10.4122/1.1000000716
Zhang, Fan
Fan
Zhang
zhangf@ornl.gov
Yeh, Gour-Tsyh
Gour-Tsyh
Yeh
gyeh@mail.ucf.edu
Parker, Jack
Jack
Parker
parkerjc@ornl.gov
Brooks, Scott
Scott
Brooks
brookssc@ornl.gov
Pace, Molly
Molly
Pace
pacem@ornl.gov
Kim, Young-Jin
Young-Jin
Kim
kimy1@ornl.gov
Jardine, Philip
Philip
Jardine
jardinepm@ornl.gov
Zhang, Fan
Fan
Zhang
zhangf@ornl.gov
An Integrated Media, Integrated Processes Watershed Model – WASH123D: Part 8 – Reactive Chemical Transport in Subsurface Media
XVI International Conference on Computational Methods in Water Resources
2006
2006
A watershed system includes river/stream networks, overland regions, and subsurface
media. This paper presents a reaction-based numerical model of reactive chemical
transport in subsurface media of watershed systems. Transport of M chemical species
with a variety of chemical and physical processes is mathematically described by M
partial differential equations (PDEs). Decomposition via Gauss-Jordan column
reduction of the reaction network transforms M species reactive transport equations
into M reaction extent-transport equations (a reaction extent is a linear
combination of species concentrations), each involves one and the only one linearly
independent reaction. Thus, the reactive transport problem is viewed from two
different points of view. Descirbed with a species-transport equation, the
transport of a species is balanced by a linear combinations of rates of all
reactions. Described by a reaction extent-transport equation, the rate of a linear
independent reaction is balanced by the transport of the linear combination of
species. The later description facilitates the decoupling of fast reactions from
slow reactions and circumvent the stiffness of reactive transport problems. This
is so because the M reaction extent-transport equations can be approximated with
three subsets of equations: NE algebraic equations describing NE fast reactions
(where NE is the set of linearly independent fast/equilibrium reactions), NKI
reactive transport equations of kinetic-variables involving no fast reactions
(where NKI is the number of linearly independent slow/kinetic reactions), and NC
transport equations of components involving no reaction at all (where NC = M – NE –
NKI is the number of components). The elimination of fast reactions from reactive
transport equations allows robust and efficient numerical integration. The model
solves the PDEs of kinetic-variables and components rather than individual chemical
species, which reduces the number of reactive transport equations and simplifies
the reaction terms in the equations. Two validation examples involving simulations
of uranium transport in soil columns are presented to evaluate the ability of the
model to simulate reactive transport with reaction networks involving both kinetic
and equilibrium reactions. A hypothetical three-dimensional example is presented to
demonstrate the model application to a field-scale problem involving reactive
transport with a complex reaction network.