Long-Term Bioremediation of a Chlorinated Solvents Plume in a Fractured
Limestone Aquifer using a Dechlorinating Bioreactor
Charlotte E. Riis M.S., Anders G. Christensen M.S., and Henrik H. Nielsen B.A., NIRAS;
Marc van Bemmel M.S., Bioclear; Henrik Østergaard M.S., The Capital Region of
Denmark
Abstract
Enhanced Reductive Dechlorination (ERD) has been implemented at a former dry
cleaning facility in order to create a horizontal biological barrier beneath a contaminated
clay layer aimed at preventing the contamination from spreading in a down gradient
direction. ERD is implemented as a two-step strategy: First, an active phase aimed at
cleaning up the existing plume in the upper part of the limestone aquifer by substrate
amendment and bioaugmentation using a dechlorinating bioreactor. The active phase has
been operated for 11 weeks from September to November 2006. During this period no
clogging of the infiltration wells has occurred. Substrate has been evenly distributed
throughout the treatment area. The bioreactor has continuously generated dechlorinating
biomass, allowing a fast and efficient distribution of bacteria in the aquifer. The redox
conditions have changed from sulphate reducing to highly methanogenic conditions and
the degree of dechlorination has increased during the active phase. An active biological
treatment zone with high concentrations of Dehalococcoides Ethenogenes (Dhc) and
substrate has thus been created in the targeted area beneath the source area.
The active phase will be followed by a long-term semi-passive phase aimed at
maintaining the dechlorinating activity in the bioreactive zone. This is done through
periodic substrate injections. The purpose of the bioreactive zone is to degrade the
contaminants infiltrating to the limestone aquifer from the overlying source zone, thereby
constituting a horizontal biological barrier beneath the source area. The anticipated
frequency of substrate amendments in the passive phase is 1-1½ years. On-going
monitoring will determine when substrate reinjection will be necessary. Data from the
first seven months of passive phase monitoring show that the dechlorinating activity in the
treatment area is still high. The TOC levels and methane concentrations have decreased
markedly after the conclusion of the active phase, indicating a possible flushing of the
treatment area in the highly fractured part of the limestone. The results suggests that the
substrate has diffused into the limestone matrix and that the Dhc are capable of sticking to
the aquifer material and are not immediately being transported away from the treatment
area by the ground water flushing through. No sign of recontamination has been observed
within the first seven months of monitoring and the effect of the substrate seems to
persist. This project demonstrates an alternative method for distributing substrate and
biomass in a fractured aquifer and to achieve high concentrations of dechlorinating
bacteria in the aquifer within a limited timeframe.
Introduction
At many contaminated sites, the source area is partially or completely inaccessible
due to its location under buildings, at great depth or near sensitive installations. In such
81 to 91
cases, complete source removal is not achievable and a long term remediation effort must
be initiated instead. For several years, pump & treat has been the method of choice at
most sites fitting this description. Based on the recent progress in enhanced reductive
dechlorination (ERD), ERD is thought to be a viable – and a more cost-effective alternative to pump & treat at such sites. This project demonstrates an alternative method
for distributing substrate and biomass in a fractured aquifer and to achieve high
concentrations of dechlorinating bacteria in the aquifer within a limited timeframe.
Field site description
The site situated in a residential area in Central Copenhagen is a former dry cleaning
facility. The geology at the site consists of 10 meters of clayey till with occasional sand
stringers underlain by fractured limestone. The upper 4 meters of the limestone is highly
fractured. Results from a flowlog in a 20 m deep well show that 90 % of the flow
originates from the upper 4 meters of the limestone, of which 55 % originates from the
upper one meter of the limestone. There is thus limited flow in the limestone aquifer at
depths below 14 m below surface level (bsl). The limestone constitutes the primary
aquifer. The site is located within the catchment area of a water supply.
The dry cleaning activities have caused a significant PCE-contamination in the clayey
till. Due to limited access at the site, only a small part of the contamination has been
excavated. The contamination has spread to the limestone aquifer. A recent, extensive
groundwater extraction at a neighbouring property has caused a large part of the
contamination in the limestone to be removed, resulting in low concentrations of
chlorinated daughter products in the limestone aquifer. Recontamination of the aquifer is
expected to occur within a reasonable timeframe due to the large PCE mass in the
overlying clayey till. The source area is not accessible for clean-up due to the poor access
conditions at the site. Therefore remedial activities are limited to long term cut-off or
containment methods.
