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Responses on each topic area are compiled and are also available in the rulemaking docket.
Docket ID no. EPA-HQ-OW-2008-0390 1

Dan Evans

Praxair Services, Inc.

3755 N. Business Center Dr.

Tucson, AZ 85705

(520) 545-0216

dan_evans@praxair.com

VIA CERTIFIED MAIL

November 17, 2008

Water Docket

Environmental Protection Agency

Mailcode: 2822T

1200 Pennsylvania Avenue, NW

Washington D.C. 20460

RE: Docket ID No. EPA-HQ-OW-2008-0390: Federal Requirements under the 

Underground Injection Control (“UIC”) Program for Carbon Dioxide (“CO2”) 

Geologic Sequestration (“GS”) Wells

Praxair Services Inc. (“Praxair Services”) thanks the U.S. Environmental Protection 

Agency (“EPA”) for the opportunity to submit the following comments relating to 

the UIC program for CO2 GS wells. Praxair Services is a wholly-owned subsidiary of 

Praxair, Inc. (“Praxair”), a Fortune 300 company that supplies atmospheric, process 

and specialty gases, high-performance coatings, and related services and technologies 

to a diverse global customer base. Industries served include: aerospace, food and 

beverages, chemicals, refining, healthcare, semiconductors, ore and gas production, 

primary metals and metal fabrication, as well as other areas of general industry. 

Praxair’s sales in 2007 were $9.4B with 28,000 employees and operations in more than 

40 countries. 

While Praxair’s primary products include oxygen, nitrogen, hydrogen, carbon dioxide, 

helium, and argon, its wholly-owned subsidiary, Praxair Services offers a variety of 

services for tanks, piping, and other applications. Praxair Services provides cleaning, 

purging, drying, displacing, leak detection, hydrotesting, inspecting and cathodic 

protection services for piping networks and storage tanks. More specifically, Praxair 

Services has significant technology, knowledge, and experience in the use of chemical 

tracers in monitoring the subsurface movement of fluids developed and improved 
Docket ID no. EPA-HQ-OW-2008-0390 2

over many years. Praxair Services has also participated in several projects utilizing 

perfluorocarbons (“PFCs”) in tracer studies of geologic formations. 

In its draft UIC rule for CO2 GS wells, EPA requests specific comments regarding the 

applicability, costs, and potential impacts on human health and ecosystems (including 

the potential impact on global warming) of PFCs as tracers. As an entity with 

considerable experience in this area, Praxair Services is submitting comments on this 

proposed rule for the purpose of providing data that indicates that such risks are 

minimal in the context of PFC tracer use in this application.

Background: Praxair Services Use of PFC Tracers in GS Well Operation and 

Monitoring

The general use of PFCs as tracers dates back to the 1980s and is a well-established 

practice that experts view as an environmentally safe, reliable and effective method for 

monitoring the movement of fluids both below and above ground. Praxair Services 

has been conducting tracer tests since the mid-1980s. In that time, Praxair Services 

has conducted thousands of successful tests in the U.S. and Canada. In its experience 

with tracers for a variety of applications, including GS well monitoring, Praxair 

Services believes this application of tracers can be an effective tool in accomplishing 

the goals of the UIC program.

Specific to the UIC program, Praxair Services has participated in PFC studies of fluid 

movement in a number of areas. These include various CO2injection projects, a 

mixing study in a salt cavern and several surface monitoring tests to detect gas releases 

from a natural gas storage field. These tests, as well as those conducted by others,

have demonstrated the value of using tracers, including PFCs, for monitoring fluid 

movement in conjunction with CO2sequestration projects. 

Praxair Services has and is currently participating in tests with the U.S. Department of 

Energy (DOE), including the on-going pilot study at Plant Daniel in Mississippi and 

the completed Frio sequestration project in Texas. In the Frio test, PFC tracers were 

utilized to monitor surface releases of the CO2injected into a geologic storage 

formation over the course of several months. We discuss results from the tests and 

specific recommendations regarding proper and careful application of tracers in 

subsequent sections of these comments.

