Tritium Concentrations in Vegetation as a Function of Distance from a Low-Level Radioactive Waste Site at Los Alamos National Laboratory

P.R. Fresquez; L.M. Vasquez-Tator; and E.A. Lopez (Los Alamos National Laboratory)

Low-level radioactive wastes have been buried at Area G on Mesa del Buey at Los Alamos National Laboratory (LANL) since 1957. Wastes are buried in pits, trenches, or shafts and then covered with fill material. Tritium (3H), a radioactive isotope of hydrogen, is one of the main contaminants in waste materials at Area G and has been consistently detected in biotic and abiotic samples collected within and around the disposal site year after year. The purpose of this study was to determine the extent of 3H migration from the perimeter of Area G using vegetation as an investigative tool. Piñon (Pinus edulis) and juniper (Juniperus monosperma) trees, in particular, are excellent indicators of subterranean 3H migration. Roots of these trees have been measured to depths greater than 6 meters (20 ft) in fractures of (volcanic) tuff material that underlies the shallow clay loam soils (30 cm deep) of the mesa top environment at LANL. Tree branch tips, collected at chest height, were collected at various distances (approximately 10, 50, 100, 150, and 200 meters) from the perimeter of Area G in seven directions (N, NE, E, SE, S, SW, and NW). Most overstory vegetation samples collected around and away from the perimeter of Area G were very low (median was 0.45 pCi/mL) and within the regional statistical reference level (RSRL) of 0.72 pCi/mL. The RSRL is the upper-level background concentration (mean plus three standard deviations) based on (overstory) data collected over the last five years (1998 to 2003) from sites away from the influence of LANL operations. The only overstory samples that exceeded the RSRL were the ones collected on the south side of Area G nearest the 3H disposal shafts. Tritium concentrations at those locations, 10 meters (33 ft) and 50 meters (164 ft) away from the perimeter fence line, were measured at 6,700 and 58 pCi/mL, respectively. From there, concentrations of 3H in overstory vegetation significantly decrease with distance and, around the 145 meter (476 ft) mark, the concentration (0.42 pCi/mL) fell to within the RSRL.

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