The primary pathway by which radionuclides can move off the INL Site is through the air and for this reason the air pathway is the primary focus of monitoring on and around the INL Site. Samples for particulates and iodine-131 (131I) gas in air were collected weekly for the duration of the quarter at 16 locations using low-volume air samplers. Moisture in the atmosphere was sampled at four locations around the INL Site and analyzed for tritium. Air sampling activities and results for the second quarter of 2008 are discussed below. A summary of approximate minimum detectable concentrations (MDCs) for radiological analyses and DOE Derived Concentration Guide (DCG) (DOE 1993) values is provided in Appendix B.
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 2008 (Figure 2). Three of these samplers are located on the INL Site, nine are situated off the INL Site near the boundary and six have been placed at locations distant to the INL Site. Samplers are divided into INL Site, Boundary and Distant groups to determine if there is a gradient of radionuclide concentrations, increasing towards the INL Site. Each replicate sampler is relocated every other year to a new location. At the start of 2008, one replicate sampler was moved to Blue Dome (a Boundary location) and one was moved to Atomic City (also a Boundary location). An average of 13,697 ft3 (388 m3) of air was sampled at each location, each week, at an average flow rate of 1.36 ft3/min (0.04 m3/min). Particulates in air were collected on membrane particulate filters (1.2 µm pore size). Gases passing through the filter were collected with an activated charcoal cartridge.
Figure 2. Low-volume air sampler locations.
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.
The weekly particulate filters collected during the quarter for each location were composited and analyzed for gamma-emitting radionuclides. Selected composites were also analyzed by location for 90Sr, 238Pu, 239/240Pu and 241Am as determined by a rotating quarterly schedule.
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 INL Site, Boundary, and Distant locations for the second
quarter of 2008 are shown in Figure 3. Gross alpha data are tested for
normality prior to statistical analyses, and generally show no
consistent discernable distribution. Box and whisker plots are commonly
used when there is no assumed distribution. Each data group in Figure 3
is presented as a box and whisker plot, with a median (small red
square), a box enclosing values between the 25th and 75th percentiles,
and whiskers representing the non-outlier range. 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 fourth quarter indicates
that the outlier values were not due to mistakes in collection,
analysis, or reporting procedures, but rather reflect natural
variability in the measurements. The outlier values lie within the range
of measurements made within the past several years. Thus, rather than
dismissing the outliers, they were included in the subsequent
statistical analyses.
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 INL Site, Boundary and Distant locations. The use of nonparametric tests, such as Kruskal-Wallis, gives less weight to outlier 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. Figure 3 graphically shows that the gross alpha measurements made at INL Site, Boundary and Distant locations are similar for the second quarter. If the INL Site were a significant source of offsite contamination, concentrations of contaminants could be statistically greater at Boundary locations than at Distant locations. However, in the second quarter no statistical differences were found.
Comparisons of gross alpha concentrations were made for each month of the quarter (Figures 4 – 6). Again the Kruskal-Wallis test of multiple independent groups was used to determine if statistical differences exist between INL Site, Boundary and Distant data groups.
No statistical differences in gross alpha concentrations between groups were noted during any month of the quarter (Table D-1).
As an additional 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. INL Site sample results were not included in this analysis because the onsite data, collected at only three locations, are not representative of the entire INL Site and would not aid in determining offsite impacts. In the second quarter, there was one week (April 23) where a statistical difference existed between the two sample groups (Table D-2). The Boundary group was statistically greater than the Distant group during this week but no unusual pattern seems present in the data.
Gross beta results are presented in
Table C-1. Gross beta
concentrations in air for INL Site, Boundary and Distant locations for
the second quarter of 2008 are shown in Figure 7. The data were tested
and found to be neither normally nor log-normally 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. 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 Figures 8 – 10. Statistical data are presented in Table D-1. There were no statistical differences in gross beta between groups for any month during the quarter.
Comparison of weekly Boundary and Distant gross beta data sets, using the Mann Whitney U test, showed a statistical difference between Boundary and Distant measurements during one week (June 4) in the second quarter (Table D-2). In this case, the Distant stations were statistically greater than the Boundary stations. Analysis of individual station data for this week does not indicate any consistent pattern or unusual levels of activity.
No 131I was detected in any of the charcoal cartridge batches collected during the second quarter of 2008. Weekly 131I results for each location are listed in Table C-2 of Appendix C. Gamma spectrographic analysis is also done with the 131I analysis. Cesium-137 was not detected in any of the 26 measured batches of cartridges. The analytical laboratory considers occasional detections of this nuclide a result of the materials used in the charcoal filters.
Weekly filters for the second quarter of 2008 were composited by location. All samples were analyzed for gamma-emitting radionuclides, including 137Cs (see Table C 3, Appendix C). No manmade gamma-emitting radionuclides were detected.
Composites were also analyzed for 90Sr,
238Pu, 239/240Pu and 241Am (see
Table C 3, Appendix C.)
Americium-241 was reported on one composite and Plutonium-239/240 was
reported on four composites. The values found were generally within the
range of detections during the past several years, although the 241Am
result from the Experimental Field Station was toward the upper end of
the range. For reference, the reported value was just over 0.1 percent
of the DOE Derived Concentration Guide for 241Am.