14C dating of Gorleben groundwater

14C dating of Gorleben groundwater

G. Buckaua,*, R. Artingera, S. Geyerb, M. Wolfc, P. Fritzb, J.I. Kima

aInstitut fuÈr Nukleare Entsorgungstechnik, Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, GermanybUfZ-Umweltforschungszentrum Leipzig-Halle, D 06120 Halle, Germany

cGSF-National Research Center for Environment and Health, Institute of Hydrology, D-85764 Neuherberg, Germany

Received 19 July 1998; accepted 31 May 1999

Editorial handling by W.M. Edmunds

Abstract

Previous attempts to apply 14C for dating of groundwater in the Gorleben aquifer system has given results withcon¯icting 3H and stable isotope data and hydrological estimates. 14C model ages of 1±10 ka have been found for3H containing recharge water, up to 31 ka for groundwater with Holocene stable isotope signatures and 6±10 ka for

groundwater at 35 m depth. In this paper it is shown, that the reasons are assumption of to high 14C concentrationin recharge groundwater and not correcting for the in¯uence of 14C dilution by dissolved inorganic C (DIC) frommicrobiologically mediated mineralization of organic components in deep sediments. To overcome these di�culties a

new approach is applied evaluating the site-speci®c 14C source term (including the in¯uence of nuclear atmospherictesting), and the already previously used overall dilution of DIC. Closed system conditions are assumed and noisotopic fractionation is considered. For most of the groundwaters, the 14C ages achieved by the present method are

in agreement with 3H, stable isotopes and hydrological estimates. It is shown that down to approximately 140 mdepth no 14C decay can be detected. Situations are also discussed, either where the 14C method is not applicable(shallow peat-bog like groundwater) or does not yield reliable groundwater ages (brines at <200 m depth). # 2000Elsevier Science Ltd. All rights reserved.

1. Introduction

Within the German program for nuclear waste dis-posal, the Gorleben salt dome has been investigated asa candidate site for more than 20 a. To ensure long-

term safety, amongst others, the radiological impactfrom a release of long-lived radionuclides into theoverlieing aquifer system needs to be assessed.

Thereby, adequate understanding of the hydrologicalsituation is required. For this purpose of hydrological

modeling, groundwater dating is essential. Despitelarge e�orts, there is still a great deal of uncertaintyabout the origin and age of the Gorleben ground-

waters. Groundwater dating by stable isotopes, Noblegas temperatures and 14C lead to inconclusive or con-tradicting results. The aim of this paper is to showthat, contrary to previous experiences, 14C ground-

water dating can be performed on many of thegroundwaters. It is also the aim to show where this isnot the case due to lack in understanding of origin and

geochemical conditions.Application of the 14C groundwater dating relies on

dissolved inorganic C (DIC) as a groundwater tracer.

The age is calculated based on the di�erence betweenthe 14C concentration of DIC in a given groundwater

Applied Geochemistry 15 (2000) 583±597

0883-2927/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PII: S0883-2927(99 )00071-2

* Corresponding author. Tel.: +49-7247-824-461; fax: +49-

7247-826-998.

E-mail address: [email protected] (G. Buckau).

and its concentration at the time of the recharge (the

source term). This approach encounters two majorproblems, namely (i) quanti®cation of the source term,and (ii) decrease of the 14C concentration by mixing

with DIC from various sources (generally with zero14C concentration) due to geochemical reactions in thecourse of the groundwater evolution.

The 14C concentration in DIC upon primaryrecharge is normally set to the level of atmospheric

CO2 prior to nuclear atmospheric testing, i.e. around100 pmc ( percent modern C referred to 94.9 % of theactivity concentration of the NBS oxalic acid standard

in 1950) (Pearson and White, 1967; Pearson and Han-shaw, 1970). In the atmosphere, a deviation of the 14C

concentration from 100 pmc is expected from changesin the earth's magnetic ®eld and activity of the sun.Climatic changes may also in¯uence the 14C concen-

tration in di�erent global reservoirs (atmosphere, bio-sphere and oceans) (Stuiver et al., 1991; Stuiver andBraziunas, 1993). Consequently, the 14C age may devi-

ate from the actual groundwater age (Pearson et al.,1983). The di�erences increase steadily to greater than

3 by 25 ka BP. For groundwater dating, however, suchdi�erences are negligible compared to other uncertain-ties. A much more dramatic impact resulted from the

atmospheric nuclear testing leading to a peak valueapproaching 200 pmc in the northern hemisphere

around 1963±1966 (Moser and Rauert, 1983; Levinand Kromer, 1997). Therefore, it is di�cult to deter-mine the 14C age of the primary recharge of a pre-at-

mospheric nuclear testing groundwater. A directin®ltration of DIC from atmospheric CO2 via precipi-tation is much less than dissolution of soil-CO2, which

originates from root-respiration and turnover of soilorganic material, including humus and peat. The 14C

concentration of primary recharge, i.e. DIC of organicorigin at shallow depth, depends on the 14C concen-trations of such di�erent sources and their contribution

to DIC. The 14C concentration of DIC from root-res-piration will directly re¯ect the atmospheric concen-tration. Through turnover of relatively old humus or

peat, the 14C concentration of primary recharge DIC islowered (Taylor, 1998).

Once the primary recharge DIC is generated, di�er-ent processes may lead to its isotopic modi®cation, es-pecially dilution by various sources. Published

examples on such processes are: (1) dissolution of 14Cfree sedimentary carbonate by the primary recharge

Cic acid (MuÈ nnich, 1957; Ingerson and Pearson, 1964);(2) on repeated dissolution and precipitation, the car-bonate sediments may contain 14C (Geyh, 1972; Fritz

et al., 1978); (3) ion exchange of Ca2+ and Mg2+

against Na+ in clay minerals, leading to dissolution ofCaCO3 (Fritz and Mozeto, 1981); (4) incongruent dis-

solution of dolomite (Matthess, 1990); (5) degassing ofCO2 (Eichinger, 1981); (6) dissolution of CaSO4 result-

ing in calcite precipitation through excess concen-

trations of Ca2+ (Eichinger, 1981); (7) changes insaturation concentrations via weathering of feldspar(Vogel and Ehhalt, 1963); (8) dissolution of sedimen-

tary carbonate by organic acids, such as humic andfulvic acids (Vogel and Ehhalt, 1963); (9) admixing ofvolcanic or magmatic CO2 (Fritz and Mozeto, 1981;

