1

Recharge rates to deep aquifer layers estimated with

39Ar, 85Kr and 14C data: A case study in Odense

(Denmark)

Roland Purtschert

University of Bern,

Switzerland

Troels Bjerre,

Johann Linderberg

VCS

Denmark

Klaus Hinsby

Geological Survey of

Denmark and Greenland

Starting point, Objectives

Deep groundwater

Fast circulation, residence times of decades → vulnerabe

Slower circulation, RT of centuries -> less vulnerable?

Gra

ph

ics, U

SG

S

Increasing interest in pre-modern

groundwater due to

Overexploitation

Contamination

Effects of climate change

Investigation of potential and

limitations of tracers beyond the 50

years age limit.

Determination of recharge rates,

residence times and renewal rates

Combination of tracer methods and

numerical modelling

0 5 10 kilometers

Study site: Odense river catchment, Funen Denmark

Well fields

Photo Jan Kofoed Winther

Total area 1046 km2

Direction of

groundwtaer flow Odense Water Ltd is one of the largest water utilities in Denmark with a

groundwater abstraction permission of ~14 mill. m³/year

The groundwater is mainly abstracted from rather shallow aquifers

(quaternary sand deposits) on 7 wellfields around the city of Odense.

The catchments of the wellfields are dominated by conventional farming

and urban areas

The groundwater table is close to the surface and the low lying areas are

generally drained by tile drains

Topography

from Hansen et al, 2009

The catchment area

is characterised by

two high areas rising

to 130 m above sea

level divided by a

wide and shallow

depression

stretching NE–SW

Hydrogeology

0 5 10kilometers

Sand/gravel

Clayey till

Sandy till

Fractured Clay

Chalk

Several ice advances and subsequent ice retreats during the Weichselian glaciation have

formed the present landscape and geology (Houmark-Nielsen & Kjær 2003)

Main aquifers are found in semi-confined units of glaciofluvial sand and gravel deposits at

varying depths overlain by glacial till.

Tertiary marls and clay forms the lower boundary of the Quaternary aquifer system

Geology of the quaternary deposits is rather complex and heterogeneous

Interconnected hydrostatigraphic units with a typical thickness of 10-15 meter

Troldborg, 2004

6

Timescales of groundwater dating methods

(10.7 yr)

(269 yr)

(Half-life)

Dating range

Meth

od

s

years

Sampling in GAB, Australia

7

85Kr-39Ar: Key data

85Kr 39Ar

Half-life: 10.7 269

Decay mode: b b

Modern isotope ratio (2008) ~3·10-11 8.1·10-16

Atoms/Liter water 58‘000 8‘700

Activity (Bq/L water) 1.2 10-4 7.1 · 10-7 → 22 decays/yr

Input function

Water sample volume:

1-2 tons! Note: Water residence times are based on a isotope ratio (39Ar/Ar, 85Kr/Kr

etc) and are therefore (rather) insensitive to

Details of recharge conditions (the addition of excess air, recharge temperature)

Degree of degassing (both in nature and during sampling)

(In contrast to 3H/3He, SF6 etc that are based on absolute concentrations)

Sampling and detection method

Sampling for 85Kr, 39Ar and 14C

Active and passive shielding in underground lab

Water degassing in the field 39Ar and 85Kr activity measurement by

Low Level Counting

Spontaneous fission

U (n)

Th

Al, Mg..(α,n)

Subsurface

Production 39K(n,p)39Ar

39Ar production

Pore-

Water

p n

n

+

+

n n

p

n n

n

p p

p

n n

p n

n

-

n

nth

nt

h

+

e-

0

-

-

-

e-

e-

0

e+

e+

n

4n n

2

p

n

-

-

-

+

-

Atmospheric Production 40Ar(n,2n)39Ar

0.104 dpm/L Argon

(100% modern)

T1/2=269 years

cosmic rays 0

50

100

150

200

250

300

0 200 400 600 800 1000

groundwater residence time (years)

