It is known that there is a certain unpredictability associated with fractured and karstified rock formations that limits the comprehension of flow and transport phenomena that take place within them. Yet the wide spreading of these rocks makes it extremely important to improve their understanding in order to protect them from natural and anthropic hazards. In fractured rock aquifers the pronounced heterogeneity of the medium due to the existence of discontinuities may create hydrodynamic conditions of difficult interpretation and therefore imply uncertainty in modeling and in planning remediation interventions. In such context the Darcy law can be adapted to flow throughout fractures by means of the well known cubic law, valid under the assumption of ideal fractures, represented by smooth and parallel plates; however its application to real fracture systems characterized by rough and variable apertures has many times proved to be oversimplified and unrealistic. The parallel plate model is no more valid at higher depth where, due to compressive stresses, the walls of the fractures are pressed together forming asperities that lead to reduction in fracture aperture and cause flow channeling. In addiction, the partial closure of fractures lead to other relevant phenomena in the vadose zone: unsaturated portions within the fractures will act as barriers to liquid flow along them in that the water will tend to flow from one matrix block to another across the partially saturated fractures. In the vadose zone, the hydrodynamic structure of a fractured rock aquifer may be totally reversed: high flux zones at near saturation become low flux zones in unsaturated conditions. At full saturation however, in fractured aquifers joints act as major conduits for water, dissolved matter and contaminants; in highly heterogeneous fracture networks, open fractures as well as bedding planes, karstic conduits or faults may give even place to preferential flow paths for ground water, contaminants in solution, and free product. The relatively large size of fracture openings permit fluid to reach high velocities and to move quickly away from a leak site, contaminating large areas in a short time. Furthermore, as contamination moves through a fractured rock aquifer, it tends to diffuse from the flowing fracture water into the rock's essentially stagnant pore water. The rock matrix itself is often able to store ground water, contaminants in solution, or free product. These processes tend both to retard the plume's advance through a fractured rock mass and to substantially increase the difficulty of purging contamination from the aquifer: the cleanup of fractured rock aquifers in some cases requires many decades, even centuries. The parameters which most strongly govern the degree to which matrix diffusion prolongs the aquifer restoration process have been detected as rock's matrix porosity, fracture spacing, matrix diffusivity, the chemical identity of the contaminant(s), and the length of time the aquifer has been contaminated. It is straightforward that an ad-hoc cleanup technique has to act on the first three parameters that have been identified as crucial within the remediation process itself. Moreover, in fractured rock aquifers numerical models used to simulate remediation processes make use of averaged values for these parameters, as it is extremely difficult to determine them accurately. Making predictions of actual cleanup times is therefore most of the times of scarce significance. As planning effective remediation interventions requires spatial and temporal predictions on contaminant propagation, it proves to be necessary to implement specific interpretations of the classical methodologies of describing fluid flow and solute transport in fractured rock aquifers. The present chapter provides an analysis of the peculiar aspects that concern the dynamics of groundwater circulation and contaminant propagation in fractured rock aquifers for the application of the ad-hoc cleanup technique. © 2012 Nova Science Publishers, Inc. All rights reserved.

Fluid flow and contaminant propagation in fractured rock aquifers

Cherubini, Claudia
2010

Abstract

It is known that there is a certain unpredictability associated with fractured and karstified rock formations that limits the comprehension of flow and transport phenomena that take place within them. Yet the wide spreading of these rocks makes it extremely important to improve their understanding in order to protect them from natural and anthropic hazards. In fractured rock aquifers the pronounced heterogeneity of the medium due to the existence of discontinuities may create hydrodynamic conditions of difficult interpretation and therefore imply uncertainty in modeling and in planning remediation interventions. In such context the Darcy law can be adapted to flow throughout fractures by means of the well known cubic law, valid under the assumption of ideal fractures, represented by smooth and parallel plates; however its application to real fracture systems characterized by rough and variable apertures has many times proved to be oversimplified and unrealistic. The parallel plate model is no more valid at higher depth where, due to compressive stresses, the walls of the fractures are pressed together forming asperities that lead to reduction in fracture aperture and cause flow channeling. In addiction, the partial closure of fractures lead to other relevant phenomena in the vadose zone: unsaturated portions within the fractures will act as barriers to liquid flow along them in that the water will tend to flow from one matrix block to another across the partially saturated fractures. In the vadose zone, the hydrodynamic structure of a fractured rock aquifer may be totally reversed: high flux zones at near saturation become low flux zones in unsaturated conditions. At full saturation however, in fractured aquifers joints act as major conduits for water, dissolved matter and contaminants; in highly heterogeneous fracture networks, open fractures as well as bedding planes, karstic conduits or faults may give even place to preferential flow paths for ground water, contaminants in solution, and free product. The relatively large size of fracture openings permit fluid to reach high velocities and to move quickly away from a leak site, contaminating large areas in a short time. Furthermore, as contamination moves through a fractured rock aquifer, it tends to diffuse from the flowing fracture water into the rock's essentially stagnant pore water. The rock matrix itself is often able to store ground water, contaminants in solution, or free product. These processes tend both to retard the plume's advance through a fractured rock mass and to substantially increase the difficulty of purging contamination from the aquifer: the cleanup of fractured rock aquifers in some cases requires many decades, even centuries. The parameters which most strongly govern the degree to which matrix diffusion prolongs the aquifer restoration process have been detected as rock's matrix porosity, fracture spacing, matrix diffusivity, the chemical identity of the contaminant(s), and the length of time the aquifer has been contaminated. It is straightforward that an ad-hoc cleanup technique has to act on the first three parameters that have been identified as crucial within the remediation process itself. Moreover, in fractured rock aquifers numerical models used to simulate remediation processes make use of averaged values for these parameters, as it is extremely difficult to determine them accurately. Making predictions of actual cleanup times is therefore most of the times of scarce significance. As planning effective remediation interventions requires spatial and temporal predictions on contaminant propagation, it proves to be necessary to implement specific interpretations of the classical methodologies of describing fluid flow and solute transport in fractured rock aquifers. The present chapter provides an analysis of the peculiar aspects that concern the dynamics of groundwater circulation and contaminant propagation in fractured rock aquifers for the application of the ad-hoc cleanup technique. © 2012 Nova Science Publishers, Inc. All rights reserved.
fractured aquifers, flow, transport
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11392/2381777
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