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 fourth quarter of 2010 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 fourth quarter of 2010 (Figure 2). Four of these samplers are located on the INL Site, seven are situated off the INL Site near the boundary, and seven 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. During 2010, one replicate sampler was operating in Dubois (a Distant location) and one was operating at the Van Buren gate (an INL Site location). An average of 19,017 ft3 (539 m3) of air was sampled at each location, each week, at an average flow rate of 1.89 ft3/min (0.05 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 fourth quarter of 2010 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. 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. For the quarter, there was no statistical difference noted. Figure 3 graphically shows that the gross alpha measurements made at INL Site, Boundary, and Distant locations are similar for the fourth 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.
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 fourth quarter, there was one week where a statistical difference existed between the two sample groups (Table D-2). During the week of October 6, the Distant group was statistically higher than the Boundary group. Examination of data from this week does not show any unusual pattern, with Jackson being the highest location and Mud Lake and Arco the lowest.
Gross beta results are presented in
Table C-1. Gross beta
concentrations in air for INL Site, Boundary, and Distant locations for
the fourth quarter of 2010 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. No statistical
differences were noted in the quarterly data using the Kruskal-Wallace
test (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. No statistical differences were noted during any month.
Comparison of weekly Boundary and Distant gross beta data sets, using the Mann Whitney U test, showed no statistical differences between Boundary and Distant measurements during any week of the quarter (Table D-1).
Iodine-131 was not reported above the detection level on any charcoal cartridge batches analyzed during the quarter. 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 this quarter.
Weekly filters for the fourth quarter of 2010 were composited by location. All samples were analyzed for gamma-emitting radionuclides, including 137Cs and 90Sr. No manmade gamma-emitting radionuclides or 90Sr were found on any of the composites.
Selected composites were also analyzed for 238Pu, 239/240Pu, and 241Am. Results were not yet available.