Benefits: Use of PFC Tracers in GS Well Monitoring and Operation

The benefits of using tracers in the UIC program can generally be divided into two 

categories: “Operation and Injection” and “Monitoring” of GS wells under the UIC 
Docket ID no. EPA-HQ-OW-2008-0390 3

program. In terms of value provided by such use during the operation and injection 

phases of a sequestration project, PFCs as tracers offer several unique advantages over 

other detection devices or methods. 

First, by injecting CO2 with tracers, the injecting entity can tag injections by time, 

producer and location. Among other benefits, this would allow assignment of leaking 

CO2from a shared GS well to the appropriate party during such an occurrence. The 

additional data would also allow for better information regarding potentially 

contaminated CO2and its source and better estimation regarding the test interval and 

timing of a CO2leak. Such information would likely allow entities to determine more 

quickly where CO2is leaking and to stop the leak.

Second, the use of tracers allows for the monitoring of the movement of the CO2

through the formations in the subsurface. This provides additional value as one can 

track migration of CO2in the subsurface in conjunction with surface leaks, and to the 

extent action is needed to stop or alter migration, entities would be able to react more 

quickly and more precisely when such events occur.

In terms of monitoring CO2after injection, PFCs as tracers also allow for better 

accuracy at a relatively low cost when determining if and where a CO2leak is 

occurring. 

Because tracers can be detected at low concentrations when tests are conducted 

appropriately, even small leaks can be detected accurately in a reasonable period of 

time. Further, the specialized field equipment used by experienced tracer test teams is 

extremely reliable, and due to the benefits associated with the ability to tag different 

CO2streams, the location of a leak can be determined quickly before large releases of 

CO2can occur.

Moreover, PFCs as tracers are critical to, and effective in, a successful on-going 

monitoring program. Once the tracer is injected into a well with a set amount of CO2

there are no on-going tracer injection requirements for that given quantity of CO2. 

The only on-going cost and operational task is the periodic re-testing of the above 

ground or subsurface environments to determine if any leaks or migrations of CO2 are 

occurring. When combined with other testing methods and devices, such a 

monitoring program can detect CO2releases quickly with near certain accuracy. 

For these reasons and others, Praxair Services believes tracers are well suited to meet 

the goals of the UIC program for GS wells.

Section III-A-4: Proposed Injection Well Operating Requirements: Tracers
Docket ID no. EPA-HQ-OW-2008-0390 4

· Cost effectiveness of tracer monitoring programs

In response to the EPA concern about the cost of PFC tracers discussed in the 

proposed rule, Praxair Services offers the following comments.

Praxair Services believes the cost associated with the use of PFCs as tracers is 

manageable for the majority of applications for which tracers are suitable in the UIC 

program, but recognizes that their effectiveness depends on the application and 

concentration within the GS well. 

Praxair Services calculates that the PFC tracers can be used effectively at part per 

billion (ppb) levels in the geologically stored CO2to obtain the desired result. 

Estimated costs for the tracers themselves range from $80 to $400 per pound. This 

would equate to the addition of 1 ppb of tracer adding $2 to $10 to the cost of 

injecting 1,000 tons of CO2. Such a cost distribution would represent less than 0.04% 

of average estimates for the total injection and storage costs for a typical GS project.

· Elimination of “False positives”

In response to the EPA request for comments in the proposed rule regarding a 

concern about the risk of “false positives” for tracers used in this application, Praxair 

Services offers the following comments.

During its 24-year history, Praxair Services has come to realize that so called “false 

positives” in tracer tests are in reality only unidentified sources of a leak of the tracer. 

Identification of the source is the most critical piece of a successful tracer test as it 

simplifies and confirms the interpretation of the test results.

