Mark Williams, CU-Boulder

Dating with Isotopes

AGE-DATING BASICS

• The term "age" sometimes creates the impression that the number represents a simple piston flow transit time of a small water parcel. • Despite the prevalent use of this term, isotope hydrologists understand that the water sample measured represents the integrated travel time

through that aquifer or other water body

• "age" and "mean residence time" are used interchangeably.

Radioactive Isotopes

Radioactive isotopes are nuclides (isotope-specific atoms) that have unstable nuclei that decay, emitting alpha, beta, and sometimes gamma rays.

Such isotopes eventually reach stability in the form of nonradioactive isotopes of other chemical elements, their "radiogenic daughters."

Decay of a radionuclide to a stable radiogenic daughter is a function of time measured in units of half-lives.

TritiumHelium-3Carbon-14Sulphur-35Lead http://www.sahra.arizona.edu/programs/isotopes/

Isotopes I’ll emphasize today

http://www.sahra.arizona.edu/programs/isotopes/

35S

http://www.sahra.arizona.edu/programs/isotopes/

How can I date recent groundwaters (<100 years)?

Sulphur-35 (35S) TritiumHelium-3Lead (Pb)

35S: APPLICATIONS FOR WATERSHED HYDROLOGY

1.ESTIMATE AGE OF WATER

• Very effective for time scale less than one year

• Few other environmental tracers can do this

2. DISCRIMINATE “NEW” vs. “OLD” WATER SOURCES

• Particularly good for identifying new snow/rain in groundwater

3.DATE AGE OF SULFATE

• Date age of atmospheric –deposited sulfate less than one year old

4.DISCRIMINATE ATMOSHPERIC FROM GEOCHEMICAL SOURCES OF SULFATE

Sulfur-35 (35S) IN THE ENVIRONMENT

Radioactive isotope of sulfate Half-life of about 87 days Produced by spallation of argon atoms in the atmosphere by

cosmic rays

18ArN=22 O2 SO2

SO42-

Cosmic Rays

35SO42- 35SO4

2-

35 SN=16

35S: UNITS AND VALUES

UNITS: Generally reported asmillebecquerels per Liter (mBq/L)

millebecquerels per mgSO4 (mBq/ mgSO4)

CONCENTRATIONS:Snowfall 60 mBq/LSnowmelt 20 mBq/L because of decay of snowpackRain(Summer) 100 mBq/L

FACTORS- extent of atmospheric mixing of stratospheric air into troposphere; greatest in summer

35S: Collection and Analysis

Sample Collection Need 1-20 Liters of sample (depending on amount of SO4

2-) pass sample through ion exchange resin in the field elute SO4

2- from resin with barium chloride final volume 100 ml

Sample Analysis Liquid scintillation counting (same as tritium) Count twice, about 4 months apart as part of QA/QC Potential problem: other radioactive sources

35S: Cost

About $400/sample Ain’t cheap!

• Dr. Robert MichelChief of the Tritium LabUSGS Menlo Park, CaliforniaPh 650/329-4547, (rlmichel@usgs.gov)

• University of Waterloo Environmental Isotope Laboratory– http://www.science.uwaterloo.ca/research/eilab/

– Tracing sources of streamwater sulfate during snowmelt using S and O isotope ratios of sulfate and S-35 activity, Shanley JB, Mayer B, Mitchell MJ, et al. BIOGEOCHEMISTRY V76 N1 Pp: 161-18, 2005

Use of cosmogenic S-35 for comparing ages of water from three alpine-subalpine basins in the Colorado Front Range, Sueker JK, Turk JT, Michel RL, GEOMORPHOLOGY V27 N1-2 pp61-74, 1999

TRITIUM (3H)

Radio isotope of hydrogenTritium decays to a rare, stable isotope of helium (3He)

by beta emission.Produced primarily by

a) cosmic rays spallation of nitrogen produces about 3.5 kg at steady state (around 11 TU

today)

b) nuclear weapons testing has resulted in approximately 80 kg of tritium at this

time

Units: Tritium Units (TU) 1TU = 1 3H per 1018 hydrogen atoms

TRITIUM SPALLATION IN ATMOSPHERE

14NN=7 + 3H

atmospheric

O2

Cosmic Rays

12 CN=6

3H20

TRITIUM CONCENTRATIONS IN PRECIPITATION

Hydrological Applications

Dating water sourcesTracer

Can separate groundwater (eg aquifer) that has waters of multiple ages

Hydrology

Sources directly fed by recent rainwater/snowmelt will contain the same tritium values as that rainwater/snowmelt

Trapped aquifers will have no tritium (older than 60 years)

Water traveling slowly through aquifers will have reduced tritium (< 10 TU) or elevated tritium from bomb spike in the 1960’s

http://homepages.uni-tuebingen.de/wolfgang.siebel/pdffiles/aeg_5.pdf

http://homepages.uni-tuebingen.de/wolfgang.siebel/pdffiles/aeg_5.pdf

Age-dating using tritium decay rates

Nt = N0e-t

ln (2/ T(1/2))

• T(1/2) is the half-lifeN = Number of atoms0 = initial time t = at some time “t”T(1/2)) = 12.33 years

General Guidelines for Tritium Ages

<0.8 TU0.8-4 TU5-15 TU15 - 30

TU>30 TU>50 TU

submodern (prior to 1950s)mix of submodern and modernmodern (<5 to 10 years)some bomb tritiumrecharge in the 1960's to 1970'srecharge in the 1960's

TRITIUM: SAMPLE COLLECTION

Need 1L of waterglass or HDPE (glass only if stored)no filteringseal bottles after collectionEasy and simple

