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Using WPhast to Model the Central Oklahoma Aquifer—Transport and Chemistry

Using WPhast to Model the Central Oklahoma Aquifer—Transport and Chemistry. Chemistry: okchem.chem.dat Flow and transport: WPhast or okchem.trans.dat. David Parkhurst, Ken Kipp, and Scott Charlton. Chemistry file: okchem.chem.dat.

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Using WPhast to Model the Central Oklahoma Aquifer—Transport and Chemistry

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  1. Using WPhast to Model the Central Oklahoma Aquifer—Transport and Chemistry Chemistry: okchem.chem.dat Flow and transport: WPhast or okchem.trans.dat David Parkhurst, Ken Kipp, and Scott Charlton

  2. Chemistry file: okchem.chem.dat • Numbers associated with reactants are used in the okchem.trans.dat file • Initial conditions • Boundary conditions (solutions only) • SELECTED_OUTPUT and USER_PUNCH define PHAST chemical output • okchem.h5 (Model Viewer file) • okchem.xyz.chem (spreadsheet file) • Needs to be in the directory where the .wphast file is saved

  3. PHAST Chemistry Output • SELECTED_OUTPUT • Select items from lists • Total concentration, molalities of species, amounts of equilibrium_phases, many others • USER_PUNCH • Use Basic functions to calculate and print data • For example mg/kgw instead of mol/kgw

  4. SELECTED_OUTPUT File name applies to PHREEQC calculations PHAST results go to .h5 and .xyz.chem 1.Reset all to false 2. Set pH to true

  5. USER_PUNCH • Imbedded Basic interpreter can be used to calculate data to be written to the selected-output file • Basic functions are defined in manual under the keyword RATES • -headings provides column headings • PUNCH statements write data items

  6. USER_PUNCH: Used here to write total concentrations in mg/kgw and ug/kgw Headings User defined calculations and PUNCH statements Valid arguments for functions Function definitions

  7. Example Selected-Output File

  8. Exercise: Make the okchem.chem.dat file from your PHREEQC exercises • Define arsenic chemistry • Define a brine as solution 1 Remove iron from the definition • Define SURFACE 1 to be in equilibrium with the brine • Define EXCHANGE 1 to be in equilibrium with the brine • Define EQUILIBRIUM_PHASES 1 to have calcite and dolomite • Define solution 2 to be a carbonate ground water • Define SELECTED_OUTPUT and USER_PUNCH to print pH; mg/kgw Ca, Mg, Na, C(4), Cl, S(6); ug/kgw As • Save the file in the phast directory as okchem.chem.dat

  9. okchem.wphast • We will do everything in the .wphast file • Could use File|Export to generate .trans.dat file • Conservative transport • Reactive transport • Numerical methods

  10. Changes for Conservative Solute Transport • Save As… • Solute Transport/Flow Only • Dispersivities • Distribution of initial solutions • Boundary Conditions—Associated solutions • Time step and simulation length

  11. Save As • Save model—File|Save As… ok_wphast\ okchem.wphast

  12. Enable Solute Transport Double click on SOLUTE_TRANSPORT in tree Set “Solute transport and chemistry”

  13. Transport Calculations Require Dispersivity Parameters Double click on Media “default” in tree • Longitudinal 4000 • Horizontal 50 • Vertical 50

  14. Set Chemical Conditions • All chemistry (except associated solutions) is defined by the Chemistry Initial Conditions • Double click on CHEMISTRY_IC default in tree Chemical reactants have been defined in okchem.chem.dat SOLUTION 1—Brine SOLUTION 2—Recharge • Initial condition for entire model domain is brine

  15. Define Associated Solution for all Boundaries • If flow is into model region through a boundary, the composition must be defined. • The composition is termed the “Associated Solution” • Associated solutions may change at specified times • For Oklahoma, precipitation flux enters the region. The associated solution for the flux boundary condition is SOLUTION 2—water equilibrated with CO2, O2, calcite, and dolomite. • All other BC’s should be outflow, but just in case, we define SOLUTION 2 as the Associated Solution for each boundary.

  16. Assign SOLUTION 2 to Each Boundary Condition Double click on each boundary condition in tree—Set associated solution to “2” • Flux • Leaky south • Leaky north • Specified head • Little River • Point 1 (first) • Point 2 (last) • North Fork River • Point 1 (first) • Point 3 (last)

  17. Set Print Frequencies for HDF File • Double Click PRINT_FREQUENCY • HDF_Chemistry (As, pH, etc) every 2000 years • Heads and velocities will be saved once for steady flow by default

  18. Set Time Step and Simulation Length • Set 2,000 year time step • Set 100,000 year simulation period Double Click TIME_CONTROL

  19. Running the Simulation • Save • Required files in directory with okchem.wphast • okchem.chem.dat • PhreeqcI includes database in okchem.chem.dat file, otherwise phast.dat • Run the model

  20. Visualize Results • ModelViewer • File|New—c:\student\ok_wphast\okchem.h5 • View Cl at 100000 yr • Show|Solid • Show|Color Bar • Tools|Geometry—40X for Z • Tools|Color Bar—Logrithmic

  21. Enable Reactions Double click on CHEMISTRY_IC

  22. Run Simulation • Save • Run the model

  23. View Arsenic • Tools|Data—As • Tools|Animation • Set time to 0 years • Run • See “Options” tab to slow down animation • Tools|Crop

  24. Numerical Methods • Refine simulations • Maximum principle • Backward in Time B-I-T • Upstream in Space U-I-S • No oscillations • No numerical dispersion • Centered in Time C-I-T • Centered in Space C-I-S • Oscillations possible

  25. Exercise 1: Refine Grid and Time Step • Double number of nodes in X and Y directions • Set time step to 1000 yr • Keep Backward in time, upstream in space weighting (SOLUTION_METHOD)

  26. Exercise 2: Refine Grid and Time StepCentered in Time + Centered in Space • Double number of nodes in X and Y directions • Set time step to 1000 yr • Set weighting factors to 0.5 for time and space

  27. Numerical Dispersion For B-I-T + U-I-S Max Y velocity ~10 m/yr, Specified dispersivity is 4000 m

  28. Base Case Simulation B-I-T U-I-S 6000 m spacing 2000 yr time step

  29. Refined Simulation B-I-T U-I-S 3000 m spacing 1000 yr time step

  30. Further Refined Simulation B-I-T U-I-S 1500 m spacing 100 yr time step

  31. Criteria for Avoiding Oscillations Vertical velocity may be 10 m/yr; dispersivity 50 m or

  32. Refined Simulation C-I-T C-I-S 3000 m spacing 1000 yr time step

  33. Refined Simulation: 20000 yr B-I-T U-I-S 3000 m spacing 1000 yr time step

  34. Refined Simulation: 20000 yrYikes! That’s a lot of arsenic C-I-T C-I-S 3000 m spacing 1000 yr time step

  35. Refined Simulation: 20000 yr C-I-T C-I-S 3000 m spacing 25 yr time step

  36. Refined Simulation: 20000 yr C-I-T C-I-S 3000 m spacing 10 yr time step

  37. Numerical Methods Conclusions • C-I-T + C-I-S gives best answer if you can afford it • Still need to consider operator splitting error • B-I-T + U-I-S will give reasonable answers • Need to consider likely effects of numerical dispersion • In all cases, need to look at grid and time-step convergence

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