Solute transport performance analysis of equivalent apertures in a single undisturbed basaltic fracture

Abstract

Four bench-scale solute injection experiments were conducted in a natural undisturbed basaltic fractured core of the Serra Geral Formation (SGF) (Brazil) to assess the performance of the equivalent apertures on conservative solute transport. Fracture aperture is particularly important in controlling solute transport in fractured-rock aquifers. The spatial heterogeneity of fracture apertures complicates solute transport modeling. Scientific researchers and mainly hydrogeology professionals still adopt a single value for the fracture aperture field, i.e., an equivalent aperture. However, there is a lack of information about the most suitable equivalent aperture associated with solute transport phenomena, especially for basaltic fractured aquifers. The objective of this study was to evaluate the performance of three types of equivalent apertures on conservative solute transport in a basaltic aquifer sample. The fracture aperture field was measured using the advanced high-resolution and nondestructive technique, x-ray micro-computed tomography. The performance of the equivalent apertures was evaluated using an analytical solution of the one-dimensional advection-dispersion equation (ADE) compared to observed breakthrough curves. As a result, the arithmetic mean of the measured aperture field was 111.74 ± 54.26 μm. The analytical solution of ADE coupled with mass balance aperture was able to predict breakthrough behavior, especially for low volumetric flow rates. This study demonstrates that the mass balance aperture should be adopted as an equivalent aperture instead of the hydraulic and friction loss equivalent aperture when inertial forces of macroscopic flow are negligible. The findings provide a step forward towards better understanding of groundwater contamination risk in basaltic fractured aquifers.