SUMMARY OF AVAILABLE INFORMATION RELATED
TO THE OCCURRENCE OF VINYL CHLORIDE AND
GROUND WATER AS A TRANSFORMATION PRODUCT
OF OTHER VOLATILE ORGANIC CHEMICALS
Science Applications
International Corporation
8400 Westpark Drive
McLean, Virginia 22102
EPA Contract No. 68-01-7166, Work Assignment 5
SAIC Project No. 2-813-07-545-05
EPA Project Officer
Mr. Willi am Coniglio
October 9, 1985

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TABLE OF CONTENTS
Page
1.	INTRODUCTION	1
2.	OCCURRENCE OF VINYL CHLORIDE IN
GROUND WATER DRINKING WATER SUPPLIES	2
2.1	OFFICE OF DRINKING WATER OCCURRENCE ESTIMATES	2
2.2	ADDITIONAL OCCURRENCE INFORMATION	3
3.	INFORMATION ON THE CO-OCCURRENCE OF
VINYL CHLORIDE WITH OTHER VOCs	5
4.	TRANSFORMATION OF TRICHLOROETHYLENE AND TETRACHLORO-
ETHYLENE TO VINYL CHLORIDE IN GROUND WATER ENVIRONMENTS	10
REFERENCES	16

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1. INTRODUCTION
In 1982 and 1983, the EPA Office of Drinking Water prepared a series of
assessments on the occurrence of several volatile organic chemicals (VOCs) in
public drinking water supplies. In those assessments, the occurrence of each
VOC in drinking water was considered to be due primarily to environmental
releases associated with the industrial and commercial uses of that substance
and/or the disposal of wastes containing that substance. Subsequent to the
completion of those assessments, information became available to suggest that
the presence of certain VOCs in ground water may be due, at least in part, to
biologically mediate ¦ ^ansformations of other VOCs.
Most notable among these potential transformation processes is the
degradation of trichloroethylene and tetrachloroethylene (through one or more
diehloroethylene isomer intermediate) to vinyl chloride. The formation of
vinyl chloride from trichloroethylene and tetrachloroethylene in ground water
is a focal point of concern because vinyl chloride is a potent carcinogen
(having a drinking water risk level approximately two orders of magnitude
greater than that of trichioroethyene, and because trichloroethylene and
tetrachloroethylene are the two most frequently found VOC contaminants in
ground water drinking water supplies.
The purpose of this document is to summarize the available information
relevant to assessing the extent to which vinyl chloride may be occurring in
ground water based drinking water supplies as a transformation product.
Section 2 summarizes the available information on observed chloride occurrence
in ground water drinking water supplies. Section 3 examines the co-occurrence
of vinyl chloride with other VOCs. Section 4 reviews information relevant
to the transformation of certain VOCs to vinyl chloride in ground-water
envi ronitieias.
1

