Inverse Geochemical modeling of groundwater with special emphasis on arsenic Sharanya Shanbhogue Geochemistry 428/628 12/09/2010

Overview Case Study Scope Inverse Geochemical Modeling (PHREEQC- GEOL 628) Common Ion Effect Iron-Arsenic Model Conclusions

Case Study –Zimapan Valley, Mexico Location of Study Area What’s going on? High Concentrations of Arsenic (As) in groundwater. Possible reasons: Leaching of mine tailings. Dissolution of As rich smelter and subsequent infiltration. Interaction of Groundwater with As-bearing rocks.

Groundwater Chemistry Modeling suggests presence of As in samples. Origin of As: Aresenopyrite, scorodite, and tennantite minerals. Concentrations of species obtained from Detzani-Muhi wells Concentration Input(mmol / L) Detzanf Muhi Alkalinity 4.296 4.337 As 6.994*10-3 13.35*10-3 Ca 3.023 1.737 Fe 3.224*10-3 3.9408*10-3 Mg 0.4033 0.555 SO4 1.494 0.9102

“Common I(r)on Effect” Iron(Fe) may effect Arsenic reaction. Reactions: FeS2+ 3.5O2+ H2O = Fe2+ + 2SO42-+ 2H+ FeAsS + 3.25O2+ H2O = Fe2+ + SO42- + H3AsO4 Another groundwater example: Ca+2 release---> gypsum(CaS04)dissolution Calcite(CaC03) precipitation Common ion: Ca

Eh-pH Diagram for As-Fe-O-H-S system As in Groundwater Eh-pH Diagram for As-Fe-O-H-S system This graph shows that the As minerals present in the well are “NOT STABLE” as a result they will dissolve. Rationale: As is supposedly originating from Arsenopyrite/Scorodite Stable forms: HAsO42- and H2AsO4-

Impact As concentration in municipal water was 0.3 mg /L El-Muhi deep well 1 mg/L WHO standard 0.01 mg/L People consumed water directly from As polluted wells. High As concentrations in their drinking water in India and Bangladesh. The interaction of the underlying As-rich aquifers with organic material creates reducing conditions and mobilizes As by a complex sequence of reactions.

SCOPE Inverse geochemical modeling of water data to establish a suitable rationale for interaction between As-bearing rocks and groundwater. Effect of other species on Arsenic release.

Inverse Modeling Need to Know Inverse modeling attempts to determine sets of mole transfers of phases that account for changes in water chemistry between one or a mixture of initial water compositions and a final water composition. Solid to Solution (dissolution, exchange) Initial Solution Final Solution Solution to Solid (precipitation, exchange) Need to Know Initial Solution Final Solution Reacting Phases gases, water

Example How much calcite precipitates? 2% CO2 atm CO2 Initial Solution Final Solution  (mg/kg)   Na 12 4 Ca 49 11 Mg 3 Cl 17 HCO3- 104 15 2% CO2 atm CO2

Reactions FeS2+ 3.5O2+ H2O = Fe2+ + 2SO42-+ 2H+ (pyrite) ∆H =-294 kcal/mol log k =208.46 FeAsS + 3.25O2+ H2O = Fe2+ + SO42- + H3AsO4 (Arsenopyrite) ∆H –324 kcal/mol log k = 198.17

PHREEQC Modeling Open PHREEQCi Right Click on the Screen Properties tab will pop up 1.Go to the database scroll down and choose the required database.

Input Data 1.Input data in PHREEQc 1.PHREEQC –WATEQ4F. dat doesn’t know what Arsenopyrite is!

Modifying the database Go to the database (WATEQF.dat). Access the text file. Under phases: Add the Arsenopyrite reaction. Save the file as GEOL628.dat. Now this database will understand Arsenopyrite and its related species. Use GEOL628.dat for further modeling.

Saturation Indices(SI’s) Arsenolite, Arsenopyrite, Ca3(AsO4)2:4w, Fe(OH)3(a), Fe3(OH)8, Goethite, Hematite, Maghemite, Magnetite, Scorodite, Siderite, Siderite Anhydrite, Aragonite, Artinite, As2O5(cr), As2S3(am), As_native, Brucite, Calcite, CH4(g), Claudetite, CO2(g), Dolomite,Dolomite(d), Epsomite, FeS(ppt), Greigite, Gypsum, H2(g), H2O(g), H2S(g), Huntite, Hydromagnesite, JarositeH, Mackinawite, Magnesite, Melanterite, Nesquehonite, O2(g), Orpiment, Portlandite, Pyrite, Realgar, Sulfur

Iron and Arsenic 3Fe2++ 2HAsO42− = Fe3(AsO4)2+2H+ log_k= −15.9 Hypothesis: Fe As Ramos at al., (2009), J. Phys. Chem. C, 113 (33), 14591–14594 Lenoble et al, (2005), Journal of Hazardous Materials, 123: 31

Iron and Arsenic & PHREEQC Idea : To model addition of Fe in the well to see the changes that occur. PHREEQC Modeling: Add Fe as new phase using the modified database (GEOL 628). Output Status: Failed – Errors The Problem: ?

Conclusions As can naturally occur in groundwater. Inverse Modeling results suggest that most of the saturated minerals are those containing Fe. Literature suggested that Fe is used to immobilize As. My attempts to model the addition of NZVI (Fe0 )to groundwater for As remediation FAILED!

References Ramos at al., (2009), J. Phys. Chem. C, 33:14591–14594 Lenoble et al, (2005), Journal of Hazardous Materials, 123: 262-268. Sharif et al., (2008), Journal of hydrology, 350: 41-55 Kim et al., (2000), Environ. Sci. Technol, 34: 3094-3100 Armienta et al., (2001), Environmental Geology, 40: 571-581

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