Eichinger, 1981); (10) microbiologically mediated CH4

generation (Barker et al., 1979); and (11) microbiologi-

cally mediated oxidation of organic C via dissolved O2

or nitrate (Stumm and Morgan, 1996) or SO4 (Ceder-strom, 1946; Pearson and White, 1967; Pearson and

Hanshaw, 1970). Thereby, DIC and dissolved organicC (DOC) are generated in groundwater (Buckau et al.,

2000).In order to cope with the di�erent sources leading to

modi®cation of the isotopic C composition of DIC,

di�erent models have been developed which considerrelevant chemical processes and di�erences in the 13Csignatures. The simplest approach of chemical mixing

takes account of the dissolution of sedimentary car-bonates by primary recharge DIC (MuÈ nnich, 1957;

Ingerson and Pearson, 1964; Tamers, 1975). Extendedchemical mixing models utilize the chemical speciationby PHREEQE (Parkhurst et al., 1980) modi®ed for

appraisal of the impact of DIC on the 14C content(Wolf and Rhode, 1992; Wolf et al., 1993). The sim-

plest isotopic mixing model utilizes di�erent 13C con-centrations of DIC from primary recharge and theadmixed DIC from carbonate sediments. Such an

approach is applicable only for the two source systemwhen their 13C signatures are known. More complexisotopic mixing models attempt to account for ad-

ditional impacts from isotopic fractionation and isoto-pic exchange (Mook, 1976; Reardon and Fritz, 1978;

Fontes and Garnier, 1979). Some models combine iso-topic mixing with hydrochemical groundwater evol-ution (Wigley et al., 1978; Plummer et al., 1990, 1994).

One source of additional DIC in groundwater isthe mineralization/oxidation of sedimentary organic

C (SOC). The groundwater age indicative primaryrecharge DIC and the 14C free DIC from mineraliz-ation of SOC originate from organic sources and thus

cannot be distinguished from each other by the 13Ccontent. Therefore, in aquifer systems where this pro-cess is of considerable magnitude, radiocarbon dating

based on 13C to quantify the primary recharge DICbecomes falsi®ed. In above models this process is not

accounted for. If su�cient data are available, includingchemical composition of groundwater, isotopic compo-sition of dissolved C containing species and mineralogy

of sediments, the in¯uence of such processes can beaccounted for along a given ¯ow-path (Aravena et al.,1995). In the Gorleben aquifer system adequate miner-

alogical data are missing. Furthermore, detailed knowl-edge of the hydrological situation is not available and

G. Buckau et al. / Applied Geochemistry 15 (2000) 583±597584

considerable mixing of groundwater is expected

(Buckau et al., 1999). Therefore, in this paper another

approach is used.

As mentioned above, the aim of the paper is to

develop a method for 14C groundwater dating of

the Gorleben groundwater but also to show where

lack of information makes such dating unreliable.

For this purpose, the impact of mineralization of SOC

is accounted for by regarding the overall dilution of

DIC from recharge to a given groundwater, irrespec-

tive of the DIC source (Ingerson and Pearson, 1964;

Pearson and White, 1967; Pearson and Hanshaw,

1970; Wigley et al., 1978; Pearson, 1991). The

approach furthermore makes use of a site-speci®c 14C

source term, taking into account impact of atmos-

pheric nuclear testing. The result is plausible ground-

water ages instead of the excessive ages found by other

approaches.

2. The Gorleben aquifer system

The Gorleben aquifer system is situated above a Per-mian salt dome at Gorleben in the North German

Plain (Lower Saxony) (Fig. 1). The aquifer systemextends to 280 m below the ground surface in

reworked and deformed Miocene and Pleistocene gla-cial sediments. Below approximately 200 m depth the

so-called Gorleben channel is found with brines indirect contact with the salt dome. The main recharge

area is located south of the salt dome. Signi®cant 3Hcontents <1 TU (i.e. groundwater with ages <40 a)

are only found in groundwater samples down to adepth of 25 m (Suckow, 1993). After sinking to lower

aquifer layers, groundwater is expected to ¯ow mainlytowards the NW. At depths greater than approxi-

mately 200 m, the Gorleben channel becomes a pre-ferred ¯owpath running to the NE. The sediments

Fig. 1. Gorleben site with sampling locations.

G. Buckau et al. / Applied Geochemistry 15 (2000) 583±597 585

contain local Miocene brown coal and Pleistocene peat

deposits and groundwater has a�ected the mineraliz-

ation of SOC. By this process, DOC and DIC are sim-

ultaneously generated resulting in enhanced DICconcentrations in conjunction with DOC concen-

trations reaching <200 mg C/L (Fig. 2). The process

is driven by microbiologically mediated SO4 reduction

leading to enhanced DOC concentrations and SO4 de-pletion, especially in groundwaters between 50 and 170

m (Fig. 3). Details can be found in Buckau et al.

(1999, 2000).

In this paper, the groundwaters are classi®ed as (see

Table 1; Artinger et al., 1999): (i) recharge; (ii) tran-

sition; (iii) enhanced DOC; (iv) channel brines; and (v)

uplift-mixing groundwater. Five recharge groundwatersfrom depths <25 m have signi®cant 3H content and

DOC concentrations of 2.2 mg C/L or less. Two tran-

sition groundwaters from depths of around 30 and 70

m have low DOC concentrations similar to therecharge samples, but no signi®cant 3H content. Chan-

nel brines (3 samples, depths <200 m) also have low

DOC concentrations. Two groundwaters at depths of

65±73 m show moderately enhanced DOC concen-trations, and another six groundwaters at 35±137 mdepth have highly enhanced DOC concentrations.These eight groundwaters are called enhanced DOC

groundwaters. The uplift-mixing Gohy-341 is found atshallow depth (10±13 m). Contrary to the aboverecharge groundwaters, however, it has a high DOC

concentration (64 mg C/L). The salt content of thisgroundwater is dominated by NaCl originating fromsalt dome dissolution at great depth. However, this

groundwater also has some 3H and a 14C concen-tration indicative of recent C sources. Therefore, thisgroundwater is considered as a mixture of old deepgroundwater and recent recharge.

