Dating with Isotopes Mark Williams CUBoulder AGEDATING BASICS

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/

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

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

How can I date recent groundwaters (<100 years)? Sulphur-35 (35 S) Tritium Helium-3 Lead (Pb)

35 S: 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 (35 S) IN THE ENVIRONMENT w Radioactive isotope of sulfate w Half-life of about 87 days w Produced by spallation of argon atoms in the atmosphere by cosmic rays Cosmic Rays 18 Ar N=22 35 SO 24 35 S N=16 SO 4 2 - O 2 SO 2 35 SO 24

35 S: UNITS AND VALUES UNITS: Generally reported as millebecquerels per Liter (m. Bq/L) millebecquerels per mg. SO 4 (m. Bq/ mg. SO 4) CONCENTRATIONS: Snowfall 60 m. Bq/L Snowmelt 20 m. Bq/L because of decay of snowpack Rain(Summer) 100 m. Bq/L FACTORS- extent of atmospheric mixing of stratospheric air into troposphere; greatest in summer

35 S: Collection and Analysis Sample Collection w w Need 1 -20 Liters of sample (depending on amount of SO 42 -) pass sample through ion exchange resin in the field elute SO 42 - from resin with barium chloride final volume 100 ml Sample Analysis w Liquid scintillation counting (same as tritium) w Count twice, about 4 months apart as part of QA/QC w Potential problem: other radioactive sources

35 S: 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 V 76 N 1 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 V 27 N 1 -2 pp 61 -74, 1999

TRITIUM (3 H) Radio isotope of hydrogen Tritium decays to a rare, stable isotope of helium (3 He) 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) 1 TU = 1 3 H per 1018 hydrogen atoms

TRITIUM SPALLATION IN ATMOSPHERE Cosmic Rays 14 N N=7 atmospheric O 2 12 C N=6 + 3 H 3 H 20

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

Age-dating using tritium decay rates Nt = N 0 e-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 1950 s) 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 1 L 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 3 H 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

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

3 He/3 H age: Precise age determination By measuring 3 H together with its daughter 3 He, more precise “apparent” ages can be determined Importantly, you do not have to know the initial value of tritium

3 H and 3 He/3 H Ages, Rising River The measured 3 H 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 3 He/3 H age is 20. 5 years (Rose et al 1995), which implies a groundwater age of about 8 years using the exponential model (Manga, 2001).

3 He/3 H age: Not all roses There a number of corrections that need to be made For example, the measured 3 He must be corrected for atmospheric 3 He that is dissolved at the time of recharge. There are standard methods of dealing with these necessary corrections

3 He/3 H 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

3 He/3 H 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 210 Pb to date recent deposition of snow, lake sediments, etc. 210 Pb 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 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 234 U/238 U 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 18 O at $40/sample much less expensive than tritium

Carbon-14 (14 C) date groundwaters 100 to 1, 000 years in age

Carbon-14 (14 C) and Hydrology Radiocarbon dating of groundwater provides a mechanism to monitor, understand control exploitation of an aquifer. 14 C 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, 14 C measurements can provide insight into: groundwater flow paths recharge areas and sources of recharge.

Carbon-14 (14 C): 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 (14 C): Costs Radiometric Counting: $200 -$300/sample AMS: $400 -$2400/sample Eg Lawrence-Livermore lab

Forensic Hydrology gone bad: 129 I, 36 Cl, 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 129 I and 36 Cl, 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

129 I and 36 Cl gone wrong He dates around 35, 000 years old 14 C dates around 35, 000 years old 129 I and 36 Cl dates wrong. Why? 4 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 129 I and 36 Cl 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