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2. OCCURRENCE OF VINYL CHLORIDE IN GROUND WATER DRINKING WATER SUPPLIES
2.1 OFFICE OF DRINKING WATER OCCURRENCE ESTIMATES
The information available through mid-1983 on the occurrence of vinyl
chloride in ground water drinking water supplies was presented by EPA's Office
of Drinking Water in a document prepared in support of the June 12, 1984,
Federal Register Notice (USEPA 1984) publication of the proposed rule on VOCs
under the Safe Drinking Water Act (Letkiewicz et al. 1983). The relevant
information from that document is summarized below.
Vinyl chloride was included as an analyte in three Federal drinking water
surveys. In Phase II of the National Organic Monitoring Survey conducted May-
July 1976, 18 ground water supplies were sampled for vinyl chloride; none was
detect^ 'minimum quantifiable limit - 0.1 ug/1 ). In the National Screening
Program conducted between 1971 and 1981, vinyl chloride was observed in 2 of
12 ground water supplies sampled at concentrations of 9.4 ug/1 and 66 ug/1
(quantification limit = 0.2 ug/1). In the Ground Water Supply Survey con-
ducted in 1981, vinyl chloride was found in 7 of 929 public water supplies
examined at concentrations ranging from 1.1-8.4 ug/1 (minimum quantitation
1 imit = 1.0 ug/1).
Based on those limited data, it was estimated that vinyl chloride occurs
at levels above 1.0 ug/1 in 0.06% of the nation's ground water public drinking
water supplies. To place that value in perspective, the most frequently
observed VOCs in ground water public water supplies are trichloroethylene
(3.4%), tetrachloroethylene (3.2%), and 1,1,1-trichloroethane (2.9%). It
should be noted that the frequencies of occurrence of those substances are for
levels of 0.5 ug/1 or more. Consequently, a more directly comparable estimate
of national vinyl chloride occurrence at or above 0.5 ug/1 would probably be
higher than 0.06%, but the value cannot be estimated from existing data.
Limited state data available from New Jersey and New York were also pre-
sented in the Office of Drinking Water report. In New Jersey, vinyl chloride
was reported to be present in 6 of 21 ground water drinking water samples
taken from Fair Lawn and Mahwah, New Jersey. The concentrations ranged from
1.1-51 ug/1. No vinyl chloride was observed in 425 samples taken at other
locations in New Jersey. Vinyl chloride was not detected in 125 finished
water ground water samples taken in Mew York.
2

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The Federal survey data and state data presented in the 1983 occurrence
document did not provide any details relevant to assessing the sources of con-
tamination observed. As noted previously, it was assumed then that the vinyl
chloride observed in drinking water was related to industrial uses and/or
waste management practices.
2.2 ADDITIONAL OCCURRENCE INFORMATION
Mel 1 o (1984) summarized the results of a 1982 ground water survey con-
ducted in three states in Region VII (Kansas, Missouri, and Nebraska) to
measure VOC levels. Vinyl chloride was observed in only 1 of 208 public water
supplies studied, at a concentration of 1.2 ug/1 (quantitation limit = 1.0
ug/1).
Brass (1985) provided data from the Office of Drinking Water's Packed
Tower Aeration Project showing vinyl chloride to be present at seven locations
at concentrations ranging from 1-640 ug/1. No other details were provided.
Vincent (1984) examined the occurrence of VOCs, including vinyl chloride,
in the ground water drinking water supplies in Rroward, Dade, and Palm Beach
counties of south Florida. Vinyl chloride was observed in finished water in
one supply in Palm Beach county (2 ug/1) and one in Dade county (2 ug/1). The
most frequently observed VOC (other than trihalomethanes) was cis-Ztrans-1,2-
dichloroethyl ene.
Vincent (1984) sought to identify the sources of the VOCs observed in
this study, and could find no reasonable industrial, commercial, or waste dis-
posal source to explain the presence of the observed vinyl chloride. Vincent
(1984) referenced research at Florida International University (discussed in
more detail in Section 4 of this document) to suggest that the observed vinyl
chloride -- as well as cis-A^8-l,2-dichloroethylene, 1,1-dichloroethylene,
and 1,2-dichioroethane -- were biodegradation products of trichloroethylene,
tetrachloroethylene, and 1,1,l-trichloroethane, all solvents with considerable
use and release in the study areas.
Vincent (1984) also noted other locations (in Long Island, New York;
eastern Pennsylvania; and California) where trichloroethylene contamination of
ground water occurred and vinyl chloride was subsequently detected. No
details were provided, however, except that in all cases a contributing factor
3

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appeared to be bacterial contamination from septic tank sludges or other bio-
logical sludges.
4

