the bulk of TOC in contaminated and
uncontaminated subsurface soils and aquifer materials. The
volatile subfraction of POC can be determined quantitatively
when minimally disturbed sub-cores are preserved immediately
in the field. Methanol and acid addition (i.e., HCI, NaHSO4) to pH
2 are adequate preservatives for specific volatile organic
compound determinations. An interlaboratory round-robin test to
improve acidification and removal methods for carbonates in
total carbon using sulfurous acid (H2SO3) showed sensitivity to
several factors. Thesefactors include: operator care, acidstrength
and carbon content, and particularly, the incomplete removal of
inorganic carbon at high total carbon to organic carbon ratios.
Stable isotopic characteristics of NVOC from fuel contaminated
and organic-enriched environments were found to be quite
sensitive to the stable isotopic signatures of natural organic
matter. The extractability of POC by a range of high to medium
Department of Civil and Environmental Engineering, University of Michigan, Ann
Arbor, Ml 48109-2099.
'Office of Environmental Chemistry, Illinois State Water Survey, Champaign, IL
61820.
Departments of Geology and Chemistry, Western Michigan University,
Kalamazoo, Ml 49008.
polarity solvents resulted in the observations that relatively little
POC was extractable and water extracted comparable amounts
to 1:1 mixtures of 0.01M KOH in methanol:toluene.
Introduction
Organic matter in subsurface systems is a complex mixture of
natural organic substances, fossil fuels and a variety of synthetic
compounds. The transport and fate of organic contaminants is
quite dependent on the nature and distribution of organic carbon
in general.
Dispersion, sorption and degradation are processes which affect
organic compound transport and fate. The estimation of the
influence of these processes depends heavily on the quantitative
determination of fractions of organic carbon in soils and aquifer
materials (Powell et al., 1989). Conventional contaminant
analytical methods have focused on constituents in fuels and
synthetic mixtures (e.g., solvents, plasticizers and other
chemicals) (Keith, 1991). Methods for determining volatile and
non-volatile organic carbon (i.e., VOC and NVOC) in dissolved
(DOC) and particulate (POC) fractions have seen relatively little
attention in the literature or practice of subsurface environmental
chemistry (Thurman, 1985).
Methods for the determination of major carbon subfractions, as
well as the specific organic compounds of which they are
composed, must be based on quantitative preservation,
separation, and analytical methods which lend themselves to
routine practice. In this way, the roles, identity, and fates of
specific organic contaminants may be incorporated into process-
level hydrogeological investigations.
-------
The present study was organized around the analytical
determination of organic carbon fractions. Each fraction was
related to the matrix it was associated with given its volatility,
extractability/ polarity and its probable origin as identified by the
stable isotopic characteristics of the carbon.
This operational categorization of total carbon is shown in
Figure 1. Corresponding separation and analytical methods to
selected categories in Figure 1 are shown in Table 1.
The primary objectives of the study address aspects of Figure 1
and Table 1 which are central to the routine application of carbon
fractionation methods. These objectives were:
1) Refinement of the acidification step (i.e., TIC removal)
techniques for the quantitative determination of non-volatile
organic carbon (NVOC ) in aquifer materials. Testing of the
methodology in an interlaboratory round-robin trial. This
objective addresses problems associated with Category 1
and 2 analyses.
2) Evaluation of in-field preservation techniques for sub-cores
of split-spoon or piston cores of subsurface materials coupled
with methods to determine VOCpandNVOCp at the elemental
and specific compound level. This objective addresses
issues involved in Category 3.
3) Initial development of an extractability procedure to
characterize the leachability of various fractions of organic
matter by varying polarity solvents as shown in Category 4.
and,
4) Evaluation of established stable carbon isotope methods to
determine their potential to distinguish contaminant vsnafura/
organic carbon in subsurface materials on the basis of 13C/
12C ratios. These experiments pertain to the origin of organic
fractions in Category 5.
The approach to these objectives focused on aquifer materials
from reasonably well characterized fuel, solvent or organic
leachate contaminated as well as uncontaminated sites. Most of
these sites exhibited glacial orfluvioglacial geologic materials of
low organic carbon content. Volatile organic compounds are
among the most common ground-water contaminants and
represent significant problems in quantitative sampling and
analysis.
Experimental Procedures
Site Descriptions
The sites from which aquifer solid or ground-water samples were
collected are listed in Table 2. Most of the samples were collected
by opportunity in the course of collaboration with other researchers.
