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

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

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

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

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

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

Example Selected-Output File

Exercise: Make the okchem.chem.dat file from your PHREEQC exercises 1. Define arsenic chemistry 2. Define a brine as solution 1 Remove iron from the definition 3. Define SURFACE 1 to be in equilibrium with the brine 4. Define EXCHANGE 1 to be in equilibrium with the brine 5. Define EQUILIBRIUM_PHASES 1 to have calcite and dolomite 6. Define solution 2 to be a carbonate ground water 7. Define SELECTED_OUTPUT and USER_PUNCH to print pH; mg/kgw Ca, Mg, Na, C(4), Cl, S(6); ug/kgw As 8. Save the file in the phast directory as okchem.chem.dat

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

Changes for Conservative Solute Transport 1. Save As… 2. Solute Transport/Flow Only 3. Dispersivities 4. Distribution of initial solutions 5. Boundary Conditions—Associated solutions 6. Time step and simulation length

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

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

Transport Calculations Require Dispersivity Parameters Double click on Media “default” in tree  Longitudinal 4000  Horizontal50  Vertical50

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 Chemical reactants have been defined in okchem.chem.dat SOLUTION 1—Brine SOLUTION 2—Recharge  Initial condition for entire model domain is brine

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 CO 2, O 2, 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.

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)

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

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

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

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

Enable Reactions Double click on CHEMISTRY_IC

Run Simulation  Save  Run the model

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

Numerical Methods  Refine simulations –Maximum principle  Backward in TimeB-I-T  Upstream in SpaceU-I-S  No oscillations –No numerical dispersion  Centered in Time C-I-T  Centered in Space C-I-S  Oscillations possible

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)

Exercise 2: Refine Grid and Time Step Centered 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

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

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

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

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

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

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

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

Refined Simulation: yr Yikes! That’s a lot of arsenic  C-I-T  C-I-S  3000 m spacing  1000 yr time step

Refined Simulation: yr  C-I-T  C-I-S  3000 m spacing  25 yr time step

Refined Simulation: yr  C-I-T  C-I-S  3000 m spacing  10 yr time step

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