OHSUOregon Health &

Science University

Conceptual Framework for Evaluating the Impact of Inactive Wells on

Public Water Supply (PWS) Wells

Rick JohnsonOregon Health & Science University

OHSUOregon Health &

Science University

Aquitard

PWS WellInactive Well

•Downward vertical gradients are often created near PWS wells in confined aquifers

•Many inactive (e.g., irrigation) wells are screened across aquitards

•Under these conditions the inactive wells will leak water to the confined aquifer

OHSUOregon Health &

Science University

8000 m

100 m

3000 m

Aquitard=10 m thick

PWS Well

Unconfined Aquifer

Confined Aquifer

Numerical Model of Inactive Well Impacts

Inactive Well

OHSUOregon Health &

Science University

PWS Well

10-year capture zone

8000 m

3000

m

Key Definition: 10-Year Capture Zone Area (CZA)

OHSUOregon Health &

Science University

0

2

4

6

8

10

12

14

16

0 1.5 3 4.5Pumping Rate (mgd)

10-Y

ear

Cap

ture

Zon

e A

rea

(km

2)

High

Low

Medium

OHSUOregon Health &

Science University

0.00

0.04

0.08

0.12

0.16

0.20

0 500 1000 1500 2000

Distance from PWS Well

Fra

ctio

n of

PW

S W

ell F

low

High

Low

Contribution of water to PWS from inactive well significant!

OHSUOregon Health &

Science University

00.020.040.060.080.1

0.120.140.16

0 500 1000 1500 2000

Distance from PWS Well (m)

Ver

tica

l Hyd

rau

lic G

rad

ien

t

Low

High

Why is the inactive well contribution “constant” for different pumping rates?

One reason is that vertical gradient increases as pumping increases

OHSUOregon Health &

Science University

CONCLUSIONS FOR A SINGLE PWS WELL

1. Flow through even a single inactive well can be a significant fraction of the total flow through a PWS well

2. The contribution of the inactive well to total flow is roughly constant

3. The contribution of the inactive well does not depend strongly on the location of the inactive well within the capture zone of the PWS well.

OHSUOregon Health &

Science University

Of course, the real world is often more complicated….

OHSUOregon Health &

Science University

To examine the effect of multiple wells, we can use the simplified case of a continuously-operating PWS well surrounded by 8 seasonally-inactive irrigation wells

8000 m

3000

m

PWS Well

•PWS well flow = 1.5 mgd

•Irrigation wells on a 1-km grid each pumping at 0.75 mgd for 6 months of each year

OHSUOregon Health &

Science University

8000 m

3000

m

Summertime recharge due to irrigation

•Assume wintertime recharge•Assume summertime recharge from irrigation in 1 km2 area surrounding each well

10-year capture zone area

OHSUOregon Health &

Science University

0

0.05

0.1

0.15

0.2

0 500 1000 1500 2000

Distance from PWS Well (m)

Fra

ctio

n o

f P

WS

Wel

l F

low

As with the single well case, the inactive well has a significant impact on total PWS Well flow

OHSUOregon Health &

Science University

0.00

0.20

0.40

0.60

0.80

1.00

0 500 1000 1500 2000

Distance from PWS Well (m)

Fra

ctio

n of

Lea

ked

Mas

s R

etai

ned

in C

onfin

ed A

quife

r

How much water leaked into the confined aquifer is pumped out by irrigation wells? Almost None!

OHSUOregon Health &

Science University

0

0.05

0.1

0.15

0.2

0 500 1000 1500 2000

Distance from PWS Well (m)

Fra

ctio

n of

Irr

igat

ion

Wat

er

Com

ing

from

Con

fined

Aqu

ifer

Why does so much mass remain in the confined aquifer?

Because most of the irrigation water comes from the unsaturated aquifer

OHSUOregon Health &

Science University

0

2

4

6

8

10

0 500 1000 1500 2000

Distance from PWS Well (m)

Vo

lum

e L

eak

ed

/ Vo

lum

e P

um

pe

d

fro

m C

on

fin

ed A

qu

ife

r

Another way of looking at this is that the volume of water leaked to the confined aquifer is 2-6 times greater than volume pumped from the confined aquifer.

OHSUOregon Health &

Science University

CONCLUSIONS FOR A PWS WELL AND MULTIPLE SEASONALLY-INACTIVE IRRIGATION WELLS

1. The 10-year CZA is reduced somewhat by the irrigation wells

2. The fraction of the total PWS well flow coming from a single irrigation well is still significant.

3. The fraction does not vary much with distance from the CWS well

4. Nearly all of the water leaking into the confined aquifer during inactive periods remains in the confined aquifer.

OHSUOregon Health &

Science University

Next Steps:

To put the magnitude of the inactive well problem in perspective:

1. We need to better understand the density of inactive wells in the vicinity of PWS wells

The kind of detailed well-distribution data shown for the York case is being examined at other locations by NAWQA

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Science University

Next Steps (cont):

2. We need to better understand the probability that inactive wells are delivering contaminants to the confined aquifer and the PWS well.

• Water quality data at the PWS and from monitoring wells is helping to determine this

• Environmental tracers from the PWS can be used to estimate the potential impact of inactive wells

OHSUOregon Health &

Science University

Thanks to the USGS NAWQA Program for supporting this work