Abstract. Extended tailing of tracer breakthrough is often observed in pulse injection tracer tests conducted in fractured geologic media. This behavior has been attributed to diffusive exchange of tracer between mobile fluids traveling through channels in fractures and relatively stagnant fluid between fluid channels, along fracture walls, or within the bulk matrix. We present a field example where tracer breakthrough tailing apparently results from nondiffusive transport. Tracer tests were conducted in a fractured crystalline rock using both a convergent and weak dipole injection and pumping scheme. Deuterated water, bromide, and pentafluorobenzoic acid were selected as tracers for their wide range in molecular diffusivity. The late time behavior of the normalized breakthrough curves were consistent for all tracers, even when the pumping rate was changed. The lack of separation between tracers of varying diffusivity indicates that strong breakthrough tailing in fractured geologic media may be caused by advective transport processes. This finding has implications for the interpretation of tracer tests designed to measure matrix diffusion in situ and the prediction of contaminant transport in fractured rock. IntroductionPulse injection tracer tests conducted in fractured rocks typically result in a recovered concentration history (breakthrough) that is highly skewed to later times, particularly when compared to advection and dispersion in unconsolidated porous media. The most common explanation for this "breakthrough tailing" is that while some of the tracer moves quickly through open channels, a significant fraction of the tracer is delayed by diffusive exchange with the rock matrix. Tracer mass that moves primarily through open channels results in an early peak in concentration, while the tracer that is heavily influenced by diffusive exchange results in a low concentration "tail" over an extended period of time. The exchange of mass between relatively mobile fluid in the fracture and relatively immobile fluid in the rock matrix is usually called "matrix

[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.

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Predicting hydrologic behavior at regional scales requires heterogeneous data that are often prohibitively expensive to acquire on the ground. As a result, satellite-based remote sensing has become a powerful tool for surface hydrology. Subsurface hydrology has yet to realize the benefits of remote sensing, even though surface expressions of ground water can be monitored from space. Remotely sensed indicators of ground water may provide important data where practical alternatives are not available. The potential for remote sensing of ground water is explored here in the context of active and planned satellite-based sensors. Satellite technology is reviewed with respect to its ability to measure ground water potential, storage, and fluxes. It is argued here that satellite data can be used if ancillary analysis is used to infer ground water behavior from surface expressions. Remotely sensed data are most useful where they are combined with numerical modeling, geographic information systems, and ground-based information.

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The efficiency of contaminant biodegradation in ground water depends, in part, on the transport properties of the degrading bacteria. Few data exist concerning the transport of bacteria in saturated bedrock, particularly at the field scale. Bacteria and microsphere tracer experiments were conducted in a fractured crystalline bedrock under forced-gradient conditions over a distance of 36 m. Bacteria isolated from the local ground water were chosen on the basis of physicochemical and physiological differences (shape, cell-wall type, motility), and were differentially stained so that their transport behavior could be compared. No two bacterial strains transported in an identical manner, and microspheres produced distinctly different breakthrough curves than bacteria. Although there was insufficient control in this field experiment to completely separate the effects of bacteria shape, reaction to Gram staining, cell size, and motility on transport efficiency, it was observed that (1) the nonmotile, mutant strain exhibited better fractional recovery than the motile parent strain; (2) Gram-negative rod-shaped bacteria exhibited higher fractional recovery relative to the Gram-positive rod-shaped strain of similar size; and (3) coccoidal (spherical-shaped) bacteria transported better than all but one strain of the rod-shaped bacteria. The field experiment must be interpreted in the context of the specific bacterial strains and ground water environment in which they were conducted, but experimental results suggest that minor differences in the physical properties of bacteria can lead to major differences in transport behavior at the field scale.

Time-lapse ground-penetrating-radar ͑GPR͒ surveys exploit signal-amplitude changes to monitor saline tracers in fractures and to identify groundwater flow paths. However, the relationships between GPR signal amplitude, phase, and frequency with fracture aperture and fluid electrical conductivity are not well understood. We used analytical modeling, numerical simulations, and field experiments of multifrequency GPR to investigate these relationships for a millimeter-scale-aperture fracture saturated with water of varying salinity. We found that the response of lower-frequency radar signals detects changes in fluid salinity better than the response of higher-frequency signals. Increasing fluid electrical conductivity decreases low-frequency GPR signal wavelength, which improves its thin-layer resolution capability. We concluded that lower signal frequencies, such as 50 MHz, and saline tracers of up to 1 S/m conductivity are preferable when using GPR to monitor flow in fractured rock. Furthermore, we found that GPR amplitude and phase responses are detectable in the field and predictable by EM theory and modeling; therefore, they can be related to fracture aperture and fluid salinity for hydrologic investigations of fractured-rock flow and transport properties.

6Periodic hydraulic experiments were conducted in a five-spot well cluster completed in a single 7 bedding plane fracture. Tests were performed by using a winch-operated slug (submerged solid 8 cylinder) to create a periodic head disturbance in one well and observing the phase shift and 9 attenuation of the head response in the remaining wells. Transmissivity (T) and storativity (S) 10 were inverted independently from head response. Inverted T decreased and S increased with 11 oscillation period. Estimated S was more variable among well pairs than T, suggesting S may be 12 a better estimator of hydraulic connectivity among closely spaced wells. These estimates 13 highlighted a zone of poor hydraulic connection that was not identified by a constant rate test 14 conducted in the same wells. Periodic slug tests appear to be a practical and effective technique 15 for establishing local scale spatial variability in hydraulic parameters. 16 1 Introduction 17 Flow channelization has long been recognized as a hallmark of groundwater flow in fractured 18 bedrock systems [Tsang and Neretnieks, 1998]. The physical nature of the problem is described 19 simply through the cubic law which dictates that water flow rate is related cubically to the local 20 fracture aperture. Prediction of flow in a natural bedrock system is not so simple, however, as 21 the distribution of aperture and its interaction with water flow is highly variable even within a 22 single fracture. Understanding flow in bedrock consequently requires site specific hydraulic 23 characterization to be conducted. 24Typical pumping and slug test configurations are not well suited to bedrock environments. 25Because of the small water storage in bedrock, the hydraulic radius of influence of a pumping 26 well extends rapidly outward implying that only the earliest drawdown contains local 27 information. Early drawdown is often dominated by well-bore storage and formation damage 28 effects in open boreholes. Slug test responses are weighted more toward local hydraulics but are 29 even more sensitive to borehole influences. 30 A periodic hydraulic test potentially overcomes some of the limitations of pumping and slug 31 tests. Periodic (also called harmonic, oscillatory, or sinusoidal) tests are conducted by creating 32 an oscillating head in one well and observing the corresponding oscillatory head response in one 33 or more observation wells. Because the head signal is in a constant state of transience, periodic 34 tests highlight the influence of formation storativity on drawdown response. The repeatability of 35 the transience allow initial effects of well bore storage and pump priming to be isolated. Most 36 interestingly, periodic tests are capable of interrogating different portions of the formation 37without the addition of observations wells. This is because the spatial weighting of hydraulic 38 response to transmissivity (T) and storativity (S) is sensitive to the frequency of the head 39 oscillation [Cardiff et al., 2013;Renner and Messar, 2006]. Periodi...