USING A SORBING TRACER TO BETTER CHARACTERIZE DIFFUSION PROCESSES IN FRACTURED MEDIA

In recent years, many researchers have conducted tracer tests using multiple nonsorbing tracers with different diffusion coefficients to demonstrate and parameterize matrix diffusion in fractured media. However, in a rigorous licensing environment, skeptical peer reviewers have suggested that differences in the responses of nonsorbing tracers may be the result of diffusion into stagnant free water in fracture networks rather than true matrix diffusion. It is difficult to refute this possibility on the basis of nonsorbing tracer responses alone.

Los Alamos National Laboratory has participated in three field tracer tests in fractured media in which we used a weakly-sorbing cation-exchanging tracer, lithium ion, in addition to nonsorbing tracers to demonstrate matrix diffusion. Our rationale was that if diffusion occurred only into free water in the fracture system, lithium would be minimally retarded because of the small rock surface area available for sorption. Also, any retardation resulting from sorption to fracture surfaces would be expressed primarily as an arrival time delay. However, if diffusion occurred into the matrix, where the rock surface area to water volume ratio is large, lithium ion would be significantly retarded relative to the nonsorbing tracers, and this retardation would be expressed primarily as an attenuation in peak concentration rather than a time delay.

In all three field tests, the tracer responses were consistent with lithium ion sorbing in the matrix; in fact, the lithium responses could not be explained in any other way. Furthermore, it was possible to simultaneously quantify lithium sorption occurring in the matrix and onto fracture surfaces. In conclusion, the use of a weakly sorbing tracer in conjunction with multiple nonsorbing tracers helped verify that the diffusion deduced from nonsorbing tracer responses was truly occurring in the matrix and not just in stagnant free water in the fractured media.