In situ biodegradation of groundwater contaminants

Regulated processes for biodegrading halogenated organic compounds in an aqueous subsurface environment through stimulation of bacteria are provided. The processes provide an electron donor source to anaerobic dehalogenating bacteria as a stimulus for biodegradation of the contaminants. When necessary to limit the biological formation of vinyl halide monomer, the processes provide for a sulfate reducing environment in the region of biological activity by the addition of an inorganic sulfate. One embodiment provides for converting the aqueous subsurface environment from anaerobic dehalogenating conditions to aerobic conditions. Another embodiment provides for converting the aqueous subsurface environment from anaerobic dehalogenating conditions to anaerobic methanogenic conditions followed by conversion to aerobic conditions in order to completely degrade the contaminants to carbon dioxide and water.

BACKGROUND AND SUMMARY OF THE INVENTION 
SUMMARY OF THE INVENTION 
This invention provides an in situ process for the controlled anaerobic 
degradation, of contaminants in subsurface aquifers. More specifically, 
the present invention provides a controlled in situ process which utilizes 
indigenous anaerobic and aerobic bacteria to biodegrade halogenated 
organic compounds in subsurface aquifers. 
BACKGROUND OF THE INVENTION 
Halogenated solvents, typically used for dry cleaning and degreasing, are 
some of the most common and potentially hazardous ground water pollutants. 
Left untreated in the environment, these pollutants can remain unchanged 
for periods of fifty years or more. However, through application of the 
present invention, these pollutants may be biologically degraded to 
innocuous organic compounds or in one embodiment of the invention, to even 
carbon dioxide and water in a rapid, economical and environmentally sound 
manner. 
One commonly used technique for decontaminating an aquifer is the "pump and 
treat" method. As practiced, this method requires a series of extraction 
and injection wells in the contaminated aquifer. Contaminated water is 
drawn from the aquifer through an extraction well, treated to remove or 
degrade the contaminant, then returned to the aquifer through the 
injection wells. This method is expensive, requires extended time periods 
for treatment and in some instances creates additional waste. 
BACKGROUND ART 
Recently attempts have been made to biodegrade these contaminants through 
bacterial action using aerobic bacteria as disclosed in U.S. Pat. Nos. 
4,713,343 and 4,749,491. While aerobic degradation techniques have 
achieved some limited success in degrading the initial contaminants, such 
processes can produce end products as toxic or more toxic than the 
starting compound. Additionally, the application of aerobic processes is 
limited as such bacteria are unable to degrade certain compounds even 
after extended periods of exposure. 
Through activation and control of anaerobic bacteria, alone or in 
conjunction with other bacteria, the present invention overcomes the 
drawbacks of the previous treatment techniques. 
GENERAL DISCLOSURE OF THE INVENTION 
In general, the present invention provides a regulated process for the 
anaerobic biodegradation of halogenated organic compounds in subsurface 
aquifers. The process of the present invention provides an electron donor 
to indigenous reducing bacteria as a stimulant for biodegradation. In 
order to limit the production of vinyl halide monomer, a sulfate reducing 
environment is maintained in the region of biological activity. 
The present invention also provides a regulated process for biodegrading 
halogenated compounds utilizing indigenous anaerobic reducing and 
anaerobic methanogenic bacteria and other anaerobic, aerobic and 
facultative bacteria. Process steps include: determining the initial 
concentration of contaminants and continued monitoring of contaminant 
concentrations; stimulating biological activity by providing an electron 
donor source to the bacteria, and limiting the formation of lesser 
halogenated compounds by maintaining a sulfate reducing environment in the 
region of biological activity through the addition of sulfate upon the 
detection of an increased concentration of vinyl halide monomer. 
After eliminating the multi-halogenated compounds, the system is converted 
to an aerobic environment by supplying a source of oxygen to activate the 
indigenous aerobic bacteria. The aerobic bacteria continue biodegradation 
of the remaining halogenated compounds to give innocuous organic 
compounds, water and presumably a halogen salt as the final end products. 
In one embodiment of the invention, this process includes the addition of 
bacteria to the contaminated aquifer when the requisite bacteria are 
missing or insufficient. 
The present invention further provides a regulated process for 
biodegradation of halogenated compounds to give innocuous, environmentally 
compatible end products such as carbon dioxide, water and presumably a 
halogen salt. The process utilizes anaerobic dehalogenating bacteria and 
anaerobic methanogenic bacteria to biodegrade the halogenated compounds to 
unsubstituted end products. Following reduction of the halogenated 
compounds to unsubstituted compounds, further degradation to carbon 
dioxide and water is achieved by supplying a source of oxygen to the 
subsurface environment to activate aerobic bacteria which may be 
indigenous or added to the system, if necessary or desired. 
