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

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

Study area

0 1 20.5 Kilometers

0 1 20.5 Miles

Potential contaminant sources

RCRA, LST, NPDES

Water / wastewater

Landfill / livestock

Superfund

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!.

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Study area

0 1 20.5 Kilometers

0 1 20.5 Miles

Recharge zones

Study area

0 1 20.5 Kilometers

0 1 20.5 Miles

Recharge zones

Multi-node pumping wells

Study area

0 1 20.5 Kilometers

0 1 20.5 Miles

Recharge zones

Observation wells

FP5 FP4FP3

FP1

Study area

0 1 20.5 Kilometers

0 1 20.5 Miles

Recharge zones

Transect

FP5 FP4FP3

FP1

Hydrogeology

unconfinedsand

upper confinedsand

Electrical Resistivitylogs

Well screens

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

unconfinedsand

upper confinedsand

Model discretization

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

Simulated age animation

1009080706050403020100

Ag

e, in

yea

rs

Cross-sectional view

Simulated age animation

1009080706050403020100

Ag

e, in

yea

rs

Map view cutaway

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 - brclark@usgs.govMatt Landon – landon@usgs.govLeon Kauffman – lkauff@usgs.govGeorge Hornberger – gzhornbe@usgs.gov

4/12/2005