Basalt groundwater, composition

Radionuclide transport in natural waters is strongly dependent on sorption, desorption, dissolution, and precipitation processes. The first two sections discuss laboratory investigations of these processes. Descriptions of sorption and desorption behavior of important radionuclides under a wide range of environmental conditions are presented in the first section. Among the sorbents studied are basalt interbed solids, granites, clays, sediments, hydrous oxides, and pure minerals. Effects of redox conditions, groundwater composition and pH on sorption reactions are described. 

The groundwater transport of radionuclides through waterbearing interbed layers in the Columbia River basalt formation will be controlled by reactions of the radionuclides with groundwater and interbed solids. These interactions must be understood to predict possible migration of radionuclides from a proposed radioactive waste repository in basalt. Precipitation and sorption on interbed solids are the principle reactions that retard radionuclide movement in the interbeds. The objective of the work described herein was to determine the sorption and desorption behavior of radionuclides important to safety assessment of a high-level radioactive waste repository in Columbia River basalt. The effects of groundwater composition, redox potential, radionuclide concentration, and temperature on these reactions were determined. 

Changes in Analog Groundwater Composition The data in Figure 2 for the first three analog experiments (which used unaltered waste form, bentonite, and basalt cores) show that the groundwater exiting the basalt fissure reaches a steady-state composition for [Na+] and [Ca +] soon after the start of the experiment ... 

A steady-state groundwater composition will be achieved in a fairly short time period (20-25 days for the laboratory analog experiments). This steady state is one where the [Ca +] and [K+] are increased above their initial concentrations, and where the [Na+] and pH are decreased The reason for these groundwater compositional changes are likely due to the hydrolysis and alteration of minerals on the basalt surface ... 

Average composition of groundwaters from Columbia River basalts (525 samples). 

Sufficient DO data were not obtained from basalt-synthetic Grande Ronde groundwater experiments to allow determination of a definitive rate law. A first order kinetic model with respect to DO concentration was assumed. Rate control by diffusion kinetics and by surface-reaction mechanisms result in solution composition cnanges with different surface area and time dependencies (32,39). Therefore, by varying reactant surface area, determination of the proper functional form of the integrated rate equation for basalt-water redox reactions is possible. 

The 8 0 values of whole rock samples of Sill 1 in Fig. 13.29 vary up-section from -i-4.4%o near the lower contact to higher values in the midseclion ranging from -f5.20%o to -f6.47% and then decline toward -t4.35%at the upper contact. These data are within the range of 8 0 values of continental basalt elsewhere in the world (Hoefs 1997 Faure 2001). The stratigraphic variation of 8 0 within Sill 1 is attributable to differences in the mineral composition of the rocks or to the enrichment of the rocks in 0 as a result of isotope exchange with heated groundwater. 

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