Radioactive decay of 3H produces the noble gas helium-3 (3He). Tritium measurements alone can be used to locate the depth of the. Section 3 – Dating and Isotope Hydrology, Institute of Joint Geoscientific Research, .. means that the 14C age of DIC in groundwater from the same depth is. chemical and isotopic processes have taken place and before any decay Active CO2 may reach the water table in several ways, including dissolution of CO2.
Once a new geochemical method fails to reach fruition, even if this is due largely to circumstance rather than its merits, the huge investment of time and labor necessary to reinvent it raises a high barrier. It takes a researcher with a strong vision of the potential reward to take the risk of losing that investment.
Chlorine in Fossil Rat Urine: Tracer Serendipitous In the early s I received a telephone call from an external reviewer for the U. Department of Energy plan of investigations at Yucca Mountain. The reviewer told me about the planned collection of 36Cl samples from the exploratory shaft under the mountain.
Ratios higher than this value were considered indicative of very rapid infiltration of 36Cl fallout from nuclear weapons testing in the s.
I had no connection with the Yucca Mountain project, but the reviewer had contacted me as an independent authority on hydrological applications of 36Cl. When asked why she was so concerned about this particular aspect of the proposed investigations, she replied that it was considered the single most critical component of the entire range of research at the site, since the widespread presence of bomb 36Cl would show that unsaturated fluxes were much greater than anticipated.
In fact, when the 36Cl results came in Fabryka-Martin et al.
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It was because a long-term history of much higher cosmogenic 36Cl production had in the meantime been established through an entirely independent and quite unconventional research project.
This shift was attributed to increased 14C production during that period, resulting from an increased cosmic-ray flux, which in turn was attributed to a weakened dipole geomagnetic field Mazaud et al. After reading that paper, I realized that 36Cl could potentially address the problem because it has a very short residence time in the atmosphere and essentially falls directly out onto the land surface. A record of 36Cl deposition should therefore be much closer to a direct record of variations in cosmogenic production.
The difficulty was finding an archive in which 36Cl deposition would be preserved, inasmuch as it is extremely mobile in the presence of water. United States Betancourt et al. This experiment was proposed to the NSF and, after some initial difficulties, was funded.
Midden samples from western and southern Nevada were obtained through the cooperation of Peter Wigand at the Desert Research Institute in Reno. The initial results were successful, and in a few years a sketchy preliminary history of 36Cl deposition was reconstructed. The major feature of this reconstruction was a pattern of relatively low ratios during the Holocene i. The significance of this result was immediately apparent to those involved in the Yucca Mountain project.
Basics of 3H/3He dating
Page 98 Share Cite Suggested Citation: Furthermore, it is obvious that this temporal variation in 36Cl fallout can serve as a hydrological tracer in many other similar circumstances. The contrast with the development of the CFC tracer method could hardly be more striking. Why was the secular variation 36Cl tracer springing into existence just as it was needed, while the CFC tracer sat on the shelf for years after its capabilities had been highlighted?
The most obvious answer has to do with research funding. Although the basic idea would undoubtedly have made a fine target for one of Senator Proxmire's ''Golden Fleece" awards were he still alive, reviewers and program managers were able to see that it was a means to obtain critical data that could not be had otherwise. Other factors played a role.
The "art" of 36Cl analysis was maintained through long-term funding of the PRIME Lab accelerator mass spectrometry NSF facility at Purdue University and at my own laboratory, overcoming the inevitable start-up problems that stall the acquisition of real results. Reflections The state of research on environmental tracers in ground water hydrology is flourishing. New tracers are being developed, tracer results are being applied more and more widely as integral parts of hydrological investigations, and results of environmental tracer studies are being applied to fields outside hydrology.
What can be done to promote continued growth of the field? Two case studies with remarkably contrasting histories were examined in order to reflect upon this question.
