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Christelle MARLIN, professor Hydrogeology, hydrogeochemistry
UMR “GEOPS” CNRS - Université Paris-Sud, Orsay, France
Hydrochemistry
UE traceurs chimiques et isotopiques dans les eaux
naturelles
Water-rock interaction
Dissolution / precipitation of minerals
Gases
2. BASIC KNOWLEDGE IN
HYDRO GEOCHEMISTRY
2.1 CONCENTRATIONS
Concentrations and activity
Concentration – mg.L-1 or µg.L-1
– mg.kg-1 (=ppm) or µg.kg-1 water (=ppb)
– mol.L-1 or mol.kg-1 water
– meq.L-1 = mmol.L-1 x charge (easy to verify if sum cations = sum anions)
Charge Balance
Electrical neutrality provides good check on
analytical error
Charge Balance Error – CBE
CBE = SmcZc - SmaZa
SmcZc + SmaZa
Where: m = molar concentration of major solutes
z =charge of cation (c) or anion (a)
Charge Balance
!!
!!
Concentrations and activity
Concentration – mg.L-1 or µg.L-1
– mg.kg-1 (=ppm) or µg.kg-1 water (=ppb)
– mol.L-1 or mol.kg-1 water
– meq.L-1 = mmol.L-1 x charge
Activity – “effective concentration”
Ion-ion and ion-H2O interactions (hydration layer) cause
number of ions available to react chemically ("free" ions) to be
less than the number present
Concentration can be related to activity using the
activity coefficient g, where a = g (c)
Until now we have assumed that activity, a, is equal to concentration, c, by
setting g = 1 when dealing withdilute aqueous solutions…
But ions don’t behave ideally . . .
Concentration related to activity using the activity coefficient g, where [a] = g (c)
The value of g depends on:
– Concentration of ions and charge in the solution
– Charge of the ion
– Diameter of the ion
– Ionic strength, I = concentration of ions and charge in solution
I = 1/2 Smizi2
» where mi = concentration of each ion in moles per L,
zi = charge of ion
Activity coefficient gz 1 as concentrations 0 and tend to be <1 except for brines
Solution Models Debye-Hückel Equation
Physical model based on
electrostatic interactions
At higher ionic strength, use
extended Debye-Hückel equation
Davies Equation
for higher ionic strengths (<0.5) I
I
IAz
IBa
IAz
IAz
ii
o
ii
ii
3.0)(1
)(log
)(1
)(log
)(log
2/1
2/12
2/1
2/12
2/12
g
g
g
where I is the ionic strength of the solution as defined above; z is the charge of the ion
whose activity coefficient is being calculated; A and B are constants whose values depend
on the dielectric constant of the solvent and the temperature; and a is the effective diameter
of the ion in the solution in Å.
Values of constants tables
Na+
SO42-
(from Garrels and Christ, 1965)
I
g i
Sea
wate
r
Rivers, lakes,
groundwater
Brines
Ca2+
2.2 MAJOR CONSTITUENTS (INORGANIC, ORGANIC, DISSOLVED GASES)
2.2 MAJOR CONSTITUENTS (INORGANIC, ORGANIC, DISSOLVED GASES)
Major elements
Major elements
– 4 anions
– 4 cations
– Silica (SiO2)
– 90% of the total
Mineralisation
Minor elements
Trace elements
Global composition of groundwater
Global composition of groundwater
European guidelines for drinking water quality
Standards
Goldschmidt diagram
anions
pH-Eh conditions
pH-Eh conditions
Classical pH
range in
natural
waters : 6 –
8.5
Oxydized
environment
Reduced
environment
Main input of GW Precipitation
Acid rain destruction de la biomasse
ou dissolution des carbonates
Acid rain
SO2 + H2O + ½ O2 2H+ + SO42-
Acid rain
SO2 + H2O + ½ O2 2H+ + SO42-
même produits
que ceux de la
mise en solution
de H2SO4
Sources for ions in natural waters water-rock interaction geochemical baseline
2.2 MAJOR CONSTITUENTS (INORGANIC, ORGANIC & DISSOLVED GASES)
Organic constituents
Humic acid (HA)
Fulvic Acid(FA)
(less mobile)
(mobile)
COT & DCO COT = Carbone organique total. Somme du Carbone de nature
organique dans les matières dissoutes et en suspension dans l’eau.