3. Methodology

For 14C determination, carbonate-free NaOH andBaCl2. H2O was added to at least 50 l of water col-

lected in polyethylene containers ¯ushed with N2 or Arto exclude atmospheric CO2. The closed containerswere allowed to stand overnight prior to separation of

BaCO3 precipitate under inert-gas. After CO2 gener-ation from BaCO3 and benzene synthesis (Eichinger etal., 1980), 15 to 60 mg Butyl-PDB scintillator (Packard

& Co.) was added to 1 to 4 mL benzene, and 14C wasmeasured by liquid scintillation counting (Quantulus,LKB-Wallac). The experimental detection limit is

Fig. 2. DOC versus DIC concentrations in Gorleben ground-

water. Through mineralization of sedimentary organic C,

DOC (consisting mainly of humic and fulvic acids) and DIC

are in situ generated. The direct relationship between DOC

and DIC falls into two groups; at lower pH additional sedi-

mentary carbonate is dissolved (for details, see Artinger et al.,

1999; Buckau et al., 1999, 2000) (®gure from Buckau et al.,

1999).

Fig. 3. SO4 depletion and in situ generation of DOC in con-

junction with mineralization of sedimentary organic C.

Shaded lines show values/region where SO4 concentrations

are expected from mixing of deep brines and recharge ground-

water. SO4 depletion and enhanced DOC concentrations are

found especially between 50 and 170 m (®gure from Buckau

et al., 1999, where details can be found). Lower limit for sul-

phate reduction from Appelo and Postma (1996).


Table

1

Sampled

Gorleben

groundwaters,samplingdepths,

pH,DIC

concentration

and

isotopic

characterization

ofgroundwaterand

DIC

.Analyticalerrors

(2s);

2H:21-,

18O:

20.15-

and

13C:2

0.4

-

Sample

No.

Sample

Sampling

depth

(m)

pH

DOC

(mgC/L)

d2H

(-)a

d18O

(-)a

3H (TU)b

22s

DIC

(mmol/L)

d13C

(-)c

14C

(DIC

)

(pmc)

d22s

Rechargearea

1Gohy-181

15±18

6.4

0.5

ÿ62.3

ÿ8.852

0.08

38.7

22.7

0.67

ÿ20.6

45.7

20.9

2Gohy-411

17±20

7.0

2.2

ÿ61.7

ÿ8.422

0.11

20.7

21.5

1.54

ÿ17.9

54.2

23.3

3Gohy-421

10±13

6.0

1.9

ÿ62.2

ÿ8.612

0.06

20.0

21.5

0.84

ÿ23.1

68.5

24.5

4Gohy-611

21±24

8.4

1.4

ÿ62.0

ÿ8.542

0.19

8.0

20.7

1.06

ÿ13.3

26.5

21.7

5Gohy-711

6±9

6.0

0.9

ÿ64.4

ÿ9.082

0.09

29.2

22.1

0.59

ÿ22.4

70.4

21.4

Transitionarea

6Gohy-182

70±73

8.0

0.8

ÿ63.6

ÿ8.902

0.04

<0.28

1.07

ÿ13.4

27.9

20.6

7Gohy-201

30±35

7.8

0.9

ÿ62.3

ÿ8.852

0.28

<1.1

0.79

ÿ13.6

26.5

22.1

EnhancedDOC

8Gohy-412

65±68

7.6

7.6

ÿ57.6

ÿ8.40

<0.23

3.66

ÿ6.8

8.6

20.5

9Gohy-492

35±38

8.0

127

ÿ58.1

ÿ8.462

0.22

<0.17

11.69

ÿ11.1

2.1

21.2

10a

Gohy-572(I)

70±73

8.8

14.4

ÿ58.1

ÿ8.552

0.16

0.502

0.12

4.75

ÿ4.0

4.6

21.8

10b

Gohy-572(II)

70±73

````

ÿ59.8

0.582

0.19

4.75

ÿ4.1

5.2

21.1

11a

Gohy-573(I)

134±137

8.0

97.2

ÿ59.7

ÿ8.622

0.20

<0.13

8.15

ÿ9.1

4.0

20.7

11b

Gohy-573(II)

134±137

````

ÿ61.5

<0.40

8.15

ÿ9.2

3.7

20.8

12

Gohy-612

121±125

8.4

184

ÿ60.4

ÿ8.592

0.24

0.522

0.21

13.34

ÿ11.9

1.9

20.8

13

Gohy-2211

83±85

8.1

93.6

ÿ59.6

ÿ8.622

0.08

0.322

0.13

9.52

ÿ10.0

5.2

20.8

14

Gohy-2227

128±130

7.8

73.4

ÿ62.4

ÿ8.852

0.10

<0.3

8.17

ÿ12.8

5.0

20.8

Channel

brines

15

Gohy-193

220±223

6.8/7.1

e3.2

ÿ72.2

ÿ9.862

0.04

<0.20

4.76

ÿ14.3

1.8

20.3

16

Gohy-514

235±238

6.3/7.0

e1.6

ÿ80.7

ÿ10.312

0.29

<0.25

3.65

ÿ12.8

5.1

21.4

17

Gohy-653

216±219

6.5/7.1

e1.3

ÿ67.7

ÿ9.482

0.07

<0.26

4.39

ÿ12.5

7.6

20.7

Uplift-m

ixing

18

Gohy-341

10±13

7.0

64.0

ÿ58.8

ÿ8.362

0.09

0.412

0.12

5.49

ÿ15.1

25.0

21.8

aRel.V-SMOW

(meanvalues

ofmultiple

samplingbetween1979and1993).

bTU

3H

unit(1

TU=

0.118Bq/L).

cRel.V-PDB.

dPercentmodernC

(94.9%

of14C

activityconcentrationin

NBSoxalicacidstandard

in1950).

epH

correctedforsaltin¯uence

accordingto

Runde(1993).


between 0.6 and 0.7 pmc (2s ) (background <0.3 cpm,counting e�ciency approximately 82 %, counting time

1000 min). 3H was determined by liquid scintillationcounting of water or gas counting of propane afterelectrolytic enrichment of 3H by a factor of approxi-

mately 20 (Eichinger et al., 1980; Wolf et al., 1981;Wolf and Singer, 1991). The detection limit for thesetwo methods are 0.7 and 0.1±0.2 TU, respectively. 13C

of DIC and 2H and 18O of the water matrix was deter-mined by mass spectrometry. Bicarbonate wasmeasured by titration and DIC calculated from known

carbonate equilibrium data. More detailed descriptionof experimental procedure can be found in (Kim et al.,1995).