In order to accurately identify a source, any tracer test relies on the design of the plan 

for collecting samples. For release detection applications, an elevated concentration 

of tracer in a surface sample is an indication of a release. There are many sources 

from which such a surface leak could be detected including:

o From the CO2container or apparatus used to inject it;

o From piping used to convey the CO2to the injection well;

o From the injection well or other wells in the vicinity of the test; or

o From the formation itself.
Docket ID no. EPA-HQ-OW-2008-0390 5

Each of these releases has a specified test method that is employed in order to ensure 

there are no “false positives”. For example, releases from the tracer container and 

injection apparatus can be controlled by careful handling. Releases from surface 

piping and wells are readily identifiable using strategic samples and testing protocol. 

Tracer emanating from the subsurface is also straight forward to identify with the 

proper methodology. For these reasons, Praxair Services believes the technical 

challenge of avoiding false positives can be successfully resolved through careful use 

with test methodology specifically developed for the UIC program applications.

Texas Tracer Demonstration: Need for Accurate Source Identification

As highlighted above, PFC tracers were added to the Gulf Coast Carbon Center’s 

CO2injection at Frio, TX. In this test, passive samples were collected to be analyzed 

for the presence of tracer after injection. Samples were collected by opening an 

absorbent tube and leaving it on the site for days or weeks. The tubes were then 

collected and analyzed.

The results from the samples collected at Frio showed elevated concentrations of 

tracers at several locations around the injection site. There was no other indication of 

CO2leakage. Because of the limited number of samples, interpretation of the source 

of tracer was difficult to determine. The data led many to assume that the detections 

were a “false positive”. 

Several months later Praxair Services conducted an additional survey. Several samples 

were again collected. Analysis indicated that the highest concentrations of tracer were 

near the observation well. A probe was then inserted into the ground to collect a 

sample of soil gas at selected depths. The depth profile sample showed a very rapid 

decrease in the concentration of tracer indicating an above ground source. Additional 

samples collected around the well head led the team to discover a faulty fitting as the 

source of the leak. 

The above example highlights a significant benefit of bringing tracer analytical 

equipment to the injection site which is the ability to quickly identify the source of any 

release of the tracer. This benefit includes the ability to:

o Locate a leaking well or well head; 

o Distinguish surface releases between the tracer container and the well 

head from subsurface releases from the well boring to the surface; and

o Identify and locate releases of tracer to the surface from the formation. 
Docket ID no. EPA-HQ-OW-2008-0390 6

If properly conducted, a tracer test will not yield a false positive. However, if not 

conducted properly, the results can be difficult to interpret and can leave a tracer 

source unidentified and improperly referred to as a false positive. Therefore, to the 

extent the rule should recognize concerns about “false positives”, we recommend that 

such concerns be represented in the context of proper injection of the tracer on a 

project-by-project basis. For this reason, organizations that participate in such tests 

should be well-established and have a demonstrated track record in successfully using 

tracer technology.

· Global Warming Potential (“GWP”) of PFC Tracers

It the proposed rule, EPA suggests concern for the GWP of PFC tracers in this 

application. Praxair Services offers the following comments.

In considering the impact of the PFC tracers on global warming, EPA should 

consider the significantly lower relative contribution of injected PFC tracers to the 

total CO2that could be released from GS locations if not detected. In addition, at the 

ppb level, the absolute amount of tracers injected is limited. Generally, there should 

be a balance between potential reductions in releases of CO2 by utilizing tracer 

technology and the contribution of any small amount of PFC to the atmosphere. 

After careful evaluation of published materials, Praxair Services believes that a 

reasonable upper limit for the global warming potentials of PFC tracers relative to 

CO2is in the range of 10,0001,2. Again, even though the contribution per molecule is 

high, the greenhouse gas contribution added by ppb levels of PFC tracers in the CO2

is negligible. For example, in the event of a release of the tracer-labeled CO2, a 1 ppb 

concentration of PFC tracer with a GWP of 10,000 would increase the greenhouse 

contribution of the release by 10 parts per million. The presence of the tracer would 

be equivalent to adding 1 additional ton of CO2to a release of 100,000 tons of CO2.

On balance, we respectfully suggest that this is an acceptable risk given the rewards 

gained in leak detection from the use of tracers.

· PFC Tracer effects on water quality, human health and ecosystems

In the proposed rule, EPA also requests comments regarding the effects of PFC 

tracer on water quality, human health and ecosystems in this application. Praxair 

Services offers the following comments.

