This comment letter response contains two parts:

(1) The original submittal from the commenter. (2) A data base printout of EPA’s response to each point raised by the commenter.

Notes about the response letters:

• The response letters were generated by a data base used to track the content of all public

comment submittals and ensure that the Agency responds to each. Each comment letter

was assigned a Letter ID that corresponds to the last four digits of the docket number of the
original comment (as posted on www.regulations.gov).

• Each response letter includes: comment text; an EPA

‐ assigned comment code; and an
individual response. These are described and explained below.

• The data base contains Comment text from the letters submitted by commenters.
Many hard copy letters were scanned into electronic format for inclusion in the data

base. As a result, there may be some typographical errors attributable to optical

character recognition issues. However, all original submittals were reviewed in the

process of drafting responses to ensure that the comment content was understood and
appropriately addressed.

• The content of each comment letter was divided into topic areas and assigned a unique

Comment ID. Additionally, one or more Comment Codes (i.e., unique topic area ID) was assigned to each discrete point raised by a commenter. This information was added to

the data base. To inform final rulemaking decisions, all comments and information

received on a given topic were reviewed by technical experts in each rule area (e.g., on

construction requirements).

• Following decisions on the final rule requirements, Individual responses were drafted
by topic area to ensure completeness and consistency of the responses with the final

rule and other responses to similar comments. Each discrete point made by a

commenter was given a unique response.

• Upon completion of comment responses, all information was entered into the data base.

Each letter response is a compilation of each comment and its individual response, compiled in the order of the original comment letter. As a result of this process, there is
repetition in the response text. The Agency believes this approach ensures completeness

and attention to each comment letter that was submitted.

• All comments in a topic area were reviewed, evaluated, and responded to concurrently.

Responses on each topic area are compiled and are also available in the rulemaking docket.Docket ID no. EPA

- HQ

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- 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 1 7 , 2008

Water Docket Environmental Protection Agency

Mailcode: 2822T 1200 Pennsylvania Avenue, NW Washington D.C. 20460

RE: Docket ID No. EPA

- HQ

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- 0390: Federal Requirements under the Undergroun d Injection Control (“UIC”) Program for Carbon Dioxide (“CO 2 

### ”) 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 rela ting to the UIC program for CO 2 

### 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 technol ogies 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 offe rs 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 spe cifically, Praxair Services has significant technology, knowledge, and experience in the use of chemical tracers in monitoring the subsurface movement of fluids developed and improvedDocket ID no. EPA

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### over many years. Praxair Services has also participated in several proj ects utilizing perfluorocarbons (“PFCs”) in tracer studies of geologic formations.

In its draft UIC rule for CO 2 

### 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 submit ting 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 a n 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 succ essful 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 CO 2 

### injection projects, a mixing study in a salt cavern and several surface monitoring tests to detect gas release s 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 CO 2 

### 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 complete d Frio sequestration project in Texas. In the Frio test, PFC tracers were

utilized to monitor s urface releases of the CO 2 

### injected into a geologic storage

formation over the course of several months. We discuss r esults from the tests and specific recommendations regarding proper and careful application of tracers in subsequent sections of these com ments

.

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 UICDocket ID no. EPA

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### program. In terms of value provided by such use during the operation and injection phases of a sequestration project, PFCs as tracers offe r several unique advantages over

other detection devices or methods.

First, by injecting CO 2 

### with tracers, the injecting entity can tag injections by time, producer and location. Among other benefits, this would allow assignment of leaking

CO 2 

### from a shared GS well to the appropriate party during such an occurrence. The additional data would also allow for better information regardin g potentially

contaminated CO 2 

### and its source and better estimation regarding the test interval and timing of a CO 2 

### leak. Such information would likely allow entities to determine more quickly where CO 2 

### is leaking and to stop the leak.

Second, the use of tracers allows for the monitoring of the movement of the CO 2 

### through the formations in the subsurface. This provides additional value as one can track migration of CO 2 

### in the subsurface in conjunction with surface leaks, and to the

extent action is neede d to stop or alter migration, entities would be able to react more quickly and more precisely when such events occur.