Enhanced reductive dechlorination strategy
ERD is implemented at the site in order to create a horizontal biological barrier
beneath the contaminated clay layer aimed at preventing the contamination from
spreading in a down gradient direction. ERD is implemented as a two-step strategy: First,
an active phase aimed at cleaning up the existing plume in the upper part of the limestone
aquifer by substrate amendment and bioaugmentation and thus creating an in-situ
bioreactive zone beneath the source area. The active phase will be followed by a longterm semi-passive phase aimed at maintaining the dechlorinating activity in the
bioreactive zone. This is done through periodic substrate injections. The purpose of the
bioreactive zone is to degrade the contaminants infiltrating to the limestone aquifer from
the overlying source zone, thereby constituting a horizontal biological barrier beneath the
source area.
Materials and methods
Active phase
A system of 2 extraction wells and 5 infiltration wells has been installed at the site
interconnected with underground piping. The extraction wells (KB102, KB104) are
placed in the central part of the contaminated area, whereas the infiltration wells (KB101,
KB105-KB108) are placed in the periphery of the contaminated area, see Figure 1. A total
flow of 2 m3/h has been extracted and reinjected. Substrate (sodium acetate and lactic
acid) and dechlorinating biomass have been delivered to the subsurface by recirculating
groundwater through an on-site dechlorinating bioreactor followed by substrate addition
before reinjecting the groundwater to the aquifer. One fourth of the extracted volume of
groundwater has been led through the bioreactor, while the rest has been passed by. All
the extracted water has been mixed and substrate has been added before reinjection. The
active phase has been operated for 11 weeks from September to November 2006.
Substrate and nutrients have been added to the infiltrated water as daily pulses, separated
in time in order to prevent well clogging. Substrate has been added to the infiltrated water
in pulses corresponding to a concentration of 100 mg TOC/L during the first 9 weeks of
operation. In the last two weeks of operation the concentration input has been increased to
600 mg TOC/L in order to achieve higher concentrations of substrate in the aquifer and
thus optimize the lifetime of the substrate in the bioreactive zone. In this way the last part
of the passive phase was used for optimizing the lifetime of the semi-passive phase.
Monitoring of the development of the bioreactive zone has been performed by
repeated water sampling from four monitoring wells (KB2-KB5) placed in the treatment
area. Previously performed borehole logging and a pumping test indicate that the
limestone in the top of the aquifer is highly fractured, almost crushed. In a larger
perspective, this part of the aquifer could be considered as a single-porous media, from a
flow point-of-view. The monitoring wells are placed at different positions relative to the
flow lines between the injection and extraction wells. The monitoring wells are thus
considered representative of the flow field in the treatment area.
A total of five monitoring campaigns have been performed. The water samples have
been analyzed for chlorinated ethenes and daughter products (including ethene and
ethane), total organic carbon (TOC), sulphate, methane and Dehalococcoides
Ethenogenes. Furthermore measurements of on-line redox parameters (oxygen, ORP,
conductivity, pH, and temperature) in the extracted water and the infiltrating water as well
as measurements of water levels in the infiltration wells have been performed
continuously throughout the active phase. The purpose of this was to check the
functionality of the bioreactor and to check whether clogging occurred in the infiltration
wells.
Figure 1. Site plan with sampling locations and recirculation system
Passive phase
Following the active phase of ERD, quarterly monitoring will demonstrate the
longevity of the substrate and the dechlorinating activity in the treatment zone. When the
substrate is depleted, recontamination will occur and substrate reinjection will be needed
in order to reactivate the reductive dechlorination in the treatment zone. The substrate
reinjection will be performed with the existing extraction/reinfiltration system by
recirculation for two weeks. Because of the well configuration chosen for this system,
recontamination can be allowed within the treatment zone. Based on the results of the
active phase and additional ground water modelling, the anticipated frequency of
substrate amendments in the passive phase is 1-1½ years.
Monitoring of the development of the dechlorinating activity and contaminant
composition has been performed on a quarterly basis following the conclusion of the
active phase. An additional monitoring well, KB6, has been installed down gradient of the
treatment area. Monitoring has been performed at three, seven and nine months after the
end of the active phase. The nine months monitoring data is not yet available. The
monitoring program of the active phase has been applied in the passive phase; however
there has been a change of laboratory and thus analysis method between the two phases.