1 IPCC/TEAP Special Report: Safeguarding the Ozone Layer and the Global Climate System, 2003.

2 Keith P. Shine, et al, “Perfluorodecalin: global warming potential and first detection in the atmosphere,” Atmospheric Environment, Volume 39, 

Issue 9, pages 1759-1763, 1/11/05.
Docket ID no. EPA-HQ-OW-2008-0390 7

The wording of the rule should reflect the demonstrated environmental safety of 

certain classes of tracers, such as PFC tracers and noble gases, as documented by 

experts in the scientific community.

PFCs are chemically and biologically inert and, to the best of our knowledge and 

belief, present little direct threat to human health or ecosystems. This limited risk has 

been recognized in approvals by the U.S. EPA and the California Air Resources 

Board (CARB) regarding the release of PFCs into the atmosphere and occupied 

buildings during tracer studies conducted by Brookhaven National Laboratory. The 

absence of known hazards is also referenced in a research paper prepared for the 

Texas Commission on Environmental Quality with respect to an injection well permit. 

The paper notes that PFC compounds have “no known human or eco-toxicity”32. 

In addition, given the application type, the levels of PFC tracers used (parts per 

billion) are likely to be negligible. If tracers do enter the environment, they are 

detectable at extremely low levels in water samples or in samples collected above an 

aquifer. Praxair Services encourages the EPA to review data from available sources 

that support the absence of known hazards from the use of very low levels of PFC 

tracers.

Section III-A-6: Proposed Plume and Pressure Front Monitoring Requirements

· Effectiveness of Tracers for CO2 detection

In the proposed rule EPA expresses concern regarding the ability to detect releases of 

CO2 by monitoring increases in the background level of CO2. The rule further 

indicates that detection of an increase in the level of CO2in the atmosphere may only 

be effective for large releases due to high and potentially variable background 

concentrations at or below the ground surface. In response, Praxair Services offers 

the following comments:

The concerns expressed by EPA in the proposed rule highlight the usefulness of 

PFCs as tracers in this application. Generally, PFC concentrations in the atmosphere 

are very low (less than 1 part in 1014) and PFC background concentrations are 

relatively constant. In addition, while there are no subsurface sources of PFCs, soils 

typically emit CO2 because the concentration in soil gas is higher than in the 

atmosphere, sometimes many times higher. Vegetation is also a local source of CO2

3 Susan D. Hovorka and Mark H. Holtz and Shinichi Sakurai and Paul R. Knox, Bureau of Economic Geology and John A. and Katherine G. Jackson, 

School of Geosciences, The University of Texas at Austin and Dan Collins and Phil Papadeas and Donald Stehli, Sandia Technologies, LLC, Report to 

the Texas Commission on Environmental Quality to Accompany a Class V Application for an “Experimental Technology Pilot Injection Well 

Frio Pilot in CO2 Sequestration in Brine-Bearing Sandstones”, page 157, December 2003.
Docket ID no. EPA-HQ-OW-2008-0390 8

in both the subsurface and surface environments. Such circumstances cause 

variability in the CO2 background concentration and that variability makes small 

changes in CO2, and their source, difficult to detect, whereas increases in PFCs are 

much simpler to identify and measure due to the low and consistent nature of 

background PFC.

For example, a surface release from a GS well with PFC tracers (at a concentration of 

1 ppb) that caused a 10% increase in background CO2 would cause the PFC 

background to increase more than 400%, a relative increase that is at least 40 times 

greater and much easier to detect and identify the source of emission. Praxair 

Services believes the use of a PFC as a tracer, when used properly and in the correct 

application, makes the determination of such a release less ambiguous and reduces the 

size of a detectable release both at the surface and subsurface levels due to such 

properties.

In summary, Praxair Services believes the effectiveness of a release monitoring 

program can be significantly improved by the use of PFC tracer technology due to the 

ability of these types of tracers to more accurately track releases at a relatively low cost 

level. We hope that the EPA will encourage the use of PFCs as tracers in its final rule 

for UIC for GS wells.