Total Carbon (TC)
Category
1 INORGANIC/ORGANIC
2 MATRIX
TIC
Total Inorganic Carbon
POC
Particulate Organic Carbon
TOC
Total Organic Carbon
DOC
Dissolved Organic Carbon
3 VOLATILITY
(~40°C)
4 EXTRACTABILTY
5 ORIGIN
1
VOCD NVOCrj
[ a) elemental, and b) specific compounds] Volatile Dissolved Organic Carbon
Solvent Extraction
I) H2O
ii) 0.01 NKCI Solution
Mi) Methanol
iv) 0.01 N KOH in Methanol: Toluene
(1:1 v/v)
13Q/12Q ratios
[ a) elemental, and b) specific compounds]
Figure 1. Operational categories of subsurface carbon.
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Table 1. Separation and Analytical Methods Corresponding to Selected Particulate Carbon Fractions
Category
Carbon Fraction
Subtraction
Separation
Analysis
1 TIC
2 POC
3 VOCp
A/VOCp
a) VOCp
(elemental)
b) VOCp
(specific compounds)
CO2 removal by acidification of
TC1>2
Combustion of residue on
acidification of TC to release
CO/2
Infield preservation of
sub-cores3'4
Volatilization at >40°C
Volatilization at >40°C
CO2 by infrared spectro-
metry or coulometry
(as above)
Combustion of off-gases
O2 to CO2 (CO2 as above)
Dynamic or static head-
space GC with selective
detectors
a) A/VOCp
(elemental)
b) A/VOCp
(specific compounds)
POC as above
Extraction of solid sub-core
with organic solvents5
POC as above
Various gas or liquid
chromatographic methods
A/VOCD
VOCp (a)
(elemental)
A/VOCp (a)
(elemental)
VOCp (b)
(specific compound)
A/VOCp (b)
(specific compound)
i) weakly-sorbed
room temperature
(extraction
solvent)
ii) weakly-sorbed/
ion-exchangeable
Hi) strongly sorbed/
Hydrogen-bonded
iv) bound/occluded5'6
H20
0.01NKCI solution
Methanol
0.01 NKOH in Methanol:Toluene
(1:1 v/v)
Volatilization at >40°C off-gas
combustion in O2 to CO2
Combustion of residue from
volatilization in O2 to CO2
GC separation of off-gas from
volatilization step followed by
on-line combustion in O2 to CO2
GC separation of solvent
extraction from 3 or 4 above
followed by on-line combustion
in O2 to CO2
Combustion of dried
sample extract at
950°C to CO2 with CO2
as determined in 1 above
Isotope-ratio mass
spectrometry of CO2
Isotope-ratio mass
spectrometry of CO2 7
Isotope-ratio mass
spectrometry of CO2
Isotope-ratio mass
spectrometry of CO2
GC = Gas Chromatography
1Powell et al. (1989).
2Caughey et al. (1995).
3Hewitt et al. (1992).
4Siegrist and Jenssen (1990).
Barcelona et al. (1995).
^Modified from Cheng (1990).
7Waasenaar et al. (1991).
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Sampling Methods
Samples of aquifer solids and ground water were collected from
six sites contaminated with mixed organic wastes or petroleum
fuel mixtures. Water samples were collected by pumping or
bailing existing monitoring wells at three underground storage
tank (LIST) sites in Houston, Texas; the former site of Casey's
Canoe Livery at Sleeping Bear Dunes State Park in Empire,
Michigan; an anaerobic treatment impoundment of meat
processing wastes in Beardstown, Illinois; a clean site at Sand
Ridge State Park in Illinois; and, a fire-training area at
decommissioned Wurtsmith AFB in Oscoda, Michigan. The
sampling sites were all in shallow unconfined aquifers which had
experienced contamination over extended time periods (i.e.,
>minimum 5 years). With the exception of the Beardstown and
Sand Ridge sites, the other sites had known BTEX (benzene,
toluene, ethylbenzene and xylenes) contamination in the ground
water.
The water samples at the LIST sites were collected by a private
consultant under the direction of Dr. Joseph Salanitro, Shell
Development Co. Aquifer solid samples were subsampled from
rig drilled cores at the Sleeping Bear Dunes site. All samples
were refrigerated at 4°C after collection and water samples were
preserved by adjustment to pH 10 with KOH. Samples for BTEX
determinations were preserved in the field with HCI to pH 2 prior
to refrigeration and transported to the laboratory.
