<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD><TITLE></TITLE> <META http-equiv=Content-Type content="text/html; charset=us-ascii"> <META content="MSHTML 6.00.6000.16674" name=GENERATOR></HEAD> <BODY> I forwarded Scott Bryan's rather negative comments on Dr. Hurwitz's previous note on dating geyser water.  Shaul was kind enough to elaborate on the previous note and explain further.  His answer follows: 1. I previously wrote a piece for the group about age dating of water and described why it is tricky and very much assumption-dependent. To that extent, here is a paragraph from the abstract of the modified <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" /><st1:City><st1:place>Rye</st1:place></st1:City> and Truesdell paper which appears in the new USGS Professional paper (<A href="http://pubs.usgs.gov/pp/1717/downloads/pdf/p1717H.pdf">http://pubs.usgs.gov/pp/1717/downloads/pdf/p1717H.pdf</A>):   <BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px"> "Derivation of this deep fluid solely from present-day recharge is problematical. The designation of source areas depends on assumptions about the age of the deep water, which in turn depend on assumptions about the nature of the deep thermal system. Modeling, based on published chloride-flux studies of thermal waters, suggests that for a 0.5- to 4-km-deep reservoir the residence time of most of the thermal water could be less than 1,900 years, for a piston-flow model, to more than 10,000 years, for a well-mixed model.".</BLOCKQUOTE> <BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px"> Currently several studies are trying to provide some constraint on the apparent age of the thermal water by using a variety of isotopic techniques.  Nevertheless, each one of these methods depends on many unconstrained assumptions.  Our hope is that integration of the different techniques will narrow the assumptions and allow a better estimate.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p></BLOCKQUOTE> <DIV><o:p> I also want to emphasize that since Rye and Truesdell have made their tritium measurements (in the 1970’s and 80’s) tritium levels in the atmosphere have decreased significantly, but modern techniques to allow detection of much lower concentrations of tritium and with a much greater precision.   In a study carried out in the past year we have detected measurable tritium in four of the five geysers we sampled (Old faithful, Aurum, Daisy, and Oblong).  In Grand except for one sample with low tritium concentrations samples were below detection limit.  So far we have analyzed 24 geyser samples and by August or September we hope to have results from ten more samples.<o:p></o:p></o:p></DIV> <DIV><o:p></o:p> </DIV> <DIV><o:p> 2. With regards to the filtering approach we used to detect seasonality of <st1:place>Old Faithful</st1:place>.  In the paper we published in the journal Geology we explained our filtering approach and applied the same rigorous criteria to all geysers analyzed.  We note that the detected seasonality and annual variability could not be random and filtering just enhanced the signal, it did not create it.   3. With regards to George Marler's paper “Seasonal changes in ground water in relation to hot spring activity” published in American  Journal of Science in 1964 – indeed the observations documented in that paper (based on limited observations) indicate that geysers have seasonal characteristics.  We did not cite this (excellent) paper because of space restrictions, but certainly highly regard it. <DIV>Shaul Hurwitz U.S. Geological Survey MS #439 345 Middlefield Rd. Menlo Park, CA 94025 Tel: (650) 329-4441 <A title=blocked::mailto:shaulh@usgs.gov href="mailto:shaulh@usgs.gov" target=blank>shaulh@usgs.gov</A> <A title=blocked::http://wwwrcamnl.wr.usgs.gov/hydrotherm/ href="http://wwwrcamnl.wr.usgs.gov/hydrotherm/" target=blank>http://wwwrcamnl.wr.usgs.gov/hydrotherm/</A> </DIV></o:p></DIV></BODY></HTML>