Should you have any questions concerning these comments, please do not hesitate to 

contact me at (520) 545-0216 (dan_evans@praxair.com). 

Respectfully submitted,

Dan Evans
Letter ID: 0144 

Commenter: Dan Evans 

Organization: Praxair Services, Inc. 

Letter ID: 0144 Comment ID: 23146 Letter Lines: 61 to 326 ; 337 to 350 

Comment Code: 4490 2.4.5 Use of tracers and potential impact on human health or ecosystems 

(seeking comment) 

Comment: In its draft UIC rule for CO2 GS wells, EPA requests specific comments regarding 

the applicability, costs, and potential impacts on human health and ecosystems 

(including the potential impact on global warming) of PFCs as tracers. As an 

entity with considerable experience in this area, Praxair Services is submitting 

comments on this proposed rule for the purpose of providing data that indicates 

that such risks are minimal in the context of PFC tracer use in this 

application. 

Background: Praxair Services Use of PFC Tracers in GS Well Operation and 

Monitoring 

The general use of PFCs as tracers dates back to the 1980s and is a wellestablished practice that experts view as an environmentally safe, reliable and 

effective method for monitoring the movement of fluids both below and above 

ground. Praxair Services has been conducting tracer tests since the mid-1980s. 

In that time, Praxair Services has conducted thousands of successful tests in 

the U.S. and Canada. In its experience with tracers for a variety of 

applications, including GS well monitoring, Praxair Services believes this 

application of tracers can be an effective tool in accomplishing the goals of 

the UIC program. 

Specific to the UIC program, Praxair Services has participated in PFC studies of 

fluid movement in a number of areas. These include various CO2 injection 

projects, a mixing study in a salt cavern and several surface monitoring tests 

to detect gas releases from a natural gas storage field. These tests, as well 

as those conducted by others, have demonstrated the value of using tracers, 

including PFCs, for monitoring fluid movement in conjunction with CO2 

sequestration projects. 

Praxair Services has and is currently participating in tests with the U.S. 

Department of Energy (DOE), including the on-going pilot study at Plant Daniel 

in Mississippi and the completed Frio sequestration project in Texas. In the 

Frio test, PFC tracers were utilized to monitor surface releases of the CO2 

injected into a geologic storage formation over the course of several months. 

We discuss results from the tests and specific recommendations regarding proper 

and careful application of tracers in subsequent sections of these comments. 

1
Benefits: Use of PFC Tracers in GS Well Monitoring and Operation 

The benefits of using tracers in the UIC program can generally be divided into 

two categories: "Operation and Injection" and "Monitoring" of GS wells under the 

UIC program. In terms of value provided by such use during the operation and 

injection phases of a sequestration project, PFCs as tracers offer several 

unique advantages over other detection devices or methods. 

First, by injecting CO2 with tracers, the injecting entity can tag injections by 

time, producer and location. Among other benefits, this would allow assignment 

of leaking CO2 from a shared GS well to the appropriate party during such an 

occurrence. The additional data would also allow for better information 

regarding potentially contaminated CO2 and its source and better estimation 

regarding the test interval and timing of a CO2 leak. Such information would 

likely allow entities to determine more quickly where CO2 is leaking and to stop 

the leak. 

Second, the use of tracers allows for the monitoring of the movement of the CO2 

through the formations in the subsurface. This provides additional value as one 

can track migration of CO2 in the subsurface in conjunction with surface leaks, 

and to the extent action is needed to stop or alter migration, entities would be 

able to react more quickly and more precisely when such events occur. 

In terms of monitoring CO2 after injection, PFCs as tracers also allow for 

better accuracy at a relatively low cost when determining if and where a CO2 

leak is occurring. 

Because tracers can be detected at low concentrations when tests are conducted 

appropriately, even small leaks can be detected accurately in a reasonable 

period of time. Further, the specialized field equipment used by experienced 

tracer test teams is extremely reliable, and due to the benefits associated with 

the ability to tag different CO2 streams, the location of a leak can be 

determined quickly before large releases of CO2 can occur. 

