United States
Environmental Protection
Agency
National Exposure
Research Laboratory
Las Vegas, NV 89193-3478
Research and Development
EPA/60Q/SR-96/145 December 1996
of
and
Robert C. Borden, Russell Todd Goin, Chin-Ming Kao, and Charlita G. Rosal
A permeable barrier system was de-
veloped to control the migration of dis-
solved contaminant plumes in ground
water. The barrier system consisted of
a line of closely spaced wells installed
perpendicular to the contaminant
plume. Each well contained concrete
briquets that released oxygen and ni-
trate at a controlled rate, enhancing
the aerobic biodegradation of dissolved
hydrocarbons in the downgradient aqui-
fer.
Laboratory batch reactor experiments
were conducted with different concretes
to identify mixtures that slowly released
oxygen over an extended time period.
Concretes prepared with urea hydro-
gen peroxide were unacceptable, while
concretes prepared with calcium per-
oxide and a proprietary formulation of
magnesium peroxide had good oxygen-
release rates that slowly declined over
a three- to six-month period.
A full-scale permeable barrier sys-
tem was constructed at a gasoline-spill
site near Leland, NC. Initially, increased
dissolved oxygen and decreased ben-
zene, toluene, ethylbenzene, and xy-
lene isomer (BTEX) concentrations in
the downgradient aquifer indicated that
oxygen released from the remediation
wells was enhancing biodegradation.
Over time, treatment efficiencies de-
clined, suggesting that the barrier sys-
tem was becoming less effective in
releasing oxygen and nutrients to the
aquifer. Field tracer tests and soil analy-
ses performed at the end of the project
indicated that the aquifer in the vicinity
of the remediation wells was being
clogged by precipitation of iron miner-
als.
This Project Summary was developed
by EPA's National Exposure Research
Laboratory, Environmental Sciences Di-
vision, Las Vegas, NV, to announce key
findings of the research project that is
fully documented in a separate report
of the same title Project Report
ordering information at back),
Introduction
The U.S. Environmental Protection
Agency (U.S. EPA) is studying the perfor-
mance of enhanced bioremediation sys-
tems to evaluate the effectiveness of the
technology. The goal of this study was to
design and monitor field performance of a
permeable barrier treatment system that
enhances the biodegradation of contami-
nated ground water passing through the
barrier. This system could serve as an
alternative method for treating contami-
nated ground water and could be less
expensive than the techniques currently
employed. The potential advantages of a
permeable barrier treatment system in-
clude low maintenance requirements, no
above-ground facilities, and in-situ bio-
degradation of contaminants with no re-
quirement for disposal of contaminated
treatment media or ground water.
Laboratory batch experiments were con-
ducted to determine the oxygen-release
characteristics of several solid peroxide-
concrete mixtures. A full-scale barrier sys-
tem was then installed at an underground
storage tank (UST) gasoline-spill site near
Leland, NC. Monitoring wells were installed
upgradient and downgradient of the bar-
rier in the contaminated portion of the
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aquifer. Ground-water samples were moni-
tored and analyzed for dissolved oxygen
(DO), individual BTEX components, pH,
and other relevant parameters to assess
the effectiveness of the barrier system.
According to the system design, high DO
and low BTEX concentrations should be
observed in the remediation wells and the
downgradient monitoring wells. At some
distance downgradient of the barrier, the
BTEX concentration should be degraded
below regulatory levels.
The full-scale permeable barrier system
examined in this study employs concrete
prepared with a proprietary formulation of
magnesium peroxide (MgO2). The concrete
is loaded into permeable filter socks and
placed in a line of fully screened polyvinyl
chloride (PVC) wells (remediation wells)
installed perpendicular to the ground-water
flow direction. When ground water passes
through this line of remediation wells, the
MgO2 in the concrete reacts with water,
producing oxygen. Indigenous microorgan-
isms then use the released oxygen to
aerobically biodegrade the petroleum hy-
drocarbons present in the ground water.
Sodium nitrate (NaNO3) may also be
added to the concrete, further enhancing
biodegradation.
Laboratory Evaluation of Solid
Peroxide Concretes
Three solid peroxide compounds, mag-
nesium peroxide (MgO2), calcium perox-
ide (CaO2), and urea hydrogen peroxide
[CO (NH2)2*H2O2], were examined for their
oxygen-releasing characteristics when in-
corporated into concrete. Close to 100%
of the initial oxygen present in the perox-
ides was recovered from the MgO2 and
CaO2 concretes. In contrast, the oxygen
recovery for the CO(NH2)2-H2O2 concrete
was very low (12%). This result indicates
that a large portion of the available oxy-
gen in the original CO(NH2)2"H2O2was lost
during preparation of this concrete.
