Transcript Niwot Ridge Synthesis - University of Colorado Boulder

Dating with Isotopes
Mark Williams, CU-Boulder
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.
Tritium
Helium-3
Carbon-14
Sulphur-35
Lead
Isotopes I’ll emphasize today
http://www.sahra.arizona.edu/programs/isotopes/
35S
http://www.sahra.arizona.edu/programs/isotopes/
http://www.sahra.arizona.edu/programs/isotopes/
How can I date recent groundwaters (<100 years)?
 Sulphur-35 (35S)
 Tritium
 Helium-3
 Lead (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
18
Ar
N=22
35SO 24
35
S
N=16
SO42-
O2
SO2
35SO 24
35S:
UNITS AND VALUES
UNITS: Generally reported as
millebecquerels per Liter (mBq/L)
millebecquerels per mgSO4 (mBq/ mgSO4)
CONCENTRATIONS:
Snowfall  60 mBq/L
Snowmelt  20 mBq/L because of decay of snowpack
Rain(Summer)  100 mBq/L
FACTORS- extent of atmospheric mixing of stratospheric air into
troposphere; greatest in summer
35S:
Collection and Analysis
Sample Collection

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

Need 1-20 Liters of sample (depending on amount of SO42-)
pass sample through ion exchange resin in the field
elute SO42- 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 Michel
Chief of the Tritium Lab
USGS Menlo Park, California
Ph 650/329-4547, ([email protected])
• 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 alpinesubalpine basins in the Colorado Front Range, Sueker JK, Turk JT, Michel RL,
GEOMORPHOLOGY V27 N1-2 pp61-74, 1999
TRITIUM (3H)
 Radio isotope of hydrogen
 Tritium 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
14
N
N=7
atmospheric O2
12
C
N=6
+ 3H
3H 0
2
TRITIUM CONCENTRATIONS IN
PRECIPITATION
Hydrological Applications
 Dating water sources
 Tracer
 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-lt
l = ln (2/ T(1/2))
• T(1/2) is the half-life
N = Number of atoms
0 = initial time
t = at some time “t”
T(1/2)) = 12.33 years
General Guidelines for Tritium Ages
 <0.8 TU
 submodern (prior to 1950s)
 0.8-4 TU
 mix of submodern and modern
 5-15 TU
 modern (<5 to 10 years)
 15 - 30 TU
 some bomb tritium
 >30 TU
 recharge in the 1960's to 1970's
 >50 TU
 recharge in the 1960's
TRITIUM: SAMPLE COLLECTION
 Need  1L of water
 glass or HDPE (glass only if stored)
 no filtering
 seal bottles after collection
 Easy and simple
TRITIUM: ANALYSIS
 liquid scintillation counting
 distill sample in Ostlund electrolysis cell to increase
concentration of 3H
 mix with scintillation cocktail
 count 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/sample
 Dr. Robert Michel
 Chief of the Tritium Lab
 USGS Menlo Park, California
 Ph 650/329-4547, [email protected]):
 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/sample
 RSMAS 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
1.40
Ground water?
Mine Water
M-1400
1.30
Monitoring Well
M-MVC1
1.20
238
U, activity ratio
Stream
W-3
Spring snow melt?
234
U/
W-11
1.10
Golf Adit
S-7C
S-7D
M-MVN4
M-MVS1
Mine water
1.00
0.00
M-MVN3
2.00
4.00
1/U, in L/ug
6.00
8.00
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 Model
Use 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/sample
 AMS: $400-$2400/sample
 Eg Lawrence-Livermore lab
Forensic Hydrology gone bad:
129I, 36Cl,
and stable isotope results from
the Fruitland Formation, CO and NM
 Determined that waters in coalbed methane deposits
were lithogenic, deposited during Laramide Orogeny
 Results do not support models of subsequent basinwide 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 old
dates around 35,000 years old
 129I and 36Cl dates wrong. Why?

14C


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 ages
Somewhat expensive
May require complex collection/post-processing
Provides unique information that can address
applied/legal questions