Testing and parameterizing a conceptual model for solute transport in a fractured granite using multiple tracers in a forced‐gradient test

Abstract

[1] A cross-hole tracer test involving the simultaneous injection of two nonsorbing solute tracers with different diffusion coefficients (bromide and pentafluorobenzoate) and a weakly sorbing solute tracer (lithium ion) was conducted in a fractured granite near an underground nuclear test cavity in central Nevada. The test was conducted to (1) test a conceptual radionuclide transport model for the site and (2) obtain transport parameter estimates for predictive modeling. The differences between the responses of the two nonsorbing tracers (when normalized to injection masses) are consistent with a dualporosity transport system in which matrix diffusion is occurring. The large concentration attenuation of the sorbing tracer relative to the nonsorbing tracers suggests that diffusion occurs primarily into matrix pores, not simply into stagnant water within the fractures. The relative responses of the tracers at late times suggest that the diffusion-accessible matrix pore volume is possibly limited to only half the total volume of the flow system, implying that the effective retardation factor due to matrix diffusion may be as small as 1.5 for nonsorbing solutes in the system. The lower end of the range of possible sorption K d values deduced from the lithium response is greater than the upper 95% confidence bound of K d values measured in laboratory sorption tests using crushed granite from the site. This result suggests that the practice of using laboratory sorption data in field-scale transport predictions of cation-exchanging radionuclides, such as 137 Cs + and 90 Sr ++ , should be conservative for the SHOAL site.

Modeling of Groundwater Flow and Radionuclide Transport at the Climax Mine sub-CAU, Nevada Test Site

“…With measured porosities of 0.01 to 0.02, the range of effective diffusivity is 1.2 x 10 -13 to 4 x 10 -12 m 2 /s. Interestingly, when fitting a field tracer test, Reimus et al (2003) reported a lumped diffusion term of b D p p ε (where b is the fracture half-aperture). Based on the combination of these values, the fracture aperture ranges from 0.6 to 8 mm.…”

“…However, the value of 9 x 10 -12 m 2 /s was calculated using an unrealistically high porosity of 0.045. Reimus et al (2003) reported that laboratory measured pore diffusivities for Shoal granites (Br -experiments) ranged from 1.2 x 10 -11 to 2 x 10 -10 m 2 /s. With measured porosities of 0.01 to 0.02, the range of effective diffusivity is 1.2 x 10 -13 to 4 x 10 -12 m 2 /s.…”

“…Effective diffusion coefficients (D e =D p ε p , where D p is the pore diffusivity and ε p is the matrix porosity) were found to range from 1 x 10 -12 to 3.7 x 10 -11 m 2 /s. Core section Reimus et al (2003) Birgersson and Neretnieks (1990) compared nonsorbing tracer diffusion coefficients in laboratory and field measurements. Pore diffusivities were found to range from 5 x 10 -12 to more than 1 x 10 -10 m 2 /s in the field and from 9 x 10 -11 to 2 x 10 -10 m 2 /s in the laboratory.…”

Development of Models to Simulate Tracer Behavior in Enhanced Geothermal Systems

“…For a thermally degrading tracer, the term -kC must be added to left side of Equation (4.1), and the term -kC m must be added to the left side of Equation (4.2), where k is the decay constant given by Equation (2.1). The transformation of Equations (4.1) and (4.2) to the Laplace domain and their subsequent Laplacedomain solution and inversion of the result back to the time domain are described by Reimus et al (2003).…”

Long-term Monitoring Plan for the Shoal Underground Nuclear Test

“…Four additional characterization wells were drilled at Shoal during summer and fall 1999; details of the drilling and well installation are in IT Corporation (2000), with testing reported in Mihevc et al (2000). A key component of the second field program was a tracer test between two of the new wells (Carroll et al, 2000;Reimus et al, 2003).…”