Several different types and sizes (con-
crete cylinders or 4-cm-diameter briquets)
of solid peroxide concrete were monitored
to determine the volume of oxygen re-
leased over time. Results of this work
were fitted to a zero-order model of oxy-
gen release versus time. Oxygen release
from the CO(NH2)2-H2O2 concrete was too
rapid to be useful in field application. Fig-
ure 1 shows a comparison of predicted
oxygen-release rates over time for differ-
ent mixtures and sizes of calcium and
magnesium peroxide concrete. The 21%
MgO2 concrete cylinders and briquets had
the slowest and most uniform release rate,
while the 14% CaO2 briquets had the most
rapid release. Where a slow constant re-
30 r
-D- 37% MgO2 briquets
-«- 37% MgO2 cylinder
-O- 21 % MgO2 briquets
21 % MgO2 cylinder
14% CaO2 briquets
200
250
300
350
Days
Figure 1. Best fit estimated lines showing variation in oxygen-release rates with time for magnesium
peroxide and calcium peroxide concrete mixes.
lease of oxygen is required, the 21% MgO2
concrete will be most useful.
Field Monitoring of the
Permeable Barrier System
Ground water upgradient and downgra-
dient of the barrier was monitored over an
18-month period to determine the barrier
system's effectiveness and identify areas
where the design could be improved. The
permeable barrier was constructed to in-
tersect the BTEX plume approximately 27
m downgradient from the former UST lo-
cation and initially consisted of a series of
15-cm~diameter (6-in) PVC wells installed
approximately 1.5 m (5 ft) on center. Each
well was screened from 0 to 3 m (10 ft)
below the water table and was designed
to release a plume of DO to enhance
biodegradation in the downgradient aqui-
fer. Preliminary modeling indicated that
plumes from each well would mix over a
6-to 15-m distance, resulting in complete
biodegradation of the BTEX plume. Field
delineation of the BTEX plume indicated
that the barrier would need to be 40 m
wide and extend approximately 3 m below
the ground-water table. Twenty remedia-
tion wells were initially installed in the
remediation line perpendicular to the plume
at a distance of 1.5 m on center. The nine
wells on the eastern half of the plume did
not receive concrete and were operated
as a control to evaluate the barrier effec-
tiveness. During the course of the project,
two major modifications were attempted
to enhance the barrier system effective-
ness: 1) the use of smaller concrete bri-
quets containing MgO2 and NaNO3, and
2) the installation of additional remedia-
tion wells.
The permeable barrier system exam-
ined in this project was designed to con-
trol the migration of dissolved gasoline
components by enhancing the aerobic bio-
degradation of these compounds in the
aquifer immediately downgradient of the
barrier. Ideally, all contaminants would be
degraded to below regulatory limits before
reaching the most downgradient monito-
ring wells. The permeable barrier exam-
ined in this project did not achieve this
objective. Table 1 lists average concen-
trations of benzene, toluene, ethylbenzene,
and total xylenes over the entire treat-
ment period. While the average concen-
trations of all BTEX components decreased
substantially with distance downgradient
from the barrier, only toluene met water
quality standards in Monitoring Well SLJ5,
25 m downgradient of the barrier system.
Figures 2a and 2b show the average
concentrations of total BTEX and DO in
monitoring wells SU7, SU13, SU14, and
SU5 for the three treatment periods and
for the total project. Total BTEX con-
centrations in wells downgradient of the
barrier are significantly lower than upgra-
dient of the barrier for each treatment
period at the 95% confidence level, indi-
cating that some loss of contaminants is
occurring. The barrier was also effective
at increasing the DO concentration in the
wells immediately downgradient of the bar-
rier.
Field tracer tests conducted at the end
of the project demonstrated that the aver-
age specific discharge in remediation wells
that received oxygen-releasing concrete
were significantly lower than in remedia-
tion wells that did not receive concrete.
The lower specific discharge is attributed
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Table 1. Average Concentrations of BTEX in Monitoring Wells Over the Entire Treatment Period
Well
SU7
SU13
SUM
SU5
NCb
Standards
Distance
from
Barrier3
-10m
+3m
+8m
+25m
Benzene
(mg/L)
2.419
0.757
1.123
0.877
0.001
Toluene
(mg/L)
8.326
2.406
3.469
0.853
1.000
Ethyl-
benzene
(mg/L)
1.391
0.383
0.595
0.272
0.029
Total
Xylenes
(mg/L)
6.060
1.627
2.366
0.745
0.400
a Negative distances are upgradient of the barrier; positive distances are downgradient.
b North Carolina
a)
Period 1
Period 2
Period 3
Average
b)
Period 1
Period 2
Period 3
Average
Figure 2. Mean (a) total BTEX concentrations and (b) dissolved oxygen concentrations in monitoring
wells for individual treatment periods and entire barrier operational period. (Note: SU5 not
included in period 2 graph because only one measurement was taken.)
to the clogging of the aquifer material im-
mediately adjoining the wells by oxidized
iron precipitates.
