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Ankan Basu Geologist Marshall Miller and Associates Bluefield, VA, 24605

Geochemical Modeling with Geochemist’s Workbench. Ankan Basu Geologist Marshall Miller and Associates Bluefield, VA, 24605. Date: April 22, 2009. Why Model?. The Earth is too big and complex for the brain. How to think about the Earth?. Make it Simple.

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Ankan Basu Geologist Marshall Miller and Associates Bluefield, VA, 24605

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  1. Geochemical Modeling with Geochemist’s Workbench AnkanBasu Geologist Marshall Miller and Associates Bluefield, VA, 24605 Date: April 22, 2009

  2. Why Model? The Earth is too big and complex for the brain How to think about the Earth? Make it Simple

  3. Introduction to Geochemical Modeling • First geochemical model was prepared in 1960. • Composition of dissolved species in sea water. • Hand calculation. • Today – we can include 1000s of reactions to predict the solubility and mobility of 40 or more elements. • Some applications: • Hydrothermal alteration • Exploration of ore deposits • Contaminant migration from mine tailings.

  4. A model is a simplified version of reality • Model is a tool. • Model must balance between reality and practicality. • Must be properly constructed. • If too simplified, it may be unrealistic. • If too detailed, it can’t be readily evaluated. • Geologic Maps : a model • How many data points should you use to build a coal thickness map?

  5. Types of common models • Models are how we organize our knowledge about the world (Goldschmidt,1958). • Geochemical models predict element abundances (concentrations) and rates of circulation between various parts of the earth. Model HeuristicModels (understand the Process) TechnologicalModels (Control the process) Geological Observations Basis for model development. Maps & Cross-Sections: Spatial Models

  6. Spatial Models • Rock Type • Porosity • Mineral Distribution • Maps and Cross-sections • Illustrate the general geometric relationship between source and contaminated area. Can you conceptualize the study area? Build your Conceptual Model

  7. Conceptual Models - Reactions • Conceptualize broad general relationships among the important factors in a geological process. • Conceptual Models lead to mathematical models that can be expressed using equations. • Theory of Equilibrium • Most important – tabulation of geochemical reactions – what are the most important reaction species? Field observation of particular mineral? A+B=C+D

  8. How to Approach? • Photographs – too much information • Sketch it! (reduces bunch of details) • How much information do you need? • Keep essentials, finalize maps/cross -sections. • Develop scheme/sampling methods etc. • Think: Model post mining water quality – how many layers of aquifers would you consider?

  9. Britannia Mine Source: http://www.agf.gov.bc.ca/clad/britannia/images/acid_mine_fig1.jpg

  10. Spatial model leading to conceptual model Pre-mining Spatial Model Post-mining Spatial Model Source: http://www.agf.gov.bc.ca/clad/britannia/background.html

  11. List of some Geochemical Modeling Software's • CHESS • CHILLER • GEOCHEM-PC • KINDISP • MINEQL+ • MINTEQA2 • PHREEQC • 1DIRECT • DISSOL • EQ3/EQ6 • GEOCHEMIST’S • WORKBENCH • Hydrogeology Program • Department of Geology • University of Illinois Low Temperature/Aqueous Phase modeling

  12. The heart of geochemical modeling is Equilibrium System • Chemical equilibrium. • Aqueous fluid + one or more mineral phases • The temperature and composition of the equilibrium state is usually known. • Pressure has minor effect in near surface conditions. • Closed system modeling – simplest model • Transport modeling – more complicated

  13. Geochemical Modeling – “How” and “Why” • How? • Understanding of geochemical system • Understanding of field geology • Thermo data! • Why? • Why do you need a model?