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3. INFORMATION ON THE CO-OCCURRENCE OF VINYL CHLORIPE WITH OTHER VOCs
When the 1983 VOC occurrence estimates were prepared by the Office of
Drinking Water, the focus of the analyses was on the frequency of occurrence
of each individual VOC independent of any other contaminant. Although some
limited analyses were conducted on the frequency of single, multiple, and
cumulative occurrences of VOCs in the Federal surveys (see USFPA 1984), no
consideration was given at that time to the co-occurrence of specific pairs or
larger groups of VOCs.
Subsequently, Brass (1985) reviewed co-occurrence information for vinyl
chloride based on data from the Ground Water Supply Survey, the Region "
Survey, and the Packed Tower Aeration Project. In all of the 15 ground wa~er
supplies, Luuoa ining vinyl chloride observed in those surveys, other VOCs were
also procc"t. The most frequently co-occurring VOCs were:
cis-/trans -1,2-dichloroethylene	15 co-occurrences
trichloroethylene	12 co-occurrences
1,1-dichloroethane	10 co-occurrences
1,1-dichloroethylene	9 co-occurrences
tetrachloroethylene	7 co-occurrences
1,1,1-trichloroethane	6 co-occurrences
In the two vinyl chloride occurrences reported for the National Screening
Program, there was co-occurrence with ois - and trans-1,2-dichloroethylene,
trichloroethyl ene, and 1,1-dichloroethane in one supply, and no co-occurrence
with any of the above VOCs in the other.
Table 1 from Brass (1985) shows the specific co-occurrence values for the
c hi or ii'.utcd aliphatic VOCs examined in the Ground Water Supply Survey. A
statistical test of the Ground Water Supply Survey co-occurrence data was con-
ducted by SAIC to identify those pairs which co-occur with a greater frequency
than would be expected based upon the individual frequency of occurrence of
each substance and assuming that co-occurrence is a random event.
The probability density function for the number of co-occurrences of any
two VOCs can be expressed as a hypergeometric distribution with parameters N
(the total number of smaples in the survey) and a and b (the number of indivi-
5

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Table 1. Co-Occurrence of Chlorinated Aliphatic VOCs in the
Ground Water Supply Survey (December 1980-December 1981)
>
O

O
1

r—

r—
OJ
r—
CO
JZ
-C
-C
-C
4->
-C
sr
u
O
o
o

o
a

• r—
• r-

c
•p~
Ss

o
o
o
o
o

>)
1
OJ
1
CNJ
1
_Q
1
\
c

t-
r-H
05
•r—
*
*
«-

r-H

i—H
o
i—H

a»


c



>
0J
L-

r--
o
4->
>)
r—»
a>
-C
jC
o

u
u
0)

o
o
L.

c_
H-
-C
o
1
u

i—H


*
a>
L_
r—H
i—
J—
Vinyl chloride
7 1 0
3
1
5
7
3
1
6
1,2-Dichloroethane
10 1
3
5
1
2
2
3
3
1, 2-Dichl oropropane
13
1
0
2
3
3
1
3
1,1-Dichloroethylene

