Matrix diffusion rates in fractured volcanic rocks at the Nevada Test Site: Evidence for a dominant influence of effective fracture apertures

Abstract

Solute matrix diffusion in saturated, fractured volcanic rock at the Nevada Test Site was evaluated from field tracer tests conducted at two different locations and from laboratory‐scale transport experiments using core samples from the two locations. The laboratory tests included 15 separate tracer transport experiments conducted in 8 fractured cores (4 from each location) and 17 diffusion cell experiments conducted in matrix material adjacent to the fractures. All of the experiments featured two nonsorbing tracers with free diffusion coefficients differing by a factor of approximately 3 to allow the effects of diffusion to be distinguished from the effects of advection, dispersion, and source‐term tailing in the experiments. When considering all the laboratory and field transport experiments collectively, the lumped mass transfer parameter that characterizes matrix diffusion rates, (sec−1/2) (where ϕm is the matrix porosity, b is the effective fracture half‐aperture, and Dm is the matrix diffusion coefficient), appears to decrease as time and length scales of observation increase. However, these decreasing trends largely disappear when the laboratory and field data are considered separately, and there is little difference in Dm values measured at different timescales in laboratory experiments using the same rocks. Also, the overall variability in ϕm and Dm in the laboratory experiments is not large enough to account for the order‐of‐magnitude smaller field‐scale values of compared to lab‐scale values. We conclude that experimentally observed trends of versus time or distance scale in saturated fractured rocks at the Nevada Test Site are dominated by differences in effective fracture apertures in the various experiments, with a tendency toward larger apertures in the field experiments. These results underscore the importance of acquiring a better understanding of the factors that control effective fracture apertures as a function of distance scale in fractured media if values of from laboratory and field tracer experiments are to be reliably extrapolated to time and distance scales in risk assessment models.