Results and discussion
Substrate and biomass distribution
The substrate concentrations in the monitoring wells are shown in Figure 2. The graph
shows, that the substrate has been distributed uniformly within the treatment area after 2
weeks of operation, with a concentration level of 120-150 mg TOC/L. Subsequently, the
concentrations continue to rise to approximately 200 mg TOC/L, showing that a build-up
of substrate is taking place in the aquifer because the substrate amendment is faster than
the substrate utilization. In the last monitoring campaign, right before the recirculation
system was shut down, the concentrations were measured at 1200-1500 mg TOC/L in 3 of
4 wells, due to the increased substrate addition. The timeframe for substrate distribution
corresponds well with the timeframe of 15 days for an even distribution, predicted with
the groundwater model (Visual ModflowPro/RT3D).
1600
End of active phase
1400
KB2
KB3
TOC (mg/L)
1200
KB4
1000
KB5
800
600
400
200
0
0
10
20
30
40
50
Weeks after start of active phase
Figure 2. Substrate concentration in the four monitoring wells as a function of time
Data from passive phase show that the substrate concentrations decrease markedly to
10-25 mg/L within the first three months after the end of the passive phase. These
concentrations are still at or above the critical concentration for sustaining the
dechlorinating activity (10 mg/L). At seven months after the active phase, the substrate
concentrations have decreased to background level. A possible explanation for the rapid
decrease in TOC levels, other than biodegradation of the substrate, could be that the
substrate has been flushed out of the system in the highly fractured zone in the top of the
limestone and/or that part of the substrate has diffused into the limestone matrix during
the active phase.
Dehalococcoides ethenogenes (cells/ml)
The concentration of Dehalococcoides Ethenogenes (Dhc) is also increasing during
the active phase – from an initial concentration of 2-7·102 Dhc/mL to a concentration of
2·105 Dhc/mL after 6 and 11 weeks of operation, respectively, see Figure 3. The
dechlorinating biomass was added to the aquifer along with the substrate. It is thus
assumed that the Dhc was evenly distributed throughout the treatment area after two
weeks of operation as was the substrate. Given that a Dhc concentration of 104 Dhc/mL is
considered to be indicative of a relatively high dechlorinating activity, the achieved
number of bacteria in the aquifer is deemed more than sufficient for sustaining
dechlorinating activity in the treatment area. The distribution of Dhc in the aquifer was
also shown by the results a high number of Dhc (2.8·105 Dhc/mL) in the extraction water
after 4 weeks of operation.
1,E+07
End of active phase
KB2
1,E+06
KB3
KB4
1,E+05
KB5
1,E+04
1,E+03
1,E+02
0
10
20
30
40
50
Weeks after start of active phase
Figure 3. Dehalococcoides Ethenogenes in the monitoring wells and in the infiltration
water as a function of time
After the active phase, the concentration of Dhc in the monitoring wells initially drop
by one order of magnitude from 105 Dhc/mL to 104 Dhc/mL, and then remain at that level
during the following four months of passive phase monitoring. Data from the first passive
phase monitoring campaign show that practically all of the Dhc present contain the
vinylchloride reductase gene. This concentration level shows that dechlorinating activity
is still taking place in the treatment area, even though the substrate concentrations have
dropped considerably. This indicates that there still is a source of TOC in the aquifer,
most likely due to substrate diffusing out of the limestone matrix. It also indicates that the
dechlorinating bacteria are able to stick to the aquifer material and are not immediately
being flushed away from the treatment zone.
The substrate addition is also reflected by the pH, see Figure 4, due to the low pH
(around 3-4) of the added substrate. The pH is decreasing throughout the active phase,
from an initial level of 7.1 to 6.2 in the last monitoring campaign. Based on the
continuous measurements on the extracted water, the pH initially drops to 6.5 during the
first 3 weeks of operation, followed by a stable period until the 10th week of operation,
where the pH drops further to about 6.2, caused by a six-fold increase in the added
substrate concentration. The conductivity (data not shown) also reflects the substrate
distribution in the aquifer due to the large content of ions in the substrate. The
conductivity is thus seen to increase concurrently with the substrate concentrations.