4. Results and discussion

Results in this paper are partly based on unpub-

lished data from `Bundesanstalt fuÈ r Geowissenschaftenund Rohsto�e` (BGR, Hannover, Germany), the `Bun-desamt fuÈ r Strahlenschutz` (BfS, Salzgitter, Germany)and `Institut fuÈ r Radiochemie der Technischen Univer-

sitaÈ t MuÈ nchen` (TU Munich, Germany).Results and discussion are divided into 5 parts: (i)

determination of groundwater age from 3H, 2H and18O data; (ii) comparison of these groundwater ageswith results from published 14C methods; (iii) decreasein the 14C concentration of DIC by dilution through

the mineralization of SOC; (iv) evaluation of thesource term, i.e. concentration and 14C content of pri-marily recharge DIC; and (v) 14C groundwater datingby the present approach.

4.1. Groundwater age ranges

Table 1 shows the results of the isotopic characteriz-ation of the groundwaters. Recharge groundwaterscontain considerable concentrations of 3H (between

approximately 20 and 40 TU) and thus ages of <40 acan be postulated. The 3H concentration of Gohy-611(8 TU) can be attributed to mixing of recent and older

groundwater. In the shallow (10±13 m) uplift-mixinggroundwater Gohy-341 3H is found in the order of 1% of the content in recently recharged groundwaters.Amongst the groundwaters at depth below 30 m, the

transition waters and channel brines have 3H concen-trations below detection limits of the applied methods.Of the enhanced DOC groundwaters, Gohy-572, -612

and -2211 have 3H concentrations between 0.32 and0.54 TU. These 3 samples are from 70±125 m abovethe salt dome. Other enhanced DOC groundwaters in

this area, from 128±137 m depth (Gohy-573 and -2227), have 3H concentrations below detection limit.The same is true for the enhanced DOC groundwaters

Gohy-412 (65±68 m) (from the SW part of therecharge area south of the salt dome) and Gohy-492

(35±38 m) somewhat north of the other enhancedDOC groundwaters (cf. Fig. 1). The good reproducibil-ity of sampling and measurement can be seen from

repeated sampling of Gohy-572 and -573 (Table 1).While the positive 3H indications for 3 of the enhancedDOC samples may indicate the presence of a young

component, it is also possible that they are due to de-®ciencies in the well casing. In summary, for the pur-pose of establishing groundwater age ranges, 3H

concentrations <15 TU are allocated to a ground-water age <40 a, <1.1 TU to <40 a and 8 TU inGohy-611 to 140 a.Another indicator for the groundwater age is the 2H

and 18O content where precipitation will mainly varyalong the global meteoric water line (GMWL(d 2H=8xd18O+10) (Craig, 1961)). Deviation may

occur through, for example fractionation via evapor-ation of surface water. In Fig. 4, the 2H and 18O con-centrations are plotted. For reference to young

recharge groundwaters (<40 a) from comparablerecharge conditions, data from the Fuhrberg aquifersystem are also shown (Kim et al., 1995). Furthermore

data from a broader study with more emphasis ondeep brines are shown (Suckow, 1993). The bulk of thedata fall close to and somewhat below the GMWL.Groundwaters with 18O signatures more positive than

approximately ÿ9.1- can be allocated to Holoceneorigin (<10 ka). Some data referring to groundwaters

Fig. 4. 2H and 18O concentrations in Gorleben groundwaters

and Fuhrberg groundwaters for reference to young recharge

groundwaters from comparable recharge conditions.


Table

2

Gorleben

groundwaters

withageranges

given

by

3H,2H

and

18O

contents

(cf.Table

1)andthegroundwaterages

determined

byapplicationofvariouspublished

andfrequently

applied

14C-D

ICmethods(from

Kim

etal.,1995)(agein

a)

Sample

No.

Sample

Agerangea

(from

3H,2H

and

18O)

Vogel,1970

(A0=

85pmc)

Tamers,1975

Ext.chem

.mix

b

Model

with

phreequedata

Isotopic

mixing

model

Modi®ed

isotopic

mixingmodel

Fritz

etal.(1989);

withmeasureddata

Eichinger

model

(1981);with

measureddata

Eichinger

model

(1981);with

phreequedata

Schaefer

model

(1989);with

phreequedata

Fontesand

Garnier

(1979)

Rechargearea

1Gohy-181

<40

5000

4000

4000

6000

5000

7000

7000

8000

4000

2Gohy-411

<40

4000

1000

1000

3000

3000

4000

4000

6000

1000

3Gohy-421

<40

2000

2000

2000

3000

3000

5000

5000

6000

2000

4Gohy-611

140

10,000

5000

5000

6000

6000

7000

7000

7000

5000

5Gohy-711

<40

2000

2000

2000

3000

2000

4000

4000

5000

2000

Transitionarea

6Gohy-182

40±10,000

9000

5000

5000

6000

6000

7000

7000

7000

5000

7Gohy-201

40±10,000

10,000

6000

6000

6000

6000

7000

7000

7000

6000

EnhancedDOC

8Gohy-412

40±10,000

19,000

15,000

15,000

9000

9000

8000

8000

11,000

15,000

9Gohy-492

40±10,000

31,000

26,000

26,000

26,000

25,000

26,000

26,000

26,000

26,000

10a

Gohy-572(I)

40±10,000

24,000

19,000

20,000

9000

9000

4000

4000

14,000

20,000

10b

Gohy-572(II)

40±10,000

23,000

18,000

18,000

8000

8000

4000

4000

13,000

19,000

11a

Gohy-573(I)

40±10,000

25,000

21,000

21,000

18,000

18,000

18,000

18,000

19,000

21,000

11b

Gohy-573(II)

