Integration of Environmental Tracer Information into ... - Bioforsk

Workshop on Flowpath Characterisation, 29 June 2010 Neuherberg

Integration of Environmental Tracer Information into Groundwater Modelling Wolfgang Kinzelbach and Fritz Stauffer

Contents • Typical problems of groundwater flow models • Information contained in environmental tracer data • Use of environmental tracer data for groundwater flow modelling • Examples • Conclusions

Groundwater Modelling

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Typical problems of groundwater flow models • Non-uniqueness of calibration with heads. • Unknown fluxes (boundary flux, recharge, leakage). • No information on travel times and porosity. • Flow field too inaccurate for prediction of solute transport. Caution: Model use often flawed. Fit of heads alone is an unreliable measure of model quality. Do not trust the fluxes! Groundwater Modelling

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Basic Dilemma

vDarcy

A

Hydraulic information:

Q = A vDarcy= A K I

Known: Cross-sectional area A, Hydraulic gradient I Poorly known: Hydraulic conductivity K Same problem prevails in numerical flow models Groundwater Modelling

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Example CFC plume Schriesheim

?

? Groundwater Modelling

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Calibration of ratio of boundary fluxes

Can Environmental Tracers help?

Yes, sometimes.


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Tracers often used in water resources management Dating tracers and streamline tracers: • Tritium 3H • Tritium-Helium ratio 3H - 3He • Krypton-85 85Kr • Chlorofluorohydrocarbons CFHC (F11, F12, F103) • Carbon-14 14C • Oxygen-18 – Deuterium 18O - 2H Evaporation tracers: • Oxygen-18 – Deuterium 18O - 2H • Chloride from sea salt aerosols Cl Groundwater Modelling

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What environmental tracers can contribute • Indication that there is recent recharge. • Age of groundwater (within certain windows). • Time of travel and possibly of remediation. • Streamline information. • Ratios of fluxes. • Porosity, if fluxes are known. • Evaporation rate in the case of chloride. Groundwater Modelling

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Use of flux information from tracers River

Steady state water balance: Well

QWell= QRiver + QLand

QRiver = ?

Compute from 3 concentrations cWell, cRiver, cLand: cWell QWell= cRiver QRiver+ cLand QLand Groundwater Modelling

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1D tracer transport vDarcy  K

Darcy flux: Tracer velocity:

u

vDarcy

e

h L 

;

L ttracer

h

ttracer:

Mean tracer travel time

h-h

K, ne L

Quotient can be determined:

L2  Groundwater e hModelling ttracer K

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Residence time in spring / well catchment Catchment area A = recharge area

Aquifer, Thickness H Porosity 

Discharge rate for steady state conditions and constant parameters:

Q AN

Recharge rate N, Tracer input conc. cin

Spring / well discharge rate Q Tracer concentration c Groundwater Modelling

Mean residence time in saturated zone:

A H e H e ttracer   Q N 11

Determination of Neckar infiltration Tritium in recharge from precipitation

Neckar Tritium in Neckar

MA

Rhein

Recharge from Precipitation

Infiltration

HD

(Th. Metzger, K. Zoellmann)


Boundary flux

First: Calibration of flow model 12

Determination of Neckar infiltration Model 2 Increased infiltration; Porosity  = 0.17 Computed Tritium conc.


13


Comparison measured/computed Tritium Model 2 Increased infiltration: Model test (calculated vs. measured)


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Computed travel time & •

•

Fast check of pore velocities feasible But not as good as direct comparison of concentrations: - Mixing is neglected - There are two sources of 3H

3H-3He

age

Ex.: Piezometer Sundgauplatz

Meas. 13.8 a Comp. 16 a


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as tracer in Upper Muschelkalk Aquifer: Situation Ausstrich Oberer Muschelkalk Grundwasserhöhengleiche im mo/mm [mNN] Flüsse Verwerfungen

200

200

0 25

Meßstelle mit Nr.

22 5

0 30

Ludwigsburg

32 350 5

Pegel mit Nr.

22 5

5 27

Steinbruch mit Nr.

kf or

Leonberg

5 27

250

est

375 400

Bl ac

Schorndorf

Schwäbisch Gmünd

Stuttgart

Bad Cannstatt springs

Renningen

Weil der Stadt 425

400

Plochingen Sindelfingen

Göppingen

450

Böblingen Bad Boll

Herrenberg

42

5

Geislingen

Bad Ditzenbach Pliezhausen Metzingen

Nagold

Tübingen

Reutlingen 40 0

18  O

Rottenburg

37 5

350

Suebian Alb 32

30 0

275

Bad Urach

5


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J. Plümacher, 1999

18O as tracer in Upper Muschelkalk Aquifer: : Observed values 18 O-18 o] ) , SMOW O ( [%

