EPA Comment

ATSDR's Response

General Comments

102

Overall, we believe that the current version of the Public Health Assessment (PHA) represents a substantial improvement over the initial draft released on December 31, 2002. In general, it is more readable, provides expanded discussions, and corrects previous numerical errors.

The Public Health Assessment Working Group (PHAWG) and Oak Ridge Health Effects Subcommittee (ORRHES) comments that ATSDR has received throughout the public health assessment process have been very helpful in improving the technical aspects and overall readability of the document.

103

EPA R4 concurs with the draft PHA conclusion that the available data does not indicate the presence of uranium releases that constitute concern for the Scarboro Community.

Even though ATSDR and EPA Region IV have distinct purposes that require different goals and processes for their assessments, the two agencies have both concluded that the releases of uranium from the Y-12 plant are not a public health hazard for the Scarboro Community (see the response to comment 127 for more details about the deliberate differences between ATSDR's public health assessment sand EPA's risk assessment).

Evaluation of Past Exposures

104

At this time, we do not agree with ATSDR's final conclusions regarding past uranium exposures. Based on our review and evaluation of the PHA, we do not agree with ATSDR's conclusions on past uranium exposures (pp. 138-139) that:

"the levels of uranium released from the Y-12 plant in the past ... would not result in harmful health effects for either adults or children living near the Y-12 plant, including the city of Oak Ridge and the Scarboro community" (lines 6-8).

After reviewing and evaluating the public comments on the PHA, ATSDR made minor changes to the public health assessment report. However, ATSDR has not changed its conclusion that past and current off-site exposures to uranium released from the Y-12 pose no apparent public health hazard because the estimated doses are not at levels expected to cause adverse health effects.

In addition, this particular comment by the EPA Office of Radiation and Indoor Air in Washington, DC contradicts EPA Region IV's overall conclusion on ATSDR's PHA. In the March 27, 2003 cover letter to ATSDR, EPA Region IV stated the following:

"EPA concurs with the assessment's conclusion that the available data does not indicate the presence of uranium releases that constitute a past, current or future health threat for the Scarboro Community."

105

We believe that ATSDR's assessment of past uranium exposures is incomplete and inadequate for several reasons. To support their conclusion, ATSDR should: (1) provide a range of exposures scenarios that includes reasonably maximally exposed individuals, along with the parameter values and equations used in the calculations; (2) calculate central estimates and confidence intervals for doses and risks, for each scenario, based on a quantitative uncertainty analysis; (3) verify and validate the empirical c/Q model used to reconstruct historical uranium air concentrations for Scarboro; and (4) collect additional surface and subsurface soil samples in undisturbed areas in and around Scarboro and analyzed for isotopic uranium activity and mass concentration. For the assessment of past exposures, these samples are needed to develop a site-specific source term for the soil exposure pathways, validate the reconstructed air concentrations, and understand the depth profile and temporal migration pattern of uranium in soil.

ATSDR's responses to each of the four recommendations follow:

1. ATSDR used the results of the State of Tennessee's uranium screening evaluation in the Task 6 report to evaluate past uranium exposures to residents living near the Y-12 plant. The Task 6 screening evaluation used a two-tiered approach: a Level I assessment that focused on an individual with the highest potential for exposure to the releases (maximally exposed individual) and a Level II assessment on an average or more typically exposed individual. Task 6 states on pages ES-9 and 3-27 that "some conservatism was maintained in the uranium concentration estimates used in Level II screening to ensure that hazards to a significant portion of the potentially exposed population were not underestimated," and that the Level II screening "may be appropriately called a Refined Level I analysis" (ChemRisk 1999). See Tables 7, 9, and 10 in the PHA for the 20 human exposure routes evaluated in the Task 6 report. The equations and parameters that were used to calculate past uranium exposure doses are provided in the Task 6 report in Appendix J and Appendix K, respectively.

In addition, the Task 6 report stated that "while other potentially exposed communities were considered in the selection process, the reference locations [Scarboro] represent residents who lived closest to the ORR facilities and would have received the highest exposures from past uranium releases... Scarboro is the most suitable for screening both a maximally and typically exposed individual" (ChemRisk 1999).

2. As stated in the title, the Task 6 report was a "Screening Evaluation of Potential Off-Site Exposure," that routinely and appropriately used several layers of conservatism and protective assumptions and approaches in estimating concentrations and doses (see ATSDR's response to recommendation 1 in comment 105 and the list of conservative aspects of the screening evaluation on pages 48 and 92 of the PHA). Task 6 states that "some level of conservatism was maintained in the uranium concentration estimates used in Level II screening to ensure that hazards to a significant portion of the potentially exposed population were not underestimated."

Also, the internationally recognized expert technical reviewers hired by ATSDR to review the Task 6 report pointed out that "the estimates made in the report tend to be on the conservative side–one expects, therefore, that (when in error) the report would tend to overestimate the extent to which exposure to uranium is a problem in the Oak Ridge area. Further refinements to the study are likely to reveal that uranium exposures are actually lower than those currently estimated" (see page G-7 of the PHA).

Since the Task 6 screening evaluation resulted in overestimated total past uranium doses well below (32 times less than) the ATSDR radiogenic comparison value, ATSDR does not believe the evaluation of Y-12 uranium releases requires a further nonconservative screening or a refined evaluation with uncertainty and sensitivity analyses.

As discussed in NCRP Commentary 14, A Guide For Uncertainty Analysis In Dose And Risk Assessments Related To Environmental Contamination, issued in 1996, if a conservatively based screening calculation is performed and this screening calculation indicates the risk is "clearly below regulatory or risk levels of concern," and the possible exposure is low, then a quantitative uncertainty analysis may not be necessary. By design, conservative screenings are "highly unlikely to underestimate the true dose or risk."

Based on this document and the technical reviewers' comments, ATSDR agrees with the Task 6 authors that a quantitative uncertainty analysis is not needed for this portion of the Oak Ridge Dose Reconstruction Project. On page D-3, the Task 6 authors state "although an uncertainty analysis of the Task 6 air source term was not within the scope of Task 6, experts interviewed during the project consider release estimates for enriched uranium to be suitable for the Task 6 screening assessment and are within an order of magnitude of actual releases" (ChemRisk 1999). The response to comment 166 provides additional details about conducting uncertainty analyses.

3. The internationally recognized independent technical reviewers hired by ATSDR to review the Task 6 report evaluated the appropriateness of the empirical c/Q model. They noted that this kind of calculation was appropriate for estimating past airborne uranium concentrations in Scarboro (see page G-8).

Also, ATSDR evaluated the empirical c/Q model used by the Task 6 team in Appendix E of the PHA. ATSDR believes the empirical c/Q model is appropriate for screening because according to ATSDR's linear regression evaluation, the c/Q model overestimates the likely annual average uranium air concentrations in Scarboro.

In addition, Auxier & Associates, Inc. in a report dated November 1998, "Scarboro Community Sampling Results: Implications for Task 6 Environmental Projections and Assumptions," compared the results of the FAMU Scarboro sampling results with the deposition estimates based on the August 1998 Task 6 results (Prichard 1998). The Auxier report concluded that the Task 6 air pathway analysis is supported by the 1998 FAMU Scarboro soil data (Prichard 1998). The report stated that the agreement between deposition inferred from soil samples and deposition predicted on the basis of Task 6 air concentrations projections is well within the uncertainties of the parameters used in the calculations (Prichard 1998). The internationally recognized independent technical reviewers hired by ATSDR commented that the analysis and conclusions of the Auxier report are compelling.

The Task 6 report also states on page 3-21 that the estimated air concentrations of uranium in Scarboro are likely to be overestimated.

4. In 1998 and 2001, FAMU and EPA Region IV, respectively, collected soil, sediment, and surface water samples in and around the Scarboro community (FAMU 1998; EPA 2003). Uranium isotopic content was conducted during both analyses. In addition, EPA Region IV collected uranium core samples from two locations in Scarboro.

Also, as stated above in ATSDR's response to recommendation 2 in comment 105, ATSDR does not believe further nonconservative screening, refined evaluation, or additional sampling for uranium in Scarboro is warranted because the estimated total past and current uranium doses based on the Task 6 screening evaluation (which used several layers of conservative and protective assumptions and approaches) are well below doses expected to cause adverse health effects.

In addition, page 19 of the EPA Region IV report states that EPA "does not propose to conduct any further environmental sampling in the Scarboro community." Page 26 of the same report states that "based on EPA's results, the Scarboro community is safe. Therefore, additional sampling to determine current exposure is not warranted" (EPA 2003). The PHA provides (on page 29) short summaries of the FAMU and EPA sampling. To expand the information presented, ATSDR added summary briefs of the EPA and FAMU reports in Appendix I of the final PHA.

Also, as mentioned previously in ATSDR's response to recommendation #3 in comment 105, the Auxier report concluded that the Task 6 air pathway analysis is supported by the 1998 FAMU Scarboro soil data. The report stated that the agreement between deposition inferred from soil samples and deposition predicted on the basis of Task 6 air concentrations projections is well within the uncertainties of the parameters used in the calculations.

The PHA is the primary public health process ATSDR uses to evaluate the public health implications of people's exposure to environmental contaminants nearby communities. The purpose of the public health assessment process is to find out whether people have been exposed or are being exposed to hazardous substances and, if so, whether that exposure is harmful, or potentially harmful, and should therefore be stopped or reduced. ATSDR used the current data collected by EPA Region IV and FAMU to evaluate the public health implications of current exposure, not to validate the past screening dose reconstruction conducted by the State of Tennessee.

106

Based on our review and evaluation of the PHA, we do no agree with ATSDR's conclusions on past uranium exposures (pp. 138-139) that:

"Despite several conservative parameters, exposure to uranium through both inhalation and ingestion pathways would result in doses below levels of health concern for radiation and chemical health effects" (lines 16-18).

