Old groundwaters
István Fórizs Ph.D.
Institute for Geochemical Research,
Hungarian Academy of Sciences
Budapest
Why should we identify old
groundwaters?
• To determine the time and place of
recharge (recharge may already be
stopped)
• Mean residence time
• Exploitation induced recharge
• To understand the geochemical and
hydrological processes
Nomenclature
• Old groundwaters are
• Paleo-groundwaters (older than 10 000 a,
infiltrated during the latest glaciation)
• Sub-modern (older than 60 a)
Stable isotopes and paleogroundwaters
• These waters were infiltrated at cooler climatic
conditions during the Ice Age.
• Their dD and d18O values are significantly more negative
than those of Holocene infiltrated ones. Temperature
effect!!
• Shift in d-excess. The effect of relative humidity of (h) air
on the primary evaporation. Characteristic for arid
regions, Eastern Mediterranean and North Africa.
• There are some areas where paleo-groundwaters postdate the glaciation, because during the Ice Age there
was a permanent ice cover. The melted water infiltrated
during the deglaciation (early Holocene), e.g. in Canada.
Example: Oman
Shift in deuterim-excess (d-excess)
• Effect of primary evaporation
• Effect of secondary evaporation
• Definition: d = dD – 8*d18O
Effect of relative humidity (h) of the air:
Primary evaporation
40
20
0
d D [‰ ]
-2 0
-4 0
Sea water
-6 0
50%
-8 0
85%
100%
-1 0 0
Global Meteoric
Water Line
-1 2 0
-1 4 0
-1 8
-1 6
-1 4
-1 2
-1 0
-8
18
-6
d O [‰ ]
-4
-2
0
2
4
Secondary evaporation
40
20
0
d D [‰ ]
-2 0
20%
-4 0
100%
-6 0
80%
40%
-8 0
-1 0 0
40%
60%
80%
60%
20%
Initial water (lake
or rain drop)
-1 2 0
GMWL
-1 4 0
-1 8
-1 6
-1 4
-1 2
-1 0
-8
18
-6
d O [‰ ]
-4
-2
0
2
4
Continental effect
vapour
vapour
vapour
rain
rain
Sea
d18O
Continent
(Triassic) Bunter sandstone, England
Bath et al. 1979
-60
-70
δ2H
-80
-90
-100
-110
-120
-16
-15
-14
-13
-12
-11
δ18O
GMWL
SPRING
RIVER
BORE
-10
Ice cores show well the
climate change
d18O [‰]VSMOW
-45
0
kor [év]
GISP2
Ice core,
Greenland
Age (year)
5000
10000
15000
20000
25000
30000
-40
-35
-30
-25
Ice cores:
Canada, Greenland, Antarctic
Chemistry and paleogroundwaters
Conceptual model of groundwater flow
Chemistry and paleo-groundwaters
• Water-rock interaction may change the chemistry of
water significatly
• Recharge area:
– low TDS
– frequently Ca-HCO3 type
• Discharge area:
–
–
–
–
high TDS
frequently Na(-Ca)-HCO3(-Cl-SO4) type
high pH
high trace element content
Groundwater dating methods
Groundwater dating methods
•
•
•
•
•
Radiocarbon: 14C
Chlorine-36: 36Cl
The uranium decay series
Helium ingrowth
Krypton-81: 81Kr
Basis of 14C age determination
• Radioactive decay (discovered by Libby in
1946, Nobel Prize).
• Half-life of 14C is 5730 a (years).
• Decay equation:
At = A0×e-lt
• A0 and At are 14C initial activity, and
activity after time ‘t’, l is decay constant.
Rearranged decay equation
t = -8267×ln(At/A0) [year]
T1/2: Half-life
Ao initial activity
Expression of 14C activity
•
14C
is expressed versus a reference, in
percent modern carbon, pmC.
• Reference is the pre-industrial 14C activity
of atmospheric CO2, that is regarded as
100%.
Source of 14C
• Natural: 147N + 10n → 146C + 11p
• Where n = neutron, p = proton
• Anthropogenic: nuclear bomb tests
starting in 1952.
Natural variation in atmospheric 14C
The calculated age
• If we disregard the natural variation in
atmospheric 14C (A0 is regarded to have
been constant, as 100%), then the
calculated age is radiocarbon years and
not in calendar years.