Thus, the present invention provides a regulated process for biodegrading 
halogenated organic compounds in an aqueous subsurface environment through 
stimulation of bacteria. This process comprises the steps of: (a) 
determining the initial concentration of halogenated contaminants 
including vinyl halide monomer in the aqueous subsurface environment; (b) 
providing an electron donor to dehalogenating bacteria as a stimulus for 
anaerobic biodegradation of said compounds; (c) establishing a sulfate 
reducing environment in the region of biological activity by the addition 
of an inorganic sulfate; (d) limiting the biological formation of vinyl 
halide monomer by maintaining a sulfate reducing environment in the region 
of biological activity; (e) monitoring said aqueous subsurface environment 
for increased concentration of di- and mono-halogenated organic compounds; 
(f) upon detection of said di-halogenated organic compounds converting 
said aqueous subsurface environment from anaerobic dehalogenating 
conditions to anaerobic methanogenic conditions; (g) allowing methanogenic 
bacteria to further biodegrade said di- and mono-halogenated compounds to 
produce unsubstituted compounds; (h) converting said biological 
environment from anaerobic to aerobic by supplying a source of oxygen to 
activate aerobic bacteria; and, (i) allowing said bacteria to continue 
aerobic degradation of said halogenated organic compounds to successive 
organic degradation products and finally to produce carbon dioxide and 
water. 
As underground geology results in a continuous flow of contaminated water 
into the biodegradation system contamination levels may fluctuate. In 
order to compensate for the changing contaminant types and levels, the 
system is capable of cycling between anaerobic and aerobic biodegradation 
as determined by the contaminants present. Further, the degradation 
process will continue as the subsurface water flows away from the 
recirculation wells.

DETAILED DESCRIPTION OF THE INVENTION 
The processes of the present invention may be used to decontaminate 
subsurface aquifers contaminated with halogenated organic compounds. 
According to one embodiment of the current invention, a process is 
provided for anaerobically biodegrading halogenated compounds in an 
aqueous subsurface environment. 
The biodegradation process is initiated by the addition of an electron 
donor source to the aquifer in order to stimulate biological activity. 
Suitable electron donor sources include any inorganic or organic compound 
capable of stimulating the reduction of the halogenated contaminants, such 
as reducing sugars or fertilizers. For example, sodium benzoate has been 
used successfully to stimulate the biodegradation of tetrachloroethylene. 
As the list of electron donors is extensive, the invention is not limited 
to the examples listed herein which are exemplary and not exhaustive of 
the suitable electron donors. Rather, the choice of an electron donor 
source may be made based upon the materials obtainable at the treatment 
site. 
In conjunction with the stimulation of the biodegradation process the 
current invention provides a mechanism for controlling the resultant end 
products. This limiting feature of the current invention is achieved by 
maintaining a sulfate reducing environment in the region of biological 
activity. As sulfate is a better electron acceptor and is preferentially 
reduced over the di-halo compound, addition of sulfate limits the 
biodegradation end products to the di-halogenated compounds. The sulfate 
may be introduced to the system as a salt such as calcium sulfate, 
potassium sulfate or any other compound which will provide a free sulfate 
ion in aqueous solution. 
The preceding process may be better understood with reference to the 
accompanying drawings and by way of the following examples. Prior to 
treating a contaminated aquifer in accordance with the procedure of the 
current invention, the aquifer must be mapped in order to determine the 
subsurface geological characteristics and the direction of ground water 
flow. The mapping may be performed using techniques well known in the art. 
After the subsurface characteristics have been determined, a series of 
feeder and sampler wells may be drilled in order to allow treatment and 
testing of the aquifer. FIG. 1 depicts one possible surface arrangement of 
feeder 15,16,17 and sampler wells 18,19,20,21 and 22. This arrangement was 
used to generate the data presented in Example 1 below. 
As indicated in FIGS. 2-5 the initial concentrations of tetrachloroethylene 
(PCE), trichloroethylene (TCE), dichloroethylene (DCE), and vinyl chloride 
(VC) were determined and monitored for each sampler well prior to 
initiating biodegradation. The concentration of sulfate was also monitored 
during the biodegradation process and is indicated in FIG. 6 for each 
sampler well. 
In FIGS. 2-6, the vertical axis represents micrograms (.mu.g) per liter (1) 
of measured material for the contaminants and micrograms (.mu.g) per 
milliliter for the sulfate. As indicated on the horizontal axis, 
contaminant concentrations were determined for the period of January, 1990 
through mid-November, 1990 and sulfate concentrations were determine 
monitored from August through October of 1990. FIG. 2 represents the 
concentration of tetrachlorethylene (PCE). FIG. 3 represents the 
concentration of trichloroethylene (TCE) . FIG. 4 represents the 
concentration of dichloroothylone (DCE). FIG. 5 represents the 
concentration of vinyl chloride (VC) and FIG. 6 represents the 
concentration of sulfate. 