To the contrary, we believe that the parameter values used and the exposure pathways evaluated in the assessment of past exposures are not overly conservative, and that the doses and risks from historical exposures may have exceeded relevant radiation protection dose limits and EPA's accepted risk range.

ATSDR acknowledges that the use of the term "parameters" is misleading, in that it seems to refer to the specific parameter values used by the Task 6 team to calculate the past exposure doses. The word "aspects" was substituted in the final release of the PHA, to encompass a broader meaning for the conservative and protective features of the past evaluation, which are described on pages 48 and 92.

ATSDR also acknowledges that the use of the term "overly" is misleading, in that it seems to imply that the conservative assumptions and approaches which led to an overestimation of concentrations and doses in Task 6 are inappropriate. ATSDR removed the word "overly" in these situations.

107

For past uranium exposures, ATSDR should address the recommendations of several previous reviewers and incorporate improvements, especially formal sensitivity and uncertainty analyses and additional soil core sampling data.

ATSDR Technical Review Process

The Agency for Toxic Substances and Disease Registry (ATSDR) had each of the Phase II Oak Ridge Health Studies documents reviewed by a group of technical experts to evaluate the quality and completeness of the studies and to determine if the studies provide a foundation on which ATSDR can base follow-up public health actions or studies, and particularly, to support its congressionally mandated public health assessment of the ORR. ATSDR will use the information from the Oak Ridge Health Studies, as well as data from the technical reviews and other studies, to develop public health assessments for the ORR.

ATSDR recognizes the great amount of oversight, technical peer review, and overall work that went into the Oak Ridge dose reconstruction project. However, ATSDR wanted an additional round of expert review of the Task 6 uranium screening evaluation to consider for its public health assessment for two reasons. First, ATSDR will not attempt to reproduce (ab initio) the work or results of the Task 6 uranium screening evaluation for its public health assessment. Such an attempt cannot be justified without substantial new information about past releases of uranium, or historic environmental sampling data or meteorological data, which ATSDR does not presently have. Secondly, Task 6 uranium screening evaluation is a technical investigation fraught with uncertainty. ATSDR believes that an independent expert review of the methods and assumptions in the Task 6 uranium screening evaluation offers the best insight into the validity and usefulness of the results for making public health decisions.

ATSDR contracted with Eastern Research Group, Inc. (ERG) of Lexington, Massachusetts, to select four expert reviewers to technically review the uranium screening evaluation Task 6 report: Melvin Carter, Nolan Hertel, Ronald Kathren, and Fritz Seiler. The reviewers were asked to comment on the study design, methods, and completeness of the uranium report, as well as the conclusions of the authors of the report.

ATSDR Note to Reader of Technical Reviewers Comments

ATSDR cautions the reader that some of the technical reviewers' comments are critical of the Task 6 uranium screening evaluation report. This does not mean that the uranium screening evaluation report is flawed or should not be used. The reviewers were not provided with a forum for group discussion or with formal access to the uranium Task 6 study authors to ask questions. Not all reviewers answered every question posed to them. Sometimes they acknowledged they were commenting outside their field of expertise and sometimes they acknowledged that they did not wish to comment outside their field of expertise. The reviewers brought their varied experience to the task, and not all reviewer comments are equally valid. Occasionally two opinions are conflicted. In such an instance (and other information being equal) ATSDR will tend to prefer comments from the reviewer who had the greater expertise in the subject area. Finally, the technical reviewers knew and acknowledged the Task 6 report was a screening evaluation of the uranium releases and not a complete dose reconstruction. ATSDR intends to evaluate each of the reviewer comments for its applicability and usefulness on its own merit and it encourages the reader to do the same.

Technical Reviewers Comments

The internationally recognized expert reviewers concluded that the uranium screening evaluation in the Task 6 report was "technically sound and applicable to decision-making," "supported by and developed on the basis of information in the reports," "conformed with established and generally accepted techniques," and had "no major or significant problems with respect to the study design or the scientific approaches used." Overall, the reviewers agreed that the screening assessment is adequate for public health decision-making (see page G-7). The technical reviewers agreed that IF it is found necessary to evaluate beyond the screening stage, additional modifications such as uncertainty and sensitivity analyses would be required for a complete dose reconstruction. However, they noted that the dose estimates tend to overestimate the extent to which exposure to uranium is a problem in the Oak Ridge area and that further refinements to the study are likely to reveal that uranium exposures are actually lower than those currently estimated (see page G-7)

Task 6 Teams Comment Regarding the Use of the Task 6 Screening Evaluation

Also, the Task 6 team noted that there are areas identified throughout the report that contribute to the overall uncertainty of the results of the screening evaluation. They state: "these areas should be examined IF the evaluation of Oak Ridge uranium releases is to proceed beyond the conservative screening stage, and on to nonconservative screening and possibly a stage of refined evaluations" (see pages 5-2 and 5-3).

ATSDR Conclusion

ATSDR concluded that since the Task 6 uranium screening evaluation routinely and appropriately used several layers of conservatism and protective assumptions and approaches that resulted in overestimated total past uranium doses that are well below (32 times less than) the ATSDR radiogenic comparison value and levels expected to cause adverse health effects, ATSDR categorizes the Y-12 plant as having no apparent public health hazard from uranium exposure and does not believe the evaluation of Y-12 uranium releases requires a further nonconservative screening, a refined evaluation with uncertainty and sensitivity analyses, or additional sampling.

Please see ATSDR's responses to comment 105 for more details on this issue.

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As pointed out earlier, ATSDR relies entirely on the Y-12 airborne and surface water uranium release data, EFPC soil concentration data, and the Level II assessment in the Task 6 Report (ChemRisk 1999) to estimate pathway-specific and total uranium radiological and chemical doses to Scarboro residents from past Y-12 uranium releases. Although it adjusts for a 70-y exposure duration and makes several statements regarding the so-called conservatisms in the evaluation, ATSDR has not improved the assessment of past exposures at Scarboro as recommended by the Task 6 team (ChemRisk 1999, p. 5-3), the Oak Ridge Health Agreement Steering Panel (ORHASP 1999, pp. 71-74), and ATSDR's consultants (PHA Appendix G). We believe that ATSDR should incorporate these recommendations, summarized in Table 6 [ATSDR note: Table 6 is provided in the notes section at the end of this table.], before it completes its evaluation of past exposures and before it makes statements regarding potential health impacts due to these exposures.

ATSDR Conclusion

Since the Task 6 uranium screening evaluation routinely and appropriately used several layers of conservatism and protective assumptions and approaches that resulted in overestimated total past uranium doses that are well below (32 times less than) the ATSDR radiogenic comparison value and levels expected to cause adverse health effects, ATSDR categorizes the Y-12 plant as having no apparent public health hazard from uranium exposure and does not believe the evaluation of Y-12 uranium releases requires a further nonconservative screening, a refined evaluation with uncertainty and sensitivity analyses, or additional sampling.

State of Tennessee's External Peer Review

The Task 6 uranium screening evaluation report underwent an external State of Tennessee's peer review prior to release by the State of Tennessee and ORHASP provided technical oversight throughout the project. The Task 6 report states that "some level of conservatism was maintained in the uranium concentration estimates used in Level II screening to ensure that hazards to a significant portion of the potentially exposed population were not underestimated." Regarding the Task 6 team's suggestions for possible follow-up work on page 5-3, the Task 6 team noted that they identified areas throughout the report that contribute to the overall uncertainty of the results of the screening evaluation. The Task 6 report states on pages 5-2 and 5-3 that "these areas should be examined if the evaluation of Oak Ridge uranium releases is to proceed beyond the conservative screening stage, and on to nonconservative screening and possibly a stage of refined evaluations" (ChemRisk 1999).

ATSDR Technical Review

Also, ATSDR had the Task 6 Report technically reviewed by an independent expert panel of internationally recognized scientists. These scientists pointed out that "the estimates made in the report tend to be on the conservative side–one expects, therefore, that (when in error) the report would tend to overestimate the extent to which exposure to uranium is a problem in the Oak Ridge area. Further refinements to the study are likely to reveal that uranium exposures are actually lower than those currently estimated" (see page G-7 of the PHA and response to comment 107). These reviewers also agreed that the screening assessment is adequate for public health decision-making (see page G-7).

ORHASP Recommendations

Of the eight recommendations presented by ORHASP on pages 71 to 74, #3, #4, and #7 pertain to uranium releases from the Y-12 plant:

ORHASP Recommendation #3 suggested that "a soil sampling program is vital to gain information relevant to the historical contamination levels in residential areas closest to the ORR plants. Any decision about additional dose reconstruction studies should be deferred until the results of the recommended soil sampling program have been obtained and carefully interpreted."

As previously mentioned in ATSDR's response to comment 1, in 1998 and 2001 FAMU and EPA Region IV, respectively, collected soil, sediment, and surface water samples from the Scarboro community (FAMU 1998; EPA 2003). These sampling programs were coordinated with stakeholders to satisfy their concerns. All FAMU samples were analyzed for mercury, gross alpha/beta content, uranium, and gamma-emitting radionuclides. About 10% of the FAMU samples were also analyzed for target compound list organics, target analyte list inorganics, strontium 90, uranium, thorium, and plutonium. All EPA Region IV samples were subjected to a full analytical scan, including inorganic metals, volatile organic compounds, semi-volatile organic compounds, radiochemicals, organochlorine pesticides, and PCBs. Uranium isotopic content was measured during both analyses. In addition, EPA Region IV collected uranium core samples from two locations in Scarboro.

The results of the FAMU and EPA Region IV sampling programs in the residential areas closest to the Y-12 plant were evaluated and interpreted by Auxier, EPA Region IV, ATSDR's technical experts reviewing the Task 6 report, and ATSDR scientists.