24
3
17
11
16
7
17
Carbon tetrachloride


30
2
4
4
7
6
1,1-Dichloroethane



41
21
28
16
26
cis/trans -1 ,?-nir hioroethyl ene




54
23
23
45
1,1,1-Trichl oroethane





78
35
45
Tetrachloroethyl ene






79
39
Trichloroethyl ene







91
Source: Brass 1985
6

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dual occurrences of each VOC in the pair). To test whether the observed
number of co-occurrences is a random phenomena (according to the probabilities
specified by the hypergeometric distribution) versus the hypothesis that the
observed co-occurrence is something other than random, a hypothesis test at a
specified significance level (usually 0.05 or 0.01) can be conducted, based on
this distribution. If the number of co-occurrences equals or exceeds a cri-
tical value determined by the parameters of the hypergeometric distribution
and the significance level, the hypothesis of randomness is rejected in favor
of non-randomness. If the number of co-occurrences does not exceed this cri-
tical value, the hypothesis of randomness cannot be rejected.
Table 2 presents the results of the analysis. Table ^ows for each
pair of VOCs the individual number of occurrences and the number of co-
occurrences. Two hypergeometric probabilities are shown. The first is the
probability of observing the specific number of co-occurrences that were in
fact observed; the second is the cumulative probability of observing the
number of co-occurrences actually seen and all possible higher numbers of co-
occurrences. The last two columns show the minimum number of co-occurrences
needed for each pair to conclude that co-occurrence was a non-random event at
the 0.05 and 0.01 level, respectively.
As indicated in Table 2, about half of the VOC pairs show frequencies of
co-occurrence in the Ground Water Supply Study that exceed the amount that
would be expected by chance at the 0.05 and 0.01 level. Only 1,2-dichloro-
propane did not co-occur with any other VOC at a significant frequency, while
carbon tetrachloride and 1,2-dichloroethane co-occurred with only one other
VOC each at a significant frequency. The remaining seven VOCs each co-
occurred with between 4 and 7 other VOCs at significant frequencies.
Vinyl chloride shows significant co-occurrence with cis/trans-1,2-di-
chloroethylene, 1,1-dic hloroethyl ene, trichloroethyl ene, and 1,1-dichloro-
ethane. However, the reasons for its co-occurrence with those substances in
the Ground Water Supply Survey cannot be determined due to lack of detailed
information concerning the sites and conditions where it was observed.
VOCs may occur together because of geographic proximity of individual
sources, common use patterns, co-occurrence in certain products or wastes as
contaminants or by-products, disposal at a common waste site responsible for
the observed ground water contamination, and/or chemical or biological trans-
7

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Table 2. Hypergeometric Probabilities of Observed Co-Occurrence of Chlorinated Aliphatic VOCs in the Ground Water Supply Survey






Cumulative hyper-
Minimum
number





Hypergeometric
geometric probability
of co-occurrences to




Observed
probabil ity
of co-occurrence greater
exceed critical value



co-occur-
of observed
than or equal to


First VOC (occurrence)

Second VOC (occurrence)

rence
co-occurrence
observed co-occurrence
0.05 level
0.01 level
Vinyl chloride (7)

1,2-Dichloroethane (10)

1
0,071059
0.07319
2
2
Vinyl chloride (7)

1,2-Dichloropropane (13)

0
0.905771
1.00000
2
2
1,2-Dichloroethane (10)

1,2-Dichloropropane (13)

1
n. 124404
0.13205
2
2
Vinyl chloride (7)

1,1-Dichloroethylene (24)

3
0.000485
0.00050
2*
3*
1,2-Dichloroethane (10)

1,1-Dichloroethylene (24)

3
0.001552
0.00162
2*
3*
1,2-Dichloropropane (13)

1,1-Dichloroethylene (24)

1
0.248059
0.29001
2
3
Vinyl chloride (7)

Carbon tetrachloride (30)

1
0.186744
0.20589
2
3
1,2-Dichloroethane (10)

Carbon tetrachloride (30)

5
0.000005
0.00001
2*
3*
1,2-Dichloropropane (13)

Carbon tetrachloride (30)

0
0.650794
1.00000
3
3
1,1-Dichloroethylene (24)

Carbon tetrachloride (30)

3
0.032913
0.03891
3*
4
Vinyl chloride (7)

1,1-Dichloroethane (41)

5
0.000003
0.00000
2*
3*
1,2-Dichloroethane (10)

1,1-Dichloroethane (41)

1
0.296339
0.36468
3
3
1,2-Dichloropropane (13)

1,1-Dichloroethane (41)

2
0.092236
0.10901
3
4
1,1-Dichloroethylene (24)

1,1-Dichloroethane (41)

17
0.000000
0.00000
4*
5*
Carbon tetrachloride (30)

1,1-Dichloroethane (41)

