In situ Measurement of the Coefficient of Molecular Diffusion in Fine-Grained Till

by Reifferscheid, Laura Jean

Diffusion dominated systems, including naturally occurring aquitards and engineered barrier systems, are important components for long-term waste containment. Diffusive transport can be quantified using the effective coefficient of diffusion (De) and the effective porosity (ne). These are empirical parameters that are commonly measured in a laboratory setting. The purpose of this research was to develop a field-based technique to perform in situ measurement of diffusive transport rates. Results from this measurement technique were evaluated by comparison with the results of conventional laboratory testing and back-analysis of a previously determined large-scale field diffusion profile.

In situ diffusion testing was performed on two wells completed in diffusion-dominated zone of a till aquitard. Laboratory diffusion cell tests were conducted on core samples obtained from the screened zones of the diffusion wells. Diffusion testing was completed using conservative isotopes of water as tracers.

A finite element model was used to back-analyze results of laboratory and field experiments to determine the De values that best describe each system. Comparison of the field results with the laboratory results obtained from this study as well as previous studies indicate the field system is a useful method for measurement of De.

Back-analysis of a previously determined large-scale field deuterium profile for this aquitard showed very precise measurement of De is not required for estimation of contaminant transport in thick till aquitard systems. More accurate measurement may be more useful for barrier systems such as clay liners or slurry walls, where the characteristics of the diffusion-dominated zone are easier to define and the transport path is shorter relative to the transport time.