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 third quarter of 2011 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 third quarter of 2011 (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 2011, 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 18,688 ft3 (529 m3) of air was sampled at each location, each week, at an average flow rate of 1.85 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 and shown in Figures 3 through 6. Gross alpha data are tested for normality prior to statistical analyses, and generally show no consistent discernable distribution. 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.
Comparisons of gross alpha concentrations were made for each month of the quarter. 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 third quarter, there were three weeks (July 6, August 17, and August 31) where a statistical difference existed between the two sample groups (Table D-2). In the first two instances, the Distant group was higher than the Boundary group, which is not indicative of a potential INL Site impact. In the third week, the Boundary group was higher than the Distant group. During this week, a wildfire was burning in the vicinity of Howe. Elevated gross alpha concentrations are sometimes noted during smoky conditions and this may have impacted some of the Boundary locations such as Howe and Blue Dome.
Gross beta results are presented in Table C-1and displayed in Figures 7 through 10. 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).
Comparisons of gross beta concentrations were made for each month of the quarter. Statistical data are presented in Table D-1. A statistical difference was found during the month of August. Analysis of the data for this month indicates that the INL Site group the lowest median concentration followed by the Boundary group. This pattern is the opposite of what would be expected if the INL Site was impacting the gross beta values.
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.
Iodine-131 was not found in any charcoal cartridges measured during the third quarter. Weekly 131I results for each location are listed in Table C-2 of Appendix C.
Weekly filters for the third quarter of 2011 were composited by location. All samples were analyzed for gamma-emitting radionuclides, including 137Cs. Results are reported in Table C-3, Appendix C.
Selected composites were also analyzed for and 90Sr, 238Pu, 239/240Pu, and 241Am. Results are not yet available.
Atmospheric Moisture Sampling
Atmospheric moisture is collected by pulling air through a column of absorbent material (molecular sieve material) to absorb water vapor. The water is then extracted from the absorbent material by heat distillation. The resulting water samples are then analyzed for tritium using liquid scintillation.
Results were only available for three atmospheric moisture samples collected during the third quarter of 2011. All of these exceeded the 3s uncertainty level for tritium, with similar results to those reported previously. All samples were significantly below the DOE DCG for tritium in air of 1 x 10-7 µCi/mLair with a maximum reported value of 14.1 x 10-13 µCi/mLair at Atomic City. Additional results will be reported the next quarterly report.