Bacteria known to work within the current processes include dehalogenating 
bacterium strain DCB-1, Desulfobactariym species and Metthanobacterium 
species, identified in Applied Environ. Microbiol., Vol 53: 2671-2674, and 
Methanosarcina species, identified in Applied Environmental Microbiology, 
54:2976-2980. However, other species are known to function and the 
invention is not to be limited by the examples provided herein. 
As will be further explained in the following example, the present 
invention provides a process for the regulated anaerobic biodegradation of 
halogenated organic compounds in an aqueous subsurface environment. The 
process stimulates biodegradation by providing an electron donor source 
for the dehalogenating bacteria. Further, the process limits the formation 
of vinyl halide monomer by maintaining a sulfate reducing environment in 
the region of biological activity. 
The examples contained herein are provided to illustrate the present 
invention and not to limit it. The applicant does not wish to be limited 
by the theory presented within the examples; rather, the true scope of the 
invention should be determine based on the attached claims. All parts and 
percentages within the examples are by weight unless otherwise specified. 
EXAMPLE 1 
An aquifer known to be contaminated with chlorinated solvents was 
geologically mapped. After determination of the subsurface characteristics 
and the direction of flow of the ground water, a series of feeder wells 
(15,16,17) and sampler wells (18,19,20,21,22) were drilled as depicted in 
FIG. 1. Following the drilling of the wells, the initial concentration of 
the subsurface contaminants was determined by GCIMS analysis according to 
standard EPA methods 8240 and ethylene 8015. As shown by FIG. 2, the 
concentration of PCE prior to the initiation of biodegradation ranged from 
about 1100 ug/L to about 2300 ug/L. FIG. 3 shows the initial concentration 
of TCE to range from about 450 to about 600 ug/L. FIG. 4 shows the initial 
concentration of DCE to range from about 340 ug/L to about 720 ug/L and 
FIG. 5 indicates VC to be virtually undetectable. 
After determining the levels of contaminants, biodegradation was initiated 
and maintained by the addition of sodium benzoate as an electron source, 
through the feeder wells. Sodium benzoate was chosen in this instance due 
to its ready availability. During addition of the sodium benzoate, the 
aquifer was constantly monitored by spectrophotometric methods in order to 
insure a sulfate reducing condition in the region of biological activity 
and when necessary, additional sulfate was added in the form of calcium 
sulfate. 
PCE degraded rapidly after stimulation of the biodegradation process. FIG. 
2 indicates that three sampler wells showed nearly undetectable levels of 
PCE within less than two months of the stimulation of the process and the 
remaining wells showed less than 400 ug/L. Likewise FIG. 3 shows a 
corresponding decrease in TCE upon initiation of biodegradation with final 
concentrations ranging from about 56 to about 280 ug/L. As further 
evidence of biological activity, FIG. 4 indicates the expected increase in 
DCE concentration due to the maintenance of sulfate reducing conditions in 
the aquifer. 
While the applicant does not wish to be limited by the theory presented 
herein, the applicant believes that the dehalogenating bacteria reduce the 
PCE and TCE leading to the resulting end product of DCE. It is also 
believed that absent sulfate reducing conditions the DCE would be further 
reduced to VC; however, as dehalogenating bacteria will preferentially 
utilize sulfate as an electron acceptor, substantially all sulfate present 
in the aquifer must be reduced to sulfide before any DCE will be reduced 
to VC. The ability of sulfate to limit if not wholly preclude the 
formation of VC is demonstrated in FIGS. 4, 5 and 6. 
As noted above, the maintenance of a sulfate reducing condition in the 
aquifer resulted in an increase in the concentration of DCE, as 
illustrated in FIG. 4. During the period of increasing concentration of 
DCE, the concentration of VC remained at undetectable levels, as shown in 
FIG. 5, and the concentration of sulfate remained relatively high as shown 
in FIG. 6. However, as the concentration of sulfate dropped as indicated 
in FIG. 6, a corresponding decrease in concentration of DCE is detected as 
the bacteria began to utilize the DCE as an electron acceptor; thereby, 
reducing the DCE to VC. The reduction of DCE leads to the corresponding 
increase in VC concentration shown in FIG. 5, with VC levels reaching as 
high as 500 ug/L. 
In certain aquifers it may be desirable to halt the biodegradation process 
upon the elimination of a majority of PCE and TCE, as in the above 
example. However, at this point the system may be converted to aerobic 
degradation by supplying a source of oxygen to the aquifer, thereby 
stimulating the degradation of DCE by aerobic bacteria. The resulting end 
products of this biodegradation process would be innocuous organic 
compounds and water. 