%m

od

ern

300%mod

200%mod

0%mod

100%mod

50%mod

Subsurface secular

equilibrium

39Ar depth profile

25 50 75 100 125 150 175 200100

80

60

40

20

0

Nort

hin

g

Easting

Odense

Holmehaven

Borreby

Lunde

Soeby

scre

en d

epth

(m

)

39Ar (% modern)

0.15m/yr

40.00

60.00

80.00

100.0

120.0

140.0

160.0

180.0

181.5

39Ar (%modern)

39Ar spatial distribution

25 50 75 100 125 150 175 200100

80

60

40

20

0

Nort

hin

g

Easting

Odense

Holmehaven

Borreby

Lunde

Soeby

scre

en d

epth

(m

)

39Ar (% modern)

0.15m/yr

40.00

60.00

80.00

100.0

120.0

140.0

160.0

180.0

181.5

39Ar (%modern)

25 50 75 100 125 150 175 200100

80

60

40

20

0

Nort

hin

g

Easting

Odense

Holmehaven

Borreby

Lunde

Soeby

scre

en

de

pth

(m

)

39Ar (% modern)

0.15m/yr

40.00

60.00

80.00

100.0

120.0

140.0

160.0

180.0

181.5

39Ar (%modern)

39Ar-85Kr depth profile

25 50 75 100 125 150 175 200100

80

60

40

20

0

Nort

hin

g

Easting

Odense

Holmehaven

Borreby

Lunde

Soeby

scre

en

de

pth

(m

)

39Ar (% modern)

0.15m/yr

40.00

60.00

80.00

100.0

120.0

140.0

160.0

180.0

181.5

39Ar (%modern)

0 10 20 30 40 50 60 70 80

100

80

60

40

20

0

mean s

cre

en d

epth

(m

)

85Kr (dpm/cc Kr)

mo

de

rn w

ate

r

39Ar-85Kr depth profile

25 50 75 100 125 150 175 200100

80

60

40

20

0

Nort

hin

g

Easting

Odense

Holmehaven

Borreby

Lunde

Soeby

scre

en d

epth

(m

)

39Ar (% modern)

0.15m/yr

40.00

60.00

80.00

100.0

120.0

140.0

160.0

180.0

181.5

39Ar (%modern)

0 2 4 6 8 10 12 14

100

80

60

40

20

0

mean s

cre

en d

epth

(m

)

85Kr (dpm/cc Kr)

DL

50

55

60

65

70

75

80

85

90

95

50 55 60 65 70 75 80 85 90 95

39Ar (%modern)

depth

(m

)

Single well age gradient

Vmean: 0.25 m/yr

Top screen

Bottom screen

Expected age distribution and dispersion in heterogeneous alluvial aquifer system

A wide rather than a piston piston-flow-like age distribution can be expected

because of

The heterogeneity of the system

The spatially distributed recharge

Mixing in the extended screen intervals of the extraction wells

WATER RESOURCES RESEARCH, VOL. 38, 2002

120

30

40

50

60

70

90120

20

30

40

507080120

0

2

4

6

8

10

12

14

16

18

20

20 40 60 80 100 120

39Ar (%modern)

85K

r (d

mp

/cc K

r)

Data

AD3

AD2

AD1

Age distribution: 39Ar and 85Kr data

0 100 200 300 400

pro

po

rtio

n

age (yrs)

(Tm=100 yr)

AD1

AD2

AD3

Assumed Age Distributions

Measured 85Kr and 39Ar activities can consistently be interpreted if dispersive mixing is

taken into account (AD2-AD3)

Modern 39Ar values in samples low in 85Kr are suspicious for produced underground

Detection limit 85Kr

Age spectra 39Ar-14C

0 500 1000 1500 2000

0

2

4

6

8

10

12

14

16

#

0 500 1000 1500 2000

0

2

4

#

39Ar (35 samples)

14C (7 samples)

350 yrs

<2000 yrs

Explanation?

