The results and interpretation of five induced‐gradient tracer tests performed at five different average interborehole fluid velocities in a single fracture in monzonitic gneiss are described. The experiments were conducted using radioactive 82Br and a fluorescent dye as conservative tracers where the tracers were pulse injected into radial convergent and injection‐withdrawal flow fields. The flow fields were established between straddle packers isolating the fracture in three boreholes over distances of 12.7–29.8 m. The tracer breakthrough curves were determined from samples of the withdrawn groundwater and were interpreted using residence time distribution (RTD) theory and two deterministic simulation models. The RTD curves of the tracer experiments were interpreted by fitting to the field data a simple advection‐dispersion model and an advection‐dispersion model with transient solute storage in immobile fluid zones. Both models consider the different flow field geometries associated with injection‐withdrawal and radial convergent tests. Comparison of the fits obtained by the simulation models suggest that the initial period of solute transport in single fractures is advection dominated and with increasing tracer residence time or decreasing fluid velocity, transport progresses toward more Fickian‐like behavior. During the advective‐dominated period, the transient solute storage model is shown to adequately describe the asymmetries and long tails characteristic of the fracture RTDs. Interpretation of the tracer experiments using both simulation models further suggests that induced‐gradient tracer experiments are likely to underestimate the dispersive characteristics of single fractures under natural flow conditions.
Abstract. The development of conceptual models to describe the hydrogeology of sparsely fractured media requires the characterization of the properties of discrete fractures at the field scale. In this study, a tracer experiment conducted under conditions of natural groundwater flow in a discrete fracture in an interbedded shale and limestone sequence is interpreted. This experiment, which was initiated by injecting a small quantity of tracer into the fracture plane, involved monitoring tracer movement using 27 boreholes within a 35 x 40 m area. Test data revealed a tracer plume which spread both longitudinally and transversely in the direction of mean groundwater flow. The field breakthrough curves were interpreted using a two-dimensional finite element transport model that incorporated longitudinal and transverse dispersion, diffusion into the rock matrix, and constant fracture aperture. Additional simulations which incorporated the effects of aperture variability were conducted in a Monte Carlo format by varying the spatial correlation and variance of the aperture field. The constant aperture analysis found that the mean aperture determined from the tracer experiment was approximately 20% greater than the mean aperture measured by hydraulic methods. Values of matrix porosity ranging between 1 and 3%, a constant longitudinal dispersivity of 0.1 m, and a large range in transverse dispersivity from 0.01 to 0.22 m were required to simulate the data. Trends of increasing aperture and matrix porosity with distance were observed, suggesting that tracer transport followed increasingly tortuous pathways. Although the variable aperture simulations were not fitted to the field data, the aperture field having an isotropic spatial correlation length of 0.5 m and variance of 10,000/xm 2 provided simulated plumes that appeared to be most similar to the shape and concentration of the field plume. Evidence from the hydraulic measurement of the aperture distribution suggests that this is a reasonable estimate of the natural aperture field.
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...
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