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

Wells In Need of Protection Steps in Wellhead Protection

Develop Conceptual 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 = TU Clay-Rich Till Sand and Gravel Elevation (Ft. Above Sea Level)

~10 – 20% lake water ~70% lake water

High

Shallow Buried Drift Aquifer CFC Recharge Age of years The Case of The Likely Lake

Do I Need To Worry About That River 200 Feet Away ?

Delta 18 O (per mil) Date 100’ 250’ 1,200’ 15-60% River Water At Wells Located Within 250’

Low High

OPDC CJDN CSLF Little Geologic Cover Nitrate and Pesticide Contamination

Delta 18 O (per mil) Date

How About Those Mine Pits? Dewatered Taconite Pits Keewatin, 1996

Keewatin Isotope Results, February, 1996

Keewatin, 2001

Keewatin Trends

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 Peat Thick Unsaturated Zones Minnesota River

Thick Unsaturated Zones 1.1 Tritium Units

Thick Unsaturated Zones

g

Krabenhoft, 1990

USGS

Age of Water Useful Tracers Years or lessYears or less Years to a century A century to 30,000 years Artificial Tracers, CFC, Tritium-Helium, Tritium Tritium, CFC, Tritium- Helium, Chemistry Carbon-14 and Chemistry

Tritium in Precipitation Ottawa, Canada (IAEA)

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

Confining Unit

.. Simple Capture Zone Conjunctive Delineation Surface Watershed