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Simulation of Short Circuit Flow Paths and Transient Conditions to Understand
Vulnerability of Public Supply Wells to Contamination in the High Plains Aquifer,
York, Nebraska
Brian ClarkMatt Landon
Leon KauffmanGeorge Hornberger
4/12/2005
Take-Home Messages
• Wellbores drilled through confining units can act as “short circuits”
• Transient stresses provide opportunities for pulses of flow
• Combination of transient stresses and wellbores through confining units can allow contaminants to lower layers
Study Background
USGS National Water-Quality Assessment (NAWQA) program
NAWQA topical study: Transport of Anthropogenic and Natural Contaminants (TANC) to public supply wells
Study Background
USGS National Water-Quality Assessment (NAWQA) program
TANC goal: determine controls on movement of contaminants to public
supply wells
Study area
0 1 20.5 Kilometers
0 1 20.5 Miles
Potential contaminant sources
RCRA, LST, NPDES
Water / wastewater
Landfill / livestock
Superfund
!.
!.
!.
!.
Confined and unconfined water-levels
•many wells are commonly screened in unconfined and confined layers
460
465
470
475
480
485
4/1/03 10/1/03 4/1/04 10/1/04 4/2/05 10/2/05
Wat
er-l
evel
alt
itu
de,
met
ers
ShallowUnconfined(FP3-33)
Top of UpperConfined(FP3-130)
Date
Model discretization
Specific yield0.2 to 0.25
upper confinedsand
HK values 0.3 15 m/d
Specific Storage
1 x 10-4 7 x 10-7 per meter
Fine coarse
Transient stresses
• Simulation time– Sept 1944 to Sept 2004
• Seasonal stress periods– Irrigation pumpage off during winter
Transient stresses
•Multi-node wellsallow flow through wellbores
Halford, K.J. and Hanson, R.T., 2002
ob-1
475476477478479480481482483484485
1940 1950 1960 1970 1980 1990 2000
Year
Wat
er-l
evel
alt
itu
de,
met
ers
Hydrograph comparison
Simulated
Observedunconfined
ob-2
472473474475476477478479480481482
1940 1950 1960 1970 1980 1990 2000
Year
Wa
ter-
lev
el
alt
itu
de
, m
ete
rsHydrograph comparison
Simulated
Observedconfined
ob-3
475476477478479480481482483484485
1940 1950 1960 1970 1980 1990 2000
Year
Wat
er-l
evel
alt
itu
de,
met
ers
Hydrograph comparison
Simulated
Observedconfined
ob-4
475476477478479480481482483484485
1940 1950 1960 1970 1980 1990 2000
Year
Wat
er-l
evel
alt
itu
de,
met
ers
Hydrograph comparison
Simulated
Observedconfined
Hydrograph comparison
Simulated
Observed
G-013021
477478479480481482483484485486487
1940 1950 1960 1970 1980 1990 2000
Year
Wa
ter-
lev
el
alt
itu
de
, m
ete
rs
unconfined
Hydrograph comparison
Simulated
Observed
10N-3W-4AA-1
485486487488489490491492493494495
1940 1950 1960 1970 1980 1990 2000
Year
Wat
er-l
evel
alt
itu
de,
met
ers
unconfined
YEAR MEAN MIN MAX RMSMEAN ABS
COUNT
1964 0.08 -2.33 1.54 1.47 1.20 6
1974 -0.07 -2.88 1.42 1.29 0.95 9
1984 0.10 -2.18 2.53 1.42 1.00 10
1994 0.15 -2.60 1.75 1.31 1.07 12
2004 -0.26 -3.12 2.97 1.13 0.76 43
Residual Statistics
all units in metersResiduals =observed minus simulated
Transport model sub-grid
0 1 20.5 Kilometers
0 1 20.5 Miles
13 layers(2-14)
Version ofGWT tosupport MNW
61 rows122 columns
Age & CFC boundary by layer
observation wells
Age of water vs depth below water table 0
2
4
6
8
10
12
14
16
18
20
0 10 20 30 40 50Age, in years
Dep
th b
elo
w w
ater
tab
le,
in m
eter
s
Simulated ageMeasured age
(unconfined wells only)(measured= tritium-helium ages)
CFC concentration vs depth below water table
0
5
10
15
20
25
30
35
40
45
0 1 2 3 4CFC-11, in pmol/kg
Dep
th b
elo
w w
ater
tab
le,
in m
eter
s
Simulated CFCMeasured CFC
(confined wells only)
Conclusions
• Wellbores drilled through confining units act as “short circuits” for flow
• Simulated transient stresses provide pulses of flow
• Transient stresses and wellbores through confining units allow flow and contamination to lower layers
Contact information
Brian Clark - [email protected] Landon – [email protected] Kauffman – [email protected] Hornberger – [email protected]
4/12/2005