Occurrence of Radionuclides in Residents of the Carlsbad, New Mexico Area

Introduction

Citizen volunteers from the Carlsbad, New Mexico area were monitored for internally deposited radionuclides through a project entitled "Lie Down and Be Counted" (LDBC). This project is provided as an outreach service to the public to support education about naturally occurring and man-made radioactivity present in people who live in the vicinity of the WIPP. The data collected prior to the opening of the WIPP facility (26 March 1999) serve as a baseline for comparisons with periodic follow-up measurements that are slated to continue throughout the 35-year operational phase of the WIPP. It is important to note that these data represent an interim summary (through 1 October 2000) of an ongoing study.

Participating in the LDBC consists of a lung and whole body count every two years. Volunteers are recruited through presentations to local community groups and businesses. The entire measurement process takes approximately one hour. A detailed description of the measurement protocol, analysis and instrument detection limits is provided in the CEMRC 1998 Report. In addition, the status of the project and results are updated quarterly on the CEMRC website and reported semi-annually through a newsletter distributed throughout the local community.

Results

As of 1 October 2000, 500 individuals had participated in the LDBC project. At the time the WIPP opened, 367 individuals had been measured using the in vivo protocol. This group of 367 measurements constitutes the pre-operational baseline to which subsequent results are compared. Counts performed after the opening of the WIPP are considered to be a part of the operational monitoring phase of the WIPP EM. Recounts of the original cohort began in July 1999, and 98 recounts had been performed through 1 October 2000. In addition, 133 new volunteers have participated in the program since 1 October 1999. These new volunteers form an additional baseline cohort for future comparisons.

Demographic characteristics (Table 18) of the current LDBC cohort are statistically unchanged from those reported in the CEMRC 1999 Report, and are generally consistent with those reported in the 1990 census for citizens living in Carlsbad. The largest deviations reported in the 1999 report between the LDBC cohort and 1990 census were the over-sampling of males and under-sampling of Hispanics. Since that time, slight improvements have been made with respect to the percent of males and Hispanics represented, but Hispanics are still 50% under-sampled relative to the 1990 census. During 2001, demographic characteristics of the LDBC project will be compared to results of the 2000 census, which will provide a more accurate representation of the current population. In addition, it is important to note that if the presence of a radionuclide is dependent on a subclass of interest (gender, ethnicity, etc.), valid population estimates can still be made by correcting for the proportion of under- or over-sampling for the particular subclass.

As discussed in detail in the CEMRC 1998 Report and elsewhere (Webb et al., 2000, Radiat. Prot. Dosim. 89, 183), the criterion, L_C, was used to evaluate whether a result exceeds background, and the use of this criterion will result in a statistically inherent 5% false positive error rate per pair-wise comparison (5% of all measurements will be determined to be positive when there is no activity present in the person). For the baseline measurements (N = 367), the percentage of results greater than L_C were consistent with a 5% random false positive error rate, at the 95% confidence level (1 to 9%), for all radionuclides except ²³⁴³Th via ²¹²Pb, ²³⁵U / ²²⁶Ra, ⁶⁰Co, ¹³⁷Cs, ⁴⁰K, ³⁴Mn, ²³²Th via ²²⁸Ac and ⁶⁵Zn (Table 19). As discussed in detail in the 1998 report, five of these (²³²Th via ²¹²Pb, ⁶⁰Co, ⁴⁰K, ⁵⁴Mn (²²⁸Ac interference) and ²³²Th (via ²²⁸Ac) are part of the shield-room background and positive detection is expected at low frequency. ⁴⁰K is a naturally occurring isotope of an essential biological element, so detection in all individuals is expected. ¹³⁷Cs and ²³⁵U / ²²⁶Ra are not components of the shielded room background and were observed at frequencies greater than the 95% confidence interval for the false positive error rate (discussed in more detail later). It is interesting that no result above L_C has been observed for ⁶⁵Zn in the baseline or monitoring phase. It is unlikely that this result is a statistical anomaly, since results greater than L_C were observed for all other radionuclides. This observation suggests an abnormality in the analysis algorithm at that photon energy and further investigation is underway.

For the operational monitoring counts (Table 19, N = 226), the percentage of results greater than LC were consistent with baseline at a 95% confidence level (margin of error), except for ⁶⁰Co and ²³²Th (via ²²⁸Ac). For these radionuclides, the percentage of results greater than L_C decreased relative to the baseline. This would be expected for ⁶⁰Co, since the radionuclide has a relatively short half life (5 years), and the content within the shield has decreased via decay by approximately 45% since the baseline phase of monitoring. The ²³²Th (via ²²⁸Ac) results were unexpected and further investigation is necessary to explain this observation.

