Interpreting tracer breakthrough tailing from different forced‐gradient tracer experiment configurations in fractured bedrock

Conceptual and mathematical models are presented that explain tracer breakthrough tailing in the absence of significant matrix diffusion. Model predictions are compared to field results from radially convergent, weak‐dipole, and push‐pull tracer experiments conducted in a saturated crystalline bedrock. The models are based upon the assumption that flow is highly channelized, that the mass of tracer in a channel is proportional to the cube of the mean channel aperture, and the mean transport time in the channel is related to the square of the mean channel aperture. These models predict the consistent −2 straight line power law slope observed in breakthrough from radially convergent and weak‐dipole tracer experiments and the variable straight line power law slope observed in push‐pull tracer experiments with varying injection volumes. The power law breakthrough slope is predicted in the absence of matrix diffusion. A comparison of tracer experiments in which the flow field was reversed to those in which it was not indicates that the apparent dispersion in the breakthrough curve is partially reversible. We hypothesize that the observed breakthrough tailing is due to a combination of local hydrodynamic dispersion, which always increases in the direction of fluid velocity, and heterogeneous advection, which is partially reversed when the flow field is reversed. In spite of our attempt to account for heterogeneous advection using a multipath approach, a much smaller estimate of hydrodynamic dispersivity was obtained from push‐pull experiments than from radially convergent or weak dipole experiments. These results suggest that although we can explain breakthrough tailing as an advective phenomenon, we cannot ignore the relationship between hydrodynamic dispersion and flow field geometry at this site. The design of the tracer experiment can severely impact the estimation of hydrodynamic dispersion and matrix diffusion in highly heterogeneous geologic media.

[1]  P. Reimus,et al.  Using multiple experimental methods to determine fracture/matrix interactions and dispersion of nonreactive solutes in saturated volcanic tuff , 2000 .

[2]  Sean Andrew McKenna,et al.  On the late‐time behavior of tracer test breakthrough curves , 2000 .

[3]  R. Charbeneau,et al.  Erratum: First-passage-time transfer functions for groundwater tracer tests conducted in radially convergent flow (Journal of Contaminant Hydrology PII: S0169772299000613) , 2000 .

[4]  T. Illangasekare,et al.  Intermediate‐scale experiments and numerical simulations of transport under radial flow in a two‐dimensional heterogeneous porous medium , 2000 .

[5]  K. Karasaki,et al.  A multidisciplinary fractured rock characterization study at Raymond field site, Raymond, CA. , 2000 .

[6]  M. Becker,et al.  Tracer transport in fractured crystalline rock: Evidence of nondiffusive breakthrough tailing , 2000 .

[7]  Frederick D. Day-Lewis,et al.  Identifying fracture‐zone geometry using simulated annealing and hydraulic‐connection data , 2000 .

[8]  Jesús Carrera,et al.  A comparison of hydraulic and transport parameters measured in low-permeability fractured media , 2000 .

[9]  R. Charbeneau,et al.  First-passage-time transfer functions for groundwater tracer tests conducted in radially convergent flow , 2000 .

[10]  Roy Haggerty,et al.  Solute Transport and Multirate Mass Transfer in Radial Coordinates , 2000 .

[11]  P. Jardine,et al.  Quantifying diffusive mass transfer in fractured shale bedrock , 1999 .

[12]  Gedeon Dagan,et al.  Solute transport in divergent radial flow through heterogeneous porous media , 1999, Journal of Fluid Mechanics.

[13]  Assaf P. Oron,et al.  Flow in rock fractures: The local cubic law assumption reexamined , 1998 .

[14]  C. Tsang,et al.  Flow channeling in heterogeneous fractured rocks , 1998 .

[15]  R. W. Ostensen Tracer tests and contaminant transport rates in dual-porosity formations with application to the WIPP , 1998 .

[16]  Eva Hakami,et al.  Aperture measurements and flow experiments on a single natural fracture , 1996 .

[17]  Division on Earth The Committee on Fracture Characterisation and Fluid Flow, U.S. National Research Council, Rock Fractures and Fluid Flow: Contemporary Understanding and Applications , 1996 .

[18]  S. Gorelick,et al.  Multiple‐Rate Mass Transfer for Modeling Diffusion and Surface Reactions in Media with Pore‐Scale Heterogeneity , 1995 .

[19]  Allen F. Moench,et al.  Convergent Radial Dispersion in a Double-Porosity Aquifer with Fracture Skin: Analytical Solution and Application to a Field Experiment in Fractured Chalk , 1995 .

[20]  L. Smith,et al.  Fluid velocity and path length in fractured media , 1995 .

[21]  Chin-Fu Tsang,et al.  Flow channeling in strongly heterogeneous porous media: A numerical study , 1994 .

[22]  S. Gorelick,et al.  DESIGN OF MULTIPLE CONTAMINANT REMEDIATION : SENSITIVITY TO RATE-LIMITED MASS TRANSFER , 1994 .

[23]  Y. Tsang Usage of “Equivalent apertures” for rock fractures as derived from hydraulic and tracer tests , 1992 .

[24]  C. F. Tsang,et al.  Multiple-peak response to tracer injection tests in single fractures; a numerical study : Water Resour ResV27, N8, Aug 1991, P2143–2150 , 1992 .

[25]  William H. Press,et al.  Numerical Recipes in Fortran 77 , 1992 .

[26]  Ivars Neretnieks,et al.  A Large-Scale Flow and Tracer Experiment in Granite: 1. Experimental Design and Flow Distribution , 1991 .

[27]  Ivars Neretnieks,et al.  A Large-Scale Flow and Tracer Experiment in Granite: 2. Results and Interpretation , 1991 .

[28]  TRACER TRANSPORT IN FRACTURES : ANALYSIS OF FIELD DATA BASED ON A VARIABLE-APERTURE CHANNEL MODEL , 1991 .

[29]  Chin-Fu Tsang,et al.  Multiple-Peak Response to Tracer Injection Tests in Single Fractures: A Numerical Study , 1991 .

[30]  William H. Press,et al.  Numerical Recipes: FORTRAN , 1988 .

[31]  A. Shapiro,et al.  Assessing the validity of the channel model of fracture aperture under field conditions , 1989 .

[32]  K. Raven,et al.  Interpretation of field tracer tests of a single fracture using a transient solute storage model , 1988 .

[33]  Anders Rasmuson,et al.  Radionuclide Transport in Fast Channels in Crystalline Rock , 1986 .

[34]  Bruce A. Robinson,et al.  Characterization of flow maldistribution using inlet-outlet tracer techniques: An application of internal residence time distributions , 1986 .

[35]  J. V. Tracy,et al.  Flow through fractures , 1981 .

[36]  A. Moench,et al.  A numerical inversion of the laplace transform solution to radial dispersion in a porous medium , 1981 .

[37]  Ivars Neretnieks,et al.  Diffusion in the rock matrix: An important factor in radionuclide retardation? , 1980 .