Analytical Methods
The common elements in analytical determinations which were
accomplished on categories 1, 2, 3a NVOCp, 4, and 5a, are that
they could be referenced to verifiable primary standards. These
include: National Institute of Standards Dolomite Standard
Reference Material (SRM) #88 and potassium hydrogen
phthalate. The determinations of volatile fractions 3b) VOCp and
NVOC were straightforward applications of U.S. EPA Methods
601/6d2 (Keith, 1991) for which there are well-referenced
standards. Elemental carbon determinations on VOC fractions
were done on a compound specific basis (i.e., carbon content per
compound) by static headspace capillary gas chromatography
using EPA 601/602 methods with simultaneous photoionization
and electrolytic conductivity detectors. Unknown compounds
were quantified as dichloroethylene for chlorinated aliphatics or
benzene for aromatic compounds. In all cases VOCp samples
were collected as cut-off syringe subcores (Hewitt, 1995) preserved
with 50:50 methanol:H2O or 1% NaHSO4 solution.
The details of the acidification and analysis steps for NVOCp
determinations were modified from Acton and Barker (1992) and
are reported in Caughey et al. (1995). Four aquifer material
standards of varying TOC/TC ratios were ground to pass 200
mesh. Along with National Institute of Standards SRM 88b-
Dolomite, the ground solid samples were distributed to eight
laboratories. These test materials are described in Table 3.
Table 2. Description of Study Sites
Site/(Location)
Contaminant Mixture
Geologic Materials
Asylum Lake
(Kalamazoo, Ml)
Beardstown/Sand Ridge
State Park
(Central Illinois)
Leachate
Kalamazoo-Battle Creek
Airport
Service Station Sites
(Houston, TX)
Sleeping Bear
Dunes State Park
(Empire, Ml)
Wurtsmith AFB
(Oscoda, Ml)
None
Meat processing
treatment
impoundment
Fuels and solvents from aircraft
maintenance
Motor fuels from underground
tanks
Motor/heating fuels from under-
ground tanks
Jet fuel, chlorinated solvents
from fire-training exercises
Glacial outwash sand/gravel-
postglacial alluvium1
Glacial sands with some
interbedded gravels2
Glacial sands and gravels
with fill material1
Low permeability silty
sands/clays3
Coastal lacustrine sand-
dunes4
Fluvioglacial sands/gravels
with aeolian dune deposits5
1Hydrogeology Field Course, Western Michigan University, Summer, 1992.
2 Barcelona et al., 1989.
3Personal Communication, Dr. Joseph Salanitro, Shell Research, Houston, TX.
"Westetal., 1994.
5Cummings and Twenter, 1986.
-------
Overall, they covered a wide range of TIC at low TOC contents.
The TIC in the samples was contributed by dolomite (e.g., 99.5%
for Test Material #1 (TM1) to mixed calcite and dolomite
mineralogy. Reagent grade 6% H2SO3from the same lot was also
sentto each lab afterthe carbon content of the acid was measured
andconfirmedtobelessthan 1 |ig-C/ml. Round-robin participating
laboratories were instructed to use the identical acidification
procedure employing individual samples of >0.1g for five
replicates on each of the five test materials.
Solid samples for parallel (i.e., duplicate solid portions for each
solvent) or sequential (i.e., one set of duplicate solid portions for
successive extraction by all solvents) extraction by the four
solvents were air dried, and extracted at a 2:1 ratio of
solid:extractant(i.e., ~100g/50ml) in amber glass jars with PTFE
(polytetrafluoroethyiene) lids. Extractions were conducted at
room temperature for eight hours on a reciprocating shaker. The
slurries were then centrifuged at ~2000g for an hour and then
decanted. The extractions were repeated, combined with the
previous decantate, volume adjusted and handled as water
samples for NVOC or specific organic compound determinations.
Stable carbon isotope determinations on NVOCp and TIC samples
were done by the method of Epstein et al., (1987) and CO32'
equilibration methods, respectively. Results were expressed in
conventional per mille (0/00) del (d) notation relative to the Pee
Dee Belemnite standard.
Results and Discussions
The full details of the results on each of the primary objectives of
the work are contained in literature publications. The major
highlights of the results are discussed below with reference to the
publications.
