The Reston Groundwater Dating Laboratory

CFC Frequently Asked Questions

  1. Why would I want to measure the concentrations of chlorofluorocarbons in ground water?
    See the article Tracing and Dating Young Ground Water
  2. Can I pre-pay for analyses?
    Yes. Contact the lab.
  3. How do I collect a CFC sample of ground water?
    See Sampling Method for CFCs - Bottles.
  4. How do I collect a CFC sample of surface water?
    See Surface Water Sampling.
  5. How do I collect a CFC sample of air or unsaturated-zone air?
    See Gas Sampling.
  6. Should I collect CFC samples if I see bubbles in the water?
    Generally this is ok. We have not normally experienced problems with this, unless the gas is due to air being drawn into the pump or tubing lines, in which case there could be gross contamination of old water with modern air. Normally, finely dispersed micro bubbles are not a significant problem.
  7. How long can I keep surface water vs ground water samples before sending to the lab?
    Microbial degradation of CFCs can occur in samples of surface water if the sample contains particulate organic carbon or other particulates. Please contact the lab and arrange to send surface-water samples within several days of collection. It is important that the lab be notified so that the surface water samples will be analyzed as they are received. Ground-water samples, such as from karst, that may also contain particulates, should be treated as surface water, to minimize potential for degradation of CFCs on storage.
  8. Why should I collect and return 4 - 5 bottles of water from the same well?
    The Reston Chlorofluorocarbon Laboratory will analyze 2 of your bottles. Additional bottles will be analyzed if needed. The collection sequence can give information on the possible transient nature of discharge from the well during the sampling period. In some cases, the sampling procedure may contaminate a sample, and multiple analyses may show this. Also some samples may be lost in shipping due to breakage.
  9. How long can I keep the bottles before shipping?
    Tests have shown that clear samples of ground water can normally be stored in bottles for several months or more. If longer storage is required, it is best to flame seal the water samples into borosilicate ampoules.
    For more information on this subject, please see the report on The Stability of CFCs in Ground Water Samples Archived in Borosilicate Ampoules.
  10. How do I assign a CFC age?
    See the Guidelines for Assignment of Apparent CFC Age.