TRITIUM: ANALYSIS

liquid scintillation countingdistill sample in Ostlund electrolysis cell to

increase concentration of 3Hmix with scintillation cocktailcount with a Packard CA 2000 scintillation

counter detection limit at one sigma 0.3-1.0 TU precision = 3% Lab-dependent! Be aware

TRITIUM: ANALYTICAL COST

About $150-190/sampleDr. Robert Michel

Chief of the Tritium Lab USGS Menlo Park, California Ph 650/329-4547, rlmichel@usgs.gov):

University of Waterloo Environmental Isotope Laboratory http://www.science.uwaterloo.ca/research/eilab/

Be aware of precision, accuracy, turn-around times

3H and 3He/3H Ages

In principle, the measurement of both 3H and its decay product, 3He, allows a "true" mean age (referred to hereafter as the 3He/3H age) to be obtained

3He/3H age: Precise age determination

By measuring 3H together with its daughter 3He, more precise “apparent” ages can be determined

Importantly, you do not have to know the initial value of tritium

3H and 3He/3H Ages, Rising River

The measured 3H concentration at the Rising River springs is 4.23±0.5 TU (Rose et al 1995), which implies a mean groundwater age of about 7-9 years. The measured 3He/3H age is 20.5 years (Rose et al 1995), which implies a groundwater age of about 8 years using the exponential model (Manga, 2001).

3He/3H age: Not all roses

There are a number of corrections that need to be made

For example, the measured 3He must be corrected for atmospheric 3He that is dissolved at the time of recharge.

There are standard methods of dealing with these necessary corrections

3He/3H age: Sample Collection

Samples are collected in 3/8" diameter copper tubes, clamped at both ends.

IMPORTANT: samples can only be collected from waters that have NOT mixed with the atmosphere since recharge Groundwater wells Springs

Otherwise, reset with present tritium/helium values

Need an expert to collect samples

3He/3H age: Cost

$700-1,000/sampleRSMAS Laboratory

http://www.rsmas.miami.edu/groups/tritium/

Ain’t cheap.Takes several months

Lead Isotopes

Lead: Hydrological Applications

Dating sediment cores: use 210Pb to date recent deposition of snow, lake sediments, etc. 210Pb has a half-life of 22.3 years, allowing dating within the past 100 years.

The distinct isotopic composition of lead ratios in surface and groundwaters to identify pollution sources

determining the relative importance in stream/ground water of atmospheric Pb (which concentrates in the upper soil layers) versus the Pb in groundwater that is derived from chemical weathering processes.

Uranium Isotopes: Mixing Diagram

1.00

1.10

1.20

1.30

1.40

0.00 2.00 4.00 6.00 8.00

1/U, in L/ug

234

U/238

U, activity ratio

M-1400

M-MVC1

W-3 W-11

Golf Adit

M-MVS1

S-7CS-7D

M-MVN4

M-MVN3

Ground water?

Spring snow melt?

Mine water

Mine Water

Stream

Monitoring Well

Uranium Isotopes

Can be quite handy for those dealing with uranium-related contamination problems Particularly where there is high natural levels of U

Generally plot the 234U/238U activity ratio (y-axis) versus the inverse of uranium concentrations (1/U)

The resulting diagram may show distinct source waters which can help unravel source water/flowpath sources of uranium

Exponential Flow/Box ModelUse non-radiogenic isotopes

(Plummer et al., 2001)

Box-Model Benefits

Can use any isotope to derive “recent” mean residence times

By measuring stable water isotopes in precipitation and wells/springs, we can solve for the residence time of water in the subsurface reservoir

Estimate water “age” without using radiogenic isotopes

18O at $40/sample much less expensive than tritium

Carbon-14 (14C)

date groundwaters 100 to 1,000 years in age

Carbon-14 (14C) and Hydrology

Radiocarbon dating of groundwater provides a mechanism to monitor, understand and control exploitation of an aquifer. 14C dating can help determine whether a community is

mining their water resources.

When the appropriate field measurements are collected and appropriate corrections are applied for dilution, 14C measurements can provide insight into: groundwater flow paths recharge areas and sources of recharge.

Carbon-14 (14C): Sample collection and prep

Dating groundwaters with DOC is not without methodological difficulties. Its concentration in groundwater is typically below 1 mg-C/L, which makes sampling difficult.

DOC is usually stripped from 100 L or more of groundwater, using ion exchange resins, and then eluted in the laboratory and

fractionated into humic (HA) and fulvic acid (FA) components.

The FA is then analyzed by AMS.

Carbon-14 (14C): Costs

Radiometric Counting: $200-$300/sampleAMS: $400-$2400/sample

Eg Lawrence-Livermore lab

Forensic Hydrology gone bad:129I, 36Cl, and stable isotope results from

the Fruitland Formation, CO and NMDetermined that waters in coalbed methane

deposits were lithogenic, deposited during Laramide Orogeny

Results do not support models of subsequent basin-wide groundwater migration in the Fruitland Formation

CBM extraction no potential harm to groundwater“The combined use of 129I and 36Cl, with stable

isotope studies provides valuable information as to the hydrologic history of coalbed methane deposits, as well as their potential for commercial exploitation.” Snyder et al., 2003

129I and 36Cl gone wrong

4He dates around 35,000 years old14C dates around 35,000 years old129I and 36Cl dates wrong. Why?

These isotopic dates can be “reset” Variable degrees of mixing of end-members of different

isotopic composition

Snyder and Fabryka-Martin, 2003 wrote new paper to save face after the work above showed that the Snyder et al., 2003 paper was wrong.

Be careful with 129I and 36Cl dates!

Summary

Radio-isotopes provide the ability to date the average residence time of water

Different isotopes provide different agesSomewhat expensiveMay require complex collection/post-

processingProvides unique information that can address

applied/legal questions