Updated: June 7, 2001.
Copyright © 2000 by Walt W. McNab, Concord, CA, U.S.A. 

Computer-Mediated Distance Learning

Course on

MULTISPECIES REACTIVE TRANSPORT IN GROUNDWATER

INSTRUCTOR:

WALT W McNAB

Dr Walt McNab Lawrence Livermore National Laboratory Livermore, California, U.S.A.

Throughout this course, we have relied upon PHREEQC as our computational engine, first exploring its capabilities as a ìbatchî simulator and then turning to 1-D transport.  This limitation to one dimension is fine for education purposes and is suitable for a variety of real applications in many instances.  However, a more generalized approach for 2-D or 3-D problems, with the capability to address physical heterogeneities and arbitrary boundary conditions, would certainly provide an added degree of flexibility.  The reactive transport simulator PHAST, a combination of two U.S.G.S. groundwater simulators, PHREEQC (the geochemical speciation code used in this course) and HST3D (a groundwater flow and solute transport model) is a public domain software package, currently in beta test form, that is freely available for download from via the Internet at:

Several computational platforms, including Windows 98/NT are supported.  The instructions for using the model, PHAST, are also available for download from the same web site.  In addition to familiarity with PHREEQC (which this course will have provided to a large degree), some familiarity with solute transport modeling (e.g., finite difference modeling, grid and time step definition, definition of initial and boundary conditions, etc.) is required.

The combination of these two codes provides the user with a very powerful, general-purpose reactive transport model that can solve problems characterized by: 

One-, two-, and three-dimensional groundwater flow, either steady-state or transient, under confined or unconfined conditions

Advective-dispersive-diffusive multi-species solute transport

Aqueous complexation

 Mineral precipitation/dissolution reactions, including the existence of solid solutions (consult a mineralogy textbook if you are unfamiliar with the concept of solid solutions)

Equilibrium and kinetic reaction expressions

Reactions with mineral surfaces, including complexation and ion exchange    

In this lecture, we show how PHAST can be used to solve a problem of practical interest.  The example problem is a complex one: manipulation of aquifer chemistry, via the injection of chemical reagents, to bring about the immobilization of dissolved metal contaminants.  An example input file for one of the PHAST runs, and a presentation of the problem background, model setup, and series of results for different treatment scenarios are all included.

A detailed description of PHAST, and the setup of its input files, is beyond the scope of this lecture.  These details are provided in a userís manual that can be downloaded from the above-listed website along with the PHAST model.  Briefly, the model consists of a chemical input file that is essentially a PHREEQC input file, which by now you are familiar with, and a transport input file for HST3D.

VIEW SLIDE PRESENTATION (part 1)

VIEW SLIDE PRESENTATION (part 2)

VIEW SLIDE PRESENTATION (part 3)

The specific details of the example problem are presented in the enclosed presentation and you are advised to look though them first to understand the nature of the problem.  Afterward, take a look through the two example input files for one of the scenarios discussed in the presentation to see how they are developed:

REACTION 1 Add soda for pH control

                Na2CO3       1.0

        0.05 moles in 1 steps

SAVE Solution 1

END

SOLUTION 2 Plume interior groundwater

                temp    17.1

                pH 5.71

                pe 0.0

                redox   pe

                units   mg/kgs

                Cd 19.6

                Zn 121.2

                Ca 196.8

                Mg 59.4

                K 21.2

                Na 287.5

                Fe(2)   3.1

                Fe(3)   1E-10

                Alkalinity    74.3

                Cl 374.3

                S(6)    789

EQUILIBRIUM_PHASES 2

                Hematite        0.0             0.142

                Calcite       0.0         0.0

                Dolomite 0.0         0.0

                Otavite       0.0         0.0

                Smithsonite   0.0         0.0

                Cd(OH)2       0.0         0.0

                Zn(OH)2(e)    0.0         0.0

END

SELECTED_OUTPUT

    -file                 selected.out

    -totals               Cd  Zn

    -equilibrium_phases   Calcite  Dolomite  Otavite Smithsonite Cd(OH)2 Zn(OH)2(e)

END

TITLE

                Injection of sodium carbonate into contaminated aquifer

                Test demo of PHAST

UNITS

                -time               days

                -horizontal_grid    ft

                -vertical_grid      ft

                -head               ft

                -hydraulic_conductivity   ft/d

                -specific_storage   1/ft

                -dispersivity       ft

                -well_diameter      in

                -well_flow_rate           gpm

SOLUTION_METHOD

                -iterative_solver   true

                -space_differencing 0

                -time_differencing  1

GRID

                -uniform X

                  -100. 400.  101

                -uniform Y

                  -150. 150.  61

                -uniform Z

                  0.    20.0  2

                -chemistry_dimensions     XY

                -print_orientation  XY

MEDIA

                -zone   -100. -150. 0.          400. 150. 20.0

            -Kx   file KField.dat

                  -Ky   file KField.dat

                  -Kz   file KField.dat

                  -porosity   0.25

                  -specific_storage 1e-5

                  -trans_dispersivity     10.

                  -long_dispersivity      25.

SPECIFIED_VALUE_BC

                -zone   -100. -150. 0.          -100. 150. 20.0

                  -head 1.5

                  -associated_solution    2

                -zone   400. -150. 0.          400. 150. 20.0

                  -head 0.0

                  -associated_solution    2

WELL            1

                0. 50.

                -diameter     6.

                -injection_rate     5.

                -solution     1

                -elevation    0.    20.0

WELL            2

                0. 0.

                -diameter     6.

                -injection_rate     5.

                -solution     1

                -elevation    0.    20.0

WELL            3

                0. -50.

                -diameter     6.

                -injection_rate     5.

                -solution     1

                -elevation    0.    20.0

HEAD_IC

                -zone   -100. -150. 0.          400. 150. 20.0

                  -head X     1.5 -100.   0.0 400.

STEADY_FLOW true

                -minimum_time_step  1

                -maximum_time_step  1000 

CHEMISTRY_IC

                -zone   -100. -150. 0.          400. 150. 20.0

                  -solution   2

                  -equilibrium_phases     2

TIME_CONTROL

                -time_step    1

                -time_change  60

PRINT_FREQUENCY

                -head   60

END

You are now ready to go to the next lecture (which will be the last lecture of this course: