Isotope Investigations in Wellhead Protection Areas
Jim WalshMDH
Isotopes Can Tell Us
• How old is it?
• Where did it come from?
1H16O
2H18O
Precipitation and Groundwater from Direct Infiltration Plot on Meteoric Water Line
Krabenhoft, 1990
Delta 18 Oxygen (per mil)
-12 -10 -8 -6 -4
Del
ta D
eute
rium
(pe
r m
il)
-90
-80
-70
-60
-50
Background or Upgradient Groundwater
Mixture of Background Groundwater and Surface Water
Surface Water
Perce
ntag
e of
Sur
face
wat
er P
rese
nt in
Mixt
ure
0
25
50
75
100
Figure 10. Example of mass balance approach to water mixing using oxygen-18 and deuterium. This figure shows that the mixed sample contains roughly equal parts of surface water and background groundwater.
Mass-balance approach to mixing
Wells In Need of
Protection
Steps in Wellhead Protection
DevelopConceptual
Model
Develop and Calibrate Flow Model
Delineate Wellhead Protection Area
Delineate DWSMA
Assess Vulnerability
Low
High
Low
High
Inventory and Manage Contaminant Sources
Isotope Techniques Apply To:
• Conceptual model development
• Model calibration/validation
• Vulnerability assessments
It’s A Protected Aquifer, So I Don’t Need To Worry About Surface Water. Right ?
Tritium = 18-39 TU
Clay-Rich TillSand and Gravel
Ele
vati
on (
Ft.
Above S
ea L
evel)
~10 – 20% lake water
~70% lake water
High
•Shallow Buried Drift Aquifer
•CFC Recharge Age of 15-25 years
The Case of The Likely Lake
Do I Need To Worry About That River 200 Feet Away ?
Delt
a1
8O
(per
mil)
Date
100’250’
1,200’
15-60% River Water At Wells Located Within 250’
Low
High
OPDC
CJDNCSLF
•Little Geologic Cover
•Nitrate and Pesticide Contamination
Delt
a1
8O
(per
mil)
Date
How About Those Mine Pits?
Dewatered Taconite Pits
Keewatin, 1996
Keewatin Isotope Results, February, 1996
Keewatin Isotope Results, February, 1996
KEEWATIN WATER LEVELS
1150
1200
1250
1300
1350
1400
1450
Oct-95 Mar-97 Jul-98 Dec-99 Apr-01 Sep-02DATE
EL
EV
AT
ION
MESABI CHIEF
CARLZ
WELL #1
WELL#2
Keewatin, 2001
Keewatin Trends
0 2 /9 60 8 /9 60 5 /9 8
0 9 /0 1
0 2 /9 60 8 /9 60 5 /9 8
0 9 /0 1
0 2 /9 60 8 /9 6
0 9 /0 1
0 2 /9 60 8 /9 6
0 9 /0 1
-12 -10 -8 -6D e lta 1 8 O (p e r m il , re la t iv e to S M O W )
0
10
20
30
Chl
orid
e (m
g/l)
K eew atin W ell #1 M esabi C hief
C arlz
K eew atin W ell #2
Keewatin Wellhead Protection Area With Mesabi Chief Pit Dewatered
Proposed Wellhead Protection Area With Pits Filled
A Tale of Two Wells
Till
Differing Isotope
Signatures
Also Differing:•Tritium Results•Water Chemistry (Chloride)
Well Construction Problems ?
Well Casing
Uncased Borehole
•TC Positive•Tritium = 11.6
TU
Well Casing
Uncased Borehole
•TC Negative•Tritium = 6.3 TU
Isotopes Are Your Friends For:
• Conceptual model development
• Model calibration/validation
• Vulnerability assessments
• Well construction investigations
Till
PeatThick Unsaturated Zones
Minnesota River
Thick Unsaturated Zones
1.1 Tritium Units
Thick Unsaturated Zones
g
Krabenhoft, 1990
USGS
USGS
Age of Water Useful Tracers
• Years or lessYears or less
• Years to a century
• A century to30,000 years
Artificial Tracers, Artificial Tracers, CFC, Tritium-Helium, CFC, Tritium-Helium, TritiumTritium
Tritium, CFC, Tritium-Helium, Chemistry
Carbon-14 and Chemistry
Tritium in Precipitation
1
10
100
1000 T
ritium
(T
U)
in P
recip
itation
Decary
Corr
ecte
d to 2
000
19531959
19651971
19771983
19891995
Ottawa, Canada (IAEA)
Groundwater Age Classification
• Recent Water = 3H > 10 TURecent Water = 3H > 10 TU– water entered the ground since late water entered the ground since late
1950’s1950’s
• Mixed Water = 3H between 1 and 10 Mixed Water = 3H between 1 and 10 TUTU– probably a mixture of recent and probably a mixture of recent and
vintagevintage
• Vintage Water = 3H < 1 TUVintage Water = 3H < 1 TU– water entered the ground before water entered the ground before
19531953
Source = ECA Jr., 1995
Confining Unit
..
Simple Capture Zone
Conjunctive Delineation
SurfaceWatershe
d
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