Sorption Coefficients for Radionuclides on Samples from the Water-Bearing Magenta and Culebra Members of the Rustler Formation

摘要

The sorbing characteristics of samples from the Magenta and Culebra dolomites of the Rustler Formation (which overlie the proposed site for the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico) for various radionuclides have been studied. Core samples extracted from various depths and boreholes within a given formation were selected to provide a cross section of materials which are present in the formations. They varied in composition from pure dolomite to nearly pure anhydrite. The radionuclides used in the sorption coefficient measurements (isotopes of Cs, Sr, Tc, Eu, Am and Pu) were dissolved in a simulated Culebra groundwater at concentrations of 0.1 mu Ci/m1 or less. The results indicate that the sorption coefficients (Kd's) of these water-bearing units may vary widely depending on the borehole area and over several feet in depth in a common borehole. Cesium Kd values from 0.6 to 218 ml/g and 1.2 to 68 on Magenta and Culebra samples, respectively. Strontium Kd values ranged from 1.1 to 30 on Magenta Samples and from 0.6 to 32 on Culebra samples. Variations of Eu, Pu and Am Kd's were generally less than an order of magnitude on both Culebra and Magenta samples. Technetium was not sorbed by any of the samples. With few exceptions, samples with high dolomite contents had higher sorption coefficients than those with high anhydrite concentrations. Clay content may also be a factor for some radionuclides, particularly Cs, which has been shown to sorb preferentially on clay particles in Magenta. (ERA citation 06:009659)