2
0.243872
0.38586
4
5
Vinyl chloride (7)

oie/trane-l, 2-Dic hi oroethyl ene
(54)
7
0.000000
0.00000
3*
3*
1,2-Dichloroethane (10)

aia/trana-\,2-Di chi oroethylene (54)
2
0.093964
0.11063
3
4
1,2-Dichloropropane (13)

cia/trana-\,2-Dic hi oroethyl ene
(54)
3
0.030139
0.03507
3*
4
1,1-Dichloroethylene (24)

cia/trana-\,2-Di c hioroethylene
(54)
11
0.000000
0.00000
4*
6*
Carbon tetarchloride (30)

eio/trana-1,2-Dic hi oroethylene
(54)
4
0.064931
0.09065
5
6
1,1-Dichloroethane (41)

eto/fciMrtfl-l,2-Dichi oroethylene
(54)
21
0.000000
0.00000
6*
7*
Vinyl chloride (7)

1,1,1-Trichloroethane (78)

3
0.014251
0.01558
3*
4
1,2-Dichloroethane (10)

1,1,1-Trichloroethane (78)

2
0.157708
0.20212
3
4
1,2-Dichloropropane (13)

1,1,1-Trichloroethane (78)

3
0.069893
0.08823
4
5
1,1-Dichloroethylene (24)

1,1,1-Trichloroethane (78)

16
0.000000
0.00000
5*
7*
Carbon tetrachloride (30)

1,1,1-Trichloroethane (78)

4
0.140602
0.23958
6
7
1,1-Dichloroethane (41)

1,1,1-Trichloroethane (78)

28
0.000000
0.00000
7*
9*
cie/trane-1,2-Dic hioroet hylene
(54)
1,1,1-Trichloroethane (78)

23
0.000000
0.00000
9*
11*
Vinyl chloride (7)
Tetrachloroethylene (79)

1
0.350985
0.46432
3
4
1,2-Dichloroethane (10)

Tetrachloroethylene (79)

3
0.039041
0.04590
3*
4
1,2-Dichloropropane (13)

Tetrachloroethylene (79)

1
0.382954
0.68752
4
5
1,1-Dichloroethylene (24)

Tetrachloroethylene (79)

7
0.002147
0.00261
5*
7*
Carbon tetrachloride (30)

Tetrachloroethylene (79)

7
0.007668
0.01007
6*
8
1,1-Dichloroethane (41)

Tetrachloroethylene (79)

16
0.000000
0.00000
8*
9*
oia/trane-l, 2-Dic hi oroet hyl ene
(54)
Tetrachloroethylene (79)

23
0.000000
0.00000
9*
11*
1,1,1-Trichloroethane (78)

Tetrachloroethylene (79)

35
0.000000
0.00000
12*
14*
Vinyl chloride (7)

Trichloroethylene (91)

6
0.000005
0.00000
3*
4*
1,2-Dichloroethane (10)

Trichloroethyl ene (91)

3
0.054275
0.06570
4
5
1,2-Dichloropropane (13)

Trichloroethylene (91)

3
0.095608
0. 12650
4
5
1,1-Dichloroethylene (24)

Trichloroethylene (91)

17
0.000000
0.00000
6*
7*
Carbon tetrachloride (30)

Trichloroethylene (91)

6
0.042986
0.06432
7
8
1,1-Dichloroethane (41)

Trichloroethylene (91)

26
0.000000
0.00000
8*
10*
ai a/trans-1,2-Di c hi oroet hyl ene
(54)
Trichloroethylene (91)

45
0.000000
0.00000
10*
12*
1,1,1-Trichloroethane (78)

Trichloroethylene (91)

45
0.000000
0.00000
13*
15*
Tetrachloroethylene (79)

Trichloroethylene (91)

39
0.000000
0.00000
13*
15*
~Observed co-occurrence greater than minimum number of occurrences needed to exceed critical value.