Auxier & Associates stated on page 1 of their report that "for the stated scope of the study [FAMU 1998], the number of samples met or exceeded the number recommended in the EPA Region IV guidance (USEPA, Region 4, Science and Ecosystem Support Division, Environmental Investigations Standard Operating Procedures and Quality Assurance Manual, May 1996)" (Prichard 1998). The Auxier report compared the results of the FAMU Scarboro sampling results with the deposition estimates based on the August 1998 Task 6 results. The Auxier report concluded that the Task 6 air pathway analysis is supported by the 1998 FAMU Scarboro soil data (Prichard 1998). The report stated that the agreement between deposition inferred from 1998 soil samples and deposition predicted in the 1950s on the basis of Task 6 air concentrations projections is well within the uncertainties of the parameters used in the calculations (Prichard 1998). The internationally recognized independent technical reviewers hired by ATSDR commented that the analysis and conclusions of the Auxier report are compelling.

The Auxier report also concluded that the Task 6 Scarboro soil pathway that dominates the Task 6 screening index and uses uranium concentrations from EFPC sediment samples is not supported by their evaluation of the FAMU Scarboro soil samples (Prichard 1998). The concentrations of uranium in the EFPC sediment are about an order of magnitude larger than the uranium concentrations detected in the FAMU Scarboro soil samples data (Prichard 1998). Based on Auxier's analysis, the report concludes that the uranium concentrations in the EFPC sediment are unlikely to represent past uranium concentrations in Scarboro soil (Prichard 1998). The internationally recognized independent technical reviewers hired by ATSDR stated that the Auxier report presents convincing evidence that the FAMU soil sampling data are superior to the EFPC sediment data used as surrogates for soil data in the Task 6 soil pathway assessment (Prichard 1998). One reviewer indicated that the Auxier report convinced him that Scarboro uranium soil concentrations are 10 to 100 times lower than the values used in the Task 6 soil pathway assessment.

All four of ATSDR's independent technical reviewers also expressed confidence in the soil sampling data collected by researches from FAMU. One technical reviewer considered the FAMU data clearly superior to the Task 6 EFPC sediment data for use in public health decision-making.

From page 19 of their report: EPA Region IV "does not propose to conduct any further environmental sampling in the Scarboro community" and from page 26: "based on EPA's results, the Scarboro community is safe. Therefore, additional sampling to determine current exposure is not warranted" (EPA 2003). Page 29 in the PHA provides short summaries of the FAMU and EPA sampling. To expand the information presented, ATSDR added summary briefs of the FAMU and EPA reports in Appendix I of the final PHA.

ATSDR's evaluation and the implication of the FAMU and EPA Region IV Scarboro sampling on the Task 6 screening evaluation are discussed in the Current Soil Exposure Pathway discussion under the Current Radiation Effects section (Section III.B.2.a.).

ORHASP Recommendation #4 suggested measuring atmospheric dispersion of controlled tracer releases from representative stacks and vents at Y-12 to develop more reliable estimates of dispersion of uranium released from Y-12 stakes and vents.

This issue was evaluated by Auxier & Associates, ATSDR's independent technical reviewers, and ATSDR scientists. As previously stated, the Auxier report concluded that the Task 6 air pathway analysis is supported by the 1998 FAMU Scarboro soil data. The report stated that the agreement between deposition inferred from 1998 soil samples and deposition predicted in the 1950s (on the basis of Task 6 air concentrations projections) is well within the uncertainties of the parameters used in the calculations.

Two of the technical reviewers hired by ATSDR to review the Task 6 report disagreed about whether or not the tracer dispersion study suggested in recommendation #4 by the ORHASP was warranted. One reviewer suggested that this experiment was warranted, citing the sparse distribution of air monitoring stations in the Oak Ridge area. The other reviewer thought the tracer release studies seemed somewhat excessive and suggested that the existing calculation be reworked. Also, the technical experts pointed out that "the estimates made in the report tend to be on the conservative side–one expects, therefore, that (when in error) the report would tend to overestimate the extent to which exposure to uranium is a problem in the Oak Ridge area" (see page G-7 of the PHA).

In appendix E of the PHA, ATSDR reworked the 1986 to 1995 Y-12 uranium air emissions data and the 1986 to 1995 uranium air radioactivity concentrations in Scarboro using a linear regression analysis and compared these linear regression results to the Task 6 air pathway analysis. ATSDR concluded that the annual average U 234/235 air concentrations and doses using the Task 6 analysis were probably overestimated by a factor of up to 5. Even using these overestimated uranium air concentrations, the estimated radiation dose from uranium is well below the ATSDR radiogenic cancer comparison value and would not likely result in adverse health effects.

Since the conservative Task 6 screening evaluation (see list of conservative aspects of the screening evaluation on pages 48 and 92 of the PHA) reported a total past uranium dose that was overestimated yet still below levels of health concern, there is no need to conduct further air dispersion sampling for additional dose reconstruction studies.

Recommendation #7 suggests continued monitoring of uranium contamination, reported for each isotope. The following is from the Oak Ridge Reservation Annual Site Environmental Report for 1995 (available from http://www.ornl.gov/sci/env_rpt ).

"In 1994, Y-12 Plant personnel issued Evaluation of the Ambient Air Monitoring Program at the Oak Ridge Y-12 Plant (Energy Systems 1994a) and worked with the DOE and TDEC [Tennessee Department of Environment and Conservation] in reviewing the ambient air program for applicability and usefulness of the data. There are no federal regulations, state regulations, or DOE orders that require this monitoring. All ambient air monitoring systems at the Y-12 Plant are operated as a best management practice. With the reduction of plant operations and improved emission and administrative controls, levels of measured pollutants have decreased significantly during the past several years. In addition, processes that result in the emission of enriched and depleted uranium are equipped with stack samplers that have been reviewed and approved by the EPA to meet requirements of the NESHAP regulations. ORR air sampling stations, operated by ORNL in accordance with DOE orders, are located around the reservation. Their locations ensure that areas of potentially high exposure to the public are monitored continuously for parameters of concern.

With agreement from TDEC personnel, the ambient air sampling program at the Y-12 Plant has been significantly reduced, effective at the end of 1994. All fluoride, total suspended particulates (TSP), and particulate matter less than 10 microns in diameter (PM10) sampling has been discontinued, and all but 3 of the 12 uranium samplers have been shut down. The mercury sampling program will continue to monitor ambient air level concentrations as a result of remediation and decommissioning and decontamination activities."

"Prior to 1993, the samples were analyzed for gross alpha and beta and for activity levels of specific uranium isotopes; however, in 1993, the analysis program for radionuclides was revised as described in the Environmental Monitoring Plan for the Oak Ridge Reservation (EMP) to obtain total uranium particulate and the percentage of ²³⁵U. In this manner, uranium concentrations in ambient air could be better correlated to stack emission data, which is also measured as total uranium."

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In addition to these recommendations, we recommend that ATSDR:

1. calculate doses and risks for a range of exposure scenarios specific to Scarboro, but based on similar scenarios used to assess other ORR contaminants, such as I-131;
2. calculate doses to infants and children, not just adults, using the age-dependent dose coefficients in ICRP Publications 71 and 72;
3. calculate age-averaged lifetime cancer risks using EPA's radionuclide slope factors in the Agency's Health Effects Assessment Summary Tables (HEAST) (available at http://www.epa.gov/radiation/heast/download.htm ), which are based on the risk coefficients in Federal Guidance Report No. 13 (EPA 1999); and
4. re-evaluate the chemical effects of uranium using ICRP's revised lung model and physiologically-based biokinetic models to estimate kidney content, and use the evaluation criteria discussed in Appendix M of the Task 6 report (ChemRisk 1999).

The following are ATSDR's responses to each of the recommendations:

1. Not all contaminants behave the same in the environment, nor are the receptor populations the same for all ORR contaminants. The Task 6 team developed contaminant-specific exposure pathways for each contaminant evaluated. The relevant pathways chosen for exposure to uranium are different than the scenarios for the other ORR contaminants (see Tables 7, 9, and 10 for the uranium pathways considered by the Task 6 team). Some of the pathways evaluated for iodine-131, for example, are not applicable for exposure to uranium from the Y-12 plant.



2. In its dose assessment ATSDR primarily focused on two age groups: adults and 1-year-old children. These two groups represent the most likely impacted populations who might come in contact with potentially contaminated surface soils and surface waters. Additionally, during the evaluation of other intake pathways and taking into consideration ingestion rates and body weights, ATSDR determined there were no significant differences between adults and other age groups. For example, Table 19 lists radiation doses following soil ingestion doses by a 1-year-old child and Table 23 gives doses from ingestion of soil by adult males, adult females, 12-year-old children, and 6-year-old children.



3. Following the ATSDR Cancer Framework Policy, ATSDR does not perform risk assessments. The agency, however, does recognize the importance of EPA risk assessment and risk analysis to determine if levels of chemicals at hazardous waste sites pose an unacceptable risk (as defined by regulatory standards and requirements) and to help regulatory officials make decisions in support of cleanup strategies that will ensure overall protection of human health and the environment. ATSDR acknowledges that conservative safety margins are built into EPA risk assessments and that these assessments do not measure the actual health effects that hazardous chemicals at a site have on people. For additional information, please review the framework policy that can be found at http://www.atsdr.cdc.gov/cancer.html. See the response to comment 127 for additional information distinguishing a risk assessment from a health assessment.



4. ATSDR used the most recent dose coefficients and transfer factors as published by the ICRP (supplied on their electronic database of dose coefficients) and those of the EPA (supplied on the Federal Guidance Report 13 Cancer Risk Coefficients for Environmental Exposure to Radionuclides: CD Supplement).

