Poorly known: Hydraulic conductivity K ... Example CFC plume Schriesheim ... h t.
Quotient can be determined: t tracer. : Mean tracer travel time. K, n e h h-∆h. L ...
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
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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)
Groundwater Modelling
Boundary flux
First: Calibration of flow model 12
Determination of Neckar infiltration Model 2 Increased infiltration; Porosity = 0.17 Computed Tritium conc.
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(Th. Metzger, K. Zoellmann)
Comparison measured/computed Tritium Model 2 Increased infiltration: Model test (calculated vs. measured)
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(Th. Metzger, K. Zoellmann)
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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(Th. Metzger, K. Zoellmann)
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
Groundwater Modelling
-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
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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
Groundwater Modelling
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Uncertainty in input function
•
Freiburg series selected
25
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
Groundwater Modelling
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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