40±10,000

26,000

22,000

22,000

19,000

19,000

19,000

19,000

20,000

22,000

12

Gohy-612

40±10,000

31,000

27,000

27,000

27,000

26,000

27,000

27,000

27,000

27,000

13

Gohy-2211

40±10,000

23,000

19,000

19,000

17,000

17,000

17,000

17,000

17,000

19,000

14

Gohy-2227

40±10,000

24,000

19,000

19,000

20,000

19,000

20,000

20,000

20,000

19,000

Channel

brines

15

Gohy-193

110,000

32,000

28,000

28,000

29,000

29,000

30,000

30,000

30,000

29,000

16

Gohy-514

<12,000

23,000

19,000

19,000

20,000

19,000

20,000

20,000

20,000

22,000

17

Gohy-653

110,000

20,000

16,000

16,000

16,000

16,000

17,000

17,000

17,000

18,000

Uplift-m

ixingarea

18

Gohy-341

Mix

10,000

7000

7000

8000

7000

9000

9000

9000

7000

aForthechannel

brines

ageranges

mayalsobedi�erent,dependingontheassumptionsmadeconcerning

2H

and

18O

isotopesignaturesofPleistocenerecharge(see

text).

bmix:mixture

ofold

groundwaterfrom

depth

andyoungrecharge.


with less negative isotope signatures from depths of67±121 m below sea level fall below the GMWL

(upper right part of Fig. 4) possibly indicating an ori-gin with considerable impact from surface reservoirevaporation.

With respect to de®ning the 2H and 18O isotopicrange signifying groundwaters of Pleistocene origin(<12 ka), absolute reference values are missing. From

Fig. 4 it appears reasonable to assume that 18O valuesmore negative than about ÿ11- represent Gorlebengroundwaters of Pleistocene origin with mixed ground-

waters in the intermediate region between ÿ9.1 andÿ11-. On this basis one may expect Gohy-514 to bemainly of Pleistocene origin and Gohy-193 and -653 tobe mixtures of Pleistocene and Holocene origin.

The age ranges suggested by 3H and stable isotopedata are listed in the third column of Table 2. In therecharge area, 3H containing groundwaters at 6±24 m

depth are expected to have ages <40 a, possibly witholder components in Gohy-611. The transition andenhanced DOC groundwaters between 30 and 137 m

are expected to have ages between 40 and 10 ka. Asmentioned above, a meaningful groundwater age esti-mation for the uplift-mixing groundwater Gohy-341

appears questionable. For the channel brines Gohy-193 and -653 the expected ages can be set to the tran-sition between Pleistocene and Holocene (110 ka)whereas Gohy-541 is expected to be of Pleistocene ori-

gin (<12 ka).

4.2. Comparison with the results from published 14C

methods

Groundwater ages calculated using di�erent models/methods are listed in Table 2 (Kim et al., 1995). The14C concentrations are shown in Table 1 and necessary

chemical data can be found in Artinger et al. (1999),Buckau et al. (2000) and Kim et al. (1995). Comparedto the above established age ranges, large di�erences

are found. With few exceptions the calculated ages aremuch too great. Furthermore, the 14C ages vary con-siderably, re¯ecting di�erences in the models applied.

For recharge groundwaters, calculated ages range from1 to 10 ka compared to <40 a indicated by the 3Hcontent. For the transition groundwaters Gohy-182and -201, the results do not necessarily contradict the

given groundwater age ranges. Considering that 3Hcontaining recharge groundwaters are found down to24 m depth and the recharge groundwater Gohy-201 is

found at depth of 30±35 m, the model 14C agesbetween 6000 and 10 ka appear to be very high. Theenhanced DOC groundwaters are all expected to be of

Holocene origin, whereas the 14C results presented inTable 2, with few exceptions, indicate Pleistocene ori-gin. With respect to the channel brines, the 14C results

and age limits for Gohy-514 do not contradict eachother, for Gohy-193 and -653, however, this appears

to be the case.The reasons for the large discrepancies may be

sought in two sources, namely (i) the 14C concentration

in primary recharge is normally postulated to be 100pmc in contrast to more probable lower values due tosoil-CO2 from both root respiration and turnover of

older humus; and (ii) in enhanced DOC groundwaters,in addition to 14C decay and dilution of 14C with DICof inorganic sediment origin, the 14C concentration is

lowered by the mineralization of 14C free sedimentaryorganic C (SOC).

4.3. Dilution of DIC by the mineralization of SOC

In the Gorleben groundwaters three sources of DIC

need to be considered (Fig. 5). These are (i) the pri-mary recharge which is the 14C containing age indica-tive tracer originating from organic sources in rechargewater; (ii) mineralization of organic sediments; and (iii)

dissolution of sedimentary carbonate. Many 14Cgroundwater dating methods rely to a large extent onthe 13C signature to quantify the impact of dilution by14C-free DIC. Thereby, dilution is assumed to occurthrough dissolution of carbonate sediments, normallywith d13C 1 0 for marine carbonate. Upon mineraliz-

ation of SOC, DIC of organic origin with a di�erent13C signature is introduced. Consequently, the use ofd13C 1 0 to quantify the degree of dilution with 14C

Fig. 5. Major sources of DIC in groundwater where dilution

of primary recharge is governed by dissolution of carbonate

sediments and the mineralization of sedimentary organic C.13C values can vary somewhat around the numbers given. The14C value of 1100 pmc for root respiration refers to pre-

nuclear atmospheric testing conditions.


free DIC will be erroneous. Because the mineralizationof SOC appears in conjunction with in situ generation

of aquatic humic substances, the mineralization processis expected for enhanced DOC groundwaters. Ingroundwaters where precipitation or decomposition of

aquatic humic and fulvic acids has occurred, althoughthe mineralization process may have occurred theenhanced DOC concentration cannot be used as an in-

dicator.To judge the applicability of the 13C signature for

14C groundwater dating, a scheme is applied plotting

the 13C and DIC concentrations against each other (cf.below). For this purpose the 13C concentration of pri-mary recharge DIC is required. For closed conditionsisotopic fractionation need not be regarded (Taylor,

1998) and thus primary recharge DIC will have thesame 13C concentration as the source material (d13C=ÿ 27-) (Kim et al., 1995). In Fig. 6, d13C values of

shallow groundwater from the Fuhrberg and Munichaquifer systems are plotted against pH (Kim et al.,1995), together with data from recharge and transition

water of the Gorleben aquifer system (Table 1). Fuhr-berg sediments do not contain carbonates and thusDIC is only of organic origin. Furthermore no isotopic

fractionation is seen. Due to di�erent degrees of min-eral dissolution, pH varies between values of precipi-

tation and a value of approximately 6. In the Munichaquifer system 98 % of sediments consist of calcite

and dolomite. In these shallow waters (0.5 to 23 m),the primary recharge DIC leads to dissolution of equalamounts of DIC from sediments. Consequently, these

pH neutral waters have 13C concentrations exactlybetween the two sources. In Gorleben recharge andtransition waters, pH appears to become progressively

shifted to higher values both by dissolution of carbon-ate and non- carbonate containing sediments. The