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For past uranium exposures, we believe that ATSDR has underestimated the radiation dose from the inhalation pathway.

[ATSDR note: The commenter provided several pages of tables and equations that could not easily be inserted into this table. Therefore, this comment has been truncated and the entire comment is provided in the notes section at the end of this table.][ATSDR note: The commenter summarized "the Task 6 and ATSDR pathway-specific radiation doses to residents of Scarboro community from past releases of uranium from Y-12" reviewed the procedures ATSDR employed to arrive at "a total effective dose of 155 mrem for all pathways combined." The commenter also summarized the methods used by the Task 6 team "to calculate the doses for the inhalation pathway for both Level I and Level II screening assessments."]

After reviewing the default assumptions used in these calculations, we conclude that the Level II parameter values used by the Task 6 team (and by ATSDR) for f_t (i.e., the fraction of time that a person is exposed) and f_t (i.e., the indoor/outdoor shielding factor) are not appropriate for a "typically" exposed individual. The current f_t value of 0.4 equates to an individual being exposed for only 40% of their time each day or 9.6 hr. The current f_s value of 0.3 means that the concentration of uranium isotopes in indoor air is only 1/3 of the concentration outdoors, and is based on assumption that the house is made of brick or stone.

For residential exposures, EPA's Exposure Factors Handbook recommends 50^th percentile values of 16.4 hr per day indoors and 2 hr per day outdoors (EPA/600/P-95/002Fc, August 1997, p.15-17). Since f_t is the sum of the exposures times indoors (ET_i) and outdoors (ET_o), then f_t = ET_i + ET_o = (16.4/24) + (2/24) = 0.683 + 0.083 = 0.77. For the indoor/outdoor shielding factor, f_s, we believe that a value of 0.5 is more reasonable than the current value of 0.3 and is consistent with the value used by the Task 6 team in the Level I assessment for wood houses. It is also consistent with other values reported in the literature (For example, see: BIOMASS (The IAEA Programme on Biosphere Modeling and Assessment Methods). 2000. Model Testing Using Chernobyl Fallout Data from the Iput River Catchment Area, Bryansk Region, Russia: Scenario "Iput." BIOMASS Theme 2, Environmental Releases, Dose Reconstruction Working Group. International Atomic Energy Agency, Vienna, BIOMASS/2DR/WD02.).

[ATSDR note: The commenter then substituted these values for the current default values and modified the previous equation to account for ET_i and ET_o.]

Using our suggested values for indoor and outdoor exposure times and shielding, we calculate a committed effective dose of 122 mrem for the inhalation pathway, compared with ATSDR's current value of 35 mrem. As shown in Table 1c [ATSDR note: Table 1c is provided in the notes section at the end of this table.], by adding in the doses from the other air pathways and summing the total doses for all exposure pathways, we compute a total effective dose of 242 mrem, compared with ATSDR's current value of 155 mrem.

While it is possible to use standard default assumptions from EPA's Exposure Factors Handbook, the Task 6 team decided to use site-specific parameters they felt were most appropriate to the scenarios evaluated during the Level II screening analysis.

• Local community members provided site-specific exposure information to the Task 6 team.
• ORHASP provided technical oversight throughout the project.
• The Task 6 report underwent an external technical peer review prior to release.
• ATSDR had the report technically reviewed by an expert panel of internationally recognized scientists, who agreed that the screening assessment is adequate for public health decision-making.

That said, even substituting these default parameters, EPA calculated a total effective dose of 242 mrem over 70 years, which is still well below the radiogenic cancer comparison value of 5,000 mrem over 70 years and the average annual U.S. background radiation does of 300 mrem per year from natural sources. Additionally, the one-year approximation for EPA's estimated total dose (3.5 mrem/year) is well below ATSDR's Minimal Risk Level (MRL), the NCRP guidance for public exposure of 100 mrem/year, and the EPA CERCLA cleanup level of 15 mrem/year. Therefore, even using EPA's suggested exposure parameters, the overall conclusion that the total past uranium dose is well below levels of health concern would not change.

As a final note, this comment should have been provided to the Task 6 team during the 1998 public comment period for the Task 6 report. EPA Region IV staff attended many of the ORHASP meetings.

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On page 6 (lines 9-13) of the Summary, ATSDR states: "it should be noted that several levels of conservatism were built into this evaluation of past exposures. The values ATSDR relied on to evaluate past exposure (those from the Task 6 report) came from a screening evaluation that routinely and appropriately used conservative and overly protective assumptions and approaches, which led to an overestimation of concentrations and doses."

On page 54, ATSDR repeats these statements and presents a list of five "conservative aspects" of the evaluation, along with a sentence or two summarizing each conservatism. This list is a compilation of statements made in various places throughout Section III.B.1.a for past radiation exposures. Presumably, although not stated directly, some of these so-called conservatisms also would apply to the assessment of past chemical exposures since the Task 6 team used identical exposure equations and parameter values to calculate radionuclide and chemical intakes.

Essentially, we disagree with ATSDR's assertions, some more than others. In general, we find most unsubstantiated. We address each assertion as follows: [ATSDR note: This comment is split into the following separate comments.]

The Task 6 report states on page 3-27 that "because of the scarcity of information regarding estimates of uranium concentrations in the environment over the period of interest, some conservatism was maintained in the uranium concentration estimates used in the Level II screening to ensure that hazards to a significant portion of the potentially exposed population were not underestimated" (ChemRisk 1999).

In addition, ATSDR had the report technically reviewed by an expert panel of internationally recognized scientists. The technical reviewers pointed out that "the estimates made in the report tend to be on the conservative side–one expects, therefore, that (when in error) the report would tend to overestimate the extent to which exposure to uranium is a problem in the Oak Ridge area. Further refinements to the study are likely to reveal that uranium exposures are actually lower than those currently estimated" (see page G-7 of the PHA).

See below for specific responses to each assertion.

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(1) ATSDR states on page 54: The Task 6 values that ATSDR relied on to evaluate past exposures came from a screening evaluation that routinely and appropriately used conservative and overly protective assumptions, which led to an overestimation of concentrations and doses.

The Task 6 team performed two screening assessments, Level I and Level II. The Level I assessment addressed the maximally exposed individual–a hypothetical individual with the highest potential for exposure to releases–by assuming upper-bound values for intake rates, exposure frequencies, exposure duration, soil concentrations, and other exposure parameters. For Y-12 uranium releases to Scarboro, the Task 6 team calculated a Level I screening index (i.e., a cancer incidence risk) of 1.9 x 10^-3 that exceeded the decision guide of 1 in 10,000 (1 x 10^-4) (ChemRisk 1999, p. 4-12). Consequently, the Task 6 team performed a Level II assessment designed to address an average or more typically exposed individual, assuming "considerably less conservative estimates" for various exposure parameters and uranium soil concentrations. An item-by-item comparison of the exposure parameter values used in the Level I and Level II assessments, presented in Table K-1 of Appendix K of the Task 6 report, confirms that many of the Level II values are substantially less than comparable Level I values. For soil concentrations, the Task 6 team used average values (i.e., 14 pCi/g U 234/235 and 12 pCi/g U 238) compared to maximum values (i.e., 76 pCi/g U 234/235 and 70 pCi/g U 238) for Level I, based on measurements of soil/sediment samples taken from the EFPC floodplain, assuming that the relative concentrations of uranium isotopes were equal to their natural abundances (ChemRisk 1999, p. 3-27). Even after these reductions in conservatisms, the calculated Level II screening index, 8.3 x 10^-5, was only slightly below the decision guide of 1 x 10^-4 (ChemRisk 1999, p. 4-12).

The only discussion of conservatism we could find in the Task 6 report regarding the Level II assessment was the statement made on page 3-27 (bottom) that "conservatism was probably also introduced by the use of 1980 EFPC flood measurements to represent concentrations at Scarboro, which is outside of the floodplain." The Task 6 team defends the use of average EFPC floodplain uranium concentrations as surrogates for actual Scarboro data due to insufficient and unreliable soil measurements at Scarboro and to the uncertainty concerning the level of U 235 enrichment in the soil (Presumably, this refers to the fact that, if the Task 6 team had assumed enriched rather than natural abundances of uranium isotopes, estimated soil activities and corresponding risks might be several times, and perhaps orders of magnitude, higher, depending on the levels of enrichment assumed.) (ChemRisk 1999, p. 3-28, top). To illustrate how the overall results of the assessment would differ if lower soil concentrations were assumed, the Task 6 team calculated screening indices for total uranium soil concentrations of 7 pCi/g and 2 pCi/g, again assuming natural isotopic abundances (ChemRisk 1999, p. 3-28). The resulting screening indices were 5.8 x 10^-5 assuming 7 pCi/g and 5.1 x10^-5 assuming 2 pCi/g, corresponding to 30% and 40% reductions, respectively, compared to a screening index of 8.3 x 10^-5 calculated for the Level II assessment assuming 26 pCi/g (ChemRisk 1999, p. 4-18). They note that soil reductions and screening indices do not scale proportionally because the soil pathways represent only 43% of the total screening index from U 234/235 and 51% from U 238, and because the contributions from the air and surface water pathways to the total screening index (57%) are unaffected by alternative soil concentrations (ChemRisk 1999, p. 4-19).

Based on the discussions above, we conclude that ATSDR's statement that the Task 6 team "used conservative and overly protective assumptions and approaches, which lead to an overestimation of concentrations and doses" is unfounded. Since ATSDR's evaluation of past exposures at Scarboro is based on the Task 6 Level II, not Level I, assessment, then, by extension, ATSDR's dose estimates should not be overestimated because of several layers of conservatism.

