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

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

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

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

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

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

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

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

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

Simulated Arsenic Concentrations in Central Oklahoma

Charleston, South Carolina

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

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

Simulation of an ASR Cycle

Predicted Recovery Efficiency

1, 10, 100 Year Bubbles

Phosphorus Transport at Cape Cod, MA

 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

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

PHAST Simulation of Column Experiments

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

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

MeasuredSimulated

Predicted P Load to Ashumet Pond

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

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

B-Z—Concentration with Time

B-Z Time Series of Petri Dish

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