Cyanates et thiocyanates sont également mesurés. – Cyanate, anion composé dans l'ordre d'un atome d'oxygène, d'un atome de carbone et d'un atome d'azote,
[OCN]− . Charge négative, portée principalement par l'atome d'azote. En chimie organique, le groupe
cynanate est un groupe fonctionnel.
– Thiocyanates [SCN]−.
COD (Carbone Organique Dissous) Somme du carbone
organique contenu dans la solution aqueuse après filtration à 0,45
μm.
La demande biochimique en oxygène (DBO) est la quantité
d'oxygène nécessaire pour oxyder les matières organiques
(biodégradables) par voie biologique (oxydation des matières
organiques biodégradables par des bactéries). DBO 5 5 jours
Humic substances
Organic Carbon occurs in the hydrosphere in:
natural and contaminant forms, both of which can be in
dissolved and suspended forms: – DOC = Dissolved organic carbon, includes fulvic acids (and humic acids
above pH=2).
– POC = Particulate (suspended) organic carbon. Includes humin (and
humic acids below pH=2).
DOC and POC concentration are variable in the
hydrosphere but are generally higher in waters with high
photosynthetic productivity:
e.g., ● watershed water in forested areas,
● the outflow of high photosynthesis lakes or swamps
● sewage outfalls
Organic constituents
Organic constituents Abundance variations of DOC and
POC are similar but POC is typically
lower than DOC in most of the
hydrosphere.
- Very clear (i.e., non-turbid) waters
can have relatively high DOC and
POC compared to inorganic solutes
due to photosynthesis.
- Very polluted waters can also have
high DOC and POC from the
pollutants or from enhanced
photosynthesis/ respiration (wastes
from organisms)
Organic constituents
2.2 MAJOR CONSTITUENTS (INORGANIC, ORGANIC, DISSOLVED GASES)
Important Dissolved Gases
6 important gases are dissolved in lakes, streams, gw, seas
Nitrogen
Oxygen
Carbon dioxide
Methane
Hydrogen sulfide
Ammonia
All have important functions, but differ in behavior and origin
Air Provides Some Gases
Atmosphere has enough nitrogen (78%), oxygen (21%), and carbon dioxide (0.03%) to serve as primary source
Others present only in trace amounts in atmosphere
Gas into groundwater (degassing)
Degassing of CO2 in a borehole (Greece)
Gas Solubility
At equilibrium PO2 = pO2
The relationship between PO2
and [O2] is:
PO2 = KO2[O2] where K is
the Henry's Law constant (T
dependant and salinity)
If we assume that surface water is
in equilibrium with the atmosphere,
then [O2] can be calculated from
the T and salinity dependence on
KO2.
PO2
pO2 partial pressure air
partial pressure in water
Other Gas Sources
Methane (CH4) - anaerobic breakdown of plants/animals
Hydrogen sulfide (H2S) - chemical/bacterial transformations
Ammonia (NH3) - breakdown of nitrogenous materials by bacteria, some animals
How much gas is dissolved in water?
Dependent on several
factors:
– Solubility factor
– Pressure
– Temperature
– Salinity
Solubility of
gas in water
decreases as
temperature
rises
Noble gases
Solubility of gas in water
decreases as temperature rises
Use to estimate recharge
temperature (palaeo-groudwater)
Oceans (salinity of 35‰) have lowered gas saturation values of ~ 20%
2.3 WATER TYPES
Typology of waters
Total Dissolved Solids (TDS) = sum of all ion
concentration in mg.L-1
Typology of waters
Total Dissolved Solids (TDS) = sum of all ion
concentration in mg.L-1
Typology of waters
Total Dissolved Solids (TDS) = sum of all ion
concentration in mg.L-1
Connate water = Water trapped in the pores of a rock during formation of the rock
(“fossil” water). The chemistry of connate water can change in composition throughout
the history of the rock. Connate water can be dense and saline compared with seawater.
Formation water = Water that occurs naturally within the pores of rock in a geological
formation.