Fig. 7. (a) 13C signatures plotted against the DIC concen-

trations. The dilution curve represents groundwater conditions

where mineralization of sedimentary organic C (SOC) is of

negligible magnitude. Values above the dilution curve in gen-

eral represent groundwaters considerably a�ected by the min-

eralization of SOC, accompanied by in situ generation of

dissolved organic C (DOC). (b) 13C signatures plotted against

the DIC concentrations. (a)±(d) represent di�erent possible

mixing lines (see text). Vertical shaded lines represent 0.51

mmol/L primary recharge DIC (d13C= ÿ 27-) in Gohy-181,

-182, -611 and -711, and 0.56 mmol/L DIC in Gohy-411 and -

421 (and possibly Gohy-341).

Fig. 6. 13C and pH of Gorleben recharge and transition

groundwaters and shallow groundwaters from the Fuhrberg

and Munich aquifer systems. The lower horizontal line

(d13C=ÿ 13.5-) represents equal contributions from primary

recharge DIC (d13C=ÿ 27-) and DIC from sedimentary car-

bonate (d13C=0).


trend, however, makes it a plausible assumption that

the primary recharge is not subject to isotope fraction-ation but has a d13C value of around ÿ27-.In Fig. 7a, d13C is plotted against the DIC concen-

tration for the Gorleben groundwaters. Data fallingclose to the curve representing dilution of primary

recharge DIC with 14C free DIC from marine carbon-ate sediments may be allocated to a common geochem-ical evolution history and are not signi®cantly a�ected

by the mineralization of SOC. In contrast to this,groundwaters found above this curve are eithera�ected by the mineralization of SOC or have an ori-

gin in signi®cantly di�erent recharge conditions. InFig. 7b an alternative representation of the 13C content

versus DIC is used. The 13C signature is plottedagainst the inverse of the DIC concentrations. By thisrepresentation, mixing of two DIC sources with di�er-

ent 13C signatures fall on a straight line. The disadvan-tage is that at low DIC concentrations the scale isstretched whereas at high concentrations it is com-

pressed.In Fig. 7b, a number of possible mixing processes

are shown by straight lines. Line (a) shows that therecharge and transition groundwaters Gohy-181, -182,-611 and -711 can be represented by 0.51 mmol/l pri-

mary recharge DIC which becomes diluted by DICwith d13C=0. Enhanced DOC can be represented byaddition of DIC with a d13C value of ÿ13.5- to well

developed recharge/transition water (b). Such a sourceresults from oxidation of SOC to carbonic acid fol-

lowed by dissolution of equal amounts of sedimentarycarbonate (with d13C=0). Gohy-2227 shows a morenegative d13C value consistent with the somewhat

lower pH value than the other highly enhanced DOCsamples, i.e. the carbonic acid of organic origin is notfully compensated by dissolution of sedimentary car-

bonate. The brines cluster around line (c), representingmixing of a well developed recharge/transition water

and DIC of organic origin (d13C= ÿ 27-). The chan-nel brines, however, are separated by several km dis-tance and between recharge/transition waters and the

deep channel brines enhanced DOC groundwaters arefrequently found (Buckau et al., 1999). Therefore, theassumption of an origin of the channel brines in a

common type of well developed recharge/transitionwater, without in¯uence of mineralization of SOC

during sinking to greater depths, appears very unlikely.The shallow uplift-mixing water Gohy-341 and the

recharge waters Gohy-411 and -421 can be represented

by a common mixing line. This line (d) represents mix-ing of 0.56 mmol/l primary recharge DIC and DIC

with d13C= ÿ 13.5-. Gohy-411 and -421 originatefrom further south than the other recharge waters, anda DIC contribution from shallow mineralization of or-

ganic sediments followed by dissolution of sedimentarycarbonate may occur due to local conditions. In the

uplift-mixing water Gohy-341, located west of the saltdome, this process is much more pronounced. The

high DOC concentration of this water is also indicativeof considerable mineralization of organic sediments(Buckau et al., 1999). In comparison with the

enhanced DOC groundwaters, Gohy-341 shows amuch higher 14C concentration, showing that relativelyyoung C sources are involved in the mineralization

process (Kim et al., 1995).In summary, the mineralization of SOC has nor-

mally not been taken into account and thus di�culties

in evaluation of 14C groundwater ages are encountered.In the Gorleben aquifer system this mineralization ofSOC is of considerable importance and thus needs tobe taken into account. The 13C signature can support

evaluation of the DIC inventory, however, it cannotdistinguish between age indicative primary rechargeDIC and non-14C containing DIC of organic origin

from deep sediments. Therefore, for the enhancedDOC waters, the overall DIC concentration ratiosbetween the primary recharge and total DIC ground-

water concentrations is used. In the channel brines thegeochemical history of DIC appears unclear and thus14C groundwater dating may give questionable results.

In the up-lift mixing groundwater Gohy-341, the 14Cconcentration re¯ects generation of DIC from shallowsources and thus 14C dating appears meaningless.Another problem is related to the fact that, on using

frequently applied published methods, even young 3Hcontaining groundwaters are found to have agesbetween 1 and 10 ka. This leads to the question of the

source term for the 14C groundwater dating.