While it is true that "the second level of screening was considerably less conservative than the Level I analysis," the Task 6 report states on the bottom of page 3-27 that "because of the scarcity of information regarding estimates of uranium concentrations in the environment over the period of interest, some conservatism was maintained in the uranium concentration estimates used in the Level II screening to ensure that hazards to a significant portion of the potentially exposed population were not underestimated. Conservatism was also introduced by the use of 1980 EFPC floodplain measurements to represent concentrations at Scarboro, which is outside of the floodplain. As such the second level of screening may be more appropriately called a Refined Level I analysis." As previously mentioned, the Auxier report also concluded that the Task 6 Scarboro soil pathway that dominates the Task 6 Level II screening index and uses 1980 uranium concentrations from EFPC sediment samples is not supported by their evaluation of the FAMU Scarboro soil samples (Prichard 1998). The concentrations of uranium in the EFPC sediment are about an order of magnitude greater than the uranium concentrations detected in the FAMU Scarboro soil samples. Based on Auxier's analysis, the report concludes that the uranium concentrations in the EFPC sediment are unlikely to represent past uranium concentrations in Scarboro soil (Prichard 1998). The internationally recognized independent technical reviewers hired by ATSDR stated that the Auxier report presents convincing evidence that the FAMU soil sampling data are superior to the EFPC sediment data used as surrogates for soil data in the Task 6 soil pathway assessment. One reviewer indicated that the Auxier report convinced him that Scarboro uranium soil concentrations are 10 to 100 times lower than the values used in the Task 6 soil pathway assessment. All four of ATSDR's independent technical reviewers also expressed confidence in the soil sampling data collected by researches from FAMU. One technical reviewer considered the FAMU data clearly superior to the Task 6 EFPC sediment data for use in public health decision-making.

As such, the Task 6 report states, "the second level (Level II) of screening may be more appropriately called a Refined Level I analysis. The data that are currently available are not sufficient to support a defensible analysis of average or typical exposures to members of the Scarboro community during the years from the community's inception to the present" (ChemRisk 1999).

The Task 6 report continues on the top of page 3-28, stating that "a significant factor in the decision to maintain a conservative value of soil concentration in Level II screening was the uncertainty concerning the level of U 235 enrichment in the soil... Because of this uncertainty, the concentration corresponding to... 26,000 pCi kg^-1 total uranium was used. To illustrate how the overall results of the assessment would differ if lower soil concentrations were assumed, screening indices were also calculated for soil concentrations of 7,000 and 2,000 pCi kg^-1 total uranium... This discussion gives the reader an indication of how the overall results of the assessment would change if less conservative estimates of soil concentration were used" (ChemRisk 1999).

As a note, similar language is also provided on page ES-9.

As the commenter mentioned, even using the Task 6 uranium screening assessment, the Level II screening index (8.3 x 10^-5) is 1.2 times less than the ORHASP decision guide (1 x 10^-4) and; therefore, below the threshold for consideration of more extensive health effects studies. Based on the ORHASP decision guides, the estimated Task 6 Level II screening risk from off-site exposure to Y-12 uranium is so low that further detailed study of exposures is not warranted. (See the Level II screening index on page 4-12 of the Task 6 report and the ORHASP Decision Guides on page 57 of the ORHASP report.)

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(2) ATSDR states on page 54: The majority of the total uranium dose ...is attributed to frequently eating fish from East Fork Poplar Creek and vegetables grown in contaminated soil over several years. If a person did not regularly eat fish from the creek or homegrown vegetables over a prolonged period of time (which is very probable), then the person's uranium dose would likely have been substantially lower than the estimated doses reported in this public health assessment.

ATSDR's statement makes two points: first, that frequent fish and vegetable consumption accounts for the majority of the total uranium dose, and second, that if individuals ate less EFPC-contaminated fish and vegetables, less frequently, their total dose would be substantially lower. We address each point as follows:

With respect to the first point, Table 2 [ATSDR note: Table 2 is provided in the notes section at the end of this table.] below, shows that fish and vegetable consumption combined accounts for 67% of the total effective dose from all exposure pathways and uranium isotopes for the Task 6 Level II (and ATSDR's) assessment of past Scarboro exposures, each contributing 29% and 38%, respectively.

While we acknowledge that the combined dose from these pathways constitutes the majority (i.e., 67%) of the calculated total uranium dose, we nevertheless believe that this statistic is misleading, in that it overemphasizes the importance of these pathways, since, in our opinion, it is likely an artifact of variable exposure assumptions between Level I and Level II assessments. Our view is based on a comparison of the relative contributions of fish and vegetable consumption to the total doses calculated by the Task 6 team for the Level I and Level II assessment, shown in Table 3. [ATSDR note: Table 3 is provided in the notes section at the end of this table.]

For fish consumption only, which depends solely on the water pathway exposure assumptions, we note that Level I and Level II doses remained constant, yet the relative contribution of the fish pathway to the total dose increased from 1% for Level I to 29% for Level II. This indicates to us that the apparent substantial increase in the contribution of fish consumption pathway to the total dose for the Level II assessment (i.e., 29%) is due not to a change in exposure assumptions or dose but to the reduction or elimination of doses from all other pathways. A review of the parameter values in Table K-1 of the Task 6 report (ChemRisk 1999) specific to the EFPC fish pathway confirms that the Task 6 team applied the same set of values for both assessments.

As shown in Table 3 [ATSDR note: Table 3 is provided in the notes section at the end of this table.], the vegetable consumption pathway, which accounts for 38% of the Level II total dose, derives primarily from the soil pathways (35%) and, to a lesser extent, the air pathways (3%). Comparing Level I and Level II values for the part of the vegetable pathway that derives from the air pathways, we note that while the percent contribution to total dose remained constant at 3%, the dose dropped by a factor of 31 from Level 1 to Level II. Changes in the Level I to Level II exposure parameter values specific to vegetable consumption (see Table K-1, ChemRisk 1999) account for most of this reduction (i.e., a factor of 26). For the soil pathway-dependent component, comparing Level I to Level II assessments, we note that the vegetable pathway dose fell by a factor 44 (again, almost entirely due to changes in the exposure parameter values), yet the percent contribution to total dose dropped by only a factor of 2. Taking both components of the vegetable pathway together, we see rather substantial reductions in dose (factors of 31 and 44) and only small reductions (factors of 1 and 2) in the percent contribution to the total dose. That the vegetable pathway constitutes 38% of the Level II total dose belies the fact that the dose from this pathway is roughly 40 times less than it was under Level I.

The Task 6 Level II screening analysis is an independent evaluation, and is not based on the parameters used during the Level I screening. ATSDR used the Task 6 Level II screening results to evaluate past uranium releases to the environment from the Y-12 plant and past uranium exposures to residents living near the Y-12 plant. Therefore, while it is a nice academic exercise to compare the differences between the Level I and Level II evaluation, it is irrelevant to the fact that the majority of the total uranium dose of the Level II assessment (54% of the total U 234/235 dose and 78% of the total U 238 dose) is attributed to frequently eating fish from the EFPC and eating vegetables grown in contaminated soil over several years. If a person did not regularly eat fish from the creek or homegrown vegetables over a prolonged period of time (which is very probable), then that person's uranium dose would likely have been substantially lower than the estimated doses reported in this PHA (as noted on pages 48 and 92 of the PHA).

While it is true that "the second level of screening was considerably less conservative than the Level I analysis," the Task 6 report states on the bottom of page 3-27 that "because of the scarcity of information regarding estimates of uranium concentrations in the environment over the period of interest, some conservatism was maintained in the uranium concentration estimates used in the Level II screening to ensure that hazards to a significant portion of the potentially exposed population were not underestimated" (ChemRisk 1999).

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With respect to the second point–that if individuals ate less EFPC-contaminated fish and vegetables, less frequently, their total dose would be substantially lower–we point out that the exposure parameter values used in the Task 6 Level II and ATSDR assessments for fish and vegetable consumption are already vanishingly small, when compared to comparable mean, 95^th percentile, and recommended values in EPA's Exposure Factors Handbook (EFH) (Exposure Factors Handbook, Volume II, Food Ingestion Factors, EPA/600/P-95/002Fb, U.S. Environmental Protection Agency, Office of Research and Development, Washington, DC, August 1997), as shown in Table 4, below. [ATSDR note: Table 4 is provided in the notes section at the end of this table.]

For the fish consumption pathway, we note that the assumed daily intake rate for fish caught in EFPC (i.e., 4 g/d for both Level I and Level II assessments) is slightly less than the lower-bound of the range of mean daily intake values given in Table 10-84 of the EFH for freshwater anglers, and constitutes only 20% of the recommended mean value for total daily intake (i.e., 20 g/d, EFH, Table 10-81). Assuming a mean fish serving size of 129 g (EFH, Table 10-82) and an exposure frequency of 350 d/y (ChemRisk 1999, Table K-1), the Level II daily intake rate corresponds to ~11 servings per year of contaminated fish caught in the EFPC (i.e., 4 g/d * 350 d/y ) ¸ 129 g/serving), or about one meal of contaminated fish a month. Depending on the edible portion of the fish caught, it is conceivable that all 11 servings could come from only a few fish.

On page 63, lines 11-13, of the PHA, ATSDR states: "It is ATSDR's understanding that EFPC is not a very productive fishing location and very few people actually eat fish from the creek." Yet on page 81, lines 7-9, of the PHA, ATSDR also states: "However, the creek appears to be too shallow for swimming, and the state has issued a fishing advisory for EFPC that warns people to avoid eating fish from the creek and to avoid contact with the water (ATSDR 1993)." Whether or not EFPC is or was a suitable fishing location is debatable; however, as the Task 6 team concludes: "Even though the consumption rate of fish from this source is relatively low, the concentration in EFPC and the accumulation of uranium in fish elevate the significance of this pathway" (ChemRisk, 1999).