Chemical type – Piper diagram
Chemical type – Piper diagram
Chemical type (cont.) – example (Piper diagram)
Traitement de nombreuses analyses
Cloutier et al., 2008 (Canada)
Chemical type – Stiff diagram
Chemical type – Stiff diagram
Available environmental geochemical tracers
Dissolved elements/compounds :
Major : Na, K, Ca, Mg, Cl, HCO3, SO4, NO3, SiO2
Minor and trace : Sr, Br, F, B, Fe, Mn, …
Conservative or controlled (reactive) ?
Controlled by saturation
Dissolved elements/compounds :
Major : Na, K, Ca, Mg, Cl, HCO3, SO4, NO3, SiO2
Minor and trace : Sr, Br, F, B, Fe, Mn, …
Controlled by redox
Dissolved elements/compounds :
Major : Na, K, Ca, Mg, Cl, HCO3, SO4, NO3, SiO2
Minor and trace : Sr, Br, F, B, Fe, Mn, …
Controlled by cation exchange
Dissolved elements/compounds :
Major : Na, K, Ca, Mg, Cl, HCO3, SO4, NO3, SiO2
Minor and trace : Sr, Br, F, B, Fe, Mn, …
The most conservative tracers
Dissolved elements/compounds :
Major : Cl
Minor and trace : Br, (B)
Main tracers used to study
the mixing process
and origin of salinity
2.4 EQUILIBRIUM
Main chemical reactions
Equilibrium
Non equilibrium
Saturation
Index (SI) =
log(Q/K)
SI<0
SI>0
SI=0 Q =K Chemical equilibrium
Exemple : Saturation with respect to fluorite
3. MAIN REACTIONS
Main chemical reactions
Acid/ base
Water-rock interactions
– Precipitation/dissolution
– Weathering of al-silicates (hydrolysis reactions)
– Sorption (adsoption/desorption)
Redox
Main chemical reactions
Acid/ base
Water-rock interactions
– Precipitation/dissolution
– Weathering of al-silicates (hydrolysis reactions)
– Sorption (adsoption/desorption)
Redox
Important acids/bases in natural waters
- Acid-base reaction implies
H+ or OH-
HA = H+ + A-
Ex. : H2CO3 = H + + HCO3 -
- Species that produce H+
=acid
- Species accepting H+ = base
(conjugate base)
Carbon Dioxide
CO2 increasing in concentration in
atmosphere
High solubility
Follows solubility laws (pressure, temp.)
Many sources other than atmosphere:
rainwater, runoff, groundwater, respiration,
decomposition in sediments, crust/mantle
degassing
Carbon Dioxide
CO2 behaves much differently than other
gases once it dissolves in water
Exists in equilibrium with many additional
forms of carbon
CO2 + H2O = H2CO3
H2CO3 = HCO3- + H+
HCO3- = CO3
2- + H+
Carbonic acid
Bicarbonate
Carbonate
Ko = 10-1.5
K1 = 10-6.4
K2 = 10-10.3
K at 25°C
Carbon system
Putting it all together
Sensitive to changes in pH
Low pH - left side dominates
High pH - right side dominates
CO2 + H2O = H2CO3 = HCO3- + H+ = CO3
2- + 2H+
CO2 + H2O = H2CO3 = HCO3- + H+ = CO3
2- + 2H+
Addition of CO2 via respiration pushes
equilibrium to left and lowers pH
Respiration
Photosynthesis
Removal of CO2 via photosynthesis pulls
equilibrium to right and raises pH
Putting it all together
<CH2O> + O2 = CO2 + H2O
pH range for most
natural waters
Open system with respect CO2
Closed system with respect CO2
Silica
H4SiO4 (silicic acid)
is the dominant
species for dissolved
Si
Silicic acid
decomposes above
pH 9 (rare)
Sometines, SiO2 is
used instead of
H4SiO2
Analogy with CO2 in
solution
Disslved silica
hightly dependant
on temperature
Main chemical reactions
Acid/ base
Water-rock interactions
– Precipitation/dissolution
– Weathering of al-silicates (hydrolysis reactions)
– Sorption (adsoption/desorption)
Redox
Carbonates
Important system
20% of globe surface
most groudwater ressources
Main carbonate minerals
Limestone outcrop
Dual porosity
Microporosity (a)
Macroporosity (faults
karst) (b)
(a)
(b)
CaCO3 + H2O + CO2 = 2 HCO3- + Ca2+
In most natural waters, CO2 combines with
alkali metals or alkaline earth metals to
form bicarbonates (pH 6-8.5) and cabonates (pH>8.5)
calcite
Aggressive CO2 dissolves CaCO3 and drives equation to right.
