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Isotope Investigations in Wellhead Protection Areas. Jim Walsh MDH. Isotopes Can Tell Us. How old is it? Where did it come from?. Precipitation and Groundwater from Direct Infiltration Plot on Meteoric Water Line. Krabenhoft, 1990. Mass-balance approach to mixing.

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isotopes can tell us
Isotopes Can Tell Us
  • How old is it?
  • Where did it come from?
slide6

Steps in Wellhead Protection

Wells In Need of Protection

slide7

Develop

Conceptual

Model

isotope techniques apply to
Isotope Techniques Apply To:
  • Conceptual model development
  • Model calibration/validation
  • Vulnerability assessments
slide15

Elevation (Ft. Above Sea Level)

Tritium = 18-39 TU

Clay-Rich Till

Sand and Gravel

slide17

~70% lake water

~10 – 20% lake water

slide20

The Case of The Likely Lake

  • Shallow Buried Drift Aquifer
  • CFC Recharge Age of 15-25 years
slide26

15-60% River Water At Wells Located Within 250’

100’

250’

1,200’

Delta18O (per mil)

Date

slide29

Low

High

slide30

Little Geologic Cover

  • Nitrate and Pesticide Contamination

OPDC

CSLF

CJDN

how about those mine pits
How About Those Mine Pits?

Dewatered Taconite Pits

Keewatin, 1996

slide43

Differing Isotope Signatures

  • Also Differing:
  • Tritium Results
  • Water Chemistry (Chloride)
slide45

TC Positive

  • Tritium = 11.6 TU

Well Casing

Uncased Borehole

slide47

Well Casing

  • TC Negative
  • Tritium = 6.3 TU

Uncased Borehole

isotopes are your friends for
Isotopes Are Your Friends For:
  • Conceptual model development
  • Model calibration/validation
  • Vulnerability assessments
  • Well construction investigations
slide50

Thick Unsaturated Zones

Peat

Till

Minnesota River

slide51

Thick Unsaturated Zones

1.1 Tritium Units

slide59

Age of Water Useful Tracers

  • Artificial Tracers, CFC, Tritium-Helium, Tritium
  • Tritium, CFC, Tritium-Helium, Chemistry
  • Carbon-14 and Chemistry
  • Years or less
  • Years to a
  • century
  • A century to
  • 30,000 years
slide61

Tritium in Precipitation

Ottawa, Canada (IAEA)

slide62

Groundwater Age Classification

  • Recent Water = 3H > 10 TU
    • water entered the ground since late 1950’s
  • Mixed Water = 3H between 1 and 10 TU
    • probably a mixture of recent and vintage
  • Vintage Water = 3H < 1 TU
    • water entered the ground before 1953
  • Source = ECA Jr., 1995
slide64

Surface

Watershed

Simple Capture Zone

.

.

Conjunctive Delineation