William E. Sanford

Mobilization of transuranic radionuclides from disposal trenches by natural organic matter

Abstract Transuranic (TRU) radionuclides in groundwater at the Oak Ridge National Laboratory migrate rapidly and with little retardation of the radionuclides over distances of 80 m. Several interacting hydrogeochemical processes contribute to the observed releases of actinides ( 244 Cm and 241 Am) from the shallow unlined disposal trenches, through the highly weathered, fractured shale (saprolite) and to the surface-water seeps at White Oak Creek. Major releases are promoted when seasonal fluctuations in the water table permit groundwater to contact actinide-contaminated waste. Local recharge of stormwater into the trenches appears to permit minor releases, perhaps due to transient saturation within the trenches but above the local water table. Although the hydrogeology of the site permits contact of the TRU waste with the groundwater, the expected inorganic species of the actinides should strongly adsorb to the layer silicates and mineral oxides of the shale saprolite. Yet the timing of the actinide releases relative to when rising groundwater intercepts the trenches suggests that actinide transport is rapid, and the relative magnitude of peak actinide levels in wells near the trenches and at downgradient seeps suggests that there is very limited retention of the actinides by the formation. Based on anion exchange chromatography of the groundwater and geochemical modeling, the mobilization and transport of the actinides is demonstrated to result from complexation of the actinides by natural organic matter (NOM). Storm events contribute to mobilization by promoting hydrologic links between the TRU waste and groundwater, and by increasing the concentration of NOM in the mobile soil and groundwater. This study demonstrates that even in formations characterized by abundant mineral phases known to strongly adsorb actinides, the actinides can be transported essentially conservatively as NOM complexes.

Upward migration of radio-cesium and strontium in a sand-filled lysimeter

The upward migration of 134Cs, 137Cs, and 90Sr was observed in a silica sand-filled lysimeter at the Field Lysimeter Investigations: Low Level Waste Data Base Development experiment site at Oak Ridge National Laboratory. The source of the radionuclides first observed on the surface was identified from isotopic analysis as being from the buried waste. Cores of the sand were collected and analyzed for the vertical distribution of the radionuclides. Results of analyses revealed that pulses (elevated levels) in the activity of the Cs and Sr radioisotopes occurred at the same depths. During the sectioning of the sand core collected from directly above the buried waste form it was discovered that a fine root from an unidentified plant was present throughout all but the upper few centimeters of the core. Because the upward migration was unexpected, information that may lead to the determination of a definitive mechanism of migration was not preserved. The distribution of the radionuclides coupled with the presence of the root suggest that Cs and Sr migrated upward in the evapotranspiration stream of the root. Further study must be undertaken to confirm this phenomenon. Upward migration of radionuclides as observed here could result in direct exposures and offsite releases from underground storage facilities.

Helium and Neon Groundwater Tracers to Measure Residual DNAPL: Laboratory Investigation

A laboratory investigation was conducted to evaluate the applicability of dissolved He and Ne as partitioning tracers for detecting and quantifying nonaqueous phase liquid (NAPL) in the saturated zone. Based on the results of batch experiments, the equilibrium NAPL–water partition coefficients ( K N,W ) of these tracers for two common dense NAPLs (DNAPLs), tetrachloroethene (PCE) and trichloroethene (TCE), are: K PCE,W = 1.28 and K TCE,W = 2.42 for He, and K PCE,W = 1.84 and K TCE,W = 3.24 for Ne. Tracer partitioning is linear across the range of concentrations tested, and appears to be linear even near aqueous solubility limits of the gases. Multiple partitioning tracer tests (PTTs) were conducted in columns, and residual TCE saturations ( S TCE ) ranging from 4.7 to 10.5% were successfully measured by the tracers. Sensitivity analysis for the column experiments indicates that random tracer-measurement error of up to ±20% had little effect on results; however, accurate characterization of the tail region of the tracer curves is particularly important. Therefore, the low analytical detection limits possible with dissolved He and Ne (4 to 5 orders of magnitude below aqueous solubility) may permit better tracer curve characterization than commonly used alcohol partitioning tracers, and is a notable advantage for these tracers. Due to their high Henry9s Law constants, these gases will also partition into trapped air present in the tracer sweep zone. Equations are presented for estimating both trapped air and NAPL saturation for PTTs where three phases are present (water, trapped air, and residual NAPL). The results of this investigation provide a basis for field-scale application of dissolved He and Ne as groundwater partitioning tracers.

Headwater regions — Physical, ecological, and social approaches to understand these areas: introduction to the special issue

1 ESS-Watershed Science, Colorado State University, Fort Collins, CO 80523-1476, USA 2 Geographisches Institut, Abt. Kartographie, GIS & Fernerkundung, Georg-August-Universität Göttingen, 37077 Göttingen, Germany 3 Cooperative Institute for Research in the Atmosphere, Fort Collins, CO 80523-1375, USA 4 Geospatial Centroid, Colorado State University, Fort Collins, CO 80523-1101, USA 5 Natural Resources Ecology Laboratory, Fort Collins, CO 80523-1499, USA 6 Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, CO 80309-0450, USA 7 Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA

Results after ten years of field testing low-level radioactive waste forms using lysimeters

The Field Lysimeter Investigations: Low-Level Waste Data Base Development Program is obtaining information on the performance of radioactive waste forms. Ion-exchange resins from a nuclear power station were solidified into waste forms using portland cement and vinyl esterstyrene. These waste forms are being tested to obtain information on survivability of waste forms in a disposal environment and to develop a low-level waste data base. Radionuclide releases from those waste forms during the first 9 years of sampling have been evaluated. Also, upward migration of radionuclides was recently discovered. Lastly, lysimeter data are applied to a performance assessment source term model.The Field Lysimeter Investigations: Low-Level Waste Data Base Development Program is obtaining information on the performance of radioactive waste forms. Ion-exchange resins from a commercial nuclear power station were solidified into waste forms using portland cement and vinyl esterstyrene. These waste forms are being tested to: (a) obtain information on performance of waste forms in typical disposal environments, (b) compare field results with bench leach studies, (c) develop a low-level waste data base for use in performance assessment source term calculations, and (d) apply the DUST computer code to compare predicted cumulative release to actual field data. The program, funded by the Nuclear Regulatory Commission (NRC), includes observed radionuclide releases from waste forms in field lysimeters. The purpose of this paper is to present the experimental results of two lysimeter arrays over 10 years of operation, and to compare those results to bench test results and to DUST code predicted releases. Further analysis of soil cores taken to define the observed upward migration of radionuclides in one lysimeter is also presented.