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WHY USE?

I want students to interpret geochemical reactions that occur along flow paths

FREE SOFTWARE

Web site: http://water.usgs.gov/software/

Piper Plot GenerationGW_CHARTVisualize and discuss differences in water chemistry

Thermodynamic equilibrium softwarePHREEQCiSpeciate water (applied thermodynamics)Calculate charge imbalanceIdentify over-saturated and under-saturated minerals

CREATE PIPER PLOT WITH GW_CHART

All boxes must be filled, no red boxes

Select number of data points and units at the bottom left

All boxes must be filled (no red boxes) Add up values for TDS if needed

Program will calculate CO32- from HCO3

- at specified Temperature and pH

Must enter HCO3- first

Record symbols on a separate paper

GRAPHICS OPTIONS

Format graph with hammer tool Show numbers (values on grid lines) Make symbols proportional to TDS Add grid lines Set color individually

Legend Add names

Plot Use landscape view

HOW TO INTERPRET

Dominant cation and dominant anion characteristics of the water

Differences and similarities between waters based on ion relationships (equivalence)

TDS comparisons

Changes occurring along a flow path

BEFORE USING PHREEQCI

Review thermodynamics

Look at database to see thermodynamic data

Find good geochemical dataset Major Cations, Major Anions, HCO3, Si, Fe, Al, Mn Redox pair (Fe2/Fe3, NH4/NO3)

PHREEQCI

General Give Description Enter pH, pe, and temperature State how redox should be determined (pe or pair)

Individual element input Change default units to ug/L Enter values Change units for anions (Cl, NO3, SO4) Advanced: allow model to calculate pH using charge imbalance

Highlight “Simulation 1”

Click on little black dot

Save

EXECUTE PROGRAM

Highlight title

Run

Change database if you like for more parameters Click on box beside database Select “minteq.dat” Click “Start” and then “Dismiss”

OUTPUT FILES

Description of Solution pH and redox (if you allow model to calculate) Ionic strength Percent Error from charge imbalance (< 10%)

Distribution of Species Individual component concentrations and activity coefficients (user

specifies model) What is dominant Fe species in solution?

Saturation indices Negative are under-saturated minerals (dissolving?) Positive are over-saturated minerals (precipitating?)

CAUTIONS

Lots of uncommon minerals in the database

Model based exclusively on thermodynamic equilibrium Infinite time Does not consider biological influence

ADVANCED TECHNIQUES

Allow PHREEQCi to calculate redox conditions based on known redox pairs

Allow PHREEQCi to calculate pH based on charge imbalance

Surface adsorption

Cation exchange

Equilibrium phase (solid or gas)

Mix solutions

Define reactions