Plant and environment interactions. Simplified method for detecting tritium contamination in plants and soil
2003
behave nearly identically in the subsurface and can move in both the liquid and vapor phases (Phillips, Cost-effective methods are needed to identify the presence and 1994). distribution of tritium near radioactive waste disposal and other conGround water and soil water monitoring can provide taminated sites. The objectives of this study were to (i) develop a simplified sample preparation method for determining tritium condetailed information needed to assess risks associated tamination in plants and (ii) determine if plant data could be used with waterborne radionuclides in the subsurface, but as an indicator of soil contamination. The method entailed collection the high costs of equipment, installation, and mainteand solar distillation of plant water from foliage, followed by filtration nance of these monitoring systems limits the number and adsorption of scintillation-interfering constituents on a graphiteof sampling locations. Thus, cost-effective methods are based solid phase extraction (SPE) column. The method was evaluated needed to identify the presence and distribution of conusing samples of creosote bush [Larrea tridentata (Sesse & Moc. ex tamination. Such methods should be capable of providDC.) Coville], an evergreen shrub, near a radioactive disposal area ing an early warning of contaminant releases. In addiin the Mojave Desert. Laboratory tests showed that a 2-g SPE column tion, the methods should be of sufficient accuracy to was necessary and sufficient for accurate determination of known facilitate collection of plume-scale data and optimized tritium concentrations in plant water. Comparisons of tritium concentrations in plant water determined with the solar distillation–SPE placement of more sophisticated monitoring equipment method and the standard (and more laborious) toluene-extraction at contaminated sites. method showed no significant difference between methods. Tritium Previous studies at a humid site (Maxey Flats, Kenconcentrations in plant water and in water vapor of root-zone soil tucky; precipitation 1000 mm yr 1) showed that trees also showed no significant difference between methods. Thus, the could serve as indicators of subsurface tritium flows solar distillation–SPE method provides a simple and cost-effective way away from a radioactive waste site (Rickard and Kirby, to identify plant and soil contamination. The method is of sufficient 1987; Kalisz et al., 1988). Trees at the Kentucky site accuracy to facilitate collection of plume-scale data and optimize were rooted in shallow colluvium on steeply sloping placement of more sophisticated (and costly) monitoring equipment hillsides, and sandstone bedrock limited the vertical at contaminated sites. Although work to date has focused on one penetration of roots and water to a depth of about 1 to desert plant, the approach may be transferable to other species and environments after site-specific experiments. 4 m. Subsurface flow was reported to primarily occur as water movement along the top of the sandstone and through horizontal fractures in the sandstone. Rickard and Kirby (1987) sampled trees by collecting sap from L radioactive and mixed wastes are disposed maple (Acer spp.) and by collecting and solar-distilling primarily by shallow land burial in trenches that water from foliage of oak (Quercus spp.) and hickory are filled with waste and previously excavated soil. The (Carya spp.). Kalisz et al. (1988) sampled trees by colhazard to public health following burial is considered lecting and microwave-distilling water from leaves. In to exist for 500 yr (United States Nuclear Regulatory both studies, the collected liquid then was distilled in Commission, 1989). Thus, there is ongoing concern the laboratory to remove constituents that can interfere about the release and fate of radionuclides in the enviwith tritium analysis by liquid scintillation counting. Disronment near contaminated sites. In humid environtillation was selected as the interference removal ments, transport by ground water is considered the main method because it is a common laboratory technique. radionuclide migration pathway and, in arid environWhen sample volumes are large (e.g., 20–100 mL), the ments, transport by soil water and vapor is considered laboratory distillation is relatively simple (W.H. Rickthe main migration pathway (Siefken et al., 1982). Triard, personal communication, 2001). However, when tium, the only radioactive isotope of hydrogen, is one sample volumes are small (5–10 mL), microdistillation of the most common radionuclides disposed at waste equipment is required and the need for more rigorous burial sites. Water (H2O) and tritiated water (3HHO) handling and cleaning procedures can greatly slow the sample preparation process, especially when the number B.J. Andraski, M.J. Johnson, and C.J. Mayers, U.S. Geological Survey, of samples is large (J.A. Volpe, personal communica333 West Nye Lane, Room 203, Carson City, NV 89706. M.W. Sandtion, 2001). strom, U.S. Geological Survey, Denver Federal Center, P.O. Box 25046 MS-407, Lakewood, CO 80225. R.L. Michel, J.C. Radyk, and Recent research in an arid environment (Amargosa D.A. Stonestrom, U.S. Geological Survey, 345 Middlefield Road, MSDesert near Beatty, Nevada; precipitation 108 mm yr 1) 434 and MS-421, respectively, Menlo Park, CA 94025. Received 24 indicates that tritium movement away from the waste June 2002. *Corresponding author (andraski@usgs.gov). Abbreviations: SPE, graphite-based solid phase extraction. Published in J. Environ. Qual. 32:988–995 (2003).
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