Field measurement of radial solute transport in fractured rock

Abstract

The results of a tracer experiment conducted in a single fracture are interpreted using a semianalytical model which accounts for advective dispersion, matrix diffusion, mixing in the test zones, and for tortuosity. The experiment was conducted using a conservative tracer which was injected into a steady divergent flow field. Arrival of tracer was monitored in an array of 13 boreholes intersecting the fracture over a square area of approximately 30 rn on a side. The arrival of tracer was detected in 11 of the 13 boreholes. Simulation of the transport process suggests that matrix diffusion may play a significant role in the migration of solutes. Contrary to previously published results, the relative influence of advective dispersion was found to be increasingly diminished with increasing scale. Fracture apertures determined from the results of the tracer experiment were found to be in general agreement with apertures calculated from independent hydraulic tests. The individual advective processes that contribute to advective dispersion remain unresolved on the basis of this experiment. IntroductionThe process of solute transport in fractured rock is of significant importance in many groundwater environments. In particular, knowledge of the rate of migration and degree of dispersion of groundwater contaminants from hazardous waste and industrial sites underlain by fractured rock is important for the protection of underground and surface water supplies. There are currently a large number of conceptual models which can be used to simulate and predict solute migration in fractures with widely varying outcomes. The reason for these discrepancies is the paucity of reliable data with which to evaluate the conceptual models.The processes involved in the transport of solutes within a discrete fracture are several, with different processes predominant at the microscopic and macroscopic scales. Several models have been proposed for the simple advective dispersion process at the microscopic scale [Karadi et al., 1972;Horne and Rodriguez, 1983;Hull, 1985]. In this case, the longitudinal spreading of a solute between idealized parallel plates occurs as a result of the parabolic distribution of velocity across the width of the fracture (i.e., pure advection). In addition, molecular diffusion perpendicular to the direction of flow but within the confines of the fracture can also be considered. This is known as Taylorian dispersion, the concept for which was originally developed for pipe flow [Taylor, 1953]. Analytical expressions developed for this model incorporate longitudinal dispersivity at, as a function of mean velocity and the square of the fracture aperture. Calculations using typical values of fracture aperture, mean velocity, and molecular diffusion yield Peclet numbers (Pe = L/at,, where L is the distance along the flow path) of very large magnitude (i.e., >500). According to the review of Gelhar et al. [1992], values of Pe (converting at, to Pe using the expression given above) on the order of 2-4 are most common for e...

Phase I Contaminant Transport Parameters for the Groundwater Flow and Contaminant Transport Model of Corrective Action Unit 97: Yucca Flat/Climax Mine, Nevada Test Site, Nye County, Nevada, Revision 0

“…Numerical models and analytical solutions presenting analyses of the effect of matrix diffusion during transport in fractured geologic media have been reported by Neretnieks (1980), 1981), Tang et al (1981), Noorishad and Mehran (1982), Huyakorn et al (1983a and b), Feenstra , Grisak and Pickens (1981), Neretnieks et al (1982), Moreno et al (1985), , ; and for field tracer tests by Jones et al (1992), Novakowski and Lapcevic (1994), IT (1998), , and.…”

Field-Scale Effective Matrix Diffusion Coefficient for Fractured Rock:results From Literature Survey

“…In addition, in some of the tests, the bacteriophage and microsphere were injected simultaneously or separately with chemical tracers to separate the effects of matrix diffusion from advection and dispersion [Champ and Schroeter, 1988;Bales et al, 1989;McKay et al, 1993b;Becker et al, 1999;Reimus and Haga, 19991. At some geologic sites, tracer tests were conducted at different scales to investigate scale effects of transport parameters [Novakowski and Lapcevic, 1994;Frost et al, 1995;Himmelsbach et al, 1998;Jardine et al, 1999;Maloszewski et al, 1999;Baurnle, 20031. Most of all the field tracer tests have been analyzed using analytic or numerical models considering matrix diffusion [e.g., Brettmann et al, 19931, and their reported values of field-scale matrix diffusion coefficients were again used as entries to our data set.…”

Characterization of preferential flow paths between boreholes in fractured rock using a nanoscale zero-valent iron tracer test

“…Tracer tests have long been used to evaluate the crosshole connectivity in fractured rock (Chen 1986;Novakowski and Lapcevic 1994;Sharifi Haddad et al 2015); however, the test results have not shown the exact location of the tracer inlet in the observation well. Nanoscale zero-valent iron (nZVI) has been widely applied to in-situ remediation of groundwater contamination (Wang and Zhang 1997;Varanasi et al 2007).…”