USING FREE GEOCHEMICAL SOFTWARE FROM THE U.S. GEOLOGICAL SURVEY DEVIN CASTENDYK STATE UNIVERSITY OF NEW YORK, ONEONTA

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

FREE SOFTWARE Web site: Piper Plot Generation GW_CHART Visualize and discuss differences in water chemistry Thermodynamic equilibrium software PHREEQCi Speciate water (applied thermodynamics) Calculate charge imbalance Identify 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 CO 3 2- from HCO 3 - at specified Temperature and pH  Must enter HCO 3 - 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