Correted 14C ages (F&G Model)

Geochemical correction of 14C activities

0

50

100

200

300

400

500

600

800

1000

50000

20

40

60

80

100

120

20 25 30 35 40 45 50 55 60

14C (pmC)

39A

r (%

mo

dern

)

decay curve

piston flow

ages

2 Komponentmixing?

Geochemical Evolution

Geochemical correction

Two component mixing is not consistent (or at least very unlikely) within the

hydro-geologial context

Geochemical correction models are not sufficient to eliminate the discrepancy

between 39Ar and 14C ages

Diffusive exchange with stagnant zones (Sudicky, 1981)

5000

1000

800

600

500

400

300

200

100

50

0

0

20

40

60

80

100

120

20 25 30 35 40 45 50 55 60

14C (pmC)

39A

r (%

mo

dern

)

+ diffusion

decay curve

piston flow

ages

Time Steady State

b

a

L

Aquifer(active flow zone)

Aquitard (stagnant)

DL

Averages estimates (System of parallel layers; Sanford, 1997)

Porosity (Active & stagnant) : 0.3

Thickness flow zone: 20 m

Thickness stagnant zone: 40 m

Diffusioncoeff. (14C): 3.15 10-3 m2/yr

Diffusioncoeff. (39Ar): 2.6 10-3 m2/yr

Aquifer layers Aquitard layers

Mixing-Dispersion

0

50

100

200

300

400

500

600

800

1000

50000

20

40

60

80

100

120

20 25 30 35 40 45 50 55 60

14C (pmC)

39A

r (%

mo

dern

)

+ diffusion + dispersion

decay curve

piston flow

ages

0 100 200 300 400

pro

po

rtio

n

age (yrs)

(Tm=100 yr)

AD1

AD2

AD3

0

20

40

60

80

100

120

0 500 1000 1500 2000

Ac

tiv

ity

Time

Dispersive mixing reduces the

apparent decay rate of 39Ar relative to 14C

Subsurface Production of 39Ar

5000

1000

800

600

500

400

300

200

100

50

0

100

190

280

370

460550

10002000

0

20

40

60

80

100

120

20 25 30 35 40 45 50 55 60

14C (pmC)

39A

r (%

mo

dern

)

+ diffusion + dispersion

+ underground production

decay curve

piston flow

ages

Tracer model

ages

Assumed subsurface

secondary equilibrium:

30%modern

Summary: The combined contribution of isotope exchange with the aquifer

rocks, diffusive exchange with aquitards, dispersion and eventually underground

production resolves the discrepancy between 39Ar and 14C ages: The resulting

age span ranges between recent and 500-700 years (and not up to 2000 years

as indicated by 14C data)

Age spectra

0 100 200 300 400 500 600 700 800

0

5

10

15

20

240±270

80±84

196±90

0 100 200 300 400 500 600 700 800

0

5

10

15

20

Corrected 14

C-39

Ar tracer ages

Uncorrected 39

Ar ages

0 200 400 600 800

0

5

10

15

20

years

Numerical particle tracking ages

Age spectra

0 100 200 300 400 500 600 700 800

0

5

10

15

20

240±270

80±84

196±90

0 100 200 300 400 500 600 700 800

0

5

10

15

20

Corrected 14

C-39

Ar tracer ages

Uncorrected 39

Ar ages

0 200 400 600 800

0

5

10

15

20

years

Numerical particle tracking ages

Similar age range

Discrepancy at the lower age

limit

-60

-59

-58

-57

-56

-55

-54

-53

-8.7 -8.6 -8.5 -8.4 -8.3 -8.2 -8.1 -8.0 -7.9

d18

O (‰)

d2H

(‰)

0

10

20

30

40

50

60

70

80

90

100

-8.60 -8.50 -8.40 -8.30 -8.20 -8.10 -8.00 -7.90

d18

O (‰)

mean

dep

th b

elo

w s

urf

ace (

m)