The same chemical type may be obtained by Ca-Feldpar/water weathering
Carbonate system
CaSi2Al2O8 + 8H2O + 2CO2 = 2Al(OH)3+ 2H4SiO4 + 2HCO3- + Ca2+
3 ways for writing calcite dissolution
Buffer System
Little change in
pH despite
additions of lots
of acids or
base, as long as
supply of
carbonates &
bicarbonates
holds out
CaCO3 + H2O + CO2 = 2 HCO3- + Ca2+
pCO2 versus Ca2+ (saturation calcite)
Stalactites
CO2 (g) CO2 (aq)
CO2 (aq) + H2O H2CO3
H2CO3 HCO3- + H+
Sum :CO2 (g) + H2O HCO3- + H+
PCO2 35∙10-4 atm CaCO3 + H+ → Ca2+ + HCO3
-
PCO2 3.5∙10-4 atm
CO2 (g) + H2O + CaCO3 Ca2+
+ HCO3-
Chalk aquifer (UK)
Or FeS2 + 4.25 O2 + 2.5 H2O + CaMg(CO3)2
= Fe(OH)3 + Ca2+ + Mg2+ + 2SO42- + 2HCO3
-
Al-Silicates
Total concentration of aluminum (AlT) in solution, as a
function of pH, for a solution in equilibrium with gibbsite.
Incongruent dissolution of Al-silicates
Multiple secondary minerals
According to
climate
conditions
According to
chemical
composition
of the water
that causes
the
weathering
Formula From Conditions
illite KAl2(AlSi3O10 )(OH) 2
K-Feld, mica
blanctemperate, hydrothermal
smectite (Al,Mg)4(Si8O20 )(OH) 2biotite,
amphiboles
sub-tropical to temperate +
hydrothermal
chlorite(Mg,Fe)6-x(Al,Si)xSi4-x
Alx(OH)10
biotite,
amphibolestemperature + hydrothermal
kaolinite Al2Si2O5(OH)4 K-feld(sub-) tropical, high weathering
process, even dissolution of quartz!
Stability diagram (K-Feldspar, (KSi3Al08)
Stability diagram
Anothite CaSi2Al208
Albite NaSi3Al08
Table 9-3. Mineral weatherability listed in order
of increasing resistance to weathering
Halite
Gypsum, anhydrite
Pyrite
Calcite
Dolomite
Volcanic glass
Olivine
Ca-plagioclase
Pyroxenes
Ca-Na plagioclase
Amphibo les
Na-plagioclase
Biotite
K-feldspar
Muscovite
Vermiculite, s mectite
Quartz
Kaolinite
Gibbsite, hematite, goethite
Mineral weatherability
The common-ion effect
Natural waters are very complex and we may have saturation with respect to several phases simultaneously.
Example: What are the concentrations of all species in a solution in equilibrium with both barite and gypsum?
CaSO4·2H2O Ca2+ + SO42- + 2H2O,
KSP = [Ca2+][SO42-] = 10-4.6
BaSO4 Ba2+ + SO42-,
KSP = [Ba2+][SO42-] = 10-10.0
Cont. [Ca2+][SO4
2-] = 10-4.6
[Ba2+][SO42-] = 10-10.0
Eliminate [SO42-] by substituting 10-4.6/[Ca+]:
[Ba2+]•10-4.6/[Ca2+] = 10-10.0
2) Species: Ca2+, Ba2+, SO42-, H+, OH-
H2O H+ + OH-
Kw = [H+][OH-] = 10-14
3) Mass-balance: [Ba2+] + [Ca2+] = [SO42-]
4) Charge-balance:
2[Ba2+] + 2[Ca2+] + [H+] = 2[SO42-] + [OH-]
10-4.6 + 10-10.0 = [SO42-]2
[SO42-] = (10-4.6 + 10-10.0)1/2 = 10-2.3 mol/L
[Ca2+] = 10-4.6/10-2.3 = 10-2.3 mol/L
[Ba2+] = 10-10.0/10-2.3 = 10-7.7 mol/L
Conclusion
The least soluble salt (barite, KSP=10-10), contributes a negligible amount of
sulfate to the solution. The more soluble salt (gypsum, KSP=10-4.6)
suppresses the solubility of the less soluble salt (the common-ion effect).