In terms of monitoring CO 2 

### after injection, PFCs as tracers also allow for better accuracy at a relatively low cost when determining if a nd where a CO 2 

### 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

CO 2 

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

CO 2 

### can 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 CO 2 

### there are no on

- going tracer injection requirements for that given quantity of CO 2 

### . The only on

- going cost and opera tional task is the periodic re

- testing of the above ground or subsurface environments to determine if any leaks or migrations of CO 2 

### are occurring. When combined with other testing methods and devices, such a monitoring program can detect CO 2 

### releases qui ckly 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

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- 4: Proposed Injection Well Operating Requirements: TracersDocket ID no. EPA

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### · Cost effective ness of tracer monitoring programs

In response to the EPA concern about the cost of PFC tracers discussed in the proposed rule, Praxair Service s 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 c an be used effectively at part per billion (ppb) levels in the geologically stored CO 2 

### 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 CO 2 

### . 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 E PA 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 tha t 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 tes t 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. Ther e are many sources from which such a surface leak could be detected including:

o From the CO 2 

### container or apparatus used to inject it;

o From piping used to convey the CO 2 

### to the injection well;

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

o From the formation itself.Docket ID no. EPA

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### 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 Se rvices 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 Identi fication

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

CO 2 

### 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 wa s no other indication of

CO 2 

### 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 Ser vices 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 selecte d 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.

Th e 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 o r 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

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### 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 shou ld 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

- esta blished 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 Servic es 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 CO 2 

### that could be released from GS locations if not detected. In ad dition, at the ppb level, the absolute amount of tracers injected is limited. Generally, there should be a balance between potential reductions in releases of CO 2 

### by utilizing tracer technology and the contribution of any small amount of PFC to the atmosp here.

After careful evaluation of published materials, Praxair Services believes that a reasonable upper limit for the global warming potentials of PFC tracers relative to

CO 2 

### is in the range of 10,000 1 ,2 

### . Again, even though the contribution per molecul e is high, the greenhouse gas contribution added by ppb levels of PFC tracers in the CO 2 

### is negligible. For example, in the event of a release of the tracer

- labeled CO 2 

### , a 1 ppb concentration of PFC tracer with a GWP of 10,000 would increase the greenhou se contribution of the release by 10 parts per million. The presence of the tracer would be equivalent to adding 1 additional ton of CO 2 

### to a release of 100,000 tons of CO 2 

### . O n balance, we respectfully suggest that this is an acceptable risk given the rew ards 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 t he 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

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### 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 Reso urces 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” 3 2



### .

In addition, given the application type, the levels of PFC tracers used (par ts 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 sou rces 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 CO 2 

### detection

In the proposed rule EPA expresses conce rn regarding the ability to detect release s of

CO 2 

### by monitoring increases in the background level of CO 2 

### . The rule further

indicates that detection of an increase in the level of CO 2 

### 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 applic ation. Generally, PFC concentrations in the atmosphere are very low (less than 1 part in 10 14 

### ) and PFC background concentrations are relatively constant. In addition, while there are no subsurface sources of PFCs, soils typically emit CO 2 

### because the con centration in soil gas is higher than in the atmosphere, sometimes many times higher. Vegetation is also a local source of CO 2

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 Aust in 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 Sequestratio n in Brine

- Bearing Sandstones”, page 157, December 2003.Docket ID no. EPA

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### in both the subsurface and surface environments. Such circumstances cause variability in the CO 2 

### background concentration and t hat variability makes small changes in CO 2 

### , 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 wi th PFC tracers ( at a concentration of 1 ppb ) that caused a 10% increase in background CO 2 

### 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 l evels 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 r eleases at a relatively low cost

level. We hope that the EPA will encourage the use of PF Cs 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 s ubmitted

,

Dan EvansLetter 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 im pact on human health or ecosystems (seeking comment)

Comment: In its draft UIC rule for CO2 GS wells, EP A requests specific comments regarding the applicability, costs, and potential impacts on human health and ecosystems (including the potential impact on global wa rming) of PFCs as tracers. As an entity with considerable experience in th is area, Praxair Services is submitting comments on this proposed rule for the pur pose 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 moveme nt of fluids both below and above ground. Praxair Services has been conduc ting tracer tests since the mid-1980s. In that time, Praxair Services has conduc ted thousands of successful tests in the U.S. and Canada. In its experien ce with tracers for a variety of applications, including GS well monito ring, 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 dem onstrated the value of using tracers, including PFCs, for monitoring fluid movement in conjunction with CO2 sequestration projects.

Praxair Services has and is currently pa rticipating in tests with the U.S. Department of Energy (DOE), including th e on-going pilot study at Plant Daniel in Mississippi and the completed Frio se questration project in Texas. In the Frio test, PFC tracers were utilized to monitor surface rele ases of the CO2 injected into a geologic storage formati on 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.

1Benefits: 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 pr oject, PFCs as tracer s offer several unique advantages over other de tection devices or methods.

First, by injecting CO2 with tracers, the injecting entity can tag injections by time, producer and location. Among other bene fits, this would allow assignment of leaking CO2 from a shared GS well to the appropriate part y during such an

occurrence. The additional data woul d also allow for better information regarding potentially contaminated CO 2 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 qui ckly 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 subsur face in conjunction with surface leaks,

and to the extent action is needed to st op or alter migration, entities would be able to react more quickly and more precisely when such events occur.