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formation from one to the other. The significant co-occurrences of vinyl
chloride and other VOCs noted here does not provide any insight as to the pro-
cesses involved. Furthermore, absence of observed co-occurrence does not
necessarily negate a relationship involving transformation since the starting
compound may be consumed during the process and no longer be measureable.
9

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4. TRANSFORMATION OF TRICHLOROETHYLENE AND TETRACHLOROETHYLFNE
TO VINYL CHLORIDE IN GROUND WATER ENVIRONMENTS
As noted in the preceeding section, there are a number of possible
explanations for the observed co-occurrence of vinyl chloride with other VOCs
in ground water. This section examines the available information related to
one of those explanations, namely, the transformation of other substances to
vinyl chloride. The available information suggests that tetrachloroethylene
and trichloroethylene are the likely precursors of vinyl chloride, which would
be formed via sequential dechlorination with cis-l,2-dichloroethylene, trans-
1,2-dichloroethylene, and/or 1,1-dichloroethylene as intermediates (Parsons et
al. 1985). While hypothetical pathways from other frequently observed VOCs,
such as 1,1,1-trichloroethane, can be suggested, no evidence showing their
existence was found. Therefore, only transformations involving trichloro-
ethylene and tetrachloroethylene are considered here.
The transformation of substances present in ground water may occur
through chemical or biological processes. The major types of chemical reac-
tions that can transform organic contaminants in water are oxidation and
hydrolysis. Callahan et al. (1979), primarily citing the work of Dilling et
al. (1975) indicate that neither oxidation nor hydrolysis are significant
water-related fate processes for trichloroethylene or tetrachloroethylene.
The half-lives for combined oxidation and hydrolysis at 25°C were 10.7 months
and 8.8 months for trichloroethylene and tetrachloroethylene, respectively.
Callahan et al. (1975) did not indicate what the intermediate or final pro-
ducts of oxidation or hydrolysis would be. However, oxidation and hydrolysis
reactions with unsaturated compounds such as trichloroethylene and tetra-
chloroethylene would be expected to involve addition across the carbon-carbon
double bond and not the dechlorination processes necessary to produce vinyl
chloride.
The available information on the biological degradation of tetrachloro-
ethylene and trichloroethylene indicates that there are some conditions under
which such transformations will occur and other conditions under which they
will not.
Bouwer et al. (1981) studied the aerobic and anaerobic biodegration of 1-
and 2-carbon halogenated compounds, including trichloroethylene and tetra-
10

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chloroethylene, in aqueous media. In the aerobic studies, the substances were
incubated in 160 ml serum bottles filled with an aqueous mineral medium to
which a bacterial innoculum (primary sewage effluent) was added. In the
anaerobic studies, the compounds were incubated in 160 ml bottles containing
an aqueous anaerobic medium seeded with 1 ml of a methanogenic mixed cul-
ture. Three series of bottles were prepared for each study having low,
medium, or high concentrations (approximately 10, 30, and 100 ug/1 ) of each
substance.
Under the aerobic conditions, there was no observed degradation of either
trichloroethylene or tetrachloroethylene after 25 weeks of incubation. In the
anaerobic study, small decreases in i n . hi oroethyl ene and tetrachloroethylene
concentrations relative to the controls were observed after week 12 of the
16-week incuoation period. The authors noted that the changes observed sug-
gested	slow decomposition, but that the results were not conclu-
sive. The authors did not address degradation products or mechanisms of
degradation.
In a subsequent study, Bouwer and McCarty (1983) conducted additional
anaerobic degradation experiments using conditions similar to those noted
above, except that 60 ml serum bottles were used, the methanogenic mixed cul-
ture was added at 10 ml/I (versus approximately 6 ml/1 in the previous study),
and initial concentrations of trichloroethylene and tetrachloroethylene were
200 ug/1. After approximately 8 weeks, a 40% reduction in trichloroethylene
levels was observed. For tetrachloroethylene, the concentration declined to
below the detection limit of 0.1 ug/1 after 8 weeks. The authors also
observed that after 8 weeks, the seeded cultures incubated with tetrachloro-
ethylene had 91 ug/1 of trichloroethylene present, suggesting the latter to be
a product of tetrachloroethylene biodegradation. Evidence of tetrachloro-
ethyle^o rnnvprsinn to trichloroethylene was also observed by Rouwer and
McCarty (1983) in continuous-flow studies through an aerobic column containing
methanogenic bacteria. (The methanogenic columns were acclimated to a sodium
acetate substrate for 15 months prior to the addition of tetrachloroethylene
to the influent.) After 10 weeks of acclimation to tetrachloroethylene, an 86
+ 7% steady state removal rate was observed (18 + 3 ug/1 in the influent vs.
2.6 + 1.3 ug/1 in the effluent); also, 1.2 + 0.6 ug/1 of trichloroethylene was
observed in the column effluent whereas none had been added to the influent,
11