Redox conditions
The geochemistry in the treatment area has changed to strongly reducing conditions
due to the delivery of substrate and biomass to the aquifer. A good correspondence is
observed between the alterations in the redox sensitive parameters and the changes in the
substrate concentration in the treatment zone. The sulphate concentrations were initially
low (11-19 mg/L) and drop to below detection limit (8 mg/L) within the first two weeks,
see Figure 5. The methane concentrations increase from the background level of 0.4-1
mg/L to 10-13 mg/L after two weeks of operation and then continue to increase to a level
of 20 mg/l after 6 weeks of operation, remaining at this level until the end of the active
phase, see Figure 5. Also the oxidation-reduction potential (ORP) reflects the change in
the redox conditions in the treatment zone. As shown in Figure 4, ORP decreases within
the first two weeks of the active phase and then stabilizes at a level of -250 to -350 mV in
the remainder of the active phase. The generated methanogenic conditions are optimal for
reductive dechlorination.
During the first three months of the passive phase the methane concentrations drop
markedly from 20 mg/L to around 1 mg/L. At seven months after the active phase the
methane concentrations have dropped to background level. The sulphate concentrations
remain at a very low level during the first seven months of the passive phase,
considerably lower than the background level. ORP increases slightly to a level of -200
mV, which is still indicative of strongly reducing conditions. The pH increases to the
background level of 7, which corresponds well with the decrease in substrate
concentrations. Thus, the redox conditions continue to be strictly anaerobic in the
treatment area during the passive phase monitoring.
The drop in methane concentrations is remarkable, since methane is usually very
stable under anaerobic conditions. Given the rapid decrease in substrate concentrations, it
is possible that the both substrate and methane have been flushed out of the treatment area
due to high flows in the highly fractured part of the limestone. However, both the Dhc and
the sulphate concentrations remain constant at levels indicating high dechlorinating
activity. If the treatment area is indeed being flushed through rapidly, Dhc is likely to be
sticking to the aquifer material and substrate is likely to have diffused into the limestone
matrix, from where it constitutes a durable source of TOC.
0
10
End of active phase
-50
8
-150
pH
ORP (mV)
-100
-200
6
-250
-300
4
-350
-400
2
0
5
10
15
20
25
30
35
40
45
50
Weeks after start of active phase
KB2 (mV)
KB2 (pH)
KB3 (mV)
KB3 (pH)
KB4 (mV)
KB4 (pH)
KB5 (mV)
KB5 (pH)
Figure 4. ORP and pH in the four monitoring wells as a function of time
Sulphate & methane (mg/L)
30
End of active phase
25
20
15
10
5
0
0
10
20
30
40
50
Weeks after start of active phase
KB2 Sulphate
KB3 Sulphate
KB4 Sulphate
KB5 Sulphate
KB2 Methane
KB3 Methane
KB4 Methane
KB5 Methane
Figure 5. Sulphate and methane concentrations in the four monitoring wells as a function
of time
Reductive dechlorination
As mentioned previously, the initial concentrations of chlorinated ethenes and
degradation products (VOCl) are low. However, the composition of the contaminants is
seen to change throughout the active phase in the way that the molar sum of chlorinated
compounds decreases (see figure 6) and the molar sum of the non-chlorinated compounds
(ethene and ethane) increases.
During the active phase, the degree of dechlorination has increased from 80-85 % to
around 95 %. The degree of dechlorination was initially fairly high, which reflects the fact
that the contamination mainly consisted of vinylchloride, with small amounts of cisDCE
and ethane. The initial contaminant composition, before initiating the active phase of
ERD, is thought to be due to degradation in the clayey till, rather than in the limestone.
The degree of dechlorination is thus increasing concurrently with substrate and biomass
being delivered to the treatment zone. During recirculation, degradation to some extent
occurs in the bioreactor, but since only a fourth of the extracted water was led through the
bioreactor and the rest was passed by, the increase in the degree of dechlorination is
thought to reflect an increase in the degradation activity in the treatment zone throughout
the active phase.
0,20
90%
0,15
End of active phase
80%
0,10
70%
0,05
60%
Total chlorinated ethenes
(µM)
Degree of dechlorination
100%
0,00
0
5
10
15
20
25
30
35
40
45
50
Weeks after start of active phase
KB2 %
KB2 µM
KB3 (µM)
KB3 µM
KB4 (µM)
KB4 µM
KB5 (µM)
KB5 µM
Figure 6. Degree of dechlorination in the four monitoring wells as a function of time
Following the active phase, the concentrations of chlorinated ethenes continue to
decrease to a level of 0.01 µM. Only in well KB3 a slight increase in vinylchloride is
observed at seven months after the active phase, which could be indicative of initial
recontamination. Other than that, the data reveal no sign of recontamination of the
treatment area.
