A laboratory physical model to analyse flow and transport processes in a fractured rock sample at bench scale level

Abstract

The knowledge of flow and transport phenomena in fractured rocks is very important in hydrogeologic engineering in order to optimize clean up and monitoring strategies, to carry out risk assessment and to manage interventions in aquifers.
Recently, understanding, characterizing and modeling physical and chemical interactions within fractured aquifers has acquired increasing importance, especially with regard to the question of water resources development and groundwater contamination. Sometimes the equivalent porous medium approach fails to reproduce flow and transport patterns in such complex geological formations. Critical emerging issues for fractured aquifers are the validity of the Darcian-type “local cubic law” which assumes a linear relationship between flow rate and pressure gradient to accurately describe flow patterns and of the classical advection-dispersion equation to describe the propagation of solute.
Most studies of transport through discrete fractures are still based on simpler flow models which has limited the interpretation of solute breakthrough curves.
Experimental data obtained under controlled conditions such as in a laboratory allow to increase the understanding of the fundamental physics of fuid flow and solute transport in fractures.
In this study hydraulic and tracer tests on artificially created fractured rock samples of parallelepiped (0.60×0.40×0.8m) shape have been carried out.
The volumes of water passing through different paths across the fractured sample for various hydraulic head differences and break-through curves for saline tracer pulse across different pathways have been measured.
The above experiments are aimed at understanding the relations existing between the applied boundary conditions, the geometry of the system and the occurring ow and transport phenomena.
The experimental results have shown evidence of non linearity in ow and concentration pro les that cannot be described by conven- tional solute transport models.
In fact, the classical advection-dispersion equation -used as a benchmark for comparison in a numerical model- poorly describes the experimental breakthrough curves of the tracer propagation. A comparative analysis of the obtained results has allowed to study the behavior of flow and transport in the investigated medium on the one hand, and to evaluate possible improvements to the experimental setup on the other.