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TRAINING ON SURFACE EXPLORATION STUDIES FOR GEOTHERMAL RESOURCES AND

DEVELOPMENT OF CONCEPTUAL MODELS

UNDER THE AUSPICES OF INTERIM PROJECT COORDINATION UNIT OF THE AFRICA

GEOTHERMAL CENTER OF EXCELLENCE

Principles and application of geochemistry in

geothermal resource exploration

Dr. Tobias Björn Weisenberger, Iceland GeoSurvey

(slides adepted from Iwona M. Galeczka and Finnbogi Óskarsson)

Outline

− Classification of geothermal fluids

− Primary and secondary fluids

− Fluid types and where to find them

− Tracing the origin of geothermal fluids

− Conservative components

− Stable isotopes

− Geothermal fluid sampling and analysis

CLASSIFICATION OF GEOTHERMAL FLUIDS

Classification of subsurface waters

− Meteoric water

− Oceanic water

− Evolved connate water

− Metamorphic water

− Magmatic water

− Juvenile water

Most common

components of

geothermal solutions

Sedimentary basins

Often small component

White (1986)

Main objectives of chemical studies in geothermal development

• Exploration stage

• Temperature

• Recharge (origin of fluids)

• Development stage

• Production properties (scaling, corrosion, gas)

• Environmental concerns

• Production stage

• All of the above (better information and changes)

• Changes in acidity

• Mineral saturation

Objectives of geochemical exploration

− Subsurface composition

− Temperature

− Contribution to conceptual model

− Origin and flow direction

− Reservoir location

− Boiling

− Production properties

− NCG content

− Scaling

− Corrosion

− Environmental effects

“Ingredients” of a geothermal fluid

UPPER CRUST

LOWER CRUST

MAGMA

INTRUSION

PRECIPITATION

“low”

temperature

rock

water

“high”

temperature

volatiles

Composition of geothermal fluids

− Various factors and processes control the fluid composition of geothermal systems, including:

− Initial water composition

− Meteoric or sea water

− Water-rock interaction

− Will drive component concentrations towards equilibrium with alteration minerals

− Addition of magmatic constituents

− Gases (often acidic; anions)

Dallol, Ethiopia

Constituents of geothermal fluids

− Major elements and gases (1 to >1000 ppm)− SiO2, Na, K, Mg, Ca, F, Cl, SO4, B, (Fe, Al)

− CO2, H2S, H2, N2, Ar, CH4

− Trace elements (<1 to 1000 ppb)− Various other elements including halogens (Br, I), metalloids (As, Sb), alkaline and alkaline

earth metals (Li, Cs, Rb, Sr, Ba) and transition metals (Cr, Mn, Cu, Zn, Pb etc.)

− Reactive (rock-forming) constituents− SiO2, Al, Na, K, Ca, Mg, Fe, CO2, H2S, H2

− Conservative constituents− Cl, Br, B, I, N2, Ar

Discharge data is used to compute

the deep fluid chemical composition

The most common speciation

program for this purpose is WATCH

computer code

http://www.geothermal.is/software

Geothermal fluids

− Primary fluids− The deep reservoir fluid of a geothermal system

− Low-temperature fluids may be sampled from natural manifestations

− High-temperature fluids only sampled from boreholes

− Wide range of temperatures, from <100°C to >300°C

− Liquid water or steam

− Secondary fluids− Evolve from primary fluids on the way to the surface

− Depressurisation boiling

− Phase separation

− Steam condensation

− Mixing with shallow groundwater

− Temperatures at surface ≤ 100°C

− Liquid water or steam

Types of geothermal fluids available for geochemical study

•Well discharge

• Liquid

• Two phase fluid

• Steam

• Natural manifestations

• Geothermal solutions (often boiled or cooled)

• Geothermal steam

• Gas

• Mixed waters

• Steam heated surface waters

• Production

• Separated brine

• Steam

• Condensate

• Cooling tower water

TYPES OF GEOTHERMAL FLUIDS FROM NATURAL MANIFESTATIONS

steam vents

steam heated

surface water

Mixed solutions

CO2 rich water

ascending steam

water

table

oxidation

boiled geothermal

solutions

deep fluid

Modified from: Arnórsson et al. (2007) RiM&G, v 65, pp 259-312

Primary fluids

− NaCl fluids

− Generally meteoric in origin

− Cl from <100 to <10000 ppm

− pH largely buffered by CO2

− Acid-sulphate fluids

− Contain H2SO4 and HCl of magmatic origin

− pH largely controlled by H2SO4

− High salinity fluids

− Derived from seawater

− Connate water

IDDP-1, Krafla, Iceland

Hrunalaug, Iceland

Secondary fluids

− The most common types of secondary geothermal fluids include:

− Alkaline hot springs – boiled geothermal water

− Carbonate springs – often a mixture of CO2-rich steam and non-thermal ground- and/or surface water

− Acid-sulphate pools – steam-heated non-thermal ground- and/or surface water where H2S in the steam has been oxidised to H2SO4

− Steam vents – the steam formed upon boiling

− Possible partial condensation of steam

Boiled or cooled geothermal solutions

− Quite common

− Often restricted to lower parts of geothermal fields

− Very useful for geothermometry

− Give information about production properties

− Give information about origin of fluid

− Characterized by precipitation of minerals (silica or carbonate)

− pH usually 7-10, Cl- dominant anion

− Net addition of mass and volume to surface

Hveravellir, Iceland

Boiled or cooled geothermal solutions

Haukadalur, Iceland

CO2-rich springs

• Surface water mixed

with CO2-rich steam

• Often found at the

peripheries of systems

• pH typically 4.5 to 7

• HCO3- is the dominant

anion

• Typically clear water,

often with bubbling gas

• Carbonate deposits

commonly seen

• Net addition of mass

and volume to surface

Torfajökull, Iceland

Steam heated surface water

Kamchatka, Russia

• Commonly associated with steam vents

• Often very acidic (pH < 3) due to H2S oxidation

• SO42- is the dominant anion

• Dissolved constituents derived by alteration of surface rock

• No memory of conditions in reservoir

• Acid alteration minerals include amorphous silica, kaolinite, anatase, pyrite, iron oxides

• Leaching of all elements except S

• Net loss of mass and volume

Geothermal steam

• Very common manifestation of high temperature geothermal activity

• Useful for geothermometry

• Gives information about the origin of the fluid

• May give information about the concentration of dissolved solids

Krafla, Iceland

Geothermal steam – surface alteration

pyrite

amorphous

silica + kaolinite

Fe-oxides

Ölkelduháls, Iceland

• Surface rocks commonly intensely altered around steam

vents

• H2S oxidized and forms H2SO4

• Sulfuric acid leaches most elements from the rock

• Net loss of mass and volume of rock

Types of geothermal fluids and applications

Fluid type Main application

Steam • Geothermometry

• Origin of fluid

Gas • May be useful for geothermometry

• May delinate active faults

Geothermal waters • Geothermometry

• Origin of fluid

• Production properties

Mixed waters • Geothermometry if mixing trend can

be defined

Steam heated surface waters • No memory of the geothermal

system

• Geothermal manifestation

Well discharge • Geothermometry

• Origin of fluid

• Production properties

Cl – SO4 – HCO3 ternary plot

− A general classification of water types by dominant anions is a fast and easy way of weeding out unsuitable samples

− The main water groups seen on the ternary diagramme are:

1. Neutral Cl-dominated waters, likely to represent well equilibrated fluids from major upflow zones

2. Waters with high CO2 concentration, often from the peripheries of hydrothermal systems or cooling systems

3. Highly acidic SO4 waters, formed by volcanic (HCl, SO2) or lower-temperature geothermal (H2S) gases in groundwater

SO4

Cl

HCO3

(Giggenbach, 1991)

TRACING THE ORIGIN OF GEOTHERMAL FLUIDS

Tracing of fluid origin

− Conservative components (Cl, Br, B, 2H etc.)

− Cl/B ratios− Derived from:

− Seawater and rain (ratio 1330)

− Rocks (tholeiite ratio 25-50)

− Magma (ratio low, <1)

− Have been used to track the history of solutions in high- and low-temperature fields in Iceland

− Stable water isotopes (2H and 18O)− 2H (D) relatively unaltered from precipitation

− 18O more affected by reactions with host-rock

− Other stable or radioactive isotopes (3H, 36Cl, 87Sr, 222Rn etc.)

Chloride in precipitation in Iceland

• Cl (mg/L) in Icelandic surface water, originating in precipitation

• Concentration decreases with distance from the sea and altitude

• The Cl/B ratio of seawater is preserved in precipitation

Cl/B in low-temperature systems in SW-Iceland

Arnórsson and Andrésdóttir (1995)

precipitation

Cl/B in high- and low-temp. systems in Iceland

Sea water

mixing

rock dissolution (Cl/B=30)

Magmatic gas or

high-T phase

separation

Arnórsson and Andrésdóttir

(1995)

Isotopes

− Atomic nuclei are composed of protons and neutrons− Atomic number (Z): Number of protons

− Mass number (A): Number of protons + neutrons

− Isotopes: Same atomic number, different mass numbers− from Greek: isos (same) + topos (place)

− Stable or radioactive (decay spontaneously)

Isotopic fractionation

− Isotopes of the same chemical element have almost identical physical and chemical properties

− However, because of their small mass differences, they have different reaction rates and different abundances in chemical compounds or phases that are in isotopic exchange

− Physical processes such as diffusion, evaporation, melting, condensation, etc. also produce isotopic differentiation

− All these variations in the isotopic composition, produced by chemical or physical processes, in compounds or phases, present in the same system, are called isotopic fractionation

Stable isotopes in geochemistry

−2H (D) and 18O most used

− Analysed for by mass spectrometry

− Difficult to measure absolute concentrations but easy to determine ratios (R = heavy isotope/light isotope)

− δD and δ18O reported as ‰-deviations from SMOW (Standard Mean Ocean Water)

𝛿𝑠𝑎𝑚𝑝𝑙𝑒 =𝑅𝑠𝑎𝑚𝑝𝑙𝑒

𝑅𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑− 1 × 1000‰

− δ > 0: enriched in the heavy isotope compared to standard

− δ < 0: depleted in the heavy isotope compared to standard

− Same reporting conventions for many other isotopes:− δ15N (‰ATM), δ13C (‰PDB), δ34S (‰CDT)

Isotopes in precipitation

− Sea water: D and 18O ~ 0‰ (SMOW) − Evaporation → Clouds → Precipitation

− Latitude: Lower isotope ratios at higher latitudes

− Altitude and distance from sea shore: Lower isotope ratios at higher altitudes and greater distances from shore

− Single showers: Origin of cloud, temperature of condensation

− Seasonal changes: Lower values in winter

− Long-term climatic changes: Lower values at lower temperature

− Quantitative effect: Inverse relationship with quantity of precipitation. More pronounced at low than high latitudes

Global meteoric water line

Craig (1961):

𝛿𝐷 = 8.0 ∙ 𝛿18𝑂 + 10.0

Isotopes in studies of geothermal water

− Little changes in isotope ratios after precipitation

− Local annual means for precipitation known

− Meteoric line (Craig, 1961) applies but deviations are known and local lines have been determined

− Isotope values for geothermal water suggest origin

− Mixing, water-rock interaction, condensation and age may have to be accounted for

− Fractionation constants known as function of T

• 3 rainwater stations

• 24 glacier stations

• 657 groundwater stations

Deuterium map of Iceland

Árnason, 1976

Altered meteoric thermal waters

− Related to local meteoric line; D similar but 18O displaced to higher value

− Oxygen isotopes exchange between hot rocks and deeply circulating meteoric water → higher 18O in water but lower in rock: “Oxygen isotope shift”

− Rock minerals contain little hydrogen. Hence no comparable hydrogen isotope shift

− Hydrogen exchange with epidote possible at high T

− Magnitude of 18O shift depends on original 18O of water and rock, mineralogy and texture and time of contact

− Carbonate rocks → large 18O shifts

Altered meteoric thermal waters

oxygen isotope shift

Data from Sveinbjörnsdóttir (1988)

and Karingithi (2000)

Example: Olkaria, Kenya

− The Olkaria reservoir fluids show various degrees of mixing between water from Lake Naivasha and precipitation

− Deeper wells have higher ratio of water from L. Naivasha

− Peripheral wells have higher ratio of precipitation or escarpment water

Tritium (3H) in Skagafjörður WaterS

− Tritium (3H) is a naturally occurring radioactive isotope of hydrogen (t½ = 12.3 a), activity measured in tritium units (1 TU = 1.185 Bq/L)

− Natural levels of 3H in precipitation are low, but anthropogenic 3H has increasedin the atmosphere since the 1950s (now at about 10 TU)

Data from Arnórsson

and Sveinbjörnsdóttir

(2000)

Strontium isotopes – Water-rock interaction

− In low salinity systems, dissolved Sr retains the isotope composition of the dissolved rocks

− More complicated for water with seawater component87/86Srseawater = 0.709168

− Example: A water sample from a hot spring in Gisenyi, Rwanda has 87/86Sr = 0.767 and 3H < 0.1 TU

− Likely to represent deep circulation of old groundwater

Bahati et al. (2005)

GEOTHERMAL SAMPLING AND ANALYSIS

Sampling and analytical results

− Results of chemical analysis are the basis for geochemical interpretation

− The results are only as good as the sampling and analytical techniques

− Junk in → Junk out

− Imperative that well trained personnel with insight into possible interferences perform the tasks

− Analysis in the field

− Proper sample preservation and treatment

− Correct sample steam fraction

Possible mistakes and error sources

− Improper containers

− Overuse of cleaning materials

− Confusion over reported chemical state

− Si or SiO2 (100 ppm Si = 214.3 ppm SiO2)

− Lack of, or improper treatment

− No dilution for SiO2

− Samples filtered long after collection

− Uncritical reading of instructions or formulae

− Inappropriate or imprecise analytical methods

Types of geothermal fluid samples

▪ Well discharge

▪ Liquid

▪ Two phase fluid

▪ Steam

• Production phasesamples• Separated brine

• Steam from turbines

• Condensate

• Cooling tower water

• Natural manifestations▪ Geothermal waters

(often boiled or cooled)

• Geothermal steam

• Gas

• Mixed waters

• Steam heated surface waters

To keep in mind during sampling

− Measure temperature

− Sampling pressure (critical for well samples)

− Take photographs

− GPS coordinates

− Description of site

− Estimate flow rate

− Alteration/mineralization

− Relationship to other geothermal manifestations and geology

Choice of sampling sites

− Geological setting

− Temperature

− Springs: As close to outflow as possible

− Fumaroles: Sulphur deposits and concentrated flows good guides

− Wells: Best 1.5 m away from bends or constrictions

Sampling of springs

− Important to be close to the source

− Ideal to use a peristaltic pump

− Do not use a suction pump

− Avoid turbulent flow

− May cause loss/gain of CO2 or H2S

− Changes pH

− Cool the sample for analysis of volatiles (pH, CO2, H2S, NH3)

Sampling of springs

From Arnórsson et al., 2006

Kapisya, Zambia

Sampling of fumaroles (steam vents)

− Best to collect from powerful fumaroles

− Focussed flow is ideal

− Diffuse flow through cracks is very difficult to sample

− Sulphur deposits are often a good sign

− Solfataras may be too permeable

− Absence of H2S odour is a bad sign

Yes

Yes

No

Selection of fumaroles for sampling

Þeistareykir, Iceland

Small fumaroles gathered for

flow measurement and sampling

Reykjanes, Iceland

Sampling of fumaroles

Krafla, Iceland

Two-phase well sample

Measure Ps, Ts

(gives hv and hl)