Quantitative Determination of Non-Volatile
Organic Carbon (NVOC)
Seven of the eight laboratories (designated A through G) fully
participated in the round-robin study of TIC removal methods of
NVOC determinations (Caughey et al., 1995). The details of their
execution of the round-robin procedures are summarized in
Table 4. Initially it was planned that mean reported TOC values
would be used as the target values with which laboratory accuracy
would be compared. However, the errors in the datasets were
systematically biased rather than random and this was not
possible. The pooled Total Carbon (TC), TIC and TOC (i.e.,
NVOC) results for the study are shown in Table 5. Interlaboratory
agreement was best for TC and TIC for all five test materials.
These results underscore the excellent accuracy and precision
of combustion and coulometric endpoints for CO2 quantitation.
The TOC results showed significant scatter, however, particularly
at high TIC to TOC ratios.
This study confirmed the results of previous literature contributions
citing incomplete TIC removal as the most significant source of
error in NVOC determinations. Clearly, the use of commercial
sulfurous acid does not represent the answer to this problem.
This work and more recent efforts (Heron etal., 1996) commends
the use of strong non-oxidizing mineral acid (e.g., H3PO4, HCI
etc.) for TIC removal from aquifer solids. The grinding of samples
to grain sizes less than 0.063 mm and belowis also recommended,
provided a shatterbox ratherthan a high speed rotary grinder can
be used. The principal journal publication from this work(Caughey
Table 3. Test Material Descriptions
Approximate Values
Test
Material
Description
(depth interval)
Major Mineral
by x-ray Diffraction
XRD (Percentages)
TC
(mg g-1)
TOC
(mg g-1)
TIC
(mg g-1)
1 NIST SRM 88b
2 Aquifer material core A
(76-98 cm)
3 Aquifer material core A
(262-284 cm)
4 Aquifer materials core SC
(317-415 cm)
5 Aquifer material core 40
(60-125 cm)
Dolomite, 99.5; quartz, 0.5
Quartz, 63.1; dolomite, 18.0;
feldspars, 13.6; calcite, 5.2
Quartz, 87.0; feldspars, 5.9;
dolomite, 5.5; calcite, 1.6
Quartz, 54.5; dolomite, 28.9;
calcite, 9.4; feldspars, 7.2
Quartz, 91.6; feldspars, 5.2;
dolomite, 2.6; calcite, 0.5
126.5
28.8
12.8
48.2
19.6
0.5
1.7
2.1
0.6
13.5
125.9
27.2
10.0
46.9
4.6
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Table 4. Method Details for Seven Participating Laboratories
Replicate
Lab ID Weight (mg)
Total Acid
Used (ml)
TOO
Instrument
Comments
C
D
30-90
20-30
20-50
20-30
400-800
12
3-18
3-18
5-9
UIC 5000
LECO WR-112
UIC 5000
UIC CM 120
LECO CS-225;
Dohrmann DC1800
Samples were acidified before
transfer to combustion boats
Porous combustion crucibles
leaked acid
Salt crust hindered sample acidi-
fication
Used 2M HNO3 for acidification;
determined TOC as ASOC + AIOC
F
G
250-500
80-130
9-12
9
LECO CS-444
UIC
Did not determine TIC
Table 5. Pooled Round-Robin Test Results for Carbon Determinations
Parameter
(units)
Pooled TOC mean
(mg C g-1)
Pooled TC mean
(mg C g-1)
Pooled TIC mean
(mg C g-1)
TOCEST
(mg C g-1)
TIC/TOCEST
TM 1
50.99*
(35.85)
[70%]
126.70
(1.84)
[1.5%]
125.67
(0.37)
[0.3%]
1.03
122
TM 2
5.07*
(5.79)
[114%]
28.84
(1.43)
[4.9%]
27.18
(0.74)
[2.7%]
1.66
16
Test Material
TM 3
2.75
(2.04)
[74%]
12.83
(0.78)
[6.1%]
9.97
(0.45)
[4.5%]
2.86
3.5
TM 4
11.87*
(13.84)
[117%]
48.54
(2.17)
[4.4%]
46.91
(1.00)
[2.1%]
1.63
29
TM 5
13.52
(2.16)
[16%]
19.63
(0.96)
[4.9%]
4.55
(0.92)
[20.3%]
15.08
0.30
NOTE: Asterisks indicate biased values where the estimated error was greater than 100%. Values in parentheses are standard
deviations; values in brackets are relative standard deviations.
-------
et al., 1995) provides a detailed description of the procedural
recommendations.
It should be noted that TOC errors of a factor of two or more would
have a significant impact on the value of Koc inputto an estimation
of retardation coefficients. This level of error may be routinely
observed in samples with high TIC to TOC ratios (i.e., >10) and
dolomite percentages above 15%. Practically, these analytical
problems may be expected in studies involving glacial or
carbonate aquifer solid samples.
Evaluation of In-field Aquifer Solid Preservation
Techniques for VOC Determinations
There has been a great deal of recent concurrent work on the
preferred means of preservation of VOC samples. The results of
this work reported along with those of other groups (Siegrist and
van Ee, 1994) and (Wisconsin DNR, 1994) strongly support the
following:
1) Immediate field preservation of core material in 40 ml of
headspace vials with mineral acid, methanol, or sodium
bisulfate is necessary to perform accurate VOC
determinations;
2) Syringe sub-sample collection from cores minimizes sample
disturbance and handling time which leads to higher and
more reproducible recoveries;
3) Negative bias (i.e., low results) levels are greater for
compounds which are more volatile and less strongly sorbed;
and
4) Bulk jar sampling of core materials without preservation
other than refrigeration leads to gross negative bias in VOC
determinations.
The limited results of the present study were in close agreement
with those of more systematic investigations reported above. The
primary references including Barcelona etal., 1993 and Barcelona
et al., 1995 should be consulted for complete details.
Extractability ofNVOC by Solvents of High to
Medium Polarity
Fifteen samples of aquifer materials from several sites were
taken in parallel (i.e., individual solid samples for each extraction
solution) and sequentially (i.e., single solid samples taken through
the series of extractions). The extracting solutions and the
operational leachability fraction they represent included:
Extractant
1. Distilled H2O
2. 0.01 N KCI
3. Methanol (MeOH)
4. MeOH-0.01N
(KOH/Toluene)
(1:1 V/V)
Leachability Fraction
Weakly Sorbed
Weakly Sorbed/lon Exchangeable
Strongly Sorbed/Hydrogen Bonded
Bound/Occluded
The results of these extractions are shown in Tables 6 and 7 for
the parallel and sequential methods, respectively. In general,
MeOH and the MeOH-KOH/toluene extracted more of the total
extractable carbon than the aqueous solvents (e.g., H2O and KCI
solution). For both contaminated and uncontaminated solids, the
percent carbon extractable by the aqueous based solvents,
which might be leached easily, was less than 50% of the total.
Parallel extraction tended to extract more total carbon than the
sequential method. This might be explained by the full rehydration
of the solid samples by the preceding aqueous extractions in the
sequential case which reduced the effectiveness of stronger
solvents.
The extractability of carbon from the Sleeping Bear Dunes
samples is shown graphically in Figure 2. There was a clear trend
in apparent leachability as a function of position in the flow field.
That is, source zone organic matter was less extractable than
background or downgradient samples. This may be expected
due to lower hydraulic conductivity and perhaps interconnected
pore space near fuel product masses. It was unresolved why
methanol and the alkaline methanol:toluene differed greatly in
their extractability of the hydrocarbon contaminants.
Evaluation of Stable Carbon Isotope
Characteristics of Major Carbon Fractions
In this portion of the work it was anticipated that significant
differences could be observed in the stable carbon isotopic ratios
(i.e., 13C/12C) between TIC and TOC fractions. This was because
of their likely carbonate mineral and plant matter origins,
respectively. Mineral carbonates typically show d13C values of ~0
o/oo relative to the Pee Dee Belemnite standard. Organic carbon
from fossil fuels and plants exhibit d13C values -20 to -28 o/oo. In
the d notation, this reflects depletion of 13C relative to the standard
in parts perthousand and is termed isotopically depleted (lighter).
It was also hoped that petroleum contaminated samples would
differ significantly in 13C/12C ratios from both of the above end
members and isotopic shifts (Suchomel et al., 1990) or in PIC or
POC fractions from transformation of the contaminants.
The samples for this part of the study were collected from the
Sleeping Bear Dunes, Beardstown and Sand Ridge sites which
were petroleum or meat processing contaminated and
uncontaminated, respectively.
The limited selection of sampling sites and types of contamination
did not permit a comprehensive conclusion to be drawn on the
utility of stable carbon isotope determinations to differentiate
natural organic carbon from fuel hydrocarbons in aquifer solids.