Stable isotopes d18O-d2H

18

0.56 0.8d O

T Cd C

‰ / °C18

0.28 150100

d Oh m

m ‰ (Schrag, 1996)

(Poage , 1996) 50 yr

300 yr

600 yr

Time

IPC

C 2

00

1, M

an

n e

t al, 1

99

9

Water Budget

~1046 km2

Precipitation*: 600-1000 mm (av: 840mm)

Evaporation*: 500-600 mm (av. 550)

Recharge*: ~290 mm

Shallow layers

(< 50-60 m)

Deep layers

(>50-60 m)

Drain to river, (run off,

shallow aquifers*: 203 mm

Other boundaries, storage change*

Recharge to deep layers 40 mm

(from corrected 39Ar-14C ages)

303 (100%)

Volumes Mio m3 (%)

880

-575

212 (70%)

14 (4 %) Abstraction* (2006) 17 mm

42 (13%)

35 (13%)

* Data from Hansen, 2006

Spectral GammaLog

Well U 341

U Total K Th

Well U 341

U Total K Th

2 ppm 1% 7 ppm 2 ppm 1% 7 ppm

High 39Ar values: Hypothesis

27

Sand/gravel

Clayey till

Sandy till

Fractured Clay

Chalk

CR

25 50 75 100 125 150 175 200100

80

60

40

20

0

Nort

hin

g

Easting

Odense

Holmehaven

Borreby

Lunde

Soeby

scre

en

de

pth

(m

)

39Ar (% modern)

0.15m/yr

40.00

60.00

80.00

100.0

120.0

140.0

160.0

180.0

181.5

39Ar (%modern) 85Kr free

Conclusions

The relation between 85Kr and 39Ar indicates pronounced dispersive mixing in accordance with the heterogeneous structure of the aquifer.

Uncorrected 39Ar- and 14C-model age scales differ by ~1 order of magnitude (Note: 14C model ages consider geochemical corrections).

The consideration of a whole set of processes (rather than a single one) results in a consistent 14C- and 39Ar-age range between 20 years and ~700 years.

Underground production of 39Ar is locally relevant in particular in the northern part of the study area. A general assessment of this effect needs more detailed investigation.

Combined and integrated tracer data including 39Ar allow for the estimation of flow dynamic and recharge rate of groundwater in deeper aquifer layers that are out of range of transient tracers (3H/3He, SF6 etc).

Vertical age profiles have potential as climate records on the millennium scale (instead of horizontal “flow lines” with notorious limited sampling points)

29

Thank you

Spatial variation of recharge rates

BolbroBolbroBolbroBolbroBolbroBolbroBolbroBolbroBolbro

100 5

kilometer

HolmehavenHolmehavenHolmehavenHolmehavenHolmehavenHolmehavenHolmehavenHolmehavenHolmehaven

BorrebyBorrebyBorrebyBorrebyBorrebyBorrebyBorrebyBorrebyBorreby

Nr. SøbyNr. SøbyNr. SøbyNr. SøbyNr. SøbyNr. SøbyNr. SøbyNr. SøbyNr. Søby

LindvedLindvedLindvedLindvedLindvedLindvedLindvedLindvedLindved

DalumDalumDalumDalumDalumDalumDalumDalumDalum

EksercermarkenEksercermarkenEksercermarkenEksercermarkenEksercermarkenEksercermarkenEksercermarkenEksercermarkenEksercermarken

LundeLundeLundeLundeLundeLundeLundeLundeLunde

20 40 60 80 100 120 140 160 180 200

100

80

60

40

20

0

--

13

2.3

1.5

3

7.48.4

3.1

2.3

4

7.6 1

1.4

5

1.2de

pth

(m

)

39Ar (% modern)

20 40 60 80 100 120 140 160 180 200

100

80

60

40

20

0

de

pth

(m

)

39Ar (% modern)

from Dubgaard et al, 2007

Symbol size~85Kr activity