Similar comparisons and conclusions can be made for the vegetable consumption pathway. As shown in Table 4 [ATSDR note: Table 4 is provided in the notes section at the end of this table.], the assumed Level II consumption rate, 0.2 kg/d, is for vegetables only, not for vegetables and fruit, and is at the lower bound of the range of average values listed in the EFH. Moreover, for the air pathway-component of the vegetable pathway, the actual daily intake of uranium-contaminated vegetables is actually far less, i.e., ~0.01 kg/d, when adjusted for the fraction of consumed vegetables assumed to be contaminated (0.2) and the fraction of contamination remaining on vegetables after washing (0.2), calculated as, 0.2 kg/d * 0.2 * 0.2 = 0.008 or ~0.01 kg/d. Likewise, for the soil pathway-component of the vegetable pathway, the actual daily intake of uranium-contaminated vegetables is 0.04 kg/d, when adjusted for the fraction of assumed contaminated vegetables (0.2), calculated as, 0.2 kg/d * 0.2 = 0.04 kg/d. The combined, adjusted rate (i.e., 0.01 + 0.04 = 0.05 kg/d) for home-grown vegetable consumption is small and probably underestimates historical intake rates for residents of Scarboro and other Oak Ridge communities who most likely consumed both home- and locally-grown vegetables and fruits contaminated with uranium during the years of peak releases from Y-12.

While it is possible to use standard default assumptions from EPA's Exposure Factors Handbook, the Task 6 team decided to use site-specific parameters they felt were most appropriate to the scenarios evaluated during the Level II screening analysis. The Task 6 report underwent an external technical peer review prior to release, ORHASP provided technical oversight throughout the project, and community members provided site-specific exposure information. In addition, ATSDR had the report technically reviewed by an expert panel of internationally recognized scientists, who agreed that the screening assessment is adequate for public health decision-making (see page G-7).

The following three paragraphs address issues raised in the comment and help illustrate why it is important to use site-specific parameters over standard default assumptions, when possible:

• The recommendations in EPA's Exposure Factors Handbook, Table 10-81 give the mean intake of 20.1 g/day as the amount of total fish eaten by the general population. This daily intake includes eating 14.1 g/day of marine fish and 6.0 g/day of freshwater/estuarine fish (EPA 1997). EFPC is a freshwater creek. As noted on page 10-25 of EPA's Exposure Factors Handbook, "the recommended values are 6.6 g/day for freshwater/estuarine fish..." (EPA 1997). As noted on page K-7, the Task 6 team estimated that people eat 4.0 g/day of fish from EFPC because "activity is likely to be low due to limited access, the nature of the Creek, and the availability of higher quality fisheries nearby" (ChemRisk 1999).



• The Task 6 team used a factor of 0.2 for the amount of vegetables consumed that are contaminated. However, as noted in Table 13-71 in EPA's Exposure Factors Handbook, people in the South generally tend to eat a lower fraction of home-produced vegetables (0.069) than what the Task 6 team assumed for the Scarboro community.



• Scarboro is known to have private vegetable gardens. The community has not expressed any concern over consumption of homegrown fruits.

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(3) ATSDR states on page 54: The Task 6 report noted that late in the project it was ascertained that the Y-12 uranium releases for some years used to develop the empirical c/Q (c is chi) value may have been understated due to the omission of some unmonitored release estimates. This would cause the empirical c/Q values to be overestimated and in turn would cause the air concentrations to be overestimated.

What the Task 6 report actually says is: (1) information gained late in the Task 6 project indicates that Y-12 uranium releases for some of the years used in the development of the empirical c/Q value may have been understated due to omission of some unmonitored releases; (2) the Task 6 team was not able to evaluate this new data sufficiently to warrant its use in this assessment because of time constraints; (3) if Y-12 uranium releases during years used to develop the empirical c/Q value were indeed under reported, that would mean that the associated empirical c/Q values were overestimated, and concentrations at Scarboro that were estimated using that approach were in turn overestimated; and (4) it is impossible to gauge the magnitude of any biases potentially introduced by this possible under reporting without closely evaluating the bases of the release estimates during the associated years in the 1980s and 1990s (ChemRisk 1999). As noted, the Task 6 team does not provide this new data or any supporting analyses, nor do they speculate on the magnitude of the suspected overestimate. Neither does ATSDR.

Given the lack of data or analyses, it is not advisable to speculate on how the new data might reduce the empirical c/Q value and thereby may or may not decrease reconstructed Scarboro air concentration estimates, doses, and risks. The Task 6 empirical c/Q value is a scaling factor–increasing or decreasing its value will affect air concentrations, doses, and risks proportionally. We agree with the Task 6 team that speculation about such changes are premature at this time, especially given that the uncertainties associated with all Y-12 uranium release estimates have not been quantified. Task 6 team did state that U 234/235 releases may be uncertain by a factor of about 10 and that U 238 releases may be even more uncertain.

The following is the actual quote from page 3-21 of the Task 6 report:

"In addition, information was gained late in the project that indicated that Y-12 uranium releases for some of the years used for development of the empirical c/Q value may have been understated due to omission of some unmonitored release estimates. It was not possible within the time frame of this project to evaluate the new data sufficiently to warrant its use in this assessment. If Y-12 uranium releases during years used to develop the empirical c/Q value applied in this assessment were indeed under reported, that would mean that the associated empirical c/Q were overestimated, and concentrations at Scarboro that were estimated using that approach were in turn overestimated. It is impossible to gauge the magnitude of any biases potentially introduced by this possible under reporting without closely evaluating the bases of the release estimates during the associated years in the 1980s and 1990s" (ChemRisk 1999).

ATSDR agrees that the magnitude of the overestimation is not known. However, there is no doubt that if the release estimates were understated due to omission of some unmonitored release estimates, this would cause the empirical c/Q values to be overestimated and in turn would cause the air concentrations to be overestimated (as noted on pages 48 and 92 of the PHA).

The empirical c/Q is calculated by dividing the uranium air concentration by the uranium release rate (see page 3-17 in ChemRisk 1999). If the uranium release rate was increased from unmonitored release estimates being added to it, the c/Q value would be lowered (for example, if the air concentration is 10 and the release rate is 1, c/Q would be 10; if the air concentration is 10 but the release rate is increased to 2 due to the addition of unmonitored releases, c/Q would be lowered to 5). Applying a lower c/Q value to the uranium release estimates would result in lower estimated uranium air concentrations in Scarboro.

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When this new information about potentially underestimated Y-12 uranium releases becomes available, we suggest that ATSDR, or others, should incorporate these data into a formal uncertainty analysis of radiation doses and risks. The analysis should account for the uncertainties associated with all pertinent variables, including the uranium release estimates, the measured Scarboro airborne uranium concentrations, and the empirical c/Q values.

As stated in the title, the Task 6 report was a "Screening Evaluation of Potential Off-Site Exposure" that routinely and appropriately used several layers of conservatism and protective assumptions and approaches (see list of conservative aspects of the screening evaluation on pages 48 and 92 of the PHA). Additionally, as noted above, lowering the empirical c/Q value by adding the omitted unmonitored release estimates would result in an overestimation of Scarboro air concentrations. Since the screening evaluation, which contained conservative aspects, resulted in a total past uranium dose below levels of health concern, ATSDR does not believe the evaluation of Y-12 uranium releases requires a further refinement with uncertainty and sensitivity analyses.

In addition, based on the ORHASP decision guides, the estimated Task 6 Level II screening risk from off-site exposure to Y-12 uranium is so low that further detailed study of exposures is not warranted. The Level II screening index (8.3 x 10^-5) is 1.2 times less than the ORHASP decision guide (1 x 10^-4) and; therefore, below the threshold for consideration of more extensive health effects studies. (See the Level II screening index on page 4-12 of the Task 6 report and the ORHASP Decision Guides on page 57 of the ORHASP report.)

As discussed in the NCRP Commentary 14, A Guide For Uncertainty Analysis In Dose And Risk Assessments Related To Environmental Contamination (issued in 1996), if a conservatively based screening calculation is performed and this screening calculation indicates the risk is "clearly below regulatory or risk levels of concern," and the possible exposure is low, then a quantitative uncertainty analysis may not be necessary. By design, conservative screenings are "highly unlikely to underestimate the true dose or risk."

Based on this document, ATSDR agrees with the Task 6 authors that a quantitative uncertainty analysis is not needed for this portion of the Oak Ridge Dose Reconstruction Project. On page D-3, the Task 6 authors state that "although an uncertainty analysis of the Task 6 air source term was not within the scope of Task 6, experts interviewed during the project consider release estimates for enriched uranium to be suitable for the Task 6 screening assessment and within an order of magnitude of actual releases" (ChemRisk 1999). The response to comment 166 provides additional details about conducting uncertainty analyses.

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We also suggest that the Task 6 c/Q approach or any other approach used to reconstruct historical uranium air and soil concentrations at Scarboro should be modified, as necessary, and should be verified and validated, perhaps using additional soil core sampling results and meteorological data for Scarboro, as recommended by several reviewers (see Appendix G).

Since the Task 6 Level II screening evaluation, which contained conservative aspects, resulted in a total past uranium dose below levels of health concern, ATSDR does not believe the evaluation of Y-12 uranium releases requires further validation.

Additionally, in 2001, EPA Region IV collected and analyzed core samples from two locations in Scarboro. On page 19, the EPA Region IV report states that it "does not propose to conduct any further environmental sampling in the Scarboro community." On page 26, the report states that "based on EPA's results, the Scarboro community is safe. Therefore, additional sampling to determine current exposure is not warranted" (EPA 2003). Page 29 in the PHA provides a short summary of the EPA sampling. To expand the information presented, ATSDR added a summary brief of the EPA report in Appendix I of the final PHA.

