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Conceptual Framework for Evaluating the Impact of Inactive Wells on

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Conceptual Framework for Evaluating the Impact of Inactive Wells on

Public Water Supply (PWS) Wells

Rick Johnson

Oregon Health & Science University

PWS Well

Aquitard

- 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

Numerical Model of Inactive Well Impacts

Inactive Well

PWS Well

Unconfined Aquifer

100 m

Confined Aquifer

3000 m

8000 m

Aquitard=10 m thick

Low

0.16

0.12

High

Fraction of PWS Well Flow

0.08

0.04

0.00

0

500

1000

1500

2000

Distance from PWS Well

Contribution of water to PWS from inactive well significant!

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

One reason is that vertical gradient increases as pumping increases

0.16

High

0.14

0.12

0.1

Vertical Hydraulic Gradient

0.08

0.06

0.04

Low

0.02

0

0

500

1000

1500

2000

Distance from PWS Well (m)

CONCLUSIONS FOR A SINGLE PWS WELL

- Flow through even a single inactive well can be a significant fraction of the total flow through a PWS well
- The contribution of the inactive well to total flow is roughly constant
- 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.

PWS Well

8000 m

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

- 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

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

10-year capture

zone area

3000 m

Summertime recharge due to irrigation

8000 m

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

0.2

0.15

Fraction of PWS Well Flow

0.1

0.05

0

0

500

1000

1500

2000

Distance from PWS Well (m)

0.80

0.60

in Confined Aquifer

Fraction of Leaked Mass Retained

0.40

0.20

0.00

0

500

1000

1500

2000

Distance from PWS Well (m)

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

0.15

Coming from Confined Aquifer

0.1

Fraction of Irrigation Water

0.05

0

0

500

1000

1500

2000

Distance from PWS Well (m)

Why does so much mass remain in the confined aquifer?

Because most of the irrigation water comes from the unsaturated aquifer

8

6

Volume Leaked/ Volume Pumped

from Confined Aquifer

4

2

0

0

500

1000

1500

2000

Distance from PWS Well (m)

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.

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

- The 10-year CZA is reduced somewhat by the irrigation wells
- The fraction of the total PWS well flow coming from a single irrigation well is still significant.
- The fraction does not vary much with distance from the CWS well
- Nearly all of the water leaking into the confined aquifer during inactive periods remains in the confined aquifer.

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

- 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

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