When Does Aquifer Heterogeneity Matter? When Does Aquifer Heterogeneity Matter?

Predicting the Influence of Predicting the Influence of Alternative Conceptual Models Alternative Conceptual Models

on Contaminant Plume Migrationon Contaminant Plume Migration

A classroom exercise for Teaching A classroom exercise for Teaching Hydrogeology in the 21Hydrogeology in the 21stst Century Century

Milford, Michigan

Approximate limit of Pleistocene continental glaciation

Remedial Extraction Wells --

August 2003

Municipal Water Supply Wells

Conceptual ModelConceptual Model

870’

830’

VOM PW-1,2520 gpm

NS

900’

180 ft100 ft

Downward migration driven by gravity (recharge) and pumping.

DLZ-1,2,3300 gpm

Remedial pumping wells intended to intercept contaminant plume(s)

Conceptual Conceptual Model Model

UncertaintyUncertaintyAquitard geometry poorly Aquitard geometry poorly

constrained by eight constrained by eight wells – only two of wells – only two of which penetrate > 5’.which penetrate > 5’.

S N

DLZ PW-1, 3, 2

VOM PW-1,2

Aquifer

Aquitard

(a) Regional Aquitard Model

123456789101112131415

Alternative Conceptual ModelsAlternative Conceptual Models

65432

1

78

101112131415

S N

DLZ PW-1, 3, 2

VOM PW-1,2

9

Aquifer

Aquitard

A: Regional Aquitard Model

Alternative Conceptual ModelsAlternative Conceptual Models

65432

1

78

101112131415

S N

DLZ PW-1, 3, 2

VOM PW-1,2

9

Aquifer

Aquitard

B: Extended Aquifer Model

Question:Question:

870’

VOM PW-1,2

NS

900’

DLZ PW-1, 2, 3

830’

870’

830’

VOM PW-1,2

NS

900’

DLZ PW-1, 2, 3

If the two conceptual models suggest different flow paths for the contaminant plumes, will these differences be detectable in the extraction wells?

To Answer the question…To Answer the question…

Construct two flow and transport modelsConstruct two flow and transport models Regional AquitardRegional Aquitard Extended AquiferExtended Aquifer

Calculate contaminant breakthrough Calculate contaminant breakthrough curvescurves MTBEMTBE cDCEcDCE

Note: extraction well concentrations were measured in the combined effluent stream entering the air stripper at the site.

MTBE

Layer 1 – Shallow

Day 1

MTBE

Layer 1 – Shallow

Day 365

MTBE

Layer 1 – Shallow

Day 730

MTBE

Layer 1 – Shallow

Day 1095

MTBE

Layer 1 – Shallow

Day 1460

MTBE

Layer 1 – Shallow

Day 1825

VOM-PW-1/2

DLZ-PW1/2/3

Note how the plume evolves from the initial steady state condition after the remedial wells begin pumping. What does this imply about the transport path for the plume – is it becoming shallower or deeper after pumping begins?

Contaminant Transport Model (MT3DMS) output for MTBE in a shallow model layer.

Flux-weighted Average BTCs - MTBEConceptual Model Comparison

0

1

2

3

4

5

6

7

8

9

10

0 365 730 1095 1460 1825

Time (days from 7/30/2003)

MT

BE

Co

nce

ntr

atio

n [

pp

b]

Model A: Regional Aquitard

Model B: Extended Aquifer

Observed Data

Flux-weighted Average BTCs - MTBEConceptual Model Comparison

0

1

2

3

4

5

6

7

8

9

10

0 365 730 1095 1460 1825

Time (days from 7/30/2003)

MT

BE

Co

nce

ntr

atio

n [

pp

b]

Model A: Regional Aquitard

Model B: Extended Aquifer

Observed Data

Extraction Wells

Flux-weighted Average BTCs - cDCEConceptual Model Comparison

0

1

2

3

4

5

6

7

8

9

10

0 365 730 1095 1460 1825

Time (days from 7/30/2003)

cDC

E C

on

cen

trat

ion

[p

pb

]

Model A: Regional Aquitard

Model B: Extended Aquifer

Observed Data

Flux-weighted Average BTCs - cDCEConceptual Model Comparison

0

1

2

3

4

5

6

7

8

9

10

0 365 730 1095 1460 1825

Time (days from 7/30/2003)

cDC

E C

on

cen

trat

ion

[p

pb

]

Model A: Regional Aquitard

Model B: Extended Aquifer

Observed Data

Extraction Wells

870’

VOM PW-1,2

NS

900’

DLZ PW-1, 2, 3

830’

870’

830’

VOM PW-1,2

NS

900’

DLZ PW-1, 2, 3

Implication: Small differences in flow paths may not matter when the contaminant signal is integrated across long screen intervals.

Observation: Both conceptual models agree well with observed data.

Implications for Implications for Monitoring Well Monitoring Well

NetworkNetwork

Remedial Extraction Wells --

August 2003

Municipal Water Supply Wells

RL-1

What about monitoring wells with smaller screened intervals?

Question: Question: Could a monitoring well Could a monitoring well detect migration of the detect migration of the plume in response to plume in response to remedial pumping?remedial pumping?

RL-1

RL-1

0.0

1.0

2.0

3.0

4.0

1 2 3 4 5 6 7 8 9

Sample Qtr

Co

nce

ntr

atio

n (

pp

b)

cDCEAnswer: Answer:

Yes, if it is in the Yes, if it is in the right place.right place.

Question: Question: Does one end-member Does one end-member

conceptual model better fit conceptual model better fit the data than the other?the data than the other?

Answer: Answer: You decide!You decide!

RL-1 cDCE

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 365 730 1095 1460 1825

Days (from 7/30/03)

Co

nce

ntr

atio

n [

pp

b]

RL-1 (Calculated)/cDCE RegAq Pmp

RL-1 (Calculated)/cDCE ExtAq Pmp

RL-1 (Observed)/cDCE

Extended Aquifer

Regional Aquitard

Discussion QuestionsDiscussion Questions

Can end-member models be used to Can end-member models be used to evaluate the consequences of conceptual evaluate the consequences of conceptual model uncertainty?model uncertainty?

Can uncertainty be more important for Can uncertainty be more important for some modeling purposes than others?some modeling purposes than others? Integrated breakthrough curvesIntegrated breakthrough curves Performance monitoring networks Performance monitoring networks