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 (PHREEQCGEOL 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 Asbearing rocks.
Groundwater Chemistry
• Concentrations of species
obtained from Detzani-Muhi
wells
• Modeling suggests
presence of As in
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
samples.
• Origin of As:
Aresenopyrite,
scorodite, and
tennantite minerals.
“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.
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
Initial
Solution
Final
Solution
(mg/kg)
(mg/kg)
Na
12
4
Ca
49
11
Mg
3
3
Cl
12
17
HCO3
104
15
-
atm CO2
Reactions
FeS2+ 3.5O2+ H2O
(pyrite)
∆H =-294 kcal/mol
log k =208.46
= Fe2+ + 2SO42-+ 2H+
FeAsS + 3.25O2+ H2O = Fe2+ + SO42- + H3AsO4
(Arsenopyrite)
∆H –324 kcal/mol
log 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.
Saturation Indices(SI’s)
60
40
Arsenolite, Arsenopyrite, Ca3(AsO4)2:4w, Fe(OH)3(a), Fe3(OH)8, Goethite, Hematite, Maghemite,
Magnetite, Scorodite, Siderite, Siderite
20
-20
Anhydrite
Aragonite
Arsenolite
Arsenopyrite
Artinite
As2O5(cr)
As2S3(am)
As_native
Brucite
Ca3(AsO4)2:4w
Calcite
CH4(g)
Claudetite
CO2(g)
Dolomite
Dolomite(d)
Epsomite
Fe(OH)3(a)
Fe3(OH)8
FeS(ppt)
Goethite
Greigite
Gypsum
H2(g)
H2O(g)
H2S(g)
Hematite
Huntite
Hydromagnesite
JarositeH
Mackinawite
Maghemite
Magnesite
Magnetite
Melanterite
Nesquehonite
O2(g)
Orpiment
Portlandite
Pyrite
Realgar
Scorodite
Siderite
Siderite(d)(3)
Sulfur
0
-40
-60
-80
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
• 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
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!