INVERSE GEOCHEMICAL MODELING OF

GROUNDWATER WITH SPECIAL EMPHASIS ON

ARSENICSharanya 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:1. Leaching of mine tailings.2. Dissolution of As rich

smelter and subsequent infiltration.

3. Interaction of Groundwater with As-bearing rocks.

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.

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

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

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.

Solution to Solid (precipitation, exchange)

Solid to Solution(dissolution, exchange)

gases, water

Need to KnowInitial SolutionFinal Solution

Reacting Phases

Initial Solution Final Solution

Example

2% CO2

atm CO2

How much calcite precipitates?

Initial Solution 

Final Solution (mg/kg)   (mg/kg)  

Na 12 4

Ca 49 11

Mg 3 3

Cl 12 17

HCO3- 104 15

Reactions

FeS2+ 3.5O2+ H2O = Fe2+ + 2SO42-+ 2H+

(pyrite)∆H =-294 kcal/mollog k =208.46

FeAsS + 3.25O2+ H2O = Fe2+ + SO42- + H3AsO4

(Arsenopyrite)∆H –324 kcal/mollog k = 198.17

PHREEQC Modeling

1. Open PHREEQCi

2. 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

1. Go to the database

(WATEQF.dat).

2. Access the text file.

3. Under phases: Add the

Arsenopyrite reaction.

4. Save the file as GEOL628.dat.

5. Now this database will

understand Arsenopyrite and

its related species.

6. Use GEOL628.dat for further

modeling.

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

Saturation Indices(SI’s)

Iron and Arsenic

• 3Fe2++ 2HAsO42− = Fe3(AsO4)2+2H+

• log_k= −15.9

• Fe3++HAsO42− = FeAsO4+H+

• log_k= −11.7 • Hypothesis:

Fe AsLenoble et al, (2005), Journal of Hazardous Materials, 123: 31

Ramos at al., (2009), J. Phys. Chem. C, 113 (33), 14591–14594

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

THANK YOU!