Mabton Interbed

Pomona basalt flow. A third interbed sample was taken from drilling cores of the Mabton Interbed, located between the Saddle Mountains and Wanapum Basalts. The Mabton Interbed is the first continuous, major interbed above the candidate repository horizons in the Grande Ronde Basalts. Mineralogical characteristics of the interbed materials are summarized in Table n. A more complete discussion of the characteristics of the interbed materials may be found in Reference (4). 

Procedures. Batch equilibrations of interbed solids (Mabton Interbed, Rattlesnake Ridge sandstone, or tuff), tracers, and groundwaters were used to measure radionuclide distributions between solid and liquid phases. Triplicate measurements were made for each combination of temperature, redox condition, tracer concentration, tracer type, groundwater composition, and interbed sample. Constant temperatures were maintained by placing the... 

Source Rattlesnake Ridge Interbed Mabton Interbed Rattlesnake Ridge Interbed... 

Sorption isotherms were also measured for sorption of selenium, technetium, tin, radium, uranium, neptunium, plutonium, and americium on the reference Mabton Interbed solids. The GR-1A groundwater composition was used in these experiments. Two temperatures (23°C 2°C and 60°C 1°C) were used, and both oxidizing and reducing conditions were used for each radionuclide. 

Tin and americium were so extensively sorbed under all conditions that isotherm data could not be obtained. These elements are not significantly mobile in the Mabton Interbed aquifer. Values of Freundlich constants for technetium, radium, uranium, neptunium, and plutonium are given in Table IV. The Freundlich equation did not fit the selenium sorption data very well probably because of slow sorption kinetics or precipitation. Precipitation was also observed for technetium at 23°C for concentrations above 10 7M. This is about the same solubility observed for technetium in the sandstone isotherm measurements. Linear isotherms were observed only in the case of radium sorption. In general, sorption on the Mabton Interbed was greater than on the Rattlesnake Ridge sandstone. This is probably due to the greater clay content of the Mabton standard. 

Table IV. Freundlich Constants (Molar Basis) for Radionuclide Sorption on Mabton Interbed Solids...

Figure 4. Sorption and Desorption Isotherms for (a) Selenium and (b) Technetium Sorption on Mabton Interbed Solids under Reducing Conditions.

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