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again suggesting that trichloroethylene is a biodegradation product of tetra-
chioroethylene. Further degradation products were not analyzed for.
Walker (1984) studied the fate of triehloroethylene in two different
soils (Chalmus Silty Clay Loam and Russel Silt Loam) under conditions of a
simulated spill or discharge. Trichloroethylene (5 or 10 ml) was applied to
the surface of each column, with 50 or 100 ml/day of water added to provide
continuous elution for 132 days. No significant degradation (abiotic or bio-
logical) was observed. The columns were tested to determine if nutrient
enrichment, by adding ammonia nitrogen, would enhance biodegradation pro-
cesses. Although effluent chloride levels of the enriched columns increased,
it was not significant and the corresponding trichloroethylene degradation
could not be measured. Neither cis- nor trem8-l,2-dic hi oroethyl ene was
observed in the column effluent (minimum detection limit stated to be 2 mg/1;
it is likely Luat this is a typographical error and should be 2 ug/1).
Wilson et al. (1983) examined the biological degradation of trichloro-
ethylene and tetrachloroethylene in subsurface materials taken above and below
the water table at Fort Polk, Louisiana, and Pickett, Oklahoma, described as
"fairly typical of shallow, water-table aquifers in the south-central region
of the United States." The depths from which the materials were taken were
2.1 m and 3,fi m for above and below the water table, respecitvely, at Fort
Polk; and 3.6 m and 4.8 m for above and below the water table, respectively,
at Pickett. The samples were noted to be aerobic when collected. Trichloro-
ethylene or tetrachloroethylene were added to microcosms prepared from the
subsurface material at concenrations of 600-800 ug/1. The microcosms were
incubated up to 9 weeks for the Fort Polk samples and 27 weeks for the Pickett
samples. Contaminant levels were measured at intervals during the incubation
period.
Tnere was detectable degradation of both contaminants over the study
period in some of the subsurface materials. However, the rates of degradation
were comparable to abiotic degradation rates noted by the authors as reported
by Dilling et al. (1975) (1.6 and 2.0% wk-1 for trichloroethylene and tetra-
chloroethyl ene, respectively). The authors, therefore, concluded that the
observed degradation may not have been due to biological processes. The
authors did not address the potential products or mechanisms of degradation.
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Wilson et al. (1984) reported similar loss rates of 1.0 and 1.5% wk~* for
trichloroethylene and tetrachloroethylene in similar subsurface microcosm
studies. As in the previous study, the authors could not preclude nonbio-
logical processes to account for the loss rate. In this study, the pore water
of the microcosm was examined for the potential products of biologically
mediated reductive dehalogenation, including vinyl chloride, 1,1-dichloro-
ethylene, and 1,2-dichloroethylene. None were reported to be found at concen-
trations of up to 1% of the potential parent compound.
Wilson and Wilson (1985) demonstrated that trichloroethylene could be
degraded aerobically to carbon dioxide in sandy soil that had previously been
exposed to natural gas :¦ enrich it for methanotrophic bacteria and other
organisms that oxidize tne other small alkanes present in natural gas. Using
radiolabeled [14C]-trichloroethylene, the author determined that the trans-
formation to carbon dioxide was essentially complete; they did not identify or
speculate on the nature of any intermediate products.
Parsons et al. (1985) conducted static microcosm studies to measure the
extent to which tetrachloroethylene could he biologically degraded by micro-
organisms present in the muck bed of the Florida Everglades, which serve as
the primary recharge basin for the Ricayne aquifer in southeastern Florida.
This study was specifically performed to determine whether such processes
