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LABORATORY INVESTIGATIONS AND ANALYTICAL AND NUMERICAL MODELING OF THE TRANSPORT OF DISSOLVED SOLUTES THROUGH SATURATED FRACTURED ROCK

The objective of this research was to determine the applicability of reactive tracer data obtained from laboratory tests to larger-scale field settings. Laboratory tracer tests were used to quantify transport properties in fractured volcanic ash flow tuff from southern Nevada. In a series of experiments, a pulse containing several ionic tracers was injected into four tuff cores, each containing one induced fracture oriented along the main axis. Multiple tests were also conducted at different flow velocities. Transport data from nonreactive tracers of different diffusivity allowed the separation of the effects of hydrodynamic dispersion within the fracture and molecular diffusion between flowing and nonflowing water within the systems, which was presumed to be in the fracture and bulk porous matrix, respectively. Reactive tracers were also included to estimate the sorption capacity of the tuffs. The experiments verified the importance of fracture/matrix and solute/solid interactions in the fractured tuffs. Using artificial tracers of different physical and chemical properties in the same test provided unique interpretations of the tests and minimized uncertainty in transport parameter estimates. Compared to separate field tests in the rock types, the laboratory experiments tended to overestimate the degree of diffusive mass transfer and underestimate sorption capacity due to the small scale and high tracer concentrations, respectively. The first result suggests that geometry differences between lab apparatuses and field systems precluded the direct extension of laboratory-derived transport parameters to field scales. For example, smaller-scale processes such as diffusion within the stagnant water in the fractures (“free-water diffusion”), caused by fracture aperture variability, were more important at small time scales. Because free water diffusion coefficients are larger than matrix diffusion coefficients, this led to an overestimation of the amount of diffusive mass transfer. Furthermore, laboratory diffusion cell tests provided independent estimates of matrix diffusion coefficients for the tracers, and these values were similar to those estimated for the same tracers in the field tests. Thus, the value of the diffusion coefficients in the larger-scale field tests appeared to approach their asymptotic “true” values because of the larger volume of porous rock accessed during tracer testing. The sorption capacity of the solid material was underestimated in both the laboratory and field tracer experiments. A fraction of the ion-exchanging tracer (lithium) moved through the system unretarded because the ion exchange sites on the solid phase were overwhelmed by the tracer. This happened more so in the laboratory tests due to the smaller amount of dilution in the system. These results clearly indicate that by injecting high-concentration tracers during cross-well tests, the tracer data can indicate sorption parameters smaller than those determined under lower-concentration conditions. These results indicate that one should be cautious when applying laboratoryderived tracer data to field settings. The use of multiple tracer experiments conducted over a wide range of time scales and injection concentrations will help avoid ambiguity of the derived transport parameters.