Colloid facilitated transport in fractured rock : parameter estimation and comparison with experimental data

Many contaminants in groundwater strongly interact with the immobile porous matrix, which retards their movement relative to groundwater flow. Colloidal particles, which are often present in groundwater, have a relatively small size and large specific surface area which makes it possible for them to also adsorb pollutants. The sorption of tracers to colloids may enhance their mobility in groundwater, relative to the case where colloids are not present. A class of pollutants for which colloid-facilitated transport may be of particular significance are radioactive isotopes. A major reason for why geologic repositories are considered suitable for the disposal of spent nuclear fuel is the strong affinity of many radionuclides to adsorb onto the porous matrix. Therefore, radionuclides accidentally released, would be contained in the geological media by adsorption or filtration until sufficient decay takes place. However, the presence of colloids may enhance radionuclide mobility in the groundwater, and reduce the efficiency of geologic media to act as a natural barrier

Colloid facilitated transport in fractured rocks : parameter estimation and comparison with experimental data.

Colloid-facilitated migration of plutonium in fractured rock has been implicated in both field and laboratory studies . Other reactive radionuclides may also experience enhanced mobility due to groundwater colloids. Model prediction of this process is necessary for assessment of contaminant boundaries in systems for which radionuclides are already in the groundwater and for performance assessment of potential repositories for radioactive waste. Therefore, a reactive transport model is developed and parameterized using results from controlled laboratory fracture column experiments. Silica, montmorillonite and clinoptilolite colloids are used in the experiments along with plutonium and Tritium . . The goal of the numerical model is to identify and parameterize the physical and chemical processes that affect the colloid-facilitated transport of plutonium in the fractures. The parameters used in this model are similar in form to those that might be used in a field-scale transport model

Simulations of Groundwater Flow and Radionuclide Transport in the Vadose and Saturated Zones beneath Area G, Los Alamos National Laboratory

Numerical simulations are used to predict the migration of radionuclides from the disposal units at Material Disposal Area G through the vadose zone and into the main aquifer in support of a radiological performance assessment and composite analysis for the site. The calculations are performed with the finite element code, FEHM. The transport of nuclides through the vadose zone is computed using a three-dimensional model that describes the complex mesa top geology of the site. The model incorporates the positions and inventories of thirty-four disposal pits and four shaft fields located at Area G as well as those of proposed future pits and shafts. Only three nuclides, C-14, Tc-99, and I-129, proved to be of concern for the groundwater pathway over a 10,000-year period. The spatial and temporal flux of these three nuclides from the vadose zone is applied as a source term for the three-dimensional saturated zone model of the main aquifer that underlies the site. The movement of these nuclides in the aquifer to a downstream location is calculated, and aquifer concentrations are converted to doses. Doses related to aquifer concentrations are six or more orders of magnitude lower than allowable Department of Energy performance objectives for low-level radioactive waste sites. Numerical studies were used to better understand vadose-zone flow through the dry mesa-top environment at Area G. These studies helped define the final model used to model flow and transport through the vadose zone. The study of transient percolation indicates that a steady flow vadose-zone model is adequate for computing contaminant flux to the aquifer. The fracture flow studies and the investigation of the effect of basalt and pumice properties helped us define appropriate hydrologic properties for the modeling. Finally, the evaporation study helped to justify low infiltration rates

Isotope and Phase Effects on the Proton Tautomerism in Polycrystalline Porphycene Revealed by NMR

Using high resolution solid state (15)N and (2)H spectroscopy and longitudinal relaxometry we have studied the tautomerism of porphycene in the solid state, corresponding to a double proton transfer in two cooperative hydrogen bonds. The tautomerism is degenerate above 225 K but the degeneracy is lifted below this temperature, indicating a phase transition. Thus, the high-temperature phase is characterized by a dynamic proton disorder and the low-temperature phase by a dynamic proton order. (15)N magnetization transfer experiments obtained under cross polarization (CP) and magic angle spinning (MAS) conditions reveal the presence of two nonequivalent molecules A and B in the unit cell of phase II, exhibiting slightly different equilibrium constants of the tautomerism. Rate constants of the tautomerism in phase I could be obtained by the analysis of the longitudinal (15)N and (2)H relaxation times. The former, obtained at 9.12 MHz, exhibit a T(1) minimum around 270 K and are consistent with proton transfer induced dipolar (1)H-(15)N relaxation mechanism. The latter, obtained at 46.03 MHz, exhibit a minimum around 330 K and arise from quadrupole relaxation. Within the margin of error, the rate constants of the HH and of the HD/DD tautomerism are the same, exhibiting a barrier of about 30 kJ mol(-1), as expected for an overbarrier reaction in a configuration with two compressed hydrogen bonds. By contrast, in the low-temperature phase a switch of the DD transfer kinetics into the nanosecond time scale is observed, exhibiting a non-Arrhenius temperature dependence which is typical for tunneling. This increase of the rate constants by lowering the temperature is discussed in terms of a switch from a concerted HH transfer in phase I to a stepwise transfer in phase II, where intermolecular interactions lower the energy of one of the cis-intermediates