Old groundwaters István Fórizs Ph D Institute for

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 d. D and d 18 O 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 = d. D – 8*d 18 O

Effect of relative humidity (h) of the air: Primary evaporation Sea water 50% 85% 100% Global Meteoric Water Line

Secondary evaporation 20% 100% 80% 40% 60% 80% 20% Initial water (lake or rain drop) GMWL 40%

Continental effect vapour rain Sea d 18 O Continent

(Triassic) Bunter sandstone, England Bath et al. 1979

Ice cores show well the climate change

Age (year) GISP 2 Ice core, Greenland

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-HCO 3 type • Discharge area: – – high TDS frequently Na(-Ca)-HCO 3(-Cl-SO 4) type high p. H high trace element content

Groundwater dating methods

Groundwater dating methods • • • Radiocarbon: 14 C Chlorine-36: 36 Cl The uranium decay series Helium ingrowth Krypton-81: 81 Kr

Basis of 14 C age determination • Radioactive decay (discovered by Libby in 1946, Nobel Prize). • Half-life of 14 C is 5730 a (years). • Decay equation: At = A 0×e-lt • A 0 and At are 14 C initial activity, and activity after time ‘t’, l is decay constant.

Rearranged decay equation t = -8267×ln(At/A 0) [year]

T 1/2: Half-life Ao initial activity

Expression of 14 C activity • 14 C is expressed versus a reference, in percent modern carbon, pm. C. • Reference is the pre-industrial 14 C activity of atmospheric CO 2, that is regarded as 100%.

Source of 14 C • Natural: 147 N + 10 n → 146 C + 11 p • Where n = neutron, p = proton • Anthropogenic: nuclear bomb tests starting in 1952.

Natural variation in atmospheric 14 C

The calculated age • If we disregard the natural variation in atmospheric 14 C (A 0 is regarded to have been constant, as 100%), then the calculated age is radiocarbon years and not in calendar years.

Anthropogenic impacts on atmospheric 14 C

Correction: why needed? • During the flow path 14 C 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 14 C. • Initial 14 C activity: Arecharge = q* A 0, where q is dilution factor.

• Decay equation becomes: At = q. A 0 e-lt or t = -8267×ln(At/(q. A 0)) [year]

Short introduction to carbon stable isotope geochemistry

Abundance of carbon stable isotopes 12 C = 98, 9% 13 C = 1, 1%

13 C distribution in nature

13 C in C 3, C 4 and CAM plants

Photosinthesis • C 3 plants (85%): Calvin cycle E. g. trees, cereals, legumes (bean), beet. • C 3 plants: d 13 C value is from -33 to -20 [‰]VPDB • Mean value= -27‰.

Photosinthesis • C 4 plants (5%): Hatch-Slack cycle E. g. cane, maize • d 13 C value is -16 to -9 [‰]VPDB • Mean value: -12, 5‰.

13 C in soil CO 2 • Soil CO 2 originates from decomposition of organic material and root respiration. • The pressure of soil CO 2 gas is 10 -100 times higher than the atmospheric. • A part of soil CO 2 diffuses to the atmosphere causing isotopic fractionation: the remaining CO 2 is heavier by ca. 4‰. • The d 13 C value of soil CO 2: C 3 vegetation: ≈ -23 [‰]VPDB C 4 vegetation: ≈ -9 [‰]VPDB

Carbon in water • Source: air CO 2 (d 13 C ≈ -7 [‰]VPDB), or soil CO 2 ( -9‰ — -23‰) or limestone (0± 2‰) • • Carbonate species in water CO 2(aq) (aquatic carbondioxide) H 2 CO 3 (carbonic acid) HCO 3 - (bicarbonate ion) CO 32 - (carbonate ion) } DIC

Distribution of carbonate species as a function of p. H at 25 °C Clark-Fritz 1997

Isotopic fractionation at 25 °C • Soil CO 2 • CO 2(aq) • H 2 CO 3 • HCO 3 • CO 32 - } εCO 2(aq)-CO 2(g) = -1. 1‰ } CO 2(aq) ≡ H 2 CO 3 } εHCO 3(-)-CO 2(aq) = 9. 0‰ } εCO 3(2 -)-HCO 3(-) = -0. 4‰

Fractionation factors as a function of temperature • 103 lnα 13 CCO 2(aq)-CO 2(g) = -0. 373(103 T-1) + 0. 19 • 103 lnα 13 CHCO 3(-)-CO 2(g) = 9. 552(103 T-1) + 24. 10 • 103 lnα 13 CCO 3(2 -)-CO 2(g)= 0. 87(103 T-1) + 3. 4