Conclusions
No clogging of the infiltration wells has occurred during the 11 weeks of operation of
the recirculation system in the active phase. Substrate has been evenly distributed
throughout the treatment area. The bioreactor has continuously generated dechlorinating
biomass, allowing a fast and efficient distribution of bacteria in the aquifer. The redox
conditions have changed from sulphate reducing to highly methanogenic conditions and
the degree of dechlorination has increased during the active phase. An active biological
treatment zone with high concentrations of Dehalococcoides Ethenogenes and substrate
has thus been created in the targeted area beneath the source area.
Maintenance of the reactive zone
Data from the first seven months of passive phase monitoring show that the
dechlorinating activity in the treatment area is still high with decreasing concentrations of
chlorinated ethenes, high numbers of Dehalococcoides Ethenogenes (Dhc) and low
sulphate concentrations. The TOC levels and methane concentrations have decreased
markedly after the conclusion of the active phase, indicating a possible flushing of the
treatment area in the highly fractured part of the limestone. With a high degree of flushing
of the treated volume of the aquifer, the sustained levels of sulphate and Dhc suggests that
the substrate has diffused into the limestone matrix and that the Dhc are capable of
sticking to the aquifer material and are not being transported away from the treatment area
by the ground water flushing through. The data from the passive phase stresses the need
for a more detailed study of the interaction between solutes in fractures and matrix and the
implications for substrate longevity and clean-up times in double-porous aquifers.
No sign of recontamination has been observed within the seven months monitoring
period and the effect of the substrate seems to persist. The current data set does not
provide reason for initiating substrate reinjection at this point in time. Continued
monitoring will determine when substrate reinjection will be necessary.
Acknowledgments
Funding for this study was provided by the Municipality of Copenhagen and the
Capital Region of Denmark.
References
Miljøkontrollen, Københavns Kommune, 2007. “Gl. Kongevej 31-33, Kbh. V.
Enhanced Reductive Dechlorination. Active Phase”. NIRAS A/S. February 2007. (In
Danish)
Biographical sketches
Charlotte E. Riis, NIRAS, Sortemosevej 2, 3450 Allerod, Denmark. Phone +45
48104200, email: [email protected]
Riis received her M.S. in 1999 from the Technical University of Denmark. She has since
1999 worked as a consultant and project manager at NIRAS, Denmark. Her interests
include characterization of contaminants in soil and ground water and bioremediation, in
particular natural attenuation and enhanced reductive dechlorination.
Anders G. Christensen, NIRAS, Sortemosevej 2, 3450 Allerod, Denmark. Phone +45
48104200, email: [email protected]
Christensen received his M.S. in 1988 from the Technical University of Copenhagen. He
has since 1994 been working as a consultant for NIRAS, Denmark. His interests include
in-situ remediation and flow and transport in the unsaturated zone.
Henrik H. Nielsen, NIRAS, Sortemosevej 2, 3450 Allerod, Denmark. Phone +45
48104200, email: [email protected]
Nielsen received his B.S. in 1996 from the Technical University of Denmark and his
Graduate Diploma in Business Administration (Management and Organization) in 2002.
He has since 1999 worked as a consultant and project manager at NIRAS, Denmark. His
interests include in-situ remediation and vapour intrusion control.
Marc van Bemmel, Bioclear, Rozenburglaan 13, 9727 DL Groningen, the Netherlands,
phone +31 505718455, email: [email protected]
Van Bemmel received his M.S. at the Wageningen Agricultural University in 1997 and
has worked as an in situ bioremediation specialist at Bioclear, the Netherlands since that
time. He has been project leader in research, design and operation of many in situ
bioremediation projects, especially for chlorinated solvents.
Henrik Østergaard, The Capital Region of Denmark, Kongens Vaenge 2, 3400 Hillerod,
Denmark. Phone +45 48205329, email: [email protected]
Østergaard received his M.S. in 1985 from the University of Copenhagen. He worked
from 1985 to 1986 at Velux designing test for investigating the effect of wood preserving
chemicals. From 1986 to 1986 he worked at the Municipality of Copenhagen and has
since 1988 worked at the Capital Region of Denmark (former County of Frederiksborg).
His interests include investigations and remediation of soil and ground water
contamination as well as operation of remediation systems.