A combined model for water
flow, solute transport and
biogeochemical reactions—
HP1
Xuelian Bai
Aaron Sawatzky
3 December 2008
Outline
• Introduction
• The combined HP1 model
• Examples and Applications
Introduction
• The migration of elements and contaminants
in the subsurface is affected by a multitude
of complex, interactive physical, chemical,
mineralogical and biological processes.
• All the processes are influenced by
precipitation and evapotranspiration, changes
of chemical composition or pH of soil and
dissolution/precipitation of minerals.
Objective
• The combined HP1 model can
simultaneously simulate variablysaturated transient water flow, multicomponent solute transport, speciation
and geochemical processes, including a
variety of mixed equilibrium and kinetic
reactions.
Mallants et al., 2008
Simulating water flow, transport and biogeochemical reactions in environmental
soil quality problems
Flow & transport model
HYDRUS-1D
Biogeochemical model
PHREEQC
Simulating the movement of water,
heat and multiple solutes in
variably-saturated heterogeneous
or layered soils subject to a
variety of atmospheric and other
boundary conditions
Simulating the behavior of
complex chemical systems, such
as speciation, ion exchange,
surface complexation, and
mineral precipitation/dissolution
Mallants et al., 2008
Coupling procedure
 Coupling method: non-iterative sequential approach
(weak coupling)
 Within a single time step:
 First solve water flow equation and heat transport equation
(HYDRUS);
 Then solve convection-dispersion equation for solute
transport for element master/primary species (inert
transport) (HYDRUS)
 Finally solve for each element, calculate speciations,
equilibrium reactions, kinetic reactions, … (PHREEQC)
Jacques et al., 2003
HP1 – model features
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1D FE water flow in variably-saturated media
1D FE transport of multiple solutes by CDE
1D heat transport
Mixed equilibrium / kinetic biogeochemical reactions
• Aqueous speciation (reactions in pore-water)
• Cation exchange (on clay, organic matter, …)
• Surface complexation (e.g. iron oxyhydroxides)
• Mineral dissolution / precipitation
• Any kinetic reactions (oxidation/reduction,
(bio)degradation, dissolution/precipitation)
Mallants et al., 2008
HP1 examples
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Transport of heavy metals (Zn2+, Pb2+, and Cd2+) subject to
multiple cation exchange
Transport with mineral dissolution of amorphous SiO2 and
gibbsite (Al(OH)3)
Heavy metal transport in a medium with a pH-dependent
cation exchange complex
Infiltration of a hyperalkaline solution in a clay sample
(kinetic precipitation-dissolution of kaolinite, illite, quartz,
calcite, dolomite, gypsum, …)
Long-term transient flow and transport of major cations
(Na+, K+, Ca2+, and Mg2+) and heavy metals (Cd2+, Zn2+, and
Pb2+) in a soil profile.
Kinetic biodegradation of TNT (multiple degradation
pathways)
Jacques et al., 2003
Typical application and processes
involved
 Cycling of radionuclides/metals in soil-plant systems
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Heterogeneous physical/chemical properties
Water flow under rainfall - evapotranspiration conditions
Root growth and water uptake
Microbiological growth
Degradation of organic matter with radionuclides/metal
release
• Transport/adsorption/decay
• Uptake of radionuclides/metals by plants
Mallants et al., 2008
Application fields of HP1
 Process Coupling and Interactions
Investigate the impacts of multiple coupled biogeochemical
reactions in the presence of complex flow fields and spatial
heterogeneity.
 Interpretation of Laboratory and Field Data
Provide a useful framework for interpreting experimental
results. Understand qualitative and quantitative trends and
relationships present in the data.
 Sensitivity Analysis
Permit the systematic evaluation of the impact of model
parameters (both reactive and hydrogeological), initial
conditions, and boundary conditions upon the model output.
 Integration and Synthesis
Tool for integrating all of the knowledge obtained from
simulation, sensitivity analyses, and laboratory and field
experimentation.
Mallants et al., 2008
References
• www.sckcen.be/hp1
• J. Šimůnek et al. (2006) Multicomponent geochemical transport
modeling using Hydrus-1D and Phreeqc. JAWRA.
• D. Jacques et al. (2003) The Hydrus-Phreeqc multicomponent
transport model for variably-saturated porous media: code
verification and application.
• D. Jacques et al. (2008) Modeling coupled water flow, solute
transport and geochemical reactions affecting heavy metal
migration in a podzol soil. Geoderma.
• L. Wissmeier and D.A.Barry. (2008) Reactive transport in
unsaturated soil: comprehensive modeling of the dynamic spatial
and temporal mass balance of water and chemical components.
Advances in Water Resources.