Obtain hd from X

hd = X hv + (1-X) hl

Separator

Vapour sample

Liquid sample

Adjustment

Webre separator

− A portable Webre separator is convenient for sampling two phase wells

− Typically chromium steel

− By adjusting the water level in the separator it is possible to sample each phase separately

− Low water level for gas

− High water level for water

Sampling of wells

Two phase well

sample collected

according to ÍSOR

protocol

“Non-ideal” sample

Well sampling - quality considerations

− Sampling valve should be placed about 1.5 m away from bends or constrictions

− Valve should be fully open during sampling

− Separation efficiency should be checked

− No bubbles in liquid phase (field check: pH)

− Analyse for Na or Cl in condensate (field check: conductivity)

− Sample pressure (and temperature) should be accurately recorded using calibrated instruments

− P and T should be close to the boiling curve

Sample pressure

− A good pressure measurement is imperative for future interpretation

− The steam fraction depends on the pressure

− The separator and line pressures should match when valve is fully open

− Psep < Pline indicates restricted flow

− Psep > Pline indicates boiling in the separator

− Choose a pressure gauge so that the measured pressure is within 20-80% of the range

− Low Ps increases the risk of silica precipitation

Calculation of reservoir fluid

− Typically done using speciation programmes

− WATCH, SOLVEQ, etc.

− Enthalpy of the deep liquid (ℎ𝑑𝑙) must be known or estimated

− Sampling pressure (𝑃𝑠) must be measured and preferably sampling temperature as well (𝑇𝑠)

− 𝑃𝑠 gives enthalpy of water and vapour phases at sampling conditions (ℎ𝑙 and ℎ𝑣)

− Then we can calculate the vapour fraction

𝑋 =(ℎ𝑑𝑙 − ℎ𝑙)

(ℎ𝑣 − ℎ𝑙)

which is used to combine the liquid and vapour phases

Sample treatment and preservation

− Physical methods

− Filtration

− Freezing

− Cooling

− Airtight containers

− Immediate analysis

− Chemical methods

− Acidification

− Precipitation

− Dilution

− Prevention of redox

− Gas fixation

− Ion exchange

− Extraction

Sample preservation - physical methods

− Filtration

− Removes solid particles

− Critical for samples that will be acidified

− Also necessary for ion chromatograph analyses

− May remove colloids that form in the solution

− A problem for SiO2 rich springs

− May also affect concentrations of other elements such as Al

− Should be preformed using an in-line filter

− Reduces risk of contamination from filtering equipment

− Prevents the loss of cations and trace elements to intermediate (plastic) containers

The blue color of the

water is due to

colloidal silica that

has formed due to

cooling of the liquid

from reservoir

conditions (~200°C)

Sample preservation - physical methods

− Freezing

− To stop biological activity

− Important for “nutrient” species such as phosphate, ammonia, nitrite and nitrate

− Airtight containers

− To prevent loss of volatile constituents

− CO2, H2S, NH3, etc.

− Immediate analyses

− Critical for redox sensitive species

− Sulfur (H2S, SO42-, S2O3

2-, S°), arsenic species etc.

− Also important for pH and volatile components

Sample preservation - chemical methods

− Precipitation

− Used to prevent interferences between species

− Important for SO42- in geothermal samples

H2S:

500 mmol/kg

SO42-:

20 mmol/kg

time

oxidation

H2S:

200 mmol/kg

SO42-:

320 mmol/kg

Sample preservation - chemical methods

− Precipitation

− Used to prevent interferences between species

− Important for SO42- in geothermal samples

H2S:

500 mmol/kg

SO42-:

20 mmol/kg

Addition of ZnAc2

Precipitation of ZnS

H2S:

0 mmol/kg

SO42-:

20 mmol/kg

ZnS precipitate

Sample preservation - chemical methods

− Dilution

− To prevent precipitation of amorphous silica

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 50 100 150 200 250 300 350

SiO

2(m

g/k

g)