4.4. 14C source term

For groundwater dating, the 14C concentration of

primary recharge at the time of recharge is required asa starting point. The age indicative 14C concentrationof primary recharge will be governed by the relative

contributions from young sources, namely root respir-ation re¯ecting atmospheric 14C concentrations andturnover of old humus via microbial activity. Already

in the recharge groundwaters, the 14C concentration ofDIC may decrease by dilution through dissolution ofsedimentary carbonates. This process can be quanti®edvia the di�erences in the d13C values of the two

sources.The shallow waters in the recharge area may be

divided into two di�erent groups (cf. above and

Fig. 7b). This re¯ects di�erences in present localconditions. For evaluation of a source term onemust consider that (i) the absolute di�erence in the

primary recharge DIC concentrations is not verylarge, (ii) local recharge conditions may change withtime, and (iii) considerable intermixing of ground-


water takes place in this aquifer system (Buckau et

al., 1999) and thus the origin of deep groundwater

is probably a mixture from di�erent recharge lo-

cations. For evaluation of the age indicative primary

recharge DIC, 13C is used to calculate the original

recharge contribution of organic origin in all recharge

and transition waters. In Table 3, the DIC concen-

trations and d13C values are given. Applying a d13Cvalue of ÿ27- for primary recharge and zero for sedi-

mentary carbonate, admixing of DIC of inorganic ori-

gin is corrected for. With respect to the primary

recharge DIC concentration, the results for recharge

and transition groundwaters can be summarized as

0.602 0.21 mmol/L. On proceeding similarly the 14C

value for primary recharge DIC is obtained (Table 3).

A closer look on the recharge and transition

groundwaters reveals that the calculated 14C concen-

trations of primary recharge are in¯uenced by fall-out

from atmospheric nuclear testing. In Fig. 8, the 14C

concentrations of primary recharge DIC are plotted

against the groundwater 3H concentrations. In this

®gure also data from Suckow (1993) are shown,

including several samples with low 3H concentrations.

To allow comparison of data from di�erent sampling

years (1990±1994) all 3H data have been normalized

to, 1994. Linear correlation result in a 14C concen-

tration of primary recharge, corrected for atmospheric

nuclear testing. For comparison, data from Fuhrberg

groundwaters (cf. above) are also shown. The values

are scattered around the correlation for Gorleben

recharge groundwaters and thus support the ®ndings

in Gorleben.

The data points in Fig. 8 can be divided into three

groups, (1) those resulting in above linear correlation,

(2) groundwaters not considerably in¯uenced by fall-

out, and (3) some groundwaters with signi®cant 3H

concentrations deviating considerably from liner re-

lationship with 14C. The groundwaters are sampled

within a period of 4 a and thus correction for decayshould adequately re¯ect variations in recharge 3Hconcentrations. The reasons for considerable deviations

in some samples is not clear. The overall conclusion,however, is that prior to atmospheric nuclear testingthe 14C concentration of primary recharge in the twocomparable aquifer systems Gorleben and Fuhrberg is

not even close to the frequently assumed approxi-mately 100 pmc but is somewhat above 50 pmc.In summary, in Gorleben the primary recharge is

calculated to consist of 0.6020.21 mmol/L DIC with

Table 3

Gorleben recharge and transition groundwaters: DIC concentration and its 13C and 14C signatures, and 13C and 14C concentrations

for primary recharge of DIC. Primary recharge refers to DIC excluding contribution originating from sedimentary carbonate. Cor-

rection is made via 13C with a d13C of ÿ27- for DIC of organic origin (primary recharge) and dC13 of zero for DIC originating

from sedimentary carbonate

Sample DIC

(mmol/L)

d13C(DIC)

(- (rel. PDB))

14C

(DIC)

(pmc)22s

DIC

(primary recharge)

(mmol/L)

14C

(DIC (primary recharge))

(pmc)

Gohy-181 0.67 ÿ 20.6 45.720.9 0.51 59.9

Gohy-182 1.07 ÿ 13.4 27.920.6 0.53 56.2

Gohy-201 0.79 ÿ 13.6 26.522.1 0.40 52.6

Gohy-411 1.54 ÿ 17.9 54.223.3 1.02 81.8

Gohy-421 0.84 ÿ 23.1 68.524.5 0.72 80.1

Gohy-611 1.06 ÿ 13.3 26.521.7 0.52 53.8

Gohy-711 0.59 ÿ 22.4 70.421.4 0.49 84.9

0.6020.21 67.02 14.5

Fig. 8. Impact of atmospheric testing on the 14C concen-

tration of primary recharge DIC in Gorleben groundwaters.

Open symbols are not regarded for evaluation of 3H and 14C

originating from nuclear atmospheric testing. Data from the

Fuhrberg aquifer system with comparable conditions are

shown for comparison (for explanation, see text).


a 14C content of 54.221.7 pmc, corrected for the e�ectof atmospheric nuclear testing.

4.5. Evaluation of groundwater age

As already discussed above, after recharge, the 14Cconcentration will decrease not only by decay, but alsoby dilution with DIC from dissolution of carbonate

sediment and mineralization of SOC. To visualize theimpact of overall dilution of DIC, irrespective of thedilution source, the 14C concentration of DIC is

plotted against the total DIC concentration in Fig. 9.The primary recharge is set to 54.2 pmc at a DIC con-centration of 0.60 mmol/L (see above). From this pri-

mary recharge a dilution curve is plotted for theexpected 14C concentration on the assumption that no14C decay has occurred. The results for the enhanced

DOC groundwaters fall around the dilution curve,suggesting no signi®cant 14C decay. The channel brineGohy-653 also falls on the dilution curve and thus no14C age is found. Contrary to this, Gohy-193 and -514

fall somewhat below the dilution curve. The deviationof the experimental value from the dilution curveresults in the 14C age of approximately 4 ka for Gohy-

514 and approximately 10 ka for Gohy-193.The uplift-mixing Gohy-341 falls well above the di-

lution curve re¯ecting di�erent geochemical conditions.

If applying the DIC contribution from organic origin(cf. primary recharge), 3.07 mmol/L DIC with a 14Cconcentration of 44.7 pmc is found. The high DICconcentration is the result of mineralization of organic

sediment, accompanied by a high DOC content. This

14C concentration is rather close to the 54.2231.7 pmcfound for the primary recharge of Gorleben ground-

waters. The NaCl content is similar to the enhancedDOC groundwaters Gohy-2211 and -2227 (from 80±85and 128±130 m depth, respectively and the 3H concen-

tration is similar to the enhanced DOC groundwatersGohy-572, -612 and -2211 (Table 1). Therefore, onemay assume that at least major parts of this ground-

water is relatively old. The DIC, however, is obviouslygenerated at shallow depth, and thus 14C groundwaterdating of this water is not applicable.