Fractionation: 25 °C, DIC-CO 2(soil) Clark-Fritz 1997

Fractionation: DIC-CO 2(soil) at 25 °C Clark-Fritz 1997

The pathway of 14 C to groundwater in the recharge environment

Correction methods • Statistical • Chemical mass-balance • d 13 C • 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 CO 2 m. DICrecharge q = ────── m. DICsample(final) • m = concentration in moles/liter • m. DICrecharge is measured at the recharge area or calculated from estimated PCO 2 -p. H 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 m. DICfinal = m. DICrecharge +[m. Ca 2+ + m. Mg 2+ m. SO 42 - + ½(m. Na+ + m. K+ - m. Cl-)] m = concentration in moles/liter

d 13 C mixing model 1 • Closed system model at low p. H d 13 Csample - d 13 Ccarb q = ────────, d 13 Csoil CO 2 - d 13 Ccarb Where d 13 Csample = measured in groundwater DIC d 13 Ccarb = 0 ‰ (calcite being dissolved) d 13 Csoil CO 2 = -23 ‰

d 13 C mixing model 2 • Closed system model at any p. H d 13 Csample - d 13 Ccarb q = ────────, d 13 Crecharge - d 13 Ccarb Where d 13 Crecharge = d 13 Csoil CO 2 + e 13 CDIC-CO 2(soil)

e: enrichment factor • Depends highly on p. H and on temperature e 13 CA-B = (RA / RB - 1)*1000 ‰,

Fontes-Garnier model • Open and closed system dissolution are considered • m. DICcarb = m. Ca + m. MG –m. SO 4 + ½(m. Na + m. K –m. Cl) • This DIC consists of two parts: • dissolved in open system: C-14 exchange with soil CO 2 • dissolved in closed system (C-14 dead)

• m. DICCO 2 -exch = (d 13 Cmeasxm. DICmeas - d 13 Ccarbxm. DICcarb d 13 Csoilx(m. DICmeas – m. DICcarb)/(d 13 Csoil - e 13 CCO 2(soil)- d 13 Ccarb) Ca. CO 3 • this may be negative • q. F-G = (m. DICmeas – m. DICcarb + m. DICCO 2 -exch)/ m. DICmeas

Uncertainity

(Triassic) Bunter sandstone, England Bath et al. 1979

Problem Data got on well water in Hungary • Tritium: 3 TU • d 18 O = -10, 7 [‰]VSMOW • 14 C-content: 30 pm. C • What is your opinion about this water?

Clorine-36: 36 Cl

Chlorine isotopes 35 Cl = 75. 4% stable 36 Cl = radioactive, 301 000 year half-life 37 Cl = 24. 6% stable

Sources of 36 Cl • Natural: collision of cosmic neutron and 35 Cl atom. • Subsurface or epigenic production? • Anthropogene: mostly nuclear bomb tests in sea water.

Terminology • R 36 Cl= number of 36 Cl atoms per/Cl • A 36 Cl=number of 36 Cl atoms/liter • Evaporation: – R 36 Cl = constant – A 36 Cl increase • Dissolution of „old” chlorine: – R 36 Cl decrease – A 36 Cl = constant

Decay At = A 0 e-lt

Initial activity of 36 Cl • A 0 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 36 Cl/Cl in precipitation map

• AMS is used for the measurement • Sampling is very simple • Geochemical modelling is necessary: dissolution of 36 Cl-free chlorine (this is a most problematic part) • Age range up to 1. 5 million years

Krypton-81: 81 Kr

Krypton-81: 81 Kr • 81 Kr • • is produced in the upper atmosphere by cosmic-ray-induced spallation of five heavier Kr isotopes, i. e. from 82 Kr to 86 Kr. 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: 81 Kr (cont. ) • The decay equation is: 81 Kr = 81 Kr ×e-lt t 0 • The 81 Kr concentration is expressed as number of atoms/liter • 81 Kr = 1100 atoms/L: initial value in 0 modern groundwater • E. g. 81 Kr = 900 atoms/L • t = -(ln(900/1100)/l = 66 297 a

Krypton-81: 81 Kr (cont. ) • The 81 Kr concentration can be expressed as percent of modern atmosphere (similar to 14 C) • R/Rair = (81 Kr/Kr)sample/(81 Kr/Kr)air in percent • E. g. 81 Kr = 40% • t = -(ln(40%/100%)/l = (ln(0. 4)/(3. 03*10 -6) = 302 722 a

Krypton-81: 81 Kr (cont. ) • Advantages: – Anthropogenic sources are minimal. – 81 Kr is inert (no chemical reactions envolved) • Disadvantages: – Technical difficulties, 1 or 2 labs in the world. – Limited experience (only 3 case studies worldwide)

Brines