ohsu oregon health & science university conceptual framework for evaluating the impact of...
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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
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
OHSUOregon Health &
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