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