Two studies cannot be presumed to yield any definitive patterns, but some common threads can be discerned: The first point, and by far the most important, is a reaffirmation of the societal value of basic scientific research. This idea is certainly nothing new, but given the recent spate of attacks on this proposition, it is worth reemphasizing.
Who could have predicted that the apparently straightforward and obviously applicable CFC tracer would not be successful for almost 20 years after it was first proposed, while the esoteric investigation of fossil rat urine would pay immediate practical dividends? The best criterion for predicting the utility of research directions is not an attempt to second-guess their end use, but rather some estimate of their fundamental scientific worth.
The long technique development times and the large investment in the geochemical art need to be recognized when supporting hydrological tracer research. Promising techniques may be killed by demanding immediate results. The trend toward longer NSF grant periods is a very promising development in this regard. Proposed advances in environmental tracers need to be evaluated within Page 99 Share Cite Suggested Citation: Possession of impeccable credentials in other areas of geochemistry does not qualify proposal reviewers from outside the field.
The establishment of a separate NSF program in hydrology is a major step forward from the situation at the time CFC research temporarily died.
For this step we owe a great deal to the Opportunities in the Hydrologic Sciences report and to those who invested their time in writing it. Vadosezone techniques for estimating ground-water recharge in arid and semiarid regions. Calibration of the 14C timescale over the past 30, years using mass spectrometric U-Th ages from Barbadoscorals.
Van Devender, and P.
The Last 40, Years of Biotic Change. University of Arizona Press. Combined use of groundwater dating, chemical, and isotopic analyses to resolve the history and fate of nitrate contamination in two agricultural watersheds, Atlantic coastal plain, Maryland. The alluvium and terrace system of central Oklahoma.
A tracer study of the Floridan aquifer in southeastern Georgia: Implications for groundwater flow and paleoclimate. Distribution of chlorine in the unsaturated zone at Yucca Mountain: An indicator of fast transport paths. September 26—29,Las Vegas, Nevada. Los Alamos National Laboratory.
Water movement in desert soil traced by hydrogen and oxygen isotopes, chloride, and chlorine, Southern Arizona. Geomagnetic field control of 14C production over the last 80 ky: Implications for the radiocarbon time scale. Strontium, boron, oxygen, and hydrogen isotope geochemistry of brines from basal strata of the Gulf Coast sedimentary basin, USA.
Page Share Cite Suggested Citation: Geochemical estimates of paleorecharge in the Pasco Basin: Evaluation of the chloride mass balance technique.
Regional ground-water mixing and the origin of saline fluids: Opportunities in the Hydrologic Sciences. Environmental tracers for water movement in desert soils of the American Southwest.
Chlorine in fossil rat urine: An archive of cosmogenic nuclide deposition during the past 40, years. Evaluation of liquid and vapor water flow in desert soils based on chlorine and tritium tracers and nonisothermal flow simulations.
After oxidation to HTO, it takes part in the natural water cycle. These tests which were mainly performed in the early s, led to an increase of tritium in precipitation over the continents of the northern hemisphere from roughly 5 TU to levels of the order of TU.
One TU Tritium Unit means a tritium to hydrogen ratio of Whereas the addition of bomb tritium to the environment practically eliminated the use of natural tritium as a tracer, it offered a new tool, i. If the tritium delivery as a function of time can be reconstructed, this penetration process can be used for quantitative studies of water movement through identification of the bomb peak in certain ground water bodies. However, there are natural limits to this method because tritium decay and dispersion make it increasingly difficult to identify the bomb peak in groundwater.
These problems can be overcome by using tritium in combination with its decay product 3He 3Hetrit as first suggested by Tolstykhin and Kamensky and experimentally confirmed by Torgersen et al. The total 3He concentration has a variety of sources equation 2: In this equation, only 3Hetot and 3Heeq are determined through measurements.
The total 4He concentration measured in a groundwater sample can be written as: If no terrigenic helium is contained in the groundwater sample, 3Hetrit can be calculated by using equation 4: