Geological Society of America

3 Springs and Groundwaters of the Intermountain West

Wednesday, 17 May 2006
8:00 AM - 11:40 AM

RETARDATION OF RADIONUCLIDES IN YUCCA MOUNTAIN SATURATED ALLUVIUM

Paper number: 3-9
Program: THEME SESSIONS
Time: 10:40 AM – 11:00 AM

Authors

C.M. Sedlacek Presenting Author
Los Alamos National Laboratory, Los Alamos, NM
M.K. Schulmeister
Emporia State University, Emporia, KS
P.W. Reimus
Los Alamos National Laboratory, Los Alamos, NM
C.D. Scism
Los Alamos National Laboratory, Los Alamos, NM
S.J. Chipera
Los Alamos National Laboratory, Los Alamos, NM
M. Ding
Los Alamos National Lab, Los Alamos, NM

Abstract

The saturated alluvium south of Yucca Mountain, NV represents an important barrier to radionuclide migration from the proposed high-level radioactive waste repository at Yucca Mountain. Here we present preliminary results of a study to determine whether a correlation exists between the hydraulic conductivity of the alluvium and radionuclide sorption/desorption parameters. The alluvium has considerable variability in saturated hydraulic conductivity, probably because of varying depositional environments that resulted in alternating layers of coarser and finer alluvium grain sizes. A positive correlation between conductivity and sorption would result in greater flow through more sorptive portions of the aquifer and hence more radionuclide retardation than an uncorrelated system, whereas a negative correlation would result in greater flow through less sorptive portions of the aquifer and hence less retardation.

Partitioning of ²³³U(VI) and ²³⁷Np(V) between alluvium and groundwater was measured in batch sorption experiments involving alluvium samples from four different depth intervals in a borehole located ~18 km from the proposed repository. The four intervals were previously determined to have over a two order of magnitude range of hydraulic conductivities based on laboratory permeameter tests. After a 3-day sorption testing period, the alluvium samples were exposed to a continuous flow of radionuclide-free groundwater to induce radionuclide desorption. Desorption rates as a function of time were determined by analyzing radionuclide concentrations in the water that came in contact with the alluvium.

Radionuclide partition coefficients (radionuclide sorbed per g of alluvium divided by nuclide remaining in solution per ml) after 3 days of sorption ranged from 14.56 to 18.57 ml/g for ²³³U and from 8.11 to 11.92 ml/g for ²³⁷Np, with sorption appearing to be only weakly correlated with hydraulic conductivity. Desorption rates for both radionuclides decreased by several orders of magnitude during 45,500 minutes of desorption testing, suggesting many different types of sorption sites in the alluvium with widely varying affinities for the nuclides. The slow desorption of a significant fraction of the radionuclide mass has important implications for transport over large distances in the alluvium.