Anthropogenic impacts on
atmospheric 14C
Correction: why needed?
• During the flow path 14C is diluted by
geochemical reactions:
–
–
–
–
Limestone (calcite) dissolution
Dolomite dissolution
Exchange with the aquifer matrix
Oxidation of old organics within the aquifer
• Calcite, dolomite and old organics are free of 14C.
• Initial 14C activity: Arecharge = q* A0,
where q is dilution factor.
• Decay equation becomes:
At = qA0e-lt
or
t = -8267×ln(At/(qA0))
[year]
Short introduction to carbon
stable isotope geochemistry
Abundance of carbon stable
isotopes
12C
= 98,9%
13C = 1,1%
13C
distribution in nature
13C
in C3, C4 and CAM plants
Photosinthesis
• C3 plants (85%): Calvin cycle
E.g. trees, cereals, legumes (bean), beet.
• C3 plants: d13C value is from -33 to -20
[‰]VPDB
• Mean value= -27‰.
Photosinthesis
• C4 plants (5%): Hatch-Slack cycle
E.g. cane, maize
• d13C value is -16 to -9 [‰]VPDB
• Mean value: -12,5‰.
13C
in soil CO2
• Soil CO2 originates from decomposition of
organic material and root respiration.
• The pressure of soil CO2 gas is 10-100 times
higher than the atmospheric .
• A part of soil CO2 diffuses to the atmosphere
causing isotopic fractionation: the remaining CO2
is heavier by ca. 4‰.
• The d13C value of soil CO2:
C3 vegetation: ≈ -23 [‰]VPDB
C4 vegetation: ≈ -9 [‰]VPDB
Carbon in water
• Source: air CO2 (d13C ≈ -7 [‰]VPDB),
or soil CO2 ( -9‰ — -23‰)
or limestone (0±2‰)
•
•
•
•
Carbonate species in water
CO2(aq) (aquatic carbondioxide)
H2CO3 (carbonic acid)
HCO3- (bicarbonate ion)
CO32- (carbonate ion)
}
DIC
Distribution of carbonate species as a
function of pH at 25 °C
Clark-Fritz 1997
Isotopic fractionation at 25 °C
• Soil CO2
} εCO2(aq)-CO2(g) = -1.1‰
• CO2(aq)
• H2CO3
• HCO3• CO32-
} CO2(aq) ≡ H2CO3
}
εHCO3(-)-CO2(aq) = 9.0‰
} εCO3(2-)-HCO3(-) = -0.4‰
Fractionation factors as a function
of temperature
• 103 lnα13CCO2(aq)-CO2(g) = -0.373(103T-1) + 0.19
• 103 lnα13CHCO3(-)-CO2(g) = 9.552(103T-1) + 24.10
• 103 lnα13CCO3(2-)-CO2(g)= 0.87(103T-1) + 3.4
Fractionation: 25 °C, DIC-CO2(soil)
Clark-Fritz 1997
Fractionation: DIC-CO2(soil) at 25 °C
Clark-Fritz 1997
The pathway of 14C to groundwater in the
recharge environment
Correction methods
• Statistical
• Chemical mass-balance
• d13C
• Dolomite dissolution
• Matrix exchange (Fontes-Garnier model)
Statistical model
• If we do not know anything about the
recharge area, we can use the world
average for q, which is 85% (0.85).
• 0.65 – 0.75
for karst systems
• 0.75 – 0.90
for sediments with finegrained carbonate such as loess
• 0.90 – 1.00
for crystalline rocks
Chemical mass-balance
• Closed system model: no exchange between
DIC and soil CO2
mDICrecharge
q = ───────────
mDICsample(final)
• m = concentration in moles/liter
• mDICrecharge is measured at the recharge area or
calculated from estimated PCO2-pH conditions. If
the present climate differs significantly from that
during the infiltration, then the calculation is
rather speculative.
