Drinking Water Security

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Water age dating is a simple way to test the security of groundwater aquifers, by establishing how long the water has been underground, i.e. how long since the waters were exposed to surface sources of contamination.

Deeper groundwater sources can overcome the most common problems that arise from microbiological contamination of surface waters, shallow groundwaters and spring water.

According to the Drinking-Water Standards for New Zealand:2005 (DWSNZ:2005), a groundwater source is considered secure when it can be demonstrated that it is not likely to be contaminated by pathogenic organisms by satisfying the following conditions:

  • Not directly affected by surface or climate influences
  • Abstracted from a bore head that provides satisfactory sanitary protection

Age dating yields an average age of the water as most groundwaters are mixtures of water with different ages. The important question is: What is the fraction of the water with age less than one year?

The DWSNZ:2005 specifies that this young fraction must be less than 0.005% of the water present in the aquifer. When this is the case, it is unlikely that contamination from recent sources of pollution will be a problem.

This young fraction can be determined from a series of samplings for tritium, CFCs or SF6, separated in time by two or more years. Single samplings of tritium, CFCs and SF6 can be used for less precise estimates of the young fraction, but must be confirmed by future sampling.

References:

  • Drinking Water Standards for New Zealand, 2005.Ministry of Health. http://www.mfe.govt.nz/ publications/ water/nz-drinking-water-standards-00.html
  • Morgenstern, U. 2004 Assessment of age distribution in groundwater.p. 580-587 In: Proceedings of the 2nd Asia Pacific Association of Hydrology and Water Resources Conference. Singapore: 2nd Asia Pacific Association of Hydrology and Water Resources Conference.
  • Morgenstern, U. 2004 Time delay in groundwater: assessment of age distribution. p. 35-36 In: Prior, C.A.; Faure, K. (eds) First Quaternary Techniques Workshop, 6-7 May 2004, National Isotope Centre. Lower Hutt: Institute of Geological & Nuclear Sciences Limited. Institute of Geological & Nuclear Sciences information series 63
  • van der Raaij, R.W.; Stewart, M.K.; Hawke, R.M. 2004 Groundwater age-dating in areas with complex recharge mechanisms.p. 167-168 In: The water balance : programme and abstracts for the New Zealand Hydrological Society 2004 Conference, Queenstown, 16-19 November 2004. Wellington: New Zealand Hydrological Society.
  • van der Raaij, R.W. 2003 Age-dating of New Zealand groundwaters using sulphur hexafluoride. MSc (Physical Geography) thesis, Victoria University of Wellington
  • van der Raaij, R.W.; Stewart, M.K.; Hawke, R.M. 2002 SF6 groundwater age-dating in New Zealand.p. 41-42 In: The New Zealand Hydrological Society Symposium 2002, 3-6 December 2002 : all the easy water is gone, Blenheim Country Scenic Circle Hotel... Blenheim. Wellington: New Zealand Hydrological Society.
  • van der Raaij, R.W.; Stewart, M.K. 2001 Application of CFC and SF6 dating techniques to New Zealand.p. 13 In: Just add water : New Zealand Hydrological & New Zealand Limnological Society joint conference, 20-23 November 2001 : conference programme and abstracts. Wellington, NZ: New Zealand Hydrological Society.