could account for the presence of vinyl chloride and ais - and trans-1,2-
dichloroethylene found in the ground water in southern Florida, for which no
industrial or other sources could be identified (see earlier discussion of
Vincent 1984).
In one set of studies, 10 ml of muck collected from a pristine site was
placed in each of seven 60-ml septum bottles. Surface water collected a the
same time as the muck was added to fill each bottle. A 10-ul methanol solu-
tion containing 100 ug of tetrachloroethylene was added to each bottle (the
100 ug of tetrachloroethylene was also found to have approximately 1.6 ug of
trichloroethylene present as a contaminant). Controls of muck and surface
water without tetrachloroethylene, sterile (autoclaved) muck and surface water
with tetrachloroethylene, and sterile distilled water with tetrachloroethylene
were also prepared.
Of the 100-ug tetrachioroethylene initially added to each bottle, an
average of 87 and 83 ug were measured in the water of the test and autoclaved
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control bottles, respectively, immediately after each was capped and shaken;
the remaining amount was believed to have been absorbed to the muck solids.
After 21 days incubation, the amount of tetrachloroethylene in the test
bottles had declined to 28 ug. No loss of tetrachl oroethylene was observed in
the autoclaved control or distilled water control; the unspiked muck control
yielded no chlorinated compounds.
Trie hioroethylene was observed to increase from 1.1 ug at day 0 to 4.7 ug
after 21 days; no change was observed in the autoclaved muck. In addition,
several compounds were observed in the test muck after 21 days that were not
present at day 0 or observed in the autoclaved controls (detection limit, 0.03
ug). These were cis-l,2-dich1oroethylene (3.8 ug), trans-l,2-dichloroethyl ene
(0.11 ug), vinyl chloride (1.7 ug), and 1,1-dichl oroethyl ene and/or dichloro-
methane (0.15 ug) (the latter two substances could not be distinguished with
the method used). Parsons et al. (1985) speculated that the test conditions
were anaerobic or nearly anaerobic.
Parsons et al. (1985) performed similar microcosm tests on five addi-
tional samples of muck and one of marl collected from different parts of the
Everglades. In these studies, 250 ug of tetrachl oroethyl ene was added to the
60 ml test bottles. In these assays, the levels of tetrachloroethylene trans-
formation was reported to be very low (no details provided), with only occa-
sional trace levels of chlorinated by-products formed.
Mackay et al. (1985) noted in their general discussion of biological
transformation of organic contaminants in ground water that there are many
factors that can affect the rate of biotransformation including water tempera-
ture and pH, the types of microorganisms present and their population, oxygen
levels, the substrate concentration, and the availability of electron
acceptors. Also, in some cases, native organisms may not be able to transform
a specific compound, or do so only after a period of acclimation.
The results of the several studies summarized above should be considered
in light of such factors. Some ground water conditions, such as those studied
by Parsons et al. (1985) may result in extensive biotransformation of tetra-
chl oroethyl ene and trichloroethylene to products such as vinyl chloride, while
other conditions will yield little or no degradation. Except for noting that
the processes appear to be anaerobic, the information available is not suffi-
cient to fully characterize the necessary conditions for such transformations
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to occur, nor to_ speculate on the extent to which such transformations are
occurring in U.S. ground water contaminated with trichloroethylene or tetra-
chloroethylene. However, that such processes can occur in some situations
seems clearly demonstrated by the available information.
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