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