Hydrologic Sciences Taking Stock and Looking Ahead (1998) / Chapter Skim
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4 Ground Water Dating and Isotope Chemistry
4 Ground Water Dating and Isotope Chemistry
Pages 87-100
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From page 87... ...
Such tracers have assumed new prominence in the past decade as a result of the refocusing of attention in applied ground water hydrology from questions of ground water supply, which are somewhat independent of the details of the flow path, to questions of ground water contamination, for which understanding the flow path and the nature of solute transport along it are central. Opportunities in the Hydrologic Sciences (NRC, 1991)
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From page 88... ...
This has resulted in an increased emphasis on environmental tracer methods, partly because tracers are directly relevant to predicting the movement of dissolved contaminants and partly because the time scales for flow in arid vadose zones are often so slow that information from short-term physical monitoring may be difficult to extrapolate to the longer scale appropriate for solute transport.
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From page 89... ...
Although a shallow upper zone of arid-region soils is typically hydraulically active, response time in deep desert vadose zones is on the scale of 103 to over 104 years (Phillips, 1994; Murphy et al., 1996~. Even the basics of water flow in arid-region vadose zones are still incompletely understood.
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From page 90... ...
Deep Aquifers and Regional Flow Systems Probably the most exciting development in this setting has been the adaptation of solid source mass spectrometnc methods originally developed for "hard rock" geochemistry to the investigation of heavy isotope ratios in deep ground water. At great depths the hydraulic properties are generally very poorly known and deep flow systems may as much reflect processes under ancient tectonic and climatic regimes as they do the influence of current conditions.
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From page 91... ...
CASE STUDIES IN THE DEVELOPMENT OF ENVIRONMENTAL TRACERS The 1991 National Research Council report Opportunities in the Hydrologic Sciences played a major role in formalizing research support for hydrology as a separate discipline through establishing a separate National Science Foundation (NSF) program focused on hydrology.
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From page 92... ...
They have only slight tendencies to adsorb on solid surfaces and are hence reasonably conservative in ground water. The key to their application as tracers is that, despite very low solubility in water, they are measurable at very low concentrations owing to their high electron affinity, which results in very sensitive responses in electron capture detectors attached to gas chromatographs.
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From page 93... ...
transport, and elaborate tracer tests were conducted in order to test the theories. In retrospect, much of the same information could have been obtained by examining natural flow systems and applying CFC dating to provide a framework for the interpretation of the transport of environmental tracer inputs, such as the bomb tritium pulse (Szabo et al., 1996; see Figure 4~.
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From page 94... ...
analysis of very low levels of CFCs in natural water may be well understood, being able to reliably and reproducibly measure samples at a rapid enough rate for routine research applications may take years of tinkering with detector settings, column packings, valve assemblies and materials, carrier gases, data reduction programs, and literally hundreds of other experimental variables. Such art is rarely recorded; rather it is assimilated through apprenticeship.
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From page 95... ...
Ratios higher than this value were considered indicative of very rapid infiltration of 36C1 fallout from nuclear weapons testing in the 1950s. I had no connection with the Yucca Mountain project, but the reviewer had contacted me as an independent authority on hydrological applications of 36C1.
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From page 96... ...
along the Exploratory Studies Facility (ESF) tunnel at the proposed high-level nuclear waste repository site, Yucca Mountain, Nevada.
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From page 97... ...
compared to ratios higher by almost a factor of two prior to 13,000 years ago (Plummer et al., 1997; see Figure 6~. The significance of this result was immediately apparent to those involved in the Yucca Mountain project.
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From page 98... ...
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?
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From page 99... ...
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.
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From page 100... ...
1992. Evaluation of liquid and vapor water flow in desert soils based on chlorine-36 and tritium tracers and nonisothermal flow simulations.
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