1. Inverse Geochemical modeling of groundwater with special emphasis on arsenic
Sharanya Shanbhogue
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
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

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

6. 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-

7. 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.

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 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

10. 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

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

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
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: 
Sharif et al., (2008), Journal of hydrology, 350: 41-55
Kim et al., (2000), Environ. Sci. Technol, 34:
Armienta et al., (2001), Environmental Geology, 40:

20. THANK YOU!