Age-dating basics the term "age" sometimes creates


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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.









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



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

  • 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, (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 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 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







Age-dating using tritium decay rates

  • Nt = N0e-t

  • 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

  • 0.8-4 TU

  • 5-15 TU

  • 15 - 30 TU

  • >30 TU

  • >50 TU



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, 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



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



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



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 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 old

  • 14C dates around 35,000 years old

  • 129I 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 ages

  • Somewhat expensive

  • May require complex collection/post-processing

  • Provides unique information that can address applied/legal questions




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