I. BACKGROUND §9.1
II. TREE SAMPLING §9.2
Figure 9-1: Tree-Stand Areas 1-7
Figure 9-2: Sample Locations in Area 1
Figure 9-3: Sample Locations in Areas 2-7
III. RESULTS §9.3
Figure 9-4: Free-Water Tritium in Tree Wood with Distance from NTLF Main Stack (Area 1)
Figure 9-5: Organically Bound Tritium
in Tree Wood with Distance from NTLF Main
Stack (Area 1)
Figure 9-6: Free-Water Tritium in Leaf and Duff with Distance from NTLF Main Stack (Area 1)
Figure 9-7: Organically Bound Tritium
in Leaf and Duff with Distance from NTLF Main
Stack (Area 1)
Figure 9-8: Free Water in Tree Wood, Leaf, and Duff from Areas 2-7
Figure 9-9: Organically Bound Tritium in Tree Wood, Leaf, and Duff from Areas 2-7
IV. SUMMARY §9.4
§9.1 I. BACKGROUND
Sampling of vegetation and foodstuffs can provide information regarding the presence, transport, and distribution of radioactive emissions in the environment. This information can be used to detect and evaluate changes in environmental radioactivity resulting from Berkeley Lab activities and to calculate potential human doses from consuming vegetation and foodstuffs. Possible pathways or routes for ingesting radionuclides include:
Department of Energy (DOE) guidance indicates that when the annual effective dose equivalent for the consumption of vegetation and foodstuffs is between 0.001 mSv (0.1 mrem) and 0.01 mSv (1 mrem), only a minimal vegetation and foodstuff surveillance program is required.1 Using conservative assumptions, Berkeley Lab’s maximum individual dose attributable to the consumption of locally grown vegetation and foodstuffs was well below the requirement for a minimal monitoring program. Tritium air emissions were identified as the only potentially significant contributor to these pathways.
Tritium emissions can be in the form of tritiated water vapor or tritiated hydrogen gas. The relative dose from an exposure to tritiated hydrogen gas is much less than that from an equal exposure to tritiated water. Nevertheless, in modeling and dose calculations, the Laboratory conservatively assumes that 100% of the emissions are tritiated water vapor to provide a safe over-estimate of actual dose.
Tritiated water vapor released to the environment mixes and exchanges readily with atmospheric water (e.g., precipitation, fog, vapor) and with other sources of environmental water (e.g., plant water, surface water, soil water). Within plants, tritium exists as either free-water tritium or organically bound tritium.
The Laboratory’s Environmental Monitoring Plan2 outlines the current vegetation sampling program. The objective of this portion of the program is to better understand the distribution of tritium in local vegetation.
§9.2 II. TREE SAMPLING
Berkeley Lab manages on-site trees and vegetation (and some immediately adjacent to the University of California) as part of a multi-year wildland fire task management program and its maintenance program for a fire-safe landscape.3 In the future, Berkeley Lab is considering thinning nonnative tree stands around Buildings 75, 76, and 77. See Figure 9-1.
Environmental tritium levels have been determined to be above regional background levels near the National Tritium Labeling Facility and decrease with distance from the facility stack.4 A sampling and analysis plan was developed and implemented in 1998 to characterize tritium concentrations within tree stands that might be thinned in the future.5 See Figure 9-1. Tree selection and sampling was designed to (a) provide representative samples for characterizing tritium levels within the tree stands, (b) prevent sample cross contamination, and (c) estimate field sampling variability.
Samples of wood core and chip, leaf, and duff were collected from seven tree stands and a remote location at Chabot Regional Park (approximately 20 kilometers south of Berkeley Lab). Duff consists of tree litter and other decomposing vegetation material that lies on the ground under a tree canopy. Eucalyptus and pine trees were sampled using a systematic and documented procedure. The samples were analyzed at a commercial laboratory for free-water tritium (FWT) and organically bound tritium (OBT).
To characterize tritium levels in the tree stand around the National Tritium Labeling Facility’s (NTLF) main stack (Area 1), a radial sampling pattern was selected to more easily identify tritium concentration changes and contours. See Figure 9-2. All other tree stands were sampled using an areal grid pattern that divided each stand into cells of approximately equal area. See Figure 9-3.
§9.3 III. RESULTS
The tritium results from the vegetation samples are summarized in Figures 9-4, 9-5, 9-6, 9-7, 9-8, and 9-9. A detailed listing of all tritium results is included in volume II. Tritium results for wood, leaf, and duff from a remote sampling location at Chabot Regional Park were all below or near the analytical detection limits. Nominal minimum detectable activities were 0.007 Bq/g for FWT and 0.07 Bq/g for OBT.
Within Area 1, the mean and maximum FWT levels measured in tree wood (33 samples) were 0.19 Bq/g and 0.77 Bq/g, respectively. The mean and maximum OBT levels measured in tree wood (32 samples) from Area 1 were <0.07 Bq/g and 0.32 Bq/g, respectively.
Figures 9-4 and 9-5 provide summary data on Area 1 FWT and OBT results for tree wood at specific distances from the NTLF main stack. As the figures show, both FWT and OBT levels in tree wood generally decrease with distance from the stack in Area 1. FWT levels in tree wood, however, decrease more rapidly and more consistently than OBT levels in tree wood with distance.
Area 1 FWT and OBT levels measured in leaf samples and duff samples are shown in Figures 9-6 and 9-7. Only three samples of leaf and duff were collected from Area 1 along a north-northwest transect from the NTLF main stack. Duff samples collected from Area 1 contained the highest OBT levels measured: 47 Bq/g (see Figure 9-7). OBT levels in the duff samples rapidly decrease with distance from the NTLF main stack. OBT levels measured in leaves from Area 1 were substantially lower than those in duff from the same locations.
FWT and OBT results for tree wood, lead, and duff samples from Areas 2-7 (see Figures 9-8 and 9-9) were significantly lower than the same sample types in Area 1. The mean and maximum FWT for tree wood (27 samples) in Areas 2-7 was 0.0096 Bq/g and 0.055 Bq/g, respectively. The mean and maximum OBT in tree wood (27 samples) for Areas 2-7 were both below the analytical detection limit.
§9.4 IV. SUMMARY
The results from the sampling and analysis conducted in 1998 confirm what has been previously measured and reported: tritium concentrations in vegetation are slightly elevated above regional background levels near the NTLF main stack and decrease with distance from the stack. At about 200 meters from the NTLF stack, FWT and OBT levels in vegetation are nearly indistinguishable from regional background levels. Additional sampling and analysis of tree wood, leaf, and duff in Area 1 and from other on-site areas will be performed in 1999 to supplement the information that has been gathered to date. This tritium characterization data gathered will be utilized in preparing an application for the authorization of release limits for tritium in vegetation, which will be submitted to the Department of Energy.
Routine sampling of vegetation and foodstuffs is not required under any applicable environmental regulations. Berkeley Lab undertakes voluntary sampling efforts to better understand the integrated impact of its operations on all media in the surrounding environment and to verify its overall dose-assessment program. This assessment program, which is presented in chapter 10, includes vegetation and foodstuffs as one of the contributing pathways in determining the overall impact from Berkeley Lab’s airborne radionuclides. Dose assessments performed using very conservative assumptions indicate extremely small potential impacts.