In terms of monitoring CO 2 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 c oncentrations when tests are conducted appropriately, even small leaks can be de tected accurately in a reasonable period of time. Further, th e specialized field equipment used by experienced tracer test teams is extremely reliable, a nd due to the benefits associated with the ability to tag different CO2 stream s, the location of a leak can be determined quickly before large releases of CO2 can occur.

Moreover, PFCs as tracers are critical to, and effec tive 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 subsurf ace environments to determine if any leaks or migrations of CO2 are occurring. When combined with other testing methods and devices, such a monitoring progr am can detect CO2 releases quickly with near certain accuracy.

For these reasons and others, Praxair Servi ces believes tracers are well suited to meet the goals of the UIC program for GS wells.

2Section III-A-4: Proposed Injection We ll 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 associat ed 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 commen ts in the proposed rule regarding a concern about the risk of "false pos itives" for tracers used in this application, Praxair Services o ffers 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 unidentif ied 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. Fo r 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 ap paratus 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

3container 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 th e proper methodology. For these reasons,

Praxair Services believes the technical challenge of avoiding false positives can be successfully resolved thr ough careful use with test methodology specifically developed for the UIC program applications.

Texas Tracer Demonstration: Need fo r Accurate Source Identification

As highlighted above, PFC tracers were adde d 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 afte r 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 Fr io showed elevated concentrations of

tracers at several locatio ns around the injection si te. 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 detecti ons 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 th e observation well. A probe was then inserted into the ground to collect a samp le 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 be nefit of bringing tr acer analytical equipment to the injection site which is the ability to quickly identify the source of any release of the tracer. Th is benefit includes the ability to:

o Locate a leaking well or well head; o Distinguish surface releases between th e tracer container and the well head

from subsurface releases from th e 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 no t yield a false positive. However, if not conducted properly, the results can be difficu lt 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

4reason, organizations that pa rticipate in such tests s hould 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 concer n for the GWP of PFC tracers in this application. Praxair Services o ffers 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 in jected is limited. Generally, there should be a balance between potential reductions in releases of CO2 by utilizing tracer technology and the c ontribution of any small amount of PFC to the atmosphere.

After careful evaluation of published materi als, Praxair Services believes that a reasonable upper limit for the globa l 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-label ed CO2, a 1 ppb concentration of PFC tracer with a GWP of 10,000 would increase the greenhous e 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 th is is an acceptable risk given the rewards gained in

leak detection from the use of tracers.

* PFC Tracer effects on water qualit y, human health and ecosystems

In the proposed rule, EPA also requests co mments regarding the effects of PFC tracer on water quality, hu man 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 gase s, 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 approva ls by the U.S. EPA and the California Air Resources Board (CARB) regarding the re lease of PFCs into th e atmosphere and occupied buildings during tracer studies conducted by Brookhaven National Laboratory. The absence of know n hazards is also referen ced in a research paper prepared for the Texas Commission on Enviro nmental Quality with respect to an injection well permit. The pa per notes that PFC compounds have "no known human

5or eco-toxicity"[3].

In addition, given the application type, th e 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 le vels 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 Pre ssure Front Monitoring Requirements

* Effectiveness of Trace rs for CO2 detection

In the proposed rule EPA expresses con cern 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 larg e 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 propos ed 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 typical ly 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 CO 2 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 us e of a PFC as a tracer, when used properly and in the correct application, makes the de termination of such a release less ambiguous and reduces the size of a detectable rel ease both at the

surface and subsurface levels due to such properties.

In summary, Praxair Services believes th e effectiveness of a release monitoring program can be significantly improved by th e 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.

67 1 IPCC/TEAP Special Report: Safeguarding th e Ozone Layer and the Global Climate System, 2003.

2 Keith P. Shine, et al, "P erfluorodecalin: global warming potential and first detection in the a tmosphere,"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. a nd 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 Cla ss V Application for an "Experimental Technology Pilot Injection Well Frio Pilot in CO2 Sequest ration 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 th e use of tracers at all GS sites; however, the

Director has discretion to require their use if he/she determ ines 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 pro posed by the owner or operator and approved by the Director.

If an owner or operator proposes th e use of tracers at a GS site, EP A anticipates that the specific

tracers to be used will be described in the tes ting 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 cont ext of site-specific informati on, 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 comment er. For example, studies at the Frio Brine Pilot Experiment site (Dayton, TX) used variou s types of tracers, in cluding 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 mon itoring information at injection wells as well as near-surface locations . At Frio, tracers enable d 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 a pproach 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).