LOW-VOLUME AIR SAMPLING
Radioactivity associated with airborne particulates was monitored continuously by 18 low-volume air samplers (two of which are used as replicate samplers) at 16 locations during the second quarter of 2002 (Figure 1 below). Three of these samplers are located on the INEEL, nine are situated off the INEEL near the boundary, and six have been placed at locations distant to the INEEL. Samplers are divided into INEEL, Boundary, and Distant groups to determine if there is a gradient of radionuclide concentrations, increasing towards the INEEL. Each replicate sampler is relocated every year to a new location. One replicate sampler was placed at Arco (Boundary location) and one at Howe (Boundary location) during 2002. An average of 13,168 ft3 (373 m3) of air was sampled at each location, each week, at an average flow rate of 1.3 ft3/min (0.04 m3/min). Particulates in air were collected on glass fiber particulate filters (1.2-µm pore size). Gases passing through the filter were collected with an activated charcoal cartridge.
Filters and charcoal cartridges were changed weekly at each station during the quarter. Each particulate filter was analyzed for gross alpha and gross beta radioactivity using thin-window gas flow proportional counting systems after waiting about four days for naturally-occurring daughter products of radon and thorium to decay. More information concerning gross alpha and beta radioactivity can be found in Gross versus Specific Analyses under Helpful Information.
Charcoal cartridges were analyzed for gamma-emitting radionuclides, specifically for iodine-131 (131I). Iodine-131 is of particular interest because it is produced in relatively large quantities by nuclear fission, is readily accumulated in human and animal thyroids, and has a half-life of eight days. This means that any elevated level of 131I in the environment could be from a recent release of fission products.
Gross alpha results are reported in Table C-1. Median gross alpha concentrations in air for INEEL, Boundary, and Distant locations for the second quarter of 2002 are shown in Figure 2. The data were tested for normality prior to statistical analyses. For the most part the data showed no discernable distribution. Box and whisker plots are commonly used when there is no assumed distribution. Each data group in Figure 2 is presented as a box and whisker plot, with a median, a box enclosing values between the 25th and 75th percentiles, and whiskers representing the non-outlier range. Note that outliers and extreme values are identified separately from the box and whiskers. Outliers and extreme values are atypical, infrequent, data points that are far from the middle of the data distribution. For this report, outliers are defined as values that are greater than 1.5 times the height of the box, above or below the box. Extreme values are greater than 2 times the height of the box, above or below the box. Outliers and extreme values may reflect inherent variability, may be due to errors associated with transcription or measurement, or may be related to other anomalies. A careful review of the data collected during the second quarter indicates that the outliers and extreme values were not due to mistakes in collection, analysis, or reporting procedures, but rather reflect natural variability in the measurements. The outliers and extreme values lie within the range of measurements made within the past five years. Thus, rather than dismissing the outliers, they were included in the subsequent statistical analyses. Further discussion of box plots may be found in Determining Statistical Differences under Helpful Information.
Figure 2 graphically shows that the gross alpha measurements made at INEEL, Boundary, and Distant locations are similar for the second quarter. If the INEEL were a significant source of offsite contamination, concentrations of contaminants could be statistically greater at Boundary locations than at Distant locations. Because there is no discernable distribution of the data, the nonparametric Kruskal-Wallis test of multiple independent groups was used to test for statistical differences between INEEL, Boundary, and Distant locations. The use of nonparametric tests, such as Kruskal-Wallis, gives less weight to outliers and extreme values thus allowing a more appropriate comparison of data groups. A statistically significant difference exists between data groups if the (p) value is less than 0.05. Values greater than 0.05 translate into a 95 percent confidence that the medians are statistically the same. The p-value for each comparison is shown in Table D-1. There were no statistical differences in gross alpha concentrations between groups of data during the second quarter 2002.
Comparisons of gross alpha concentrations were made for each month of the quarter (Figures 3 – 5 linked below). Again the Kruskal-Wallis test of multiple independent groups was used to determine if statistical differences exist between INEEL, Boundary, and Distant data groups. There were no statistical differences in gross alpha results between groups for any month during the second quarter (Table D-1).
As a further check, comparisons between gross alpha concentrations measured at Boundary and Distant locations were made on a weekly basis. The Mann-Whitney U test was used to compare the Boundary and Distant data because it is the most powerful nonparametric alternative to the t-test for independent samples. INEEL sample results were not included in this analysis because the onsite data, collected at only three locations, are not representative of the entire INEEL and would not aid in determining offsite impacts. The gross alpha concentrations measured at Boundary locations were not statistically greater than those measured at Distant locations in any of the thirteen weeks of data evaluated (Table D-2). More detail on the statistical tests used can be found in Determining Statistical Differences under Helpful Information.
Gross beta results are presented in Table C-1. Gross beta concentrations in air for INEEL, Boundary, and Distant locations for the second quarter of 2002 are shown in Figure 6. The data were tested and found to be neither normally nor lognormally distributed. Box and whiskers plots were used for presentation of the data. Outliers and extreme values were retained in subsequent statistical analyses because they are within the range of measurements made in the past five years, and because these values could not be attributed to mistakes in collection, analysis, or reporting procedures. As in the case of alpha activity, the quarterly data for each group appear to be similar and were determined using the Kruskal-Wallace test to be statistically the same (Table D-1).
Monthly median gross beta concentrations in air for each
sampling group are shown in
Comparison of weekly Boundary and Distant data sets, using the Mann Whitney U test, indicate no statistical differences between weekly Boundary and Distant measurements during the second quarter 2002 (Table D-2).
No 131I was detected in any of the charcoal cartridge batches collected during the second quarter of 2002. Weekly 131I results for each location are listed in Table C-2 of Appendix C Gamma spectrographic analysis is also done with the131I analysis. Cesium-137 was not detected in any of the measured cartridges.
Weekly filters for the second quarter of 2002 were
composited by location. All samples were analyzed for gamma-emitting
radionuclides, including 137Cs. Composites were also analyzed
for 90Sr, 238Pu, 239/240Pu, and 241Am.
Nine samples (composited quarterly from the individual air filters
collected weekly at Arco, Arco QA, Atomic City, Blackfoot CMS, Blue Dome,
Dubois, Main Gate, Mud Lake, and Jackson, WY) showed at least one
human-made radionuclide greater than its related 2s value (Figure 10). Occasional detection of human-made
radionuclides is not unusual and represents natural variations of these
fallout introduced radionuclides. The concentrations measured during this
quarter are consistent with those recorded in the past. All results were
far less than their respective DCGs. All results for composite filter
samples are shown in Table C-3, Appendix C.