Conceptual Models of Flow and Transport in the Fractured Vadose Zone (2001) / Chapter Skim
Currently Skimming:
Investigating Flow and Transport in the Fractured Vadose Zone Using Environmental Tracers
Investigating Flow and Transport in the Fractured Vadose Zone Using Environmental Tracers
Pages 271-294
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 271... ...
Results from these tracer applications can give insight into hydrological processes in more traditionally defined fractured vadose zones. The most generally useful tracers appear to be the stable isotopes of oxygen and hydrogen in the water molecule, tritium, halides, and chlorine-36, but there are a wide variety of potentially applicable environmental tracers.
|
From page 272... ...
SCOPE This chapter will focus on the application of environmental tracers to understanding the hydrology of fractured vadose zones. Environmental tracers are defined as solutes that are introduced into the hydrological cycle by either natural
|
From page 273... ...
Deliberately introduced tracers are of great utility for hydrological investigations in both the laboratory and the field, but these are most commonly employed in conjunction with hydraulic testing and are best considered as a component of such tests. There exists a large body of literature on the use of environmental tracers in nonfractured vadose zones, but a comprehensive review of this topic is beyond the scope of
|
From page 274... ...
Structured Agricultural Soils Farmers have been conducting environmental tracer tests in the vadose zone on a grand scale for many years. A variety of agricultural chemicals of environmental concern are routinely applied to fields and then allowed to infiltrate, under either irrigation or natural precipitation.
|
From page 275... ...
Continuum models usually adequately reproduce these curves (e.g., Bowman and Rice, 1986; Jaques et al., 1998~. Unstable Wetting Fronts When large amounts of water infiltrate into dry soils, unstable wetting fronts may develop if the soil has water-repellent surfaces or if the surficial layer is coarse (Glass et al., 1989; Hendrickx et al., 1993~.
|
From page 276... ...
This type of lumped system response is surely a close analog to the behavior of many fractured vadose zone/ aquifer systems. Numerous watershed studies have used environmental tracers to directly study preferential flow phenomena (Hammermeister, 1982; Mulholland et al., 1990; Leany et al., 1993~.
|
From page 277... ...
This response nicely illustrates the highly transient nature of the matnxlmacropore interaction that also characterizes more typical fractured rock in the vadose zone. Cave Hydrology It is the desire of every vadose fractured rock hydrologist to have a tunnel drilled beneath the research site in order to actually see and sample in detail the 60 -4 O— ~ B-Horizon Lateral Subsurface Flow Cl ............
|
From page 278... ...
To date