In Table 4, the groundwater age ranges and resultsfrom published methods (cf. Table 2) are showntogether with the present results. Also shown areresults for the enhanced DOC groundwaters applying

a conservative value of 83.1 pmc for the 14C content ofrecharge DIC (Buckau et al., 1997) and channel brinesages resulting from radiocarbon dating on aquatic ful-

vic acids (Artinger et al., 1996). In general the resultsfrom published 14C methods show unrealistically highgroundwater ages, frequently contradicting stable iso-

tope data and 3H. The present analysis shows that thetransition and enhanced DOC groundwaters at depthsbetween 30 and 137 m are so young that decay of 14C

cannot be quanti®ed accurately.Present 14C dating results for the channel brines are

not consistent with the given age limits obtained by 2Hand 18O. Application of 14C dating on aquatic fulvic

acids (Artinger et al., 1996) gives somewhat lowgroundwater ages compared to 2H and 18O data, butthe trend seems to be reasonable (Table 4). The dis-

agreement between the 14C dating on DIC and onaquatic fulvic acid leads to the conclusion that a re-liable 14C groundwater dating for the channel brines

presently is not possible. The isotope signatures in theGorleben channel show various layers of groundwaterof Holocene and Pleistocene origin and layers of var-ious salinity are assumed to ¯ow in di�erent directions.

The contradicting results of groundwater dating re¯ectthe unclear geohydrological and geochemical situationin the Gorleben channel.

5. Summary and conclusions

The developed 14C groundwater dating method

makes use of (i) a site speci®c pre-atmospheric nucleartesting primary recharge DIC as the age indicative tra-cer, and (ii) overall dilution of primary recharge DIC

in deep groundwater. The DIC concentration of thesite speci®c primary recharge and its 14C concentrationis calculated via the d13C values of recharge and tran-

sition groundwaters, that are not signi®cantly in¯u-enced by the mineralization of SOC. The impact offall-out 14C in primary recharge is corrected for via

Fig. 9. 14C concentrations of DIC in Gorleben enhanced

DOC, uplift-mixing and channel brine groundwaters plotted

against the total DIC concentration. The dilution curve rep-

resents the 14C concentrations expected for no 14C decay.


3H. The site speci®c DIC recharge, relevant for pre-at-

mospheric nuclear testing conditions, is found to be

0.602 0.21 mmol/L DIC with a 14C concentration of

54.221.7 pmc.

Application of the present approach to the

enhanced DOC groundwaters sampled between 35

and 137 m depth leads to the conclusion that these

groundwaters are so young that signi®cant 14C

decay cannot be detected. These results are very di�er-

ent from those found by application of published

methods (groundwater ages reaching up to 31 ka). For

these enhanced DOC groundwaters, the present study

leads to groundwater ages in compliance with the age

limits derived by the 2H and 18O signatures.

In the Gorleben channel, groundwater dating of

brines by di�erent methods (including the present

study) gives widely di�erent and inconsistent results.

In view of the previously known discrepancies and irre-

gularities, including in isotope data and lack of suc-

cessful modeling of hydrological conditions in the

Gorleben channel, the large di�erences in groundwater

dating results are not surprising. The present studyunderlines that a reliable groundwater dating can only

be achieved when the hydrological conditions for thestudied site are better known and adequate knowledgeof geochemical processes is available. In the shallow

peat-bog like uplift-mixing groundwater Gohy-341, the14C concentration re¯ects generation of DIC from nearsurface organic sources but not the origin of thegroundwater. Consequently groundwater dating by 14C

is not applicable.

Acknowledgements

The authors acknowledge the ®nancial support of

the `Bundesministerium fuÈ r Bildung, Wissenschaft,Forschung und Technologie`. We also acknowledge the`Bundesamt fuÈ r Strahlenschutz` for making the Gorle-

ben data base available. Thanks are due to H. Siela�and D. Wesselow of the DBE at the Gorleben site fortheir assistance in the groundwater sampling, and C.

Table 4

Age of investigated Gorleben groundwaters: ranges of groundwater age from 3H, 2H and 18O (Table 2) and results from appli-

cation of published, frequently used 14C groundwater dating methods (Table 2), results from application of primary recharge of

0.60 mmol/L DIC with a 14C concentration of 83.1 pmc (not accounting for fall-out) (Buckau et al., 1997), 14C dating of aquatic

fulvic acid (Artinger et al., 1996) and results of the present study (groundwater age in a)

Sample Sample No. Age range

(cf. Table 2)

Frequently applied methods

(cf. Table 2)

Buckau et al.,

1997

Artinger et al., 1997 This work, 1996

Recharge area

1 Gohy-181 < 40 4000±8000 recharge

2 Gohy-411 < 40 1000±6000 recharge

3 Gohy-421 < 40 2000±6000 recharge

4 Gohy-611 1 40 5000±10,000 recharge

5 Gohy-711 < 40 2000±5000 recharge

Transition area

6 Gohy-182 40±10,000 5000±9000 nsda

7 Gohy-201 40±10,000 6000±10,000 nsd

Enhanced DOC

8 Gohy-412 40±10,000 8000±19,000 3000 nsd

9 Gohy-492 40±10,000 25,000±31,000 5000 nsd

10 Gohy-572 40±10,000 4000±24,000 6000 nsd

11 Gohy-573 40±10,000 18,000±26,000 3000 nsd

12 Gohy-612 40±10,000 27,000±31,000 5000 nsd

13 Gohy-2211 40±10,000 17,000±23,000 1 0 nsd

14 Gohy-2227 40±10,000 19,000±24,000 1000 nsd

Channel brines

15 Gohy-193 1 10,000 28,000±30,000 4000 10,000

16 Gohy-514 < 12,000 19,000±23,000 11,000 4000

17 Gohy-653 1 10,000 16,000±20,000 6000 nsd

Uplift mixing

18 Gohy-341 mixb 7000±10,000 not applicable

a nsd: no signi®cant decay of 14C detected.b mix: mixture of deep and shallow groundwater where DIC is generated mainly at shallow depth.


Kardinal (Institute of Radiochemistry, TU MuÈ nchen)and H. Halder (GSF-Institute of Hydrology) for their

technical assistance. The isotope measurements werecarried out under the supervision of W. Rauert and P.Trimborn at the GSF-Institute of Hydrology. The

Authors would also like to thank F.J. Pearson Jr.(PSI, Villingen) for his valuable comments.

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14C dating of Gorleben groundwater

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