Comparison of experimental methods for estimating matrix diffusion coefficients for contaminant transport modeling

Area of Science:

• Geosciences
• Environmental Science
• Nuclear Waste Management

Background:

• Accurate estimation of matrix diffusion coefficients is crucial for understanding solute transport in geological formations.
• Traditional methods like the diffusion cell can be time-consuming and resource-intensive.
• Rock core samples from Pahute Mesa at the Nevada Nuclear Security Site (NNSS) are representative of volcanic rocks with potential relevance to nuclear waste disposal.

Purpose of the Study:

• To compare the diffusion wafer method with the traditional diffusion cell method for estimating effective matrix diffusion coefficients.
• To evaluate the accuracy, efficiency, and cost-effectiveness of the diffusion wafer method.
• To determine if the diffusion wafer method is a viable alternative for characterizing solute transport in NNSS volcanic rocks.

Main Methods:

• Conducted diffusion cell experiments using rock core samples.
• Performed diffusion wafer experiments with pre-saturated rock matrix samples.
• Analyzed and compared the effective matrix diffusion coefficients obtained from both methods.

Main Results:

• Both diffusion cell and diffusion wafer methods yielded comparable estimates for effective matrix diffusion coefficients.
• Differences between the two methods ranged from 14% to 30%, with no systematic bias observed.
• The diffusion cell method provided estimates with lower uncertainty for equivalent measurement errors, but the wafer method was faster and less expensive.

Conclusions:

• The diffusion wafer method is a simpler, quicker, and more economical alternative to the diffusion cell method.
• The agreement between the methods suggests the diffusion wafer method is suitable for estimating effective matrix diffusion coefficients.
• The diffusion wafer method's advantages make it preferable for achieving better statistical representation of rock samples in solute transport studies.