A summary of the overall data set (Table 8) suggests that distinct
differences in the d13C signatures of NVOC and PIC exist between
both the saturated and unsaturated zones at contaminated and
uncontaminated sites. Unsaturated zone d13C natural NVOC
was ~6 o/oo heavier than that in the saturated zone possibly
reflecting transformation of the original organic mixture. A recent
study by Landmeyeretal. (1996) should be consulted for the use
of d13C signatures as a function of the redox environment in which
transformations proceed.
The petroleum contaminated samples from the Sleeping Bear
site were intermediate between these values. This indicated that
the weathered fuels at this site were quite close to plant-derived
organic matter in stable carbon isotope characteristics. In general,
fossil liquid hydrocarbon mixtures as well as refined products
-------
Table 6. Average Organic Carbon In Parallel Extracts of Aquifer Solids
Extracts
Sample
1001
1002
2
3
4
5
6
7
8
9
10
11
12
13
14
15
SB Background 54E
5.5-6.0
WMU AP/NPWH
Airport Spill
SB-40' Cluster 4.9-7.4'
Carson City Ref. 16'
SB-90' Cluster 5.0-7.8'
SB-90' Cluster 2.0-4.7'
SB-40' Cluster 4.8-6.9'
SB-40' Cluster 2.0-4.1'
SB-Source Cluster 10.6-13.6'
SB-Source Cluster 7.4-10.5
ASYLUM LAKE
AL-5'
AL-15'
AL-25'
AL-35'
AL-45'
AL-55'
Total C
(mg/g)
770
1730
4600
722
1111
2433
658
8322
1020
1162
1223
615
317
806
123
123
H20
82.3
180
54.7
21.3
31.0
73.6
45.1
87
14.7
<1.0
2.7
0
11.4
34.8
16.6
-
-
17.2
KCI
(mg C/g)
47.9
123
15.0
32.0
18.7
47
110
43.5
<1.0
57
<1.0
5.8
28.6
51.3
61.3
-
-
112
Methanol
114
377
146
80.2
48
<1.0
31.3
45.9
129
12.5
11.7
5.9
31.4
65.7
37.8
-
-
36.4
KOH/
methanol
toluene
187
326
230
201
135
<1.0
37.9
229
21.0
37.6
51.2
61.2
119
163
-
-
34.0
Total
Extract Percent
(mg C/g) Extractable
431
1006
446
335
233
121
224
405
165
107
66
72.9
190
314
116
-
-
199.6
56
58
10
46
21
5
34
5
16
9
5
6*
31
51"
37
-
-
162
- = samples too low for quantitation
* replicate determinations (other chemists)
SB samples = Sleeping Bear Dunes, Empire, Michigan
range between -24 to -30 o/oo d13C. Contaminated ground water
and soil gas samples from this site were somewhat heavier -22.3
o/oo (n=1) and -22.9 + 0.1 (n=4), respectively. These samples
reflect an isotopic shift towards heavier isotopic signatures which
could be expected from microbial remineralization of either
natural plant or petroleum-related organic carbon.
It was clear from these data that though the stable isotopic
differences between plant and weathered-petroleum product
organic carbon were not overwhelming they were significant and
measurable. The work of Suchomel etal. (1990) incorporated 14C
determinations into the interpretation of stable carbon ratios and
the origin of organic matter. Their approach should be valuable
in identifying the contribution of recent or synthetic carbon in
NVOC mixtures in aquifer solids.
Conclusion
The focus of this work was the evaluation of analytical methods
to determine and characterize fractions of subsurface organic
matter. Major fractions of total organic carbon (TOC) include:
particulate organic carbon (POC) in aquifer material, dissolved
organic carbon (DOC) and both volatile (VOC) and non-volatile
(NVOC) organic carbon subfractions.
