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

Abstract

[1] 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 À2 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. INDEX TERMS:1829 Hydrology: Groundwater hydrology; 1832 Hydrology: Groundwater transport; 5104 Physical Properties of Rocks: Fracture and flow; KEYWORDS: fractured rock, matrix diffusion, tracer tests, contaminant transport, advection dispersion, channeling Citation: Becker, M. W., and A. M. Shapiro, Interpreting tracer breakthrough tailing from different forced-gradient tracer experiment configurations in fractured bedrock, Water Resour.

Evidence of non-Darcy flow and non-Fickian transport in fractured media at laboratory scale

“…In fractured rock aquifers, open fractures as well as bedding planes or faults give place to preferential flow paths for ground water, contaminants in solution, and free product to reach very quickly an exposure point (Becker and Shapiro, 2003). Recently, research has targeted physical mechanisms that can mitigate fast-path transport by delaying mass en route (Neretnieks, 1980;Haggerty and Gorelick, 1994;Ostensen, 1998;Haggerty et al, 2000;Becker and Shapiro, 2003).…”

“…Recently, research has targeted physical mechanisms that can mitigate fast-path transport by delaying mass en route (Neretnieks, 1980;Haggerty and Gorelick, 1994;Ostensen, 1998;Haggerty et al, 2000;Becker and Shapiro, 2003).…”

Saturation dependence of dispersion in porous media

“…• The observed distribution [42] of arrival times in two-dimensional simulations at the percolation threshold [38], giving also the predominant behavior of solute arrival time tails in fracture flow [43,44],…”

Mathematical Modeling of Non-Fickian Diffusional Mass Exchange of Radioactive Contaminants in Geological Disposal Formations

“…Multiscale subsurface systems often produce power-law tails in breakthrough curves [5][6][7][8], as well as in a nuclear waste repository site [9]. The breakthrough curves are not adequately described by the typical advection-dispersion with an exponential residence time (e.g., [10,11]). …”