Simulation and analysis of solute transport in 2D fracture/pipe networks: The SOLFRAC program

Abstract

The Time Domain Random Walk (TDRW) method has been recently developed by Delay and Bodin [Delay, F. and Bodin, J., 2001. Time domain random walk method to simulate transport by advection-dispersion and matrix diffusion in fracture networks. Geophys. Res. Lett., 28(21): 4051-4054.] and Bodin et al. [Bodin, J., Porel, G. and Delay, F., 2003c. Simulation of solute transport in discrete fracture networks using the time domain random walk method. Earth Planet. Sci. Lett., 6566: 1-8.] for simulating solute transport in discrete fracture networks. It is assumed that the fracture network can reasonably be represented by a network of interconnected one-dimensional pipes (i.e. flow channels). Processes accounted for are: (1) advection and hydrodynamic dispersion in the channels, (2) matrix diffusion, (3) diffusion into stagnant zones within the fracture planes, (4) sorption reactions onto the fracture walls and in the matrix, (5) linear decay, and (6) mass sharing at fracture intersections. The TDRW method is handy and very efficient in terms of computation costs since it allows for the one-step calculation of the particle residence time in each bond of the network. This method has been programmed in C++, and efforts have been made to develop an efficient and user-friendly software, called SOLFRAC. This program is freely downloadable at the URL (labo.univ-poitiers.fr/hydrasa/intranet/telechargement.htm). It calculates solute transport into 2D pipe networks, while considering different types of injections and different concepts of local dispersion within each flow channel. Post-simulation analyses are also available, such as the mean velocity or the macroscopic dispersion at the scale of the entire network. The program may be used to evaluate how a given transport mechanism influences the macroscopic transport behaviour of fracture networks. It may also be used, as is the case, e.g., with analytical solutions, to interpret laboratory or field tracer test experiments performed in single fractures.

Implementation of a Time-Domain Random-Walk Method into a Discrete Element Method to Simulate Nuclide Transport in Fractured Rock Masses

“…Some discrepancies are caused when the CTRW method is employed to the advection-dispersion equation. Aiming at removing these constraints, Delay and Bodin [39] firstly proposed a new method called time-domain random-walk (TDRW) method to simulate solute transport in heterogeneous media, which was further extended to simulate solute transport in fractured rock masses [40][41][42].…”

“…Thus, transport in the stagnant zone is ignored in this study. The nuclide transport equation in 2D fracture can be written as follows [42]:…”

“…The ADS equation can be rewritten as (19) [42], considering the change of variable ( / = ( / )( / ) = ( / )( / )):…”

Identification of marine intrusion in the plain of Collo, northeastern Algeria

“…The contours that were ranging from few hundreds of μS·cm Amongst the major chemical elements, easymeasured "Cl -" remained a priori one of the best indicators of seawater intrusion. And regardless of the lithology, chloride contents were generally low in groundwater [BODIN et al 2007]. However, chlorides encountered in large quantities in the groundwater were likely coming from: − the dissolution of natural salt by the leaching of the mineral licks; − the intrusion of seawater into the coastal areas; − water discharges of domestic and industrial wastewater.…”

“…However, chlorides encountered in large quantities in the groundwater were likely coming from: − the dissolution of natural salt by the leaching of the mineral licks; − the intrusion of seawater into the coastal areas; − water discharges of domestic and industrial wastewater. − According to BODIN et al [2007]: − near the sea, the chemical composition of the rain is loaded with chemical elements contained in the atmosphere and in the soil, which has contributed to the mineralization of the aquifer; the chloride was one of the sensitive elements; − in unconfined aquifers, the chloride concentration was linked to the precipitation of the chloride content; the concentrations generally measured in un-confined aquifers were of few mg·dm -3 and ultimately depend on distance from the coast more than lithology; − the dissolution of halite (NaCl) or the presence of salt water intrusion (coastal aquifer) could eventually lead to high levels of chlorides (several hundred mg·dm -3 to a few g·dm -3 ). By examining the contour maps of chlorides the following could be observed: − the spatial distribution of chlorides in the study area took the same pattern of the EC for both dry and wet periods; − the highest concentration were noticed in the coastal areas mainly to the northeastern area and the surroundings near the wadis of Guebli and Cherka; − the chloride concentration decreased when moving away from the sea and wadis toward the centre of the plain.…”