1. Use of Reactive-Transport Models in Field Studies: Experience with the PHAST Simulator David Parkhurst and Ken Kipp U.S. Geological Survey Denver, CO

2. Topics  The PHAST Simulator  Field Studies –Arsenic in the Central Oklahoma Aquifer –ASR in Charleston, SC –Phosphorus at Cape Cod, MA  B-Z oscillating reactions  Summary

3. PHAST  3D Reactive-Transport Simulator  HST3D—Flow and transport  PHREEQC—Chemistry  Operator splitting—Sequential Non-Iterative Approach Chemistry Transport Flow Chemistry Transport Flow

4. Flow and Transport  Point-distributed finite-difference grid  Boundary conditions –Flux –Leaky –Specified value –River –Well  Constant temperature  Constant density

5. Chemistry  Ion-association or Pitzer aqueous model  Mineral equilibrium  Surface complexation  Ion exchange  Solid solutions  Kinetics –Explicit ODE (Runge-Kutta) –Implicit ODE (CVODE)

6. Parallelization  Single processor: Flow and transport  Multiple processors: Chemistry  Data passed using MPI  10-30 processors  Model grids up to 200,000 nodes  1-100 hours of clock time  Allows field-scale modeling Transport Flow Transport Flow Chemistry Cells

7. Arsenic in the Central Oklahoma Aquifer  Arsenic mostly in confined part of aquifer  Arsenic associated with high pH  Flow: unconfined to confined back to unconfined

8. Arsenic in the Central Oklahoma Aquifer  Chemical analyses  Carbon-14 age dating  Microscopic examination of sediments  Cation-exchange measurements  Selective extractions for arsenic  Water levels  (Ground-water flow model) Available data

9. Geochemical Reactions  Brine initially fills the aquifer  Calcite and Dolomite equilibrium  Cation exchange 2NaX + Ca+2 = CaX2 + 2Na+ 2NaX + Mg+2 = MgX2 + 2Na+  Surface complexation Hfo-HAsO4- + OH- = HfoOH + HAsO4-2 Desorption at pH > 8.5

10. Simulated Arsenic Concentrations in Central Oklahoma

11. Charleston, South Carolina

12. Aquifer Storage Recovery— Charleston, SC  Well logs  2 Aquifer tests  4 ASR cycles  Conservative break-through data  Periodic chemical analyses  Quantitative X-ray mineralogy Available Data

13. Dispersion  Constant dispersivity  Dispersion adjusted by contrast in hydraulic conductivity

14. Simulation of an ASR Cycle

15. Predicted Recovery Efficiency

16. 1, 10, 100 Year Bubbles

17. Phosphorus Transport at Cape Cod, MA

18.  Column experiments—PO 4, cations, O 2  Flow and transport parameters  Mineralogy  Tracer tests  Water chemistry with time and space  Microbial processes  Isotopes Available Data—Everything

19. Reactions  Sorption—PO 4  Sorption—Cations  Mineral equilibria –Fe oxyhydroxide –Mn oxide –Fe(3) phosphate –Fe(2) phosphate  Kinetic decomposition of organic matter

20. PHAST Simulation of Column Experiments

21. Fit of Surface-Complexation Constants with UCODE Log K = 26.7 Sites = 3.0e-3 sites/L Log K = -1.8 Sites = 23.0e-3 sites/L Log K = 4.1 Log K = -7

22. Phosphorus,  mol/L Evolution of Phosphorus Plume at Cape Cod Sewage disposal during years 1-60

23. MeasuredSimulated

24. Predicted P Load to Ashumet Pond

25. Belousov-Zhabatinskii Recipe SpeciesConcentration Malonic acid 0.2 M Sodium bromate 0.3 M Sulfuric acid 0.3 M Ferroin 0.005 M

26. B-Z Definitions X [HBrO 2 ] Y [Br - ] Z[Ce(IV)] A [BrO 3 - ] B[Organic] P[HOBr] ReactionRate A + Y = X + P k 3 [H + ] 2 AY X + Y = 2P k 2 [H + ]XY A + X = 2X + 2Z k 5 [H + ]AX 2X = A + P k4X2k4X2k4X2k4X2 B + Z = 0.5 Y k 0 BZ Kinetic Rate Expressions

27. B-Z—Concentration with Time

28. B-Z Time Series of Petri Dish

29. Conclusions  Modeling results –Understanding natural systems—Oklahoma –Designing engineered systems—South Carolina –Predicting long-term effects—Massachusetts  Modeling has a weakest link –Flow—Oklahoma –Transport—South Carolina –Reactions—Massachusetts  Data requirements –Field—Aquifer tests, tracer tests, logging, chemical samples –Laboratory—column experiments, extractions, mineralogy –Resolving uncertainties is expensive  B-Z, Kindred and Celia link to biological processes