O-18 measured at 51 stations Sauerstoff - 18 - gemessen - 51 Messstellen

-8.4 -8.6 -8.8 -9.0 -9.2 -9.4 -9.6 -9.8 -10.0 -10.2 -10.4 -10.6 -10.8


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J. Plümacher, 1999

Calibration of boundary fluxes using 18O-values This determines the ratio of boundary fluxes to recharge in the 2D flow model. -8.0

Calculated 18O-Werte O

18

400

Gemessene

Gemessene Piezometerhöhen [mNN] Measured head [m]

500

300

-9.0

-10.0

200

-11.0

100 100

200

300

400

500

Berechnete Piezometerhöhen [mNN]

-11.0

Calculated head [m]

J. Plümacher, 1999


-10.0

-9.0

-8.0

18O Calculated 18 O-Werte Berechnete

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Examples from literature • Many examples, where environmental tracer data are used to check flow models and/or to improve transport models, e.g., Wei et al. (1990), Solomon and Sudicky (1991), Smethie et al. (1992), Reilly et al. (1994), Engesgaard et al. (1996), Sheets et al. (1998), Zoellmann et al. (2001), Mattle et al. (2001), Weissmann et al. (2002), Moltyaner et al. (2002), Guell and Hunt (2003), Castro and Goblet (2003), Pint et al. (2003), Troldborg et al. (2007). • Hardly any automatic joint calibration of flow and transport models using environmental tracer data. Groundwater Modelling

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And what are the problems ? • • • • •

Input function not always well known. Age window limited. No information on Darcy flux. No area of recharge. Dissolved gas tracers yield different information than solute or markers of the water molecule. • Relevant porosity may not be a constant due to dual porosity effects. • Piezometric heads show momentary situation while tracer data integrate flow over longer time. Groundwater Modelling

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Basic Dilemma Revisited

vDarcy

A

Hydraulic information:

Tracer information:

Q = A vDarcy= A K I

Q = A e u = A e L/t

Known: Area A, Gradient I, Flowpath length L, Travel time t Poorly known: K Poorly known: Eff. Porosity e No method is perfect, but more independent constraints! Groundwater Modelling

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Basic Dilemma Revisited From hydraulic data:

Qmin,hydr

Qmax,hydr

Qmin,tracer

Qmin,joint

From tracer data: Qmin,tracer

Qmax,joint

Resulting estimate Groundwater Modelling

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Baltenswil study

Onnis, 2007 Groundwater Modelling

•

Baltenswil site, Zurich (Switzerland)

•

Unconfined sandygravel aquifer.

•

No rivers / creeks

•

Several pumping wells (PW) and springs.

•

2D transient model MODFLOW, MT3D, MT3D99 23

Baltenswil study

•

Unsaturated zone thickness 5m to 60m.

•

Residence time of tracers in unsaturated zone can be much larger 24 than in saturated zone. Groundwater Modelling

Baltenswil study: Kr-85 input


•

Uncertainty in input function

•

Freiburg series selected

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Onnis, 2007

Baltenswil study: Kr-85 transport simulations

Onnis, 2007

•

100 stochastic realizations of hydraulic conductivity field, conditional to head measurements.

•

85Kr

•

transport simulations in 1D vapour transport in unsaturated zone and 2D transport in aquifer. Large variability of


85Kr

measurements in pumping well.

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Baltenswil study: Tritium input 40.00

Konstanz data

Tritium concentration [TU]

35.00

Basel data 30.00

Vaduz data Tritium decay

25.00 20.00 15.00 10.00 5.00 0.00 1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

Year

•

Uncertainty in input function.

•

Konstanz (D) series selected. Groundwater Modelling

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2008

Baltenswil study: Tritium transport simulations

Onnis, 2007

• • • •

3H

simulation do not vary much among realizations. Does it confirm hydraulic conductivity? Does it confirm effective porosity? Groundwater Modelling Impact of tritium transport in unsaturated zone.

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Baltenswil study: Helium simulations Onnis, 2007

3He

•

transport simulations in 1D unsaturated zone transport and 2D transport in aquifer.

•

Sensitivity analysis and alternative conceptual models. Groundwater Modelling

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Why are environmental tracers not used more intensively in flow modelling? • High costs. • Lack of knowledge. • Problems of tracers as discussed. • Too high expectations led to frustration.


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Conclusions and Suggestions • Environmental tracer data allow to check and improve flow models. • Environmental tracers are the only available means to estimate effective porosity and travel times on field scale. • Combination of several tracers makes the application more reliable. • Use the model to check the consistency of all data collected (even proxi-data which do not enter the model directly) and to exclude hypotheses. • Use tracer information for getting ideas, not for getting accuracy. • Give range of alternative interpretations which are consistent with all observations. Groundwater Modelling

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