Post on 28-Dec-2015
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