Dilution Tests in a Low-Permeability Fractured Aquifer: Matrix Diffusion Effect

• Published in: Ground water Vol. 45; no. 2; pp. 235 - 241
• Main Authors: Bernstein, Anat, Adar, Eilon, Yakirevich, Alexander, Nativ, Ronit
• Format: Journal Article
• Language: English
• Published: Malden, USA Malden, USA : Blackwell Publishing Inc 01.03.2007; Blackwell Publishing Inc; Ground Water Publishing Company
• Subjects: Diffusion; Groundwater; Hydrology; Methods; Models, Theoretical; Soil permeability; Velocity; Water; Water flow
• ISSN: 0017-467X; 1745-6584
• DOI: 10.1111/j.1745-6584.2006.00268.x

A point dilution test is commonly used in single-borehole tracer experiments designed to determine the Darcy velocity of a formation. This method is based on the concept that, in a borehole, a tracer's concentration declines as a consequence of the water flux. Based on theoretical simulations and field observations, this study indicates that for low-permeability, yet highly porous fractured formations, the common practice of excluding the effect of diffusive mass flux between the dissolved tracer within the borehole and the surrounding matrix may lead to significant errors in the assessment of the Darcy velocity. This conclusion was confirmed by a model adapted to simulate experimental data collected from a tracer test performed in a vertical, large-diameter (25-cm) borehole drilled along a subvertical fracture intersecting a chalk formation.