The interpretation of a tracer experiment conducted in a single fracture under conditions of natural groundwater flow

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 35×40 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 μm2 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.