Moreover, PFCs as tracers are critical to, and effective in, a successful ongoing monitoring program. Once the tracer is injected into a well with a set 

amount of CO2 there are no on-going tracer injection requirements for that given 

quantity of CO2. The only on-going cost and operational task is the periodic 

re-testing of the above ground or subsurface environments to determine if any 

leaks or migrations of CO2 are occurring. When combined with other testing 

methods and devices, such a monitoring program can detect CO2 releases quickly 

with near certain accuracy. 

For these reasons and others, Praxair Services believes tracers are well suited 

to meet the goals of the UIC program for GS wells. 

2
Section III-A-4: Proposed Injection Well Operating Requirements: Tracers 

* Cost effectiveness of tracer monitoring programs 

In response to the EPA concern about the cost of PFC tracers discussed in the 

proposed rule, Praxair Services offers the following comments. 

Praxair Services believes the cost associated with the use of PFCs as tracers is 

manageable for the majority of applications for which tracers are suitable in 

the UIC program, but recognizes that their effectiveness depends on the 

application and concentration within the GS well. 

Praxair Services calculates that the PFC tracers can be used effectively at part 

per billion (ppb) levels in the geologically stored CO2 to obtain the desired 

result. Estimated costs for the tracers themselves range from $80 to $400 per 

pound. This would equate to the addition of 1 ppb of tracer adding $2 to $10 to 

the cost of injecting 1,000 tons of CO2. Such a cost distribution would 

represent less than 0.04% of average estimates for the total injection and 

storage costs for a typical GS project. 

* Elimination of "False positives" 

In response to the EPA request for comments in the proposed rule regarding a 

concern about the risk of "false positives" for tracers used in this 

application, Praxair Services offers the following comments. 

During its 24-year history, Praxair Services has come to realize that so called 

"false positives" in tracer tests are in reality only unidentified sources of a 

leak of the tracer. Identification of the source is the most critical piece of 

a successful tracer test as it simplifies and confirms the interpretation of the 

test results. 

In order to accurately identify a source, any tracer test relies on the design 

of the plan for collecting samples. For release detection applications, an 

elevated concentration of tracer in a surface sample is an indication of a 

release. There are many sources from which such a surface leak could be 

detected including: 

o From the CO2 container or apparatus used to inject it; 

o From piping used to convey the CO2 to the injection well; 

o From the injection well or other wells in the vicinity of the test; or 

o From the formation itself. 

Each of these releases has a specified test method that is employed in order to 

ensure there are no "false positives". For example, releases from the tracer 

3
container and injection apparatus can be controlled by careful handling. 

Releases from surface piping and wells are readily identifiable using strategic 

samples and testing protocol. Tracer emanating from the subsurface is also 

straight forward to identify with the proper methodology. For these reasons, 

Praxair Services believes the technical challenge of avoiding false positives 

can be successfully resolved through careful use with test methodology 

specifically developed for the UIC program applications. 

Texas Tracer Demonstration: Need for Accurate Source Identification 

As highlighted above, PFC tracers were added to the Gulf Coast Carbon Center’s 

CO2 injection at Frio, TX. In this test, passive samples were collected to be 

analyzed for the presence of tracer after injection. Samples were collected by 

opening an absorbent tube and leaving it on the site for days or weeks. The 

tubes were then collected and analyzed. 

The results from the samples collected at Frio showed elevated concentrations of 

tracers at several locations around the injection site. There was no other 

indication of CO2 leakage. Because of the limited number of samples, 

interpretation of the source of tracer was difficult to determine. The data led 

many to assume that the detections were a "false positive". 

Several months later Praxair Services conducted an additional survey. Several 

samples were again collected. Analysis indicated that the highest 

concentrations of tracer were near the observation well. A probe was then 

inserted into the ground to collect a sample of soil gas at selected depths. 

The depth profile sample showed a very rapid decrease in the concentration of 

tracer indicating an above ground source. Additional samples collected around 

the well head led the team to discover a faulty fitting as the source of the 

leak. 

