THREE-DIMENSIONAL SIMULATION OF MASS TRANSPORT OF MULTIPLE CHEMICALS IN A DISCRETELY FRACTURED MEDIA.
N.F. GHOGOMU et R.THERRIEN
MEDEF
Département de Géologie et de Génie Géologique
Université Laval, Sainte-Foy, Québec
G1K 7P4 Canada
The transport of chemical species in groundwater systems is of interest
to a variety of earth scientists. Development of transport models that
incorporate phenomena which govern the partitioning of chemical species
between aqueous and solid phases is fundamental to understand processes
such as ore deposition, chemical diagenesis of sediments and migration
of toxic chemicals. A large number of chemical transport models, of varying
degrees of complexity, have been devised in the past. However, none of
these models considers explicitly the fractured character of three dimensional
geological systems. Incorporation of discrete fractures in a model will
provide an adequate representation of the aquifer. We present FRAC3DBC
which is a numerical model for mass transport with biochemical and chemical
reactions in a 3D discrete fractured media. While porous media is defined
in 3D, the fractures are represented by 2D planes. Bacteria growth, substrate
consumption and release of metabolic products are considered. Also taken
in consideration are equilibrium and kinetic chemical reactions. A sequential
iterative procedure is used to solve the coupled physical and chemical
transport equations. We present here solutions for simplified problems
that show some of the characteristics of the model. Reactive transport
in a one-dimensional column with a unit cross section is simulated in two
stages. In the first stage the system initially contains 2 aqueous species
A and B and one solid species AB. At time greater than zero, an incoming
fluid containing aqueous species A, C and D in known concentrations enters
the upstream boundary. The incoming fluid dissolves the solid AB and the
dissolution front is monitored. In the second stage, for the same system
we consider a possible complexation of C and B to form BC. Using finally
the same chemical and physical system, we incorporate discrete fractures
to investigate their effect on reactive transport. These tests demonstrate
the applicability of the model and the influence of fratures on reactive
transport.