Recharge rates to deep aquifer layers estimated with 39 Ar, 85 Kr and - - PowerPoint PPT Presentation

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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?

Graphics, USGS

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

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

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

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Timescales of groundwater dating methods (10.7 yr) (269 yr) (Half-life)

Dating range Methods years

Sampling in GAB, Australia

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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

50 100 150 200 250 300 200 400 600 800 1000 groundwater residence time (years) %modern

300%mod 200%mod 0%mod 100%mod 50%mod Subsurface secular equilibrium

39Ar depth profile

25 50 75 100 125 150 175 200 100 80 60 40 20

screen depth (m)

39Ar (% modern)

0.15m/yr

39Ar spatial distribution

25 50 75 100 125 150 175 200 100 80 60 40 20

screen depth (m)

39Ar (% modern)

0.15m/yr

Northing Easting

Odense Holmehaven Borreby Lunde Soeby

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 75 100 125 150 175 200 100 80 60 40 20

screen depth (m)

39Ar (% modern)

0.15m/yr 10 20 30 40 50 60 70 80 100 80 60 40 20

85Kr (dpm/cc Kr)

modern water

39Ar-85Kr depth profile

25 50 75 100 125 150 175 200 100 80 60 40 20

screen depth (m)

39Ar (% modern)

0.15m/yr 2 4 6 8 10 12 14 100 80 60 40 20

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 90 120 20 30 40 50 70 80 120

2 4 6 8 10 12 14 16 18 20 20 40 60 80 100 120

39Ar (%modern) 85Kr (dmp/cc Kr)

Data AD3 AD2 AD1

Age distribution: 39Ar and 85Kr data

100 200 300 400 proportion 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

500 1000 1500 2000 2 4 6 8 10 12 14 16

#

500 1000 1500 2000 2 4

#

39Ar (35 samples) 14C (7 samples)

350 yrs <2000 yrs

Explanation?

Correted 14C ages (F&G Model)

Geochemical correction of 14C activities

50 100 200 300 400 500 600 800 1000 5000

20 40 60 80 100 120 20 25 30 35 40 45 50 55 60

14C (pmC) 39Ar (%modern)

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

20 40 60 80 100 120 20 25 30 35 40 45 50 55 60

14C (pmC) 39Ar (%modern)

+ diffusion decay curve piston flow ages Time Steady State

b a L

Aquifer(active flow zone) Aquitard (stagnant) D L  

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

50 100 200 300 400 500 600 800 1000 5000

20 40 60 80 100 120 20 25 30 35 40 45 50 55 60

14C (pmC) 39Ar (%modern)

+ diffusion + dispersion decay curve piston flow ages 100 200 300 400

proportion

age (yrs)

(Tm=100 yr)

AD1 AD2 AD3

20 40 60 80 100 120 500 1000 1500 2000

Activity 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 50 100 190 280 370 460 550 1000 2000

20 40 60 80 100 120 20 25 30 35 40 45 50 55 60

14C (pmC) 39Ar (%modern)

+ 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

20

240±270 80±84

100 200 300 400 500 600 700 800 5 10 15 20

Corrected

14C- 39Ar tracer ages

Uncorrected

39Ar ages

200 400 600 800 5 10 15 20

Age spectra

100 200 300 400 500 600 700 800 5 10 15 20

240±270 80±84 196±90

100 200 300 400 500 600 700 800 5 10 15 20

Corrected

14C- 39Ar tracer ages

Uncorrected

39Ar ages

200 400 600 800 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

d18O (‰) d2H (‰)

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

d18O (‰)

mean depth below surface (m)

Stable isotopes d18O-d2H

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0.56 0.8 d O T C d C       ‰ / °C

18

0.28 150 100 d O h m m     ‰

(Schrag, 1996) (Poage , 1996)

50 yr 300 yr 600 yr Time IPCC 2001, Mann et al, 1999

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

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Sand/gravel Clayey till Sandy till Fractured Clay Chalk

CR

25 50 75 100 125 150 175 200 100 80 60 40 20

screen depth (m)

39Ar (% modern)

0.15m/yr

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
• ne) 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)

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Thank you

Spatial variation of recharge rates

Bolbro Bolbro Bolbro Bolbro Bolbro Bolbro Bolbro Bolbro Bolbro

10 5

kilometer

Holmehaven Holmehaven Holmehaven Holmehaven Holmehaven Holmehaven Holmehaven Holmehaven Holmehaven Borreby Borreby Borreby Borreby Borreby Borreby Borreby Borreby Borreby

• Nr. Søby
• Nr. Søby
• Nr. Søby
• Nr. Søby
• Nr. Søby
• Nr. Søby
• Nr. Søby
• Nr. Søby
• Nr. Søby

Lindved Lindved Lindved Lindved Lindved Lindved Lindved Lindved Lindved Dalum Dalum Dalum Dalum Dalum Dalum Dalum Dalum Dalum Eksercermarken Eksercermarken Eksercermarken Eksercermarken Eksercermarken Eksercermarken Eksercermarken Eksercermarken Eksercermarken Lunde Lunde Lunde Lunde Lunde Lunde Lunde Lunde Lunde 20 40 60 80 100 120 140 160 180 200 100 80 60 40 20

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2.3 1.5 3 7.4 8.4 3.1 2.3 4 7.6 1 1.4 5 1.2

depth (m)

39Ar (% modern)

20 40 60 80 100 120 140 160 180 200 100 80 60 40 20

depth (m)

39Ar (% modern)

from Dubgaard et al, 2007

Symbol size~85Kr activity