The above example highlights a significant benefit of bringing tracer analytical 

equipment to the injection site which is the ability to quickly identify the 

source of any release of the tracer. This benefit includes the ability to: 

o Locate a leaking well or well head; 

o Distinguish surface releases between the tracer container and the well head 

from subsurface releases from the well boring to the surface; and 

o Identify and locate releases of tracer to the surface from the formation. 

f properly conducted, a tracer test will not yield a false positive. However, 

if not conducted properly, the results can be difficult to interpret and can 

leave a tracer source unidentified and improperly referred to as a false 

positive. Therefore, to the extent the rule should recognize concerns about 

"false positives", we recommend that such concerns be represented in the context 

of proper injection of the tracer on a project-by-project basis. For this 

4
reason, organizations that participate in such tests should be well-established 

and have a demonstrated track record in successfully using tracer technology. 

* Global Warming Potential ("GWP") of PFC Tracers 

It the proposed rule, EPA suggests concern for the GWP of PFC tracers in this 

application. Praxair Services offers the following comments. 

In considering the impact of the PFC tracers on global warming, EPA should 

consider the significantly lower relative contribution of injected PFC tracers 

to the total CO2 that could be released from GS locations if not detected. In 

addition, at the ppb level, the absolute amount of tracers injected is limited. 

Generally, there should be a balance between potential reductions in releases of 

CO2 by utilizing tracer technology and the contribution of any small amount of 

PFC to the atmosphere. 

After careful evaluation of published materials, Praxair Services believes that 

a reasonable upper limit for the global warming potentials of PFC tracers 

relative to CO2 is in the range of 10,000 [1,2]. Again, even though the 

contribution per molecule is high, the greenhouse gas contribution added by ppb 

levels of PFC tracers in the CO2 is negligible. For example, in the event of a 

release of the tracer-labeled CO2, a 1 ppb concentration of PFC tracer with a 

GWP of 10,000 would increase the greenhouse contribution of the release by 10 

parts per million. The presence of the tracer would be equivalent to adding 1 

additional ton of CO2 to a release of 100,000 tons of CO2. On balance, we 

respectfully suggest that this is an acceptable risk given the rewards gained in 

leak detection from the use of tracers. 

* PFC Tracer effects on water quality, human health and ecosystems 

In the proposed rule, EPA also requests comments regarding the effects of PFC 

tracer on water quality, human health and ecosystems in this application. 

Praxair Services offers the following comments. 

The wording of the rule should reflect the demonstrated environmental safety of 

certain classes of tracers, such as PFC tracers and noble gases, as documented 

by experts in the scientific community. 

PFCs are chemically and biologically inert and, to the best of our knowledge and 

belief, present little direct threat to human health or ecosystems. This 

limited risk has been recognized in approvals by the U.S. EPA and the California 

Air Resources Board (CARB) regarding the release of PFCs into the atmosphere and 

occupied buildings during tracer studies conducted by Brookhaven National 

Laboratory. The absence of known hazards is also referenced in a research paper 

prepared for the Texas Commission on Environmental Quality with respect to an 

injection well permit. The paper notes that PFC compounds have "no known human 

5
or eco-toxicity"[3]. 

In addition, given the application type, the levels of PFC tracers used (parts 

per billion) are likely to be negligible. If tracers do enter the environment, 

they are detectable at extremely low levels in water samples or in samples 

collected above an aquifer. Praxair Services encourages the EPA to review data 

from available sources that support the absence of known hazards from the use of 

very low levels of PFC tracers. 

Section III-A-6: Proposed Plume and Pressure Front Monitoring Requirements 

* Effectiveness of Tracers for CO2 detection 

In the proposed rule EPA expresses concern regarding the ability to detect 

releases of CO2 by monitoring increases in the background level of CO2. The 

rule further indicates that detection of an increase in the level of CO2 in the 

atmosphere may only be effective for large releases due to high and potentially 

variable background concentrations at or below the ground surface. In response, 

Praxair Services offers the following comments: 

The concerns expressed by EPA in the proposed rule highlight the usefulness of 

PFCs as tracers in this application. Generally, PFC concentrations in the 

atmosphere are very low (less than 1 part in 1014) and PFC background 

concentrations are relatively constant. In addition, while there are no 

subsurface sources of PFCs, soils typically emit CO2 because the concentration 

in soil gas is higher than in the atmosphere, sometimes many times higher. 

