Field-scale effective matrix diffusion coefficient for fractured rock: Results from literature survey

Abstract

Matrix diffusion is an important mechanism for solute transport in fractured rock.We recently conducted a literature survey on the effective matrix diffusion coefficient, D i , a key parameter for describing matrix diffusion processes at the field scale. Forty field tracer tests at 15 fractured geologic sites were surveyed and selected for the study, based on data availability and quality. Field-scale D: values were calculated, either directly using data reported in the literature or by reanalyzing the corresponding field tracer tests. Surveyed data indicate that the effective-matrix-diffusion-coefficient factor FD (defined as the ratio of D i to the lab-scale matrix diffusion coefficient [ Dm 3 of the same tracer) is generally larger than one, indicating that the effective matrix diffusion coefficient in the field is comparatively larger than the matrix diffusion coefficient at the rock-core scale. This larger value can be attributed to the many mass-transfer processes at different scales in naturally heterogeneous, fractured rock systems. Furthermore, we observed a moderate trend toward systematic increase in the FD value with observation scale, indicating that the effective matrix diffusion coefficient is likely to be statistically scale dependent. The FD value ranges from 1 to 10,000 for observation scales from 5 to 2,000 m. At a given scale, the F ' value varies by two orders of magnitude, reflecting the influence of differing degrees of fractured rock heterogeneity at different sites. In addition, the surveyed data indicate that field-scale longitudinal dispersivity generally increases with observation scale, which is consistent with previous studies. The scaledependent field-scale matrix diffusion coefficient (and dispersivity) may have significant 1 implications for assessing long-term, large-scale radionuclide and contaminant transport events in fractured rock, both for nuclear waste disposal and contaminant remediation.

Evidence of Multi-Process Matrix Diffusion in a Single Fracture from a Field Tracer Test

“…e-mail: qzhou@eticeng.com confirmed by both single-and multi-tracer tests (e.g., Maloszewski et al, 1999). It has been found that the effective matrix diffusion coefficient (D e m ) calibrated using field tracer tests can be orders of magnitude higher than the corresponding matrix diffusion coefficient (D m ) inferred from laboratory experiments on intact rock cores (Hodgkinson and Lever, 1983;Neretnieks, 2002;Andersson et al, 2004;Liu et al, 2004;Zhou et al, 2005). The enhanced matrix diffusion has been attributed to many different mechanisms, such as infilling materials and stagnant water within fractures (Neretnieks, 2002), a degraded and altered matrix zone adjacent to fractures (Hodgkinson and Lever, 1983;Maloszewski and Zuber, 1993;Andersson et al, 2004), and connected small fractures (Wu et al, 2004).…”

Retention of Nanoparticles: From Laboratory Cores to Outcrop Scale

“…It has been proved that the effective hydrodynamic dispersion coefficient in the field is comparatively larger than the hydrodynamic dispersion coefficient in the laboratory [52][53][54][55][56][57][58]. This finding is based on a literature survey of 40 field tracer tests at 15 geologic sites.…”

A Semi-Analytical Solution for Large-Scale Injection-Induced Pressure Perturbation and Leakage in a Laterally Bounded Aquifer–Aquitard System

“…A number of numerical methods for Laplace transform inversion are available (Cohen, 2007). Among them, the methods of Stehfest (1970) andde Hoog et al (1982) have been extensively employed with semianalytical solutions developed for field pumping and tracer tests (e.g., Moench, 1985;Moridis, 1999;Zhan and Bian, 2006;Zhou et al, 2007). In this research, the de Hoog et al (1982) method was employed for numerical inversion of our analytical solutions.…”