Barite can replace gypsum because barite is less soluble than gypsum.
(in other words : gypsum dissolves while secondary barite precipitates)
2
4
6.42 10SO
Ca 2
4
0.102 10SO
Ba
2
42
4
0.10
2
4
6.4 1010 SOSOSO
,
Cation exchange
CEC
Structural substitutions result
in a charge imbalance on the
clay structure that is balanced
by addition of non-structural
ions to the interlayer region
and accounts for the cation
exchange capacity (“CEC”)
of clays
Cation exchange
Cation may be exchanged between groundwater and clay
minerals
Selectivity exists according the size and the charge of the cations
Solution (mobile element) solid (less mobile elements)
Al 3 +
Mg2+ < Ca2+ < Sr2+ < Ba2+
Na+ < K+ < Rb+
Solid
Solution
North China plain (Beijing area)
Rain water (recharge water)
should be on Na=Cl line
Redox reactions (electron transfer reactions)
pH-Pe environments
Sulfur (S)
Nitrogen (N)
Iron (Fe)
Eh=0.059.pe
Redox boundary
pE-pH diagram
U is mobile
in oxidizing
environment
Table 9-10. Origin of major aqueous s pecies in groun d water
Aqueous species Origin
Na+ NaCl d issolution (some pollution)
Plagioclase weathering
Rainwater addition
K+ Biotite weathering
K-feldspar weathering
Mg2+ Amphibole and pyroxene weathering
Biotite (and chlorite) weathering
Dolo mite weathering
Olivine weathering
Rainwater addition
Ca2+ Calcite weathering
Plagioclase weathering
Dolo mite weathering
HCO
3 Calcite and dolomite weathering
Silicate weathering
SO2
4 Pyrite weathering (some pollution)
CaSO4 dissolution
Rainwater addition
Cl- NaCl d issolution (some pollution)
Rainwater addition
H4SiO4 (aq) Silicate weathering
HCO
3
SO2
4
Argiles
Mer (10500 mg.L-1)
Engrais ( Kcl)
Non pour l’eau Mer ( 400 mg/L)
Mer (1350 mg/L)
CO2
Mer (2700 mg/L)
Mer ( 19 000 mg/L)
Engrais ( KCl)
Example
Chalk aquifer
London basin (UK)
Exemple 2
Mauritanie
APPROCHES HYDRODYNAMIQUE ET
GÉOCHIMIQUE DE LA RECHARGE DE LA
NAPPE DU TRARZA, SUD-OUEST DE LA
MAURITANIE
Ahmed Salem
Sous la direction de Christelle MARLIN et Christian LEDUC
TRA
RZA
Les zones arides et semi-arides représentent plus de 30 % des
terres émergées
13
4
Infiltration
directe Recharge
localisée
Echange
rivières / nappes
RECHARGE AQUIFERE
Variabilité spatiale et temporelle
Mécanismes de recharge en milieux semi-arides
13
5
recharge diffuse (directe)
nappe
recharge concentrée (indirecte)
nappe
nappe
nappe
Ech
an
ge
eau
de s
urf
ace e
au
so
ute
rrain
e
Climatiques
Transgressions
marines
Régime Hydro.
Transport des
particules
Digues/Barrages Déforestation
Variabilité de la recharge
Alternance périodes
Humide/sèche
Anthropiques
Niveau de base
Biseau salé
Chimie d’eau
Niveau de base
Diffusion d’eau
Chimie d’eau
Niveau de base
Eau de surface
ETP
13
6
Irrigation
Niveau de base
Eau de surface
Chimie d’eau
Mécanismes de recharge en milieux semi-arides
Principaux mécanismes de recharge de la nappe du
Trarza
Impacts climatiques et anthropiques sur la dynamique de
la nappe
Objectifs
13
7
Eau souterraine
40000 km2
Eau de très bonne
qualité
AEP de plusieurs
régions : 700 000
habitants
Eau de surface
Fleuve Sénégal
Lac de R’kiz
Lac d’Aleg
Localisation
13
8
60 000
m3.d-1
2010
Pluie mensuelle moyenne Précipitation moyenne de
1934 à 2011 :
• Rosso : 250 mm
Ecart-type : 100 mm
• Boutilimit : 150 mm
Ecart-type : 85 mm
Climat
Climat sahélien :
saison pluvieuse : Juil., Août, Sept., Oct.
