Ankan BasuGeologistMarshall Miller and AssociatesBluefield, VA, 24605

Date: April 22, 2009

Geochemical Modelingwith Geochemist’s Workbench

Why Model?The Earth is too big

and complexfor the brain

How to think about theEarth? M

ake it Sim

ple

• 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:1. Hydrothermal alteration2. Exploration of ore deposits3. Contaminant migration from mine

tailings.

Introduction to Geochemical Modeling

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?

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.

Heuristic Models(understand the Process)

Technological Models(Control the process)

Geological ObservationsBasis for model development. Maps & Cross-Sections: Spatial Models

Model

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

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?

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?

Britannia Mine

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

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

List of some Geochemical Modeling Software's

CHESS

CHILLER

GEOCHEM-PC

KINDISP

MINEQL+

MINTEQA2

PHREEQC

1DIRECT

DISSOL

EQ3/EQ6

GEOCHEMIST’S

WORKBENCHHydrogeology Program

Department of Geology

University of Illinois

Low Temperature/Aqueous Phase modeling

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

Geochemical Modeling – “How” and “Why”

• How?1. Understanding of geochemical system

2. Understanding of field geology

3. Thermo data!

• Why?Why do you need a model?

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)

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

Geochemist Workbench Essentials is a set of tools

MODULES• Rxn• Act2• Tact• SpecE8• Aqplot (graphic module) Model is a tool

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

The Geochemist’s Workbench is easy to set up.

User friendly windows program. Three basic TABS GUI or Code (Command) Steps in the program:

1. Set the basis.

2. Constrain the basis

3. Run

4. Revise

Set the Basis

Constrain the Basis

Run

Revise

The Geochemist’s Workbench basic inputs:

Setting Up•Basis•Command•Reaction

Set up is Similar for All modules

You can write your own code using GUI

Graphical User Interface Use code or use insert fields GWB Reference Manual

Paste data from spreadsheetGeochemist’s Workbench can accept data from EXCEL

Example Basis Species – All AQUEOUS

Original Basis Speciesis always an aqueousSpecies.

To change to SolidOr Gas Phase use“SWAP” command

Some common Applications

Sorption Activity-Activity Diagrams Speciation Saturation Index calculation Equilibrium Constant Calculations Solubility diagrams

Module 1: Rxn is useful to balance chemical reactions.

1. Balance Chemical Reactions

2. Calculate Equilibrium Constants

Rxn can easily Balance complex Reactions

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

LogK is LogKsp for soluble minerals

pH

We can easily determine the saturation condition of a particularMineral based on field and lab data. (for multiple assemblage – use SpecE8)

Module 2: Act2 calculates activity-activity diagrams.

1. Eh-pH diagrams

2. Solubility diagrams

3. Reaction Trace (Not available in GW 6)

4. Activity – activity diagrams

5. Stability fields of minerals

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

What is Eh Buffer?

Add reducingAgent. WhatWould happenAt the boundary

We used GWB in some projects. Real Life Example!

Stability fields forVarious Iron Species

Can you build it by Hand?

Module 3: Tact calculates Temperature-activitydiagrams.

1. Effect of temperature on mineral stability.

2. Very similar to Act2

3. Reaction Trace (Not available in GW 6)

4. Temperature – Activity diagrams

5. Temperature – Fugacity diagrams

Thermal Stability of phosphate

Tact Plots

Module 4: SpecE8 models equilibrium state of chemical system

1. Similar to Phreeqc

2. Great tool to check “accuracy” of the lab data.

3. Easy to use and output can be viewed in multiple types of graphs.

4. Equilibrium distribution of aqueous species

5. One of the most commonly used module.

Setting up SpecE8

Common ground water

Species.

Remember to add TDS for high salinity waters

TDS not requiredFor dilute solution

Module 5: Aqplot

Support for numerous units

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.

Questions?