As previously stated, the Task 6 c/Q approach was evaluated by Auxier & Associates, ATSDR's independent technical reviewers, and ATSDR scientists. (See ATSDR's response to ORHASP recommendation #4 in comment 108.)

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(4) ATSDR states on page 54: According to ATSDR's regression analysis, the method that the Task 6 team used to estimate historical uranium concentrations overestimated historical uranium 234/235 air concentrations in Scarboro by as much as a factor of 5. Consequently, airborne uranium 234/235 doses based on this method were most likely overestimated.

What ATSDR does not tell the reader is that:

1. The Task 6 team considered but rejected the use of a linear regression approach, because the ranks of the annual release estimates for U 234/235 and U 238, individually, did not always coincide with the ranks of their respective air concentrations measured at Scarboro (ChemRisk 1999, p. 3-17).
2. To account for this disparity in the ranks of releases and air concentrations, which they thought indicated some uncertainty associated with the air measurements and/or release estimates, the Task 6 team calculated 20 empirical values for c/Q, 10 each for U 234/235 and U 238, based on the 10-y data set for uranium release estimates for Y-12 and associated measured uranium air concentrations at Scarboro, from1986 to 1995 (ChemRisk 1999, p. 3-18).
3. Because statistical analysis of the data proved inconclusive, the Task 6 team combined all 20 empirically-derived c/Q values, assuming a normal distribution, and calculated a single c/Q value of 3 x 10^-7 s m^-3 corresponding to the 95% upper confidence limit of the mean (i.e., 2 x 10^-7 s m^-3) (ChemRisk 1999, p. 3-19).
4. The Task 6 team used this single, 95^th UCL c/Q value of 3 x 10^-7 s m^-3 and the Y-12 release estimates to reconstruct uranium air concentrations at Scarboro for the years 1944 to 1985 and for the years 1986 through 1995, to maintain consistency with prior years, even though measured airborne concentrations were available for this later period (ChemRisk 1999, p. 3-20).
5. The Task 6 team was aware of the limitations of their c/Q approach, due to the fact that only 10 years of monitoring data were available from Scarboro, and that these reported values were for the period 1986-1995, during which time releases from Y-12 were considerably lower than in earlier years (ChemRisk 1999, p. 5-2)

As described in Appendix E of the PHA, ATSDR performed two independent regression analyses, one each for the U 234/235 and U 238 data sets of estimated releases and measured air concentrations for the period 1986-1995. ATSDR used the resultant regression equations to predict new uranium air concentration values of U 234/235 and U 238, separately, for the same time period. Predictably, since its regression analyses are essentially "best fits" of the release and measurement data for 1986-1995, ATSDR found good agreement between their estimated values and measured values for U 234/235 and not as good agreement for the U 238 data. It concludes: "The coefficient of 0.9657 between Scarboro air concentrations and Y-12 U 234/235 emissions indicates that the regression is a very reliable estimator of historic Scarboro air radioactivity concentrations" (PHA, p. E-1, line 28 and p. E-2, lines 1-3). Conversely, because their regression correlation coefficient for U 238 was "only 0.6377," ATSDR concludes that, "the regression equation based on U 238 emissions and measured Scarboro air concentrations is not considered a reliable estimator of historic air concentrations" (p. E-2, lines 18-21).

Since Appendix E provides no data, figures, or analyses comparing measured and Task 6 estimated uranium air concentrations values for U 234/235 and U 238, we performed these comparisons, the results of which are provided in Table 5. [ATSDR note: Table 5 is provided in the notes section at the end of this table.]

As shown in this table, Task 6 uranium air concentrations are indeed higher, on average, than comparable measured values by a factor of 4 for U 234/235 only, a factor of 3 for U 238 only, and a factor of 3 for U 234/235 and U 238 combined. However, these results are entirely predictable and consistent with the Task 6 team's stated intention, namely, to apply the 95^th UCL c/Q value to estimate air concentrations for the 1986-1995 period to maintain consistency with the estimates for 1994-1985, which are based only on the c/Q approach. Since the period 1986-1995 accounts for such a small amount of the total Y-12 uranium releases (and corresponding air concentrations and air pathway-dependent radiation doses and risks), overestimation of the U 234/235 and U 238 air concentrations based on the c/Q approach is of little consequence. Moreover, it also does not necessarily follow that, if estimates of the air concentrations for 1986-1995 are indeed overestimated, then the air concentrations estimated for 1944-1985 are also overestimated, along with associated doses and risks. Finally, we note that ATSDR's regression results are heavily dependent on 2 to 3 influential data points.

Based on these considerations, we conclude that ATSDR's regression analysis: (1) only underscores the uncertainties in the release and measurement data and the limitations of any approach that uses these data to reconstruct historical uranium air concentrations at Scarboro; (2) is neither a "very reliable estimator" of historic Scarboro U 234/235 concentrations nor an unreliable estimator of U 238 concentrations, contrary to statements made; (3) is not a demonstrably better or worse approach than the Task 6 c/Q approach; and (4) should not, by itself, be relied upon to determine whether or not the c/Q approach either overestimates or underestimates air concentrations. Given ATSDR's conclusion regarding the superiority of its linear regression approach, we were surprised to discover that ATSDR did not apply it to derive revised uranium air concentrations and revised dose estimates for Scarboro.

Both the Y-12 uranium emission measurements and the station 46 (Scarboro) air concentration measurements are continuous distributions. That is, the data values may be any positive integer or non-integer number. Conversely, the annual ranking values used by the Task 6 investigators represent a discrete distribution and the values are represented by integers only. Consequently, use of the annual ranking values to assess the correlation between Y-12 uranium emissions and Scarboro uranium air concentrations is not an appropriate test and the results of that test are not valid. The uncertainty produced by that inappropriate test reflects the error of the test method rather than uncertainty in the emission and dispersion processes.

The regression analyses of the U 235 and U 238 data indicate that the U 235 emission and Scarboro air concentrations are positively correlated, but that there is more variability in the U 238 data. The variability of the U 238 data may be associated with the contribution of background U 238 to measured Scarboro air concentrations or with the greater uncertainty of the U 238 emission estimates. The very high U 235 correlation coefficient indicates that the U 235 regression equation is a better estimator of the dispersion and transport processes of Y-12 emissions and the resulting Scarboro uranium air concentrations.

Regardless of the source of the variation in the respective regression analyses, the regression equations for the U 235 and U 238 analyses clearly indicate a significant difference in the sample populations. Combining these populations into one statistical population (per the Task 6 c/Q evaluation) is inappropriate and simply compounds the uncertainty regardless of its source.

Our conclusion that the Task 6 c/Q process overestimates Scarboro uranium air concentrations is well supported by Figures E-1 and E-4. Based on these figures and the correlation coefficient of nearly 1, the regression equation is a valid estimator of the emission and dispersion processes for the 10-year period of measurement. Figures E-1 and E-4 graphically plot the specific data values for the estimated Task 6 uranium air concentrations, the measured Scarboro uranium air concentrations, and the values estimated using the regression method. Relative to Figure E-4, the Task 6 c/Q method both overestimates and underestimates the measured Scarboro concentrations and as such is not a reliable indicator of the measured values, much less the historic estimated values.

As there is no reason to assume that those dispersion processes have changed from the period of measurement (1986 to 1995) relative to the earlier period (in which emissions were higher but Scarboro air concentrations were unmeasured), the regression estimation process should be a very reliable indicator of historic uranium air concentrations.

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We suggest that ATSDR or others should do a more thorough analysis of the uncertainties for all uncertain variables, such as the uranium release estimates, the measured Scarboro airborne uranium concentrations, and the empirical c/Q values. We also suggest that the Task 6 c/Q approach, or any other approach used to reconstruct historical uranium air and soil concentrations at Scarboro, should be modified, as necessary, verified and validated, perhaps using additional soil core sampling results and meteorological data for Scarboro, as recommended by several reviewers (see Appendix G).

As stated in the title, the Task 6 report was a "Screening Evaluation of Potential Off-Site Exposure." Since the screening evaluation (which contained conservative aspects [see list of conservative aspects of the screening evaluation on pages 48 and 92 of the PHA]) resulted in a total past uranium dose below levels of health concern, ATSDR does not believe the evaluation of Y-12 uranium releases requires a further refinement with uncertainty and sensitivity analyses.

As discussed in the NCRP Commentary 14, A Guide For Uncertainty Analysis In Dose And Risk Assessments Related To Environmental Contamination (issued in 1996), if a conservatively based screening calculation is performed and this screening calculation indicates the risk is "clearly below regulatory or risk levels of concern," and the possible exposure is low, then a quantitative uncertainty analysis may not be necessary. By design, conservative screenings are "highly unlikely to underestimate the true dose or risk."

Based on this document, ATSDR agrees with the Task 6 authors that a quantitative uncertainty analysis is not needed for this portion of the Oak Ridge Dose Reconstruction Project. On page D-3, the Task 6 authors state: "although an uncertainty analysis of the Task 6 air source term was not within the scope of Task 6, experts interviewed during the project consider release estimates for enriched uranium to be suitable for the Task 6 screening assessment and are within an order of magnitude of actual releases" (ChemRisk 1999). The response to comment 166 provides additional details about conducting uncertainty analyses.

Additionally, in 2001, EPA Region IV collected and analyzed core samples from two locations in Scarboro. Page 19 their report stated that EPA Region IV "does not propose to conduct any further environmental sampling in the Scarboro community." Page 26 of their report stated that "based on EPA's results, the Scarboro community is safe. Therefore, additional sampling to determine current exposure is not warranted" (EPA 2003). Page 29 in the PHA provides a short summary of the EPA sampling. To expand the information presented, ATSDR added a summary brief of the EPA report in Appendix I of the final PHA.