  14. The “How” of Geochemical Modeling • The initial equilibrium state of the system • The mass of solvent water (1 kg by default) • The amount of any minerals in the EQ system • The fugacity of any gas at known P • Amount of dissolved components (HCO3-, Na+) • Activities of species such as H+ (pH) • Oxidation state (Eh)

  15. Geochemist Workbench 6 Essentials Version 7.0 Essential set of Geochemistry tools. Standard Version 7.0 + Reaction Path Modeling. • Professional Version 7.0 • Transport • Flow rates • Saturated/Unsaturated Flow. • Incorporate MODFLOW • More…… $799 $3499 $7999

  16. Geochemist Workbench Essentials is a set of tools • MODULES • Rxn • Act2 • Tact • SpecE8 • Aqplot (graphic module) Model is a tool

  17. Thermodynamic Datasets • Multiple Thermo Datasets available • Validity range: 0-300 o C • Most common: thermo.dat • For organic Species: thermo.comV8.R6+.dat • Thermo_phreeqc.dat • Thermo_minteq.dat Source:http://www.fkm.utm.my/~nazrikh/smj3403/news.htm

  18. Set the Basis Constrain the Basis Run Revise The Geochemist’s Workbench is easy to set up. • User friendly windows program. • Three basic TABS • GUI or Code (Command) • Steps in the program: • Set the basis. • Constrain the basis • Run • Revise

  19. The Geochemist’s Workbench basic inputs: • Setting Up • Basis • Command • Reaction • Set up is • Similar for • All modules

  20. You can write your own code using GUI • Graphical User Interface • Use code or use insert fields • GWB Reference Manual

  21. Paste data from spreadsheet Geochemist’s Workbench can accept data from EXCEL

  22. Example Basis Species – All AQUEOUS Original Basis Speciesis always an aqueous Species. To change to Solid Or Gas Phase use “SWAP” command

  23. Some common Applications • Sorption • Activity-Activity Diagrams • Speciation • Saturation Index calculation • Equilibrium Constant Calculations • Solubility diagrams

  24. Module 1:Rxn is useful to balance chemical reactions. • Balance Chemical Reactions • Calculate Equilibrium Constants

  25. Rxn can easily Balance complex Reactions

  26. Swap species to get the phases you need All default basis species are in aqueous phase. USE SWAP. Rxn also calculates the Equilibrium constant for any reaction at any Temperature. Analcime – hydrous sodium aluminum silicate

  27. pH LogK is LogKsp for soluble minerals We can easily determine the saturation condition of a particular Mineral based on field and lab data. (for multiple assemblage – use SpecE8)

  28. Module 2: Act2 calculates activity-activity diagrams. • Eh-pH diagrams • Solubility diagrams • Reaction Trace (Not available in GW 6) • Activity – activity diagrams • Stability fields of minerals

  29. Eh-pH diagrams Eh and pH are master variables Many of the dissolved species in ground water occur in more than one redox state (redox sensitive) – Fe, Mn, N, S, O, Cr etc Redox equilibrium and disequilibrium Eh buffer exist just as pH buffer controlled by various redox couples

  30. What is Eh Buffer? Add reducing Agent. What Would happen At the boundary

  31. We used GWB in some projects. Real Life Example! Stability fields for Various Iron Species Can you build it by Hand?

  32. Module 3: Tact calculates Temperature-activitydiagrams. • Effect of temperature on mineral stability. • Very similar to Act2 • Reaction Trace (Not available in GW 6) • Temperature – Activity diagrams • Temperature – Fugacity diagrams

  33. Thermal Stability of phosphate

  34. Tact Plots

  35. Module 4: SpecE8 models equilibrium state of chemical system • Similar to Phreeqc • Great tool to check “accuracy” of the lab data. • Easy to use and output can be viewed in multiple types of graphs. • Equilibrium distribution of aqueous species • One of the most commonly used module.

  36. Setting up SpecE8 Common ground water Species.

  37. Remember to add TDS for high salinity waters TDS not required For dilute solution

  38. Module 5: Aqplot

  39. Support for numerous units

  40. No model is perfect • Newton’s laws fail to predict behavior of matter at atomic level. • Formal errors – wrong assumptions • Structural errors – programming error • Computational error – due to incorrect rounding. • All models are Hypothesis.They cannot be proved TURE, but can be shown to FAIL.

  41. Questions?

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