saison sèche : Nov. à Juin Ros
so
Boutili
mit
0
20
40
60
80
100
120
J F M A M J J A S O N D
Plu
ie m
oy.
(m
m) Boutilimit
Rosso
139 saison pluvieuse
Carte piézométrique de 2011
14
0
Carte piézométrique de 1962
14
1
Répartition de la minéralisation dans la
nappe
14
2
CE (µS.cm-1) %
< 200 2
200-500 30
500-1000 35
1000-2000 22
2000-5000 8
> 5000 3
Fleuve Sénégal et ses défluents
Compositions chimiques proches entre les eaux du fleuve et
la nappe
0 20 40 60 80 100 120
Distance depuis le fleuve (km)
1
10
100
1000
10000
1E5
Teneurs
en c
hlo
rure
s (
mg.L
-1)
14
3
Transgressions marines: eau de mer reste piégée dans les
sédiments
Limite de
l’Inchirien
33000 ans BP
Limite du Nouakchottien
5500 ans BP
Limite de
l’Inchirien
33000 ans BP
Limite du Nouakchottien
5500 ans BP
Origine de fortes minéralisations : Cl- > 1000 mg.L-
1
14
4
Dilution de l’eau de mer
0,01 0,10 1,00 10,00 100,00 1000,00
Cl- (meq.L-)
0,01
0,10
1,00
10,00
100,00
1000,00
Na
+(m
eq
.L- )
Lac R'kiz
Zone près du fleuve <10 km; Cl- >1000 mg.L-
Zone près du fleuve <10 km; Cl- <1000 mg.L-
Zone loin du fleuve >10 km
1 10 100 1000 10000 100000
Cl-(mg.L-1)
0,00
0,01
0,10
1,00
10,00
100,00
1000,00
Br- (m
g.L
-1)
Zone près du fleuve <10 km, Cl- <1000 mg.L-1
Zone près du fleuve <10 km, Cl- >1000 mg.L-1
Zone loin du fleuve >10 km
36
Dissolution de la Halite
Dilution de l'eau de mer
14
5
Origine de fortes minéralisations : Cl- > 1000 mg.L-
1
Mélange eau douce
(pluie/fleuve) et une
solution marine
Limite de
l’Inchirien
33000 ans BP
Limite du
Nouakchottien
5500 ans BP
3
6 5
0 4000 8000 12000 16000 20000
Cl-(mg.L-)
-8
-6
-4
-2
0
2
4
6
δ18O
(vs V
-SM
OW
)36
Zone près du fleuve <10 km, Cl- <1000 mg.L-1
Zone près du fleuve <10 km, Cl- >1000 mg.L-1
Zone loin du fleuve >10 km
Pôle eau
de mer
Pôle
Fleuve/Pluie
Eva
po
ratio
n
Droite de mélange eau
douce-eau de mer5
25%
eau de
mer
70%
eau de
mer
14
6
Origine de fortes minéralisations : Cl- > 1000 mg.L-
1
-8 -6 -4 -2 0 2 4 6 8 10
δ18O(vs V-SMOW)
-50
-40
-30
-20
-10
0
10
20
30
δ
2H
(vs V
-SM
OW
)
Lac R'kiz
Mélange eau
douce-eau de mer
Zone près du fleuve <10 km, Cl- <1000 mg.L-1
Zone près du fleuve <10 km, Cl- >1000 mg.L-1
Zone loin du fleuve >10 km
Fleuve 2011
Pluie ponderée 2010
Eau de mer 36
5
(b)(a)
14
7
Mélange eau douce (pluie/fleuve) et une solution marine sur lequel
s’ajoute un effet d’évaporation
Origine de fortes minéralisations : Cl- > 1000 mg.L-
1