Chemical mass-balance 2
• Calculation by chemical data
mDICfinal = mDICrecharge +[mCa2+ + mMg2+ mSO42- + ½(mNa+ + mK+ - mCl-)]
m = concentration in moles/liter
d13C mixing model 1
• Closed system model at low pH
d13Csample - d13Ccarb
q = ───────────────,
d13Csoil CO2 - d13Ccarb
Where
d13Csample = measured in groundwater DIC
d13Ccarb = 0 ‰ (calcite being dissolved)
d13Csoil CO2 = -23 ‰
d13C mixing model 2
• Closed system model at any pH
d13Csample - d13Ccarb
q = ───────────────,
d13Crecharge - d13Ccarb
Where
d13Crecharge = d13Csoil CO2 + e13CDIC-CO2(soil)
e: enrichment factor
• Depends highly on pH and on temperature
e13CA-B = (RA / RB - 1)*1000 ‰,
Fontes-Garnier model
• Open and closed system dissolution are
considered
• mDICcarb = mCa + mMG –mSO4 + ½(mNa
+ mK –mCl)
• This DIC consists of two parts:
• dissolved in open system: C-14 exchange with soil
CO2
• dissolved in closed system (C-14 dead)
• mDICCO2-exch = (d13CmeasxmDICmeas - d13CcarbxmDICcarb d13Csoilx(mDICmeas – mDICcarb)/(d13Csoil - e13CCO2(soil)13C
d
carb)
CaCO3
• this may be negative
• qF-G = (mDICmeas – mDICcarb + mDICCO2-exch)/ mDICmeas
Uncertainity
(Triassic) Bunter sandstone, England
Bath et al. 1979
Problem
Data got on well water in Hungary
• Tritium: 3 TU
• d18O = -10,7 [‰]VSMOW
• 14C-content: 30 pmC
• What is your opinion about this water?
Clorine-36: 36Cl
Chlorine isotopes
35Cl
= 75.4% stable
36Cl = radioactive, 301 000 year half-life
37Cl = 24.6% stable
Sources of 36Cl
• Natural: collision of cosmic neutron and
35Cl atom.
• Subsurface or epigenic production?
• Anthropogene: mostly nuclear bomb tests
in sea water.
Terminology
• R36Cl= number of 36Cl atoms per/Cl
• A36Cl=number of 36Cl atoms/liter
• Evaporation:
– R36Cl = constant
– A36Cl increase
• Dissolution of „old” chlorine:
– R36Cl decrease
– A36Cl = constant
Decay
At = A0e-lt
Initial activity of 36Cl
• A0 is determined by the geomagnetic
latitude
• Minimum at 0 and 90 degrees
• Maximum at 40 degrees
• You must take into account the distance
from the sea
• You have to create 36Cl/Cl in precipitation
map
• AMS is used for the measurement
• Sampling is very simple
• Geochemical modelling is necessary:
dissolution of 36Cl-free chlorine (this is a
most problematic part)
• Age range up to 1.5 million years
Krypton-81: 81Kr
Krypton-81: 81Kr
•
•
•
•
•
81Kr
is produced in the upper atmosphere
by cosmic-ray-induced spallation of five
heavier Kr isotopes, i.e. from 82Kr to 86Kr.
Or by neutron capture:
80 Kr + n → 81 Kr + g
36
36
No significant subsurface production.
No appreciable anthropogenic source.
Half-life is 229 000 years.
Age range: from 35 000 to 670 000 years.
Krypton-81: 81Kr (cont.)
• The decay equation is:
81Kr = 81Kr ×e-lt
t
0
• The 81Kr concentration is expressed as
number of atoms/liter
• 81Kr0 = 1100 atoms/L: initial value in
modern groundwater
• E.g. 81Kr = 900 atoms/L
• t = -(ln(900/1100)/l = 66 297 a
Krypton-81: 81Kr (cont.)
• The 81Kr concentration can be
expressed as percent of modern
atmosphere (similar to 14C)
• R/Rair = (81Kr/Kr)sample/(81Kr/Kr)air in
percent
• E.g. 81Kr = 40%
• t = -(ln(40%/100%)/l = (ln(0.4)/(3.03*10-6) = 302 722 a
Krypton-81: 81Kr (cont.)
• Advantages:
– Anthropogenic sources are minimal.
– 81Kr is inert (no chemical reactions envolved)
• Disadvantages:
– Technical difficulties, 1 or 2 labs in the world.
– Limited experience (only 3 case studies worldwide)
Brines