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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!