The Reston Groundwater Dating Laboratory

3H/3He Noble Gas FAQ

  1. Where is tritium/helium-3 dating best suited?
    See 3H/3He Dating Background.
  2. Can I get tritium analyzed at another lab and just use the contract to determine the helium, neon and d3He?
    Yes. But this is not recommended. But to do so, just return copper tubes without requesting tritium by ingrowth, via your sample submission form. Only request analysis 1. The contract lab will report analytical results for helium, neon, and d3He. They will not interpret a 3H/3He age, even if you provide the tritium analysis. You will be responsible for calculating the 3H/3He age from the analytical data.
  3. Can I have tritium analyzed by helium ingrowth without running the helium, neon and d3He?
    Yes. Submit water samples in bottles requesting analysis 3 only.
  4. What size of socket do I need to use with my rachet to turn the bolts on the copper tubes?
    The channels come with nuts that are 13mm, ½ inch or 9/16 inch, depending on which style of channel you are sent from the contract lab. The 13mm and ½ inch nuts need deep sockets. The 9/16 inch does not need a deep socket.
  5. Where do I get the back-pressure valve?
    See 3H3He Sampling.
  6. Should I sample for 3H/3He or noble gases if there are bubbles in the water?
    No. The method is very sensitive to gas bubbles; results will be invalid. In fact, if there are any bubbles in the sample, even though you did not detect them during sampling, it is often very clear from the analyses that helium was stripped from the sample, invalidating any possible interpretation of 3H/3He age.
  7. What pumps are best for 3H/3He noble gas sampling?
    Pumps that apply a positive pressure on the water. Use of peristaltic pumps is not recommended because of potential degassing. Best results have been obtained with submersible sample pumps.
  8. Do I have to use copper tubing like we do with CFCs?
    No. Any tubing (plastic, rubber, metal) is fine. However, clear plastic is recommended so that if bubbles are present, they can be seen through the tubing.
  9. Is the 3H/3He age very sensitive to uncertainty in recharge temperature or recharge elevation?
    The solubility of helium and neon in water is not very sensitive to normal temperature variations. Often the water temperature can be used as recharge temperature in the 3H/3He age calculation. Recharge elevation is more important. One should use the elevation at the well head as the recharge elevation or the actual known recharge elevation.
  10. How long does it take to get my results back from the lab?
    Delivery can be expected within 6 months (plus helium-3 ingrowth time) of receipt of the samples at the contract lab. The helium-3 ingrowth time is normally 1-2 months. Thus, normal delivery is then approximately 7-8 months, however, laboratory equipment failure can increase the delivery time by several months.
  11. Can I pre-pay for analyses?
    Yes.
  12. Are the results put into QWDX for me?
    No.
  13. Who can I contact to ask about the status of my samples?
    email to cfc@usgs.gov.
  14. What does it mean when the contract lab reports that the sample was fractionated?
    The age, even though often calculated, is likely invalid. The term "fractionated" indicates that the sample has in some way degassed. This can happen if bubbles form during sampling. It can also happen naturally if gas bubbles form in the aquifer such as during denitrification or methanogenesis near the water table. One indication of fractionation is that Δ4He is smaller than ΔNe. The terms Δ4He and ΔNe are reported in the analysis and indicate the amount of helium and neon in the sample in excess of that in solubility equilibrium with air.
  15. What are some of the sample problems that can cause an invalid 3H/3He age?
    There are many: (1) Ends of the copper tubes are scratched or bent preventing proper vacuum seal in the extraction lab, (2) Copper tube was not completely sealed, clamps not tight enough, (3) There is too much gas in the sample, such as excess air, air leaks, or large excess terrigenic helium, (4) Sample is fractionated in some way, such as through gas stripping, (5) Sample has low tritium (<0.5TU).
  16. How can I screen my samples for large terrigenic excess helium?
    See Helium.
  17. What is the difference between the uncorrected and corrected age?
    The "uncorrected age" assumes that all the tritiogenic helium-3 in the sample is derived from tritium decay and from air. The "corrected age" accounts for the additional presence of terrigenic helium in the sample. Normally it is assumed that the 3He/4He ratio of the terrigenic helium is 2x10-8 (if of radiogenic origin). The "corrected age" should be used only if the presence of terrigenic helium is indicated in the analytical report. If the percent of terrigenic helium in the sample is low, the "uncorrected age" and "corrected age" will be nearly identical.
  18. What does a negative age mean?
    This can indicate some analytical and/or sampling problem. Loss of helium-3 due to inadequate confinement of the water sample following recharge can lead to anomalously young ages. Fractionated samples can have negative ages. Small negative ages are usually interpreted as approximately modern.