The probable cause of the poor barrier
performance was inadequate delivery of
DO to the aquifer. Assuming a 3-to-1 mass
ratio of oxygen delivered to BTEX biode-
graded, the delivered oxygen should be
sufficient to biodegrade approximately 10%
of the BTEX entering the barrier. This
problem is only partially due to clogging of
the remediation wells. Assuming no clog-
ging of the remediation wells, the maxi-
mum total BTEX concentration that this
barrier could effectively treat would be 6
mg/L.
Conclusions and
Recommendations
Concrete briquets containing either cal-
cium peroxide or a proprietary formulation
of magnesium peroxide (ORC™) have
desirable oxygen-release characteristics,
including high retention of the original oxy-
gen content and slowly declining oxygen-
release rates. Magnesium peroxide was
used in this study because of its slower
and more constant oxygen-release rate
compared to calcium peroxide. Concrete
prepared with urea hydrogen peroxide was
unacceptable for two reasons: 1) chemi-
cal assays revealed that most of the origi-
nal oxygen was lost during the preparation
of the concrete; and 2) oxygen-release
testing revealed that the oxygen that had
been retained by the concrete during
preparation was released in less than ten
days.
BTEX concentration decreased during
passage through the barrier. These reduc-
tions were statistically significant but were
not sufficient to contain the plume. BTEX
reductions on the control side of the bar-
rier were much greater than on the active
side. However, the cause of this reduction
is unknown. Consequently, it is not pos-
sible to determine whether the decline in
BTEX was due to the barrier system or
due to natural variations in BTEX concen-
tration throughout the site.
Nitrate addition enhanced the aerobic
biodegradation of BTEX as in batch reac-
tor experiments using ground water from
the site. Incorporating sodium nitrate into
the concrete briquets at 0.5 to 0.7% by
weight during the second and third treat-
ment periods of the field experiments did
not cause regulatory levels for nitrate to
be exceeded. This nitrate content should
be increased to further enhance aerobic
biodegradation and for use as an electron
acceptor after the available oxygen is de-
pleted. A small increase in the nitrate con-
tent of the concrete should not result in
any violations of water quality standards
since the maximum nitrate concentration
observed in the monitoring wells down-
gradient of the barrier was 2.9 mg/L NO3-
N, a value well below the current
ground-water standard of 10 mg/L NO3-N.
Significant concentrations of DO were
reaching wells immediately downgradient
of the permeable barrier towards the end
of this project, yet BTEX was not being
biodegraded. The lack of biodegradation
could be due to stratification within the
aquifer, which reduces mixing of oxygen-
ated- and BTEX-contaminated ground wa-
ter. In future work, variations in oxygen
and contaminant concentration with depth
should be examined to evaluate the im-
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portance of stratification on mixing and
subsequent biodegradation.
The oxygen-releasing permeable bar-
rier constructed in this project was not
fully effective in containing the hydrocarbon
plume due to two factors: 1) the high
concentration of BTEX entering the bar-
rier, and 2) the clogging of the barrier
wells by oxidized iron precipitates. The
high total BTEX concentration entering the
barrier resulted in a high demand for oxy-
gen, which was difficult to meet with a
reasonable number of remediation wells.
The high iron concentration entering the
barrier caused clogging of the remedia-
tion wells and reduced oxygen delivery to
the aquifer. Future work on oxygen-re-
leasing permeable barriers should focus
on sites with lower concentrations of bio-
degradable organics and dissolved iron.
The information in this document has
been funded wholly or in part by the United
States Environmental Protection Agency
under Cooperative Agreement Number
CR820468 to the Department of Civil En-
gineering of the North Carolina State Uni-
versity. It has been subjected to the
Agency's peer and administrative review,
and it has been approved for publication
as an EPA document. Mention of trade
names or commercial products does not
constitute endorsement or recommenda-
tion for use.
Robert C. Borden, Russell Todd Gain, and Chih-Ming Kao are with North Carolina
State University, Raleigh, NC 27695. The EPA author, Charlita G. Rosal, (also
the EPA Project Officer) is with the National Exposure Research Laboratory,
Las Vegas, NV 89193-3478.
The complete report, entitled "Enhanced Bioremediation of BTEX Using Immobi-
lized Nutrients: Field Demonstration and Monitoring," (Order No. PB97-
186290;Cost: $21.50, subject to change) will be available only from
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at
Environmental Sciences Division—Las Vegas
National Exposure Research Laboratory
U. S. Environmental Protection Agency
Las Vegas, NV 89193-3478
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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EPA/600/SR-96/145
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