FURTHER EMBODIMENTS 
In one further embodiment, the present invention utilizes indigenous 
anaerobic and aerobic bacteria. According to this embodiment, the initial 
concentration of the contaminants is first determined and is monitored 
during the degradation process. A suitable electron donor source is 
provided to the indigenous anaerobic bacteria as a stimulus for biological 
activity. Additionally, a sulfate reducing environment is maintained 
through the addition of a sulfate salt in order to preclude the formation 
of vinyl halide. Additional sulfate may be added during the degradation 
process if vinyl halide is detected. 
Once testing determines that the majority of tetra- and tri- halide 
compounds have been degraded to the di-chloro compound, the biological 
environment is converted from anaerobic to aerobic by supplying an oxygen 
source. Following this conversion, the aerobic bacteria biodegrades the 
halogenated organic compounds to produce innocuous organic compounds and 
water. 
In another embodiment, the process of the present invention utilizes 
indigenous anaerobic dehalogenating bacteria, anaerobic methanogenic 
bacteria and other indigenous bacteria to completely degrade the 
halogenated contaminants to carbon dioxide and water. According to this 
process, the initial concentration of contaminants including vinyl halide 
monomer is first determined followed by addition of an electron donor 
source to the indigenous anaerobic dehalogenating bacteria as a stimulus 
for biodegradation. Additionally, a sulfate reducing environment is 
maintained in order to limit the production of vinyl halide monomer. 
As biodegradation progresses, the system is monitored in order to detect 
increased concentrations of di-halo compounds and vinyl halide. Upon 
detection of an increased concentration of di-halo compounds, the system 
is converted from anaerobic dehalogenating conditions to anaerobic 
methanogenic conditions. This change is brought about by ceasing the 
addition of sulfate and permitting aquifer sulfate levels to drop as the 
reaction continues. 
Upon achieving methanogenic conditions, the indigenous methanogenic 
anaerobic bacteria begin biodegrading the di-halo compounds. The 
methanogens or other bacteria are allowed to continue the degradation 
process until the halogenated compounds are converted to unsubstituted 
compounds. The process upon reaching this state may be halted or allowed 
to continue aerobically. 
Once the conversion to unsubstituted compounds is complete, the system may 
be converted from anaerobic to aerobic by the addition of oxygen. After 
oxygen is added to the system, indigenous aerobic bacteria become active 
and continue the degradation process. The aerobic bacteria continually 
breakdown the organic compounds until the end products of carbon dioxide 
and water are produced. 
EXAMPLE 2 
An aquifer known to be contaminated with chlorinated solvents is 
geologically mapped. After determining the subsurface characteristics and 
the direction of ground water flow, a series of feeder and sampler wells 
are drilled. Following the drilling of the wells, the initial 
concentration of the subsurface contaminants is determined. 
After determining the levels of contaminants, biodegradation is initiated 
and maintained by the addition of a suitable electron donor source, 
through the feeder wells. During the biodegradation process, the aquifer 
is constantly monitored in order to insure a sulfate reducing condition in 
the region of biological activity and when necessary, additional sulfate 
is added. After stimulation of the biodegradation process, PCE and TCE 
rapidly degrade, producing an increased concentration of DCE due to the 
maintenance of a sulfate reducing environment. 
As noted above, the maintenance of a sulfate reducing condition in the 
aquifer will increase the DCE concentration. During the period of 
increasing concentration of DCE, the concentration of VC is preferably not 
allowed to increase and the concentration of sulfate should be maintained 
at a level to preclude biodegradation of the DCE. 
Upon detecting the increased levels of DCE due to the degradation of the 
PCE and TCE, the system is converted to anaerobic methanogenic conditions 
by allowing the sulfate concentration to drop. After the sulfate has been 
eliminated, the methanogenic bacteria utilize the DCE and VC as electron 
acceptors, thereby reducing both compounds to unsubstituted compounds. 
After the methanogenic bacteria have reduced the halogenated compounds to 
unsubstituted compounds, the system can be halted, continued or is 
converted to aerobic degradation by supplying an oxygen source to the 
aquifer. Upon addition of the oxygen source other indigenous bacteria 
begin to biodegrade the reduced compounds to successive organic 
degradation products finally producing carbon dioxide and water. 
In addition to the above embodiments discussed in Examples 1 and 2, it is 
contemplated that certain aquifers may lack the requisite bacteria to 
carry out this process. In those instances,, the requisite bacteria may be 
added to the system as the initial aquifer treatment step of the process. 
Further, other embodiments will be apparent to those skilled in the art 
from a consideration of this specification or practice of the invention 
disclosed herein. It is intended that the specification and example be 
considered as only exemplary, with the true scope and spirit of the 
invention being indicated by the following claims.