old groundwaters istván fórizs ph.d. institute for geochemical research, hungarian academy of...
TRANSCRIPT
Old groundwaters
István Fórizs Ph.D.Institute for Geochemical Research,
Hungarian Academy of SciencesBudapest
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 paleo-groundwaters
• These waters were infiltrated at cooler climatic conditions during the Ice Age.
• Their D and 18O 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 post-date 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 = D – 8*18O
-140
-120
-100
-80
-60
-40
-20
0
20
40
-18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4
18O [‰]
D [
‰]
Effect of relative humidity (h) of the air:Primary evaporation
Global Meteoric Water Line
100%85%
50%
Sea water
-140
-120
-100
-80
-60
-40
-20
0
20
40
-18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4
18O [‰]
D [
‰]
Secondary evaporation
GMWL
20%
40%60%
80%
20%
40%60%
80%
100%
Initial water (lake or rain drop)
Continental effectContinental effect
18OSea
Continent
vapour
vapour vapour
rainrain
(Triassic) Bunter sandstone, EnglandBath et al. 1979
-120
-110
-100
-90
-80
-70
-60
-16 -15 -14 -13 -12 -11 -10
δ18O
δ2H
GMWL SPRING RIVER BORE
Ice cores show well the climate change
GISP2Ice core,
Greenland
0
5000
10000
15000
20000
25000
30000
-45 -40 -35 -30 -25
18O [‰]VSMOW
kor
[év]
Age
(ye
ar)
Ice cores: Canada, Greenland, Antarctic
Chemistry and paleo-groundwaters
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-t
• A0 and At are 14C initial activity, and activity after time ‘t’, 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 + 1
0n → 146C + 1
1p
• 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-t
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: 13C value is from -33 to -20 [‰]VPDB
• Mean value= -27‰.
Photosinthesis
• C4 plants (5%): Hatch-Slack cycle
E.g. cane, maize
• 13C 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 13C value of soil CO2:
C3 vegetation: ≈ -23 [‰]VPDB
C4 vegetation: ≈ -9 [‰]VPDB
Carbon in water
• Source: air CO2 (13C ≈ -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)• CO3
2- (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
• CO2(aq)
• H2CO3
• HCO3-
• CO32-
} CO2(aq) ≡ H2CO3
}
}
}
εCO2(aq)-CO2(g) = -1.1‰
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 °CClark-Fritz 1997
The pathway of 14C to groundwater in the recharge environment
Correction methods
• Statistical
• Chemical mass-balance• 13C
• 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 fine-grained 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+ -mSO4
2- + ½(mNa+ + mK+ - mCl-)]
m = concentration in moles/liter
13C mixing model 1
• Closed system model at low pH
13Csample - 13Ccarb
q = ───────────────,13Csoil CO2 - 13Ccarb
Where13Csample = measured in groundwater DIC
13Ccarb = 0 ‰ (calcite being dissolved)13Csoil CO2 = -23 ‰
13C mixing model 2
• Closed system model at any pH
13Csample - 13Ccarb
q = ───────────────,
13Crecharge - 13Ccarb
Where
13Crecharge = 13Csoil CO2 + 13CDIC-CO2(soil)
: enrichment factor
• Depends highly on pH and on temperature
13CA-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 = (13CmeasxmDICmeas - 13CcarbxmDICcarb -
13Csoilx(mDICmeas – mDICcarb)/(13Csoil - 13CCO2(soil)-CaCO3 -
13Ccarb)
• this may be negative
• qF-G = (mDICmeas – mDICcarb + mDICCO2-exch)/ mDICmeas
Uncertainity
(Triassic) Bunter sandstone, EnglandBath et al. 1979
Problem
Data got on well water in Hungary
• Tritium: 3 TU
• 18O = -10,7 [‰]VSMOW
• 14C-content: 30 pmC
• What is your opinion about this water?
Clorine-36: 36Cl
Chlorine isotopes
35Cl = 75.4% stable36Cl = 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-t
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:
8036Kr + n → 81
36Kr + • 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:81Krt = 81Kr0×e-t
• 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)/ = 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%)/ =
-(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