Vegetation is also a local source of CO2 in both the subsurface and surface 

environments. Such circumstances cause variability in the CO2 background 

concentration and that variability makes small changes in CO2, and their source, 

difficult to detect, whereas increases in PFCs are much simpler to identify and 

measure due to the low and consistent nature of background PFC. 

For example, a surface release from a GS well with PFC tracers (at a 

concentration of 1 ppb) that caused a 10% increase in background CO2 would cause 

the PFC background to increase more than 400%, a relative increase that is at 

least 40 times greater and much easier to detect and identify the source of 

emission. Praxair Services believes the use of a PFC as a tracer, when used 

properly and in the correct application, makes the determination of such a 

release less ambiguous and reduces the size of a detectable release both at the 

surface and subsurface levels due to such properties. 

In summary, Praxair Services believes the effectiveness of a release monitoring 

program can be significantly improved by the use of PFC tracer technology due to 

the ability of these types of tracers to more accurately track releases at a 

relatively low cost level. We hope that the EPA will encourage the use of PFCs 

as tracers in its final rule for UIC for GS wells. 

6
7

1 IPCC/TEAP Special Report: Safeguarding the Ozone Layer and the Global Climate 

System, 2003. 

2 Keith P. Shine, et al, "Perfluorodecalin: global warming 

potential and first detection in the atmosphere,"Atmospheric Environment, 

Volume 39, Issue 9, pages 1759-1763, 1/11/05. 

3 Susan D. Hovorka and Mark H. Holtz and Shinichi Sakurai and Paul R. Knox, 

Bureau of Economic Geology and John A. and Katherine G. Jackson, School of 

Geosciences, The University of Texas at Austin and Dan Collins and Phil Papadeas 

and Donald Stehli, Sandia Technologies, LLC, Report to the Texas Commission on 

Environmental Quality to Accompany a Class V Application for an "Experimental 

Technology Pilot Injection Well Frio Pilot in CO2 Sequestration in Brine-Bearing 

Sandstones", page 157, December 2003. 

Individual Response: EPA agrees that tracers can be useful tools for monitoring GS projects. 

However, EPA believes—as some commenters note—that tracer use is not appropriate in all 

situations. For this reason, EPA does not require the use of tracers at all GS sites; however, the 

Director has discretion to require their use if he/she determines that the use of tracers could 

improve monitoring of the site and enhance USDW protection from endangerment. The rule will 

also allow their use if proposed by the owner or operator and approved by the Director. 

If an owner or operator proposes the use of tracers at a GS site, EPA anticipates that the specific 

tracers to be used will be described in the testing and monitoring plan, which the Director will 

evaluate in the context of other information about the GS site. The Director would assess the use 

of tracers in the context of site-specific information, including risks to USDWs. 

EPA continues to follow new and ongoing research on the safety and efficacy of tracers, 

including the Frio study mentioned by the commenter. For example, studies at the Frio Brine 

Pilot Experiment site (Dayton, TX) used various types of tracers, including PFTs, sulfur 

hexafluoride, and the noble gases krypton, neon, and xenon. These studies (e.g., Phelps and Cole, 

2008; Hovorka et al. 2005) found that tracers can provide useful CO2 monitoring information at 

injection wells as well as near-surface locations. At Frio, tracers enabled quantification of CO2 

plume behavior and provided a technique for model validity testing (McCallum et al., 2005). 

EPA will continue to track research related to tracer use. 

EPA agrees that tracers can support a multifaceted approach to monitoring GS projects, and that 

false positive detections can be managed (i.e., by the use of available test methods for detecting 

the sources of the false positives, as the commenter notes).