The margin of error could not be calculated for the ¹⁰³Ru percentage because the sample size of the operational monitoring counts was too small at the time of this summary. When sufficient operational monitoring counts are performed, the margin of error will be calculated. In addition, margins of error can not be calculated for percentages of 100 and 0 (⁴⁰K and ⁶⁵Zn, respectively), applying binomial statistics, since these percentages represent non-binomial data.

It is important to note that operational monitoring includes the counting of new individuals and the recounting of previously measured participants. Based on the data reported herein, there is no evidence of an increase in the frequency of detection of internally deposited radionuclides for citizens living within the vicinity of the WIPP, since the WIPP began receipt of radioactive waste.

⁴⁰K results were positive for all participants (n = 500), ranging from 1088 to 4462 Bq per person with an overall mean (± SE) of 2868(± 40) Bq per person. Such results are expected since K is an essential biological element contained primarily in muscle, and a theoretical constant fraction of all naturally occurring K is the radioactive isotope ⁴⁰K. The mean ⁴⁰K value for males (± SE), was 3474 (± 41) Bq per person, which was significantly greater (p < 0.0001) than that of females, which was 2156 (± 34) Bq per person. This result was expected since, in general, males tend to have larger body sizes and greater muscle content than females.

Detectable ¹³⁷Cs is present in 26.1 ± 3.5% (95% confidence level, baseline and operational monitoring counts) of citizens living in the Carlsbad area. These results are consistent with findings previously reported in CEMRC 1998 and 1999 reports and elsewhere (Webb, J. L. et al., 2000, loc. cit.). Detectable ¹³⁷Cs body burdens ranged from 5.2 to 62.9 Bq per person with an overall mean (± SE) of 10.4 (± 0.5) Bq per person. The mean ¹³⁷Cs body burden for males (± SE), was 11.1 (± 0.7) Bq per person, which was significantly greater (p = 0.02) than that of females, which was 8.9 (± 0.4) Bq per person. As previously reported (CEMRC 1998 Report; CEMRC 1999 Report; Webb, J. L. et al., 2000, loc. cit.) the presence of 137Cs was independent of ethnicity, age, radiation work history, consumption of wild game, nuclear medical treatments and European travel. Occurrence of detectable ¹³⁷Cs was associated with gender, where males had higher prevalence of ¹³⁷Cs relative to females. Presence of ¹³⁷Cs was also associated with smoking (where smokers had a higher prevalence relative to non-smokers). It is likely that the association with gender is related to the tendency for larger muscle mass in males than in females, as supported by the ⁴⁰K results. The association of ¹³⁷Cs with smoking could be related to the presence of fallout ¹³⁷Cs in tobacco, decreased pulmonary clearing capability in smokers, or other as yet unidentified factors, and further study is warranted.

As reported in the CEMRC 1998 and 1999 reports, the percentage of results greater than LC for ²³⁵U / ²²⁶Ra (12%) are significantly (although slightly) higher than the distribution-free confidence interval for a 5% random false positive error rate (1 to 9%). These data are not nearly as compelling as those for ¹³⁷Cs, but the large sample size of the current cohort tends to support the observed pattern. Although ²³⁵U and 226Ra cannot be differentiated via gamma spectroscopy, it is likely the signal is the result of ²²⁶Ra because the natural abundance of ²²⁶Ra is much greater than that of ²³⁵U. However, further study, possibly involving in vitro bioassay for the two radionuclides (the nuclides can be distinguished via alpha spectrometry), is needed. No additional investigations into this observation were conducted during this reporting period.

Table 18. Demographic Characteristics of the

"Lie Down and Be Counted" Population Sample through 1 October 2000

^aThe margin of error represents the 95% confidence interval of the observed proportion.; under complete replication of this experiment, one would expect the confidence interval to include the true population proportion 95% of the time if the sample was representative of the true population.

^bUnited States Department of Commerce, Economics and Statistics Administration, Bureau of the Census. 1990 Census of Population. Washington, DC: US Government Printing Office

^cNA = not available

Table 19. "Lie Down and Be Counted" Results through 1 October 2000

^aN = number of individuals

^bTo determine whether or not activity has been detected in a particular person, the parameter L_C is used; the _C represents the 95^th percentile of a null distribution that results from the differences of repeated, pair-wise background measurements; an individual result is assumed to be statistically greater than background if it is greater than L_C

^cThe margin of error represents the 95% confidence interval of the observed percentage; under complete replication of this experiment, one would expect the confidence interval to include the true population percentage 95% of the time, if the sample was representative of the true population

^dThese radionuclides are present in the shield background, so they are expected to be detected periodically

^eNC = not calculated; the margin of error cannot be calculated for the observed percentage because the sample size for operational monitoring counts is as yet too small; when sufficient counts are performed, the margin of error will be calculated; the margin of error for ⁴⁰K cannot be calculated since this radionuclide is present in all individuals