Temperature (°C)

amorphous silica

solubility

quartz

solubility

•Geothermal solutions are

generally in equilibrium with

quartz

•Quartz precipitation is a very

slow process

•Precipitation of amorphous silica

is a rapid process

•Cooling of geothermal solutions

will result in supersaturation of

amorphous silica

•Concentration of dissolved silica

in undiluted geothermal samples

will be underestimated

cooling

40% NaOH

Vacuum

P = 5 mbar

(Vapour pressure)

Steam: 99% H20, 0.9% CO2,

0.05% H2S, 0.02% N2,

0.02% H2, 0.001% Ar, etc

In sample bottle:

Steam condenses

CO2 and H2S dissolve in NaOH solution

CO2 + H2O + 2 NaOH ⇌ Na2CO3 + 2 H2O

H2S + NaOH ⇌ NaHS + H2OCondensed H2O

CO32-

HS-

Residual NaOH

P = 500 mbar

40% N2

40% H2

2% Ar etc

Sample preservation - chemical methods

− Gas fixation

− To increase the volume of steam collected

− For preservation of H2S

Sample treatment – overview

Phase Treatment Code To determine

Vapour None; amber glass bottle Ru 2H, 18O

2 mL 0.2 M ZnAc2 added to sample in 100 mL volumetric glass flask to precipitate sulphide

Rp SO4

None Ru Anions

0.5 mL conc. HNO3 (Suprapur) added to 100 mL sample

Ra Cations

Added to 50 mL 40% NaOH in evacuated double port bottle

Gas sample

CO2, H2S in NaOH, residual gases in gas phase, 34S in H2S

Liquid None; amber glass bottle with ground glass stopper

Ru pH, CO2, H2S, NH3

Dilution; 10 to 50 mL of sample added to 90 to 50 ml of distilled, deionised water

Rd (1:10 to 1:2)

SiO2 if > 100 ppm

None Ru Mg, SiO2 if < 100 ppm

Filtration (0.2 µm) Fu Anions

Filtration; 2 mL 0.2 M ZnAc2 added to sample in 100 mL volumetric glass flask

Fp, Fpi SO4

34S and 18O in SO4

Analysed components

Component Main

purpose

Component Main

purpose

Component Main

purpose

pH A, C, M Ca M, S CO2 T, M, E, S

Conductivit

y

Fe M, C H2S T, E, M, S

18O & D O Al M, E Ar O, T

SiO2 T, S Mg O, T N2 O, T

Cl O, A As E H2 T

B O, E SO4 A, M CH4 T

Na T, M Hg E O2 QC

K T, M Zn M, E TDS QC

A = acidity, C = corrosion, E = environmental sensitivity, M = mineral saturation,

O = origin of fluid, QC = quality control, S = scaling potential, T = temperature

Choice of analytical methods

− Available instruments

− Servicing facilities

− Trained personnel

− Comparison of methods

− Speed

− Reliability

− Cost

Most important techniques

− Gas: Titrimetry; Gas chromatography; Mass spectrometry; Radiometry

− Cations: AAS (flame, carbon furnace); FES; ICP/AES; Ion chromatography; Fluorimetry; Ion selective electrodes

− Anions: Ion chromatography; Ion selective electrodes; Spectrophotometry; Titrimetry (HCO3

-)

− Total solids: Gravimetry; Conductivity

− Trace elements: ICP/MS (in commercial laboratories)

− Isotopes: Separation + Mass spectrometry

Regarding pH

− pH is the most critical parameter for the interpreta-tion of the composition of natural waters

− Its value is a sensitive function of the activity of dissolved volatile species such as CO2, H2S and NH3

− pH is calculated from the electric potential betweenelectrodes immersed in the solution

− Very sensitive to errors during sampling and analysis

− Silica polymerisation affects pH, e.g. in weir box

− Cool inline and measure pH on site

− It is also a sensitive function of temperature

− Measure temperature at collection and analysis

Gunnarsson and

Arnórsson (2005)

Analysis of CO2 (total inorganic carbon)

