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Inverse Geochemical modeling of groundwater with special emphasis on arsenic

Sharanya Shanbhogue. Inverse Geochemical modeling of groundwater with special emphasis on arsenic. Geochemistry 428/628 12/09/2010. Overview. Case Study Scope Inverse Geochemical Modeling (PHREEQC- GEOL 628) Common Ion Effect Iron-Arsenic Model Conclusions.

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Inverse Geochemical modeling of groundwater with special emphasis on arsenic

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

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

  3. 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.

  4. Groundwater Chemistry • Concentrations of species obtained from Detzani-Muhi wells • Modeling suggests presence of As in samples. • Origin of As: Aresenopyrite, scorodite, and tennantite minerals.

  5. “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+2release---> gypsum(CaS04)dissolution Calcite(CaC03) precipitation Common ion: Ca

  6. 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-

  7. 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. Impact

  8. 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.

  9. Inverse Modeling 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) Solution to Solid (precipitation, exchange) Need to Know Initial Solution Final Solution Reacting Phases gases, water

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

  11. 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

  12. 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.

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

  14. 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.

  15. 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

  16. Iron and Arsenic • 3Fe2++ 2HAsO42− = Fe3(AsO4)2+2H+ • log_k= −15.9 • Fe3++HAsO42− = FeAsO4+H+ • log_k= −11.7 • 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

  17. 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: ?

  18. 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!

  19. 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

  20. THANK YOU!

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