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Table 7. Average Organic Carbon In Sequential Extractions of Aquifer Solids
Extracts
Sample
1001 SB Background 54E
5.5-6.0'
1002 WMU AP/NPWH
Airport Spill
2 SB-40' Cluster 4.9-7.4'
3 Carson City Ref. 16'
4 SB-90' Cluster 5.0-7.8'
5 SB-90' Cluster 2.0-4.7'
6 SB-40' Cluster 4.8-6.9'
7 SB-40' Cluster 2.0-4.1'
8 SB-Source Cluster 10.6-13.6'
9 SB-Source Cluster 7.4-10.5
ASYLUM LAKE
10 AL-5'
11 AL-15'
12 AL-25'
13 AL-35'
14 AL-45'
15 AL-55'
Total C
(mg/g)
770
1730
4600
722
1111
2433
658
8322
1020
1162
1223
615
317
806
123
123
H20
91.7
60.6
0
5.7
30.5
22.6
4.0
9.3
0
16.6
-
-
52.6
0.9
11.7
25.6
KCI
(mg C/g)
13.9
59.1
6.4
0
6.2
64.7
0
0
35.6
82.5
-
-
58.6
6.8
22.5
162
Methanol
104
75.2
17.2
6.0
65.2
82.4
32.4
46.0
53.5
45.0
-
-
67.1
44.4
110
54.5
KOH/
methanol
toluene
4.6
66.4
7.5
0
57.4
36.5
30.2
24.2
41.0
60.3
-
-
53.4
16.6
31.7
51.6
Total
Extract
(mg C/g)
214
261
31.1
11.7
159
206
66.6
79.5
130
204
-
-
232
68.7
175.9
294
Percent
Extractable
27.8
15.1
0.7
1.6
14.3
8.5
10.1
1.0
12.8
17.6
-
-
73.0
8.5
143
239
- = samples too low for quantitation.
SB = Sleeping Bear Dunes, Empire, Michigan.
POC makes up the bulk of TOC in contaminated and
uncontaminated subsurface soils and aquifer materials. The
volatile subfraction of POC can be determined quantitatively
when minimally disturbed subcores are preserved immediately
in the field. Methanol and acid addition (i.e., HCI, NaHSO4) to
pH 2 are adequate preservatives for specific volatile organic
compound determinations. An interlaboratory round-robin test to
improve acidification and removal methods for carbonates in
total carbon using sulfurous acid (H2SO3) showed sensitivity to
several factors. Thesefactors include: operator care, acid strength
and carbon content, and particularly, the incomplete removal of
inorganic carbon at high total carbon to organic carbon ratios.
The extractability of POC by a range of high to medium polarity
solvents resulted in the observations that relatively little POC was
extractable and water extracted comparable amounts to 1:1
mixtures of 0.1M KOH in methanol:toluene. Stable isotopic
characteristics of NVOC from fuel contaminated and organic-
enriched environments were found to be quite sensitive to the
stable isotopic signatures of natural organic matter.
-------
ORG. CARBON
H2O
0.1 NKCL
MeOH
MeOH-0.01NKOH/TOL
Q.
UJ
Q
O
o
BKGRD-54E
(0.1)
SOURCE (1.0)
SOURCE (1.8)
12m (0.4)
12m (1.3)
12m (1.5)
27m (0.5)
27m (2.0) §
50 100 150 200 250
Figure 2. Average Extractable Organic Carbon for Selected Sleeping Bear Dunes Samples (mg/g). Locations are designations: background,
in source area, and 12m and 27m downgradient from source, respectively. Parentheses denote depth in meters below land surface
at each location.
Table 8. Summary of Average 13C/t2C (d o/oo) Ratios in Non-Volatile Carbon Fractions
(parentheses denote relative standard derivations)
Organic Carbon n Inorganic Carbon n
Uncontaminated
Unsaturated Zone
Saturated Zone
SR
BTU
BTD
SR
BTU
-21.5
-22.5
-22.1
-27.6
-28.2
(17%) 4
(11%) 2
(5%) 6
(1.
(1.
,5%) 3
,4%) 10
-15.7 (25%) 3
-19.2 1
-22.1 (9%) 6
0.8 1
-14.1 (28%) 3
Contaminated
Saturated Zone
SR
SB
BTU
BTD
Sand Ridge.
BTD
SB
-27.1
-25.5
(6.
(0.
,3%) 4
.1%) 6
0.5 1
0.0 1
Sleeping Bear.
Beardstown
Beardstown
Upgradient.
Downgradient.
10
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Acknowledgement
TheauthorswishtothankMs. BonnieDube, Ms. Carolyn Virkhaus,
and Mrs. Debbie Patt for their assistance in the work. The
guidance and support of Candida C. West, the EPA Project
Officer, is very much appreciated.
Disclaimer
The U.S. Environmental Protection Agency through its Office of
Research and Development partially funded and collaborated in
the research described here under Cooperative Agreement No.
CR-817287 to Western Michigan University. It has been subjected
to the Agency's peer and administrative review and has been
approved for publication as an EPA document. Mention of trade
names or commercial products does not constitute endorsement
or recommendation for use.
11
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