− Must be done on-site or sample stored in air-tight glass bottles for less than a day

− Inorganic carbon is typically analysed for by a direct titration with 0.1 M HCl from pH 8.2 to 3.8

− Contributions from H2S, SiO2 and other relevant weak acids must be subtracted

− Also good to lower pH to < 3 after titration from 8.2 to 3.8, then bubble with N2 and back-titrate with 0.1 M NaOH from pH 3.8 to 8.2

− Corrects for weak acids other than H2S

Analysis of H2S

− Best to analyse immediately

− Typically analysed by titration, either by iodometry or mercuric acetate/dithizone

− Very reactive

− Metal sulphides generally insoluble and precipitated

− May be used to preserve or prevent interference

− Oxidized to S, SO2, SO32-, S2O3

2-, S4O62-, SO4

2- etc.

− Oxidation delayed by addition of hydroxide

− Reacts with organic compounds to form complexes

Analysis of SiO2

− ICP− Total silica

− Reasonable

− AAS− Total silica

− Difficulties encountered

− Spectrophotometry with molybdate− Monomeric silica.

− Dilute upon sampling or heat with NaOH to obtain total silica

− -complex, yellow, pH 3.5: High concentrations

− -complex, yellow: Time dependent. Intermediate concentrations

− Blue complex reduced with ascorbic acid: Low concentrations

Analysis of anions: F, Cl, SO4, (Br, I, NO3, ...)

− Ion chromatography is greatly preferred

− Accurate, sensitive, all anions in one run

− Need good DI water

− F in saline water may need to be analysed using ISE

− Mohr titration is accurate for Cl, but cumbersome

− Beware of interference by H2S

− Various methods available for SO4

− Titration or spectrophotometry with thorin

− Gravimetry using BaSO4

− Turbidometry commonly used, but not very good

Analysis of metals

− AAS, ICP, FES, IC

− Good techniques for main metals (Na, K, Li, Ca, Mg, Fe, Al)

− Flameless AAS preferable to flame AAS for the low Mg concentrations encountered in high-temperature geothermal waters

− Spectrophotometric methods available for Fe and Al

− Trace metals (Zn, Cd, Cu, Ni, Pb, Co, etc.) most commonly analysed by ICP-MS

− Typically at commercial laboratories

− Hg commonly analysed by AFS or AAS (gold amalgamation)

Total dissolved solids

− TDS is best determined gravimetrically

− Take a known amount of sample and dry it in a pre-weighed beaker. TDS is the weight of the residue divided by the volume of sample

− Some conductivity meters also give estimated TDS, calculated from the measured conductivity

− Only accounts for charged species (Na+, Ca2+, Cl-, HCO3-) but disregards neutral species,

most importantly H4SiO4 Underestimates TDS

Analysis of isotopes

− Atomic nuclei are composed of protons and neutrons− Atomic number (Z): Number of protons

− Mass number (A): Number of protons + neutrons

− Isotopes: Same atomic number, different mass numbers− From Greek: isos (same) + topos (place)

− Stable isotopes (D, 13C, 18O, 34S, 87Sr, etc.) analysed by mass spectrometry− Radioactive isotopes (3H, 14C, 222Rn, etc.) analysed by radiometry

Analytical quality control

− Precision

− Repeat analysis of one sample or duplicate several samples

− "Measure thrice, cut once"

− Accuracy

− Standard additions

− Different methods

− Standards or reference samples

− Ionic balance

− Mass balance (TDS, Conductivity)

− Checks

− Inter-laboratory comparisons

Conclusions

− Extreme care by trained personnel needed for sampling

− Volatiles analysed in field or soon after arrival in laboratory

− Silica most commonly by colorimetry or ICP

− AAS still most popular method for cation analysis but ICP methods are increasing their share

− Trace metals often analysed by ICP/MS in commercial laboratories

− Anions preferably by IC (F possibly by ISE)

− TDS by gravimetry rather than from conductivity

− Gases by titration and GC

− Stable isotopes by mass spectrometry

Thank you!

Dallol, Ethiopia