Strontium Attenuation in Basaltic Aquifers: Experimental Determination of Ca2+-Na+-Sr2+-K+ Exchange Between Heulandite and Aqueous Solutions

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BirdEAR-0001113The proposed study is an experimental, theoretical, and geologic investigation of zeolite-solution interaction that aims to quantify ion exchange reactions between aqueous solutions and heulandite in geologic systems. Research is based on experimental evaluation of Ca2+, Na+, Sr2+, and K+ exchange reactions in natural heulandite, a common rock-forming zeolite, known to concentrate Sr2+ in its crystal structure via ion exchange. Our goal is to provide thermodynamic data required to model reactive mass transfer between heulandite and ground-waters in basaltic aquifers, with reference to attenuation of radioactive 90Sr derived from nuclear waste respositories. This study includes binary cation exchange experiments on heulandite to determine the thermodynamic properties of the exchange reactions and crystal-chemical investigation on the effects of ion exchange on the crystal structure of heulandite employing X-ray diffraction and nuclear magnetic resonance (NMR) techniques. Finally, the credibility of the laboratory experiments and thermodynamic analyses will be tested using measured fluid and mineral compositional relations in Icelandic geothermal/metamorphic systems. The work will be conducted at Stanford University, Lawrence Livermore National Laboratory, and the Stanford Synchrotron Radiation Laboratory. The specific objectives of this study are to determine: 1) Gibbs energies and enthalpies of ion exchange reactions, 2) quantitative description of cation sites and site occupancy in heulandite as a function of composition and 3) activity-composition models for binary solid solutions in the interchannel sites in heulandite. The results will facilitate geochemical modeling of alkali- and alkaline earth-metal mass-transfer in groundwater aquifers where zeolites occur. Our findings will provide essential data to help develop heuristic and quantitative models of groundwater-zeolite interaction and the fate of 90Sr in basaltic aquifers contaminated with nuclear waste such as those below the U. S. Department of Energy sites at the Hanford Nuclear Reservation, WA, and the Idaho National Engineering Laboratories, ID, as well as in more silicic volcanic rocks at Yucca Mountain, NV, where heulandite occurs.