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(5) ATSDR states on page 54: Using the International Commission on Radiological Protection's dose conversation factors tends to overestimate the actual radiological doses due to the built-in conservative assumptions (i.e., selecting variables that typically overestimate the true but uncertain physical and biological interactions associated with radiation exposure) (for examples, see Harrison et al. 2001; Leggett 2001).

ATSDR repeats this and similar statements at several locations throughout Section III the and directs readers to Appendix F "for additional information about the conservatism built into ICRP's dose conversion factors" (PHA, p. 62, lines 10-11). Appendix E, titled A Conservative Approach in Radiological Dose Assessment, Issues Associated with Being Protective or Overestimating Radiological Dose, is three pages long. It consists of very brief discussions of ICRP dose coefficients, radiation and tissue weighting factors, detriment coefficients for workers and members of the public, and a two-paragraph summary that concludes, in part, that (a) the establishment of dose coefficients or dose conversation factors involves much uncertainty in the parameters leading to the calculation of the coefficient, and (b) because of human variability, a standardized human commonly called a "reference man" is used to estimate the radiological dose. ATSDR provides no information from the two references it cites (i.e., Harrison et al. 2001 and Leggett 2001) to substantiate its claims.

While we agree that ICRP's dose coefficients are uncertain (as pointed out Harrison et al. 2001 and Leggett 2001), we disagree with ATSDR's assertions that the ICRP intentionally incorporates overly conservative assumptions into all its models in order to derive coefficients that over predict radionuclide intakes and radiation doses. To the contrary, ICRP has, in fact, expended great effort to improve their intake, biokinetic, dosimetric, and risk models, and the reliability of their dose estimates for occupational and environmental exposures, as is clearly documented in the following ICRP Publications:

• ICRP (1989) Age-Dependent Doses to Members of the Public from Intake of Radionuclides, Part 1, ICRP Publication 56.
• ICRP (1991) 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60.
• ICRP (1992) The Biological Basis for Dose Limitation in the Skin, ICRP Publication 59.
• ICRP (1993) Age-Dependent Doses to Members of the Public from Intake of Radionuclides, Part 2, ICRP Publication 67.
• ICRP (1994a) Human Respiratory Tract Model for Radiological Protection, ICRP Publication 66.
• ICRP (1994b) Dose Coefficients for Intakes of Radionuclides by Workers, ICRP Publication 68.
• ICRP (1995a) Age-Dependent Doses to Members of the Public from Intake of Radionuclides, Part 3, ICRP Publication 69.
• ICRP (1995b) Age-Dependent Doses to Members of the Public from Intake of Radionuclides, Part 4, ICRP Publication 71.
• ICRP (1995c) Basic Anatomical and Physiological Data for Use in Radiological Protection: The Skeleton, ICRP Publication 70.
• ICRP (1996) Age-Dependent Doses to Members of the Public from Intake of Radionuclides, Part 5. Compilation of Ingestion and Inhalation Dose Coefficients, ICRP Publication 72.

Specifically, ICRP recently introduced a new respiratory tract model (ICRP 1994a) that involves considerably greater detail and physiological realism than previous models of the respiratory system. ICRP's current model of the gastrointestinal (GI) tract, which was originally developed for occupational intakes of radionuclides, has been adapted to account for environmental intakes of radionuclides by members of the public, with age-specific GI tract absorption values. ICRP's revised systemic biokinetic and dosimetry models involve parameter values that vary with age. Physiologically-based models are used for radioisotopes of calcium, iron, strontium, iodine, barium, lead, radium, thorium, uranium, neptunium, plutonium, americium, and curium, that depict loss of material by specific excretion pathways, feedback of material from organs to blood plasma, and certain physiological processes that are known to influence the distribution and translocation of the elements in the body.

Dr. Keith Eckerman–leader of the Dosimetry Research Group at Oak Ridge National Laboratory, member of several ICRP committees, including the Chair of the Task Group on Dosimetry, and principal author of all EPA Federal Guidance Documents on dose and risk coefficients confirms that the ICRP strives to use realistic models and parameter values in their calculations of dose coefficients to provide best estimates of radiation dose per unit intake or exposure (personal communication, April 2003). He notes that the degree of biological realism incorporated into each of the ICRP models is limited by practical considerations regarding the amount and quality of information available to determine actual paths of movement and parameter values for specific elements. However, he refutes ATSDR's contention that ICRP adds conservative assumptions to the dose conversion factor values to overestimate radiological doses.

Thank you for your comment. ATSDR reevaluated the appendix and incorporated changes to reflect a more accurate discussion of this issue.

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(6) ATSDR states on page 54: In evaluating the soil exposure pathway, the Task 6 team used EFPC floodplain soil data to calculate doses. Actual measured soil concentrations in Scarboro are much lower than the uranium concentrations in the floodplain soil. Consequently, the uranium doses that were estimated for the residents were overestimated because of the use of the higher EFPC floodplain uranium concentrations. The estimated doses would be much lower if they were based on actual measured soil concentrations in Scarboro.

We agree with the Oak Ridge Health Agreement Steering Panel (ORHASP 1999, pp. 52 and 72) that: (1) the results of recent sampling at Scarboro are not representative of earlier years because only surface soil was sampled; (2) collection and analysis of cores of soil from a variety of locations in and around Scarboro is needed to determine site-specific surface and subsurface soil concentrations and to check the validity of the screening calculation; (3) the depth of the core samples must be sufficient to encompass the region of downward migration of uranium in soil; (4) detailed profiles of uranium concentrations as a function of depth are necessary to understand historical patterns of contamination and migration; (5) sampling sites should be in undisturbed areas; and finally (6) any decision about additional dose reconstruction should be deferred until the results of the recommended soil sampling program have been obtained and carefully interpreted.

ATSDR agrees with ORHASP that "any decision about additional dose reconstruction studies should be deferred until the results of recommended soil sampling program have been obtained and carefully interpreted" (ORHASP 1999 page 72).

The Task 6 team, ATSDR scientists, Auxier & Associates, and ORHASP all agree that the EFPC floodplain concentrations used in the Task 6 soil pathway assessment are higher than soil concentrations found in Scarboro:

• The Task 6 report stated on the bottom of page 3-27 that "conservatism was probably also introduced by the use of 1980 EFPC floodplain measurements to represent concentrations at Scarboro, which is outside of the floodplain" (ChemRisk 1999).



• As shown in Figure 18 and Table 11 of ATSDR's PHA, the actual uranium radioactivity concentrations in Scarboro soil are approximately 8 to 22 times less than the EFPC floodplain soil concentrations.



• In 1998, DOE hired Auxier & Associates to compare the results of the Scarboro survey with relevant aspects of the Task 6 report (Prichard 1998). The report stated on page 12 that "the results of the Scarboro soil sampling are clearly relevant to the Task 6 dose projections, and by extension, the screening indices...The agreement between deposition inferred from soil samples and deposition predicted on the basis of Task 6 air concentration projections is well within the uncertainties of the parameters used in these calculations. It is concluded that the 1998 soil sampling results are very supportive of the August, 1998 Task 6 projection of the historical average concentration of uranium in air in the Scarboro Community."

Three of the technical reviewers hired by ATSDR commented on the Auxier report, describing its analysis and overall conclusions as compelling. Two reviewers stated that it presented convincing evidence that the Scarboro soil sampling data (FAMU 1998) are superior to the EFPC sediment samples used as surrogates for soil data in the Task 6 report. One reviewer indicated that the Auxier report convinced him that uranium soil concentrations are 10 to 100 times lower than the values listed in the Task 6 report. The reviewer described the Auxier report as "valuable work" that will "add the kind of information which will be needed for a risk assessment" (see page G-10 in PHA).

• ORHASP stated on page 52: "estimates of soil concentrations from uranium deposition in Scarboro (P. Voilleque, 1998) suggest that the sediment sample concentration used may have been 10 times higher than the peak concentration in Scarboro soil. Collection and analysis of cores of soil from a variety of locations have been recommended as a possible means to resolve this question and to check the validity of the screening calculation" (ORHASP 1999).

In 2001, EPA Region IV collected and analyzed core samples from two locations in Scarboro. On page 19 of its report, EPA Region IV states that it "does not propose to conduct any further environmental sampling in the Scarboro community." On page 26, the report states that: "based on EPA's results, the Scarboro community is safe. Therefore, additional sampling to determine current exposure is not warranted" (EPA 2003). Page 29 in the PHA provides a short summary of the EPA sampling. To expand the information presented, ATSDR added a summary brief of the EPA report in Appendix I of the final PHA.

In conclusion, ATSDR agrees that actual measured uranium soil concentrations in Scarboro are much lower than the uranium concentrations in the EFPC floodplain soil. Since the screening evaluation (which contained conservative aspects [see list on pages 48 and 92 of the PHA], including the use of the higher EFPC floodplain uranium concentrations to estimate exposure levels in Scarboro) resulted in a total past uranium dose well below levels of health concern, ATSDR does not believe the evaluation of Y-12 uranium releases requires additional dose reconstruction evaluation. ATSDR also agrees with EPA Region IV that additional sampling is not warranted.

For additional discussion of the ORHAP recommendation #3, see ATSDR's response to comment 108.

In addition, based on the ORHASP decision guides, the estimated Task 6 Level II screening risk from off-site exposure to Y-12 uranium is so low that further detailed study of exposures is not warranted. The Level II screening index (8.3 x 10^-5) is 1.2 times less than the ORHASP decision guide (1 x 10^-4) and; therefore, below the threshold for consideration of more extensive health effects studies. (See the Level II screening index on page 4-12 of the Task 6 report and the ORHASP Decision Guides on page 57 of the ORHASP report.)