Estimation of Effective Fracture Aperture in Glacial Tills by Analysis of Dye Tracer Penetration.

Abstract

This study advances a methodology to estimate effective apertures of fractures in glacial tills based on dye tracer infiltration tests and numerical simulations. The approach uses the visible penetration depth of the dye tracer along fracture flow paths as primary information to calculate effective fracture apertures. Further data used in the calculation are the dye tracer input concentration and retardation, the duration of the tracer injection, and the hydraulic gradient applied to control the infiltrating water fluxes. The method does not require measurement of hydraulic conductivity for the fractured till and enables direct observation of flow and transport patterns within the fractures (e.g., uniform flow and dye tracer distribution, channeling due to aperture variability, and presence of biogenic macropores in fractures). The approach was successfully verified by using the estimated effective fracture aperture values in Large Undisturbed Columns (LUCs) to consistently simulate both the observed LUC effluent breakthrough of a conservative bromide tracer and the water fluxes with the hydraulic gradient applied in the experiments. Sensitivity analyses revealed that estimation of small effective fracture apertures (<10 μm) required accurate determination of the dye tracer retardation factor. By contrast, in the case of larger effective apertures (>20 μm), the sensitivity of the estimated effective fracture aperture to variations in the porous material and solute transport parameters was low compared to the dominant sensitivity to the water flow through the fractures (cubic relation between flow and aperture). The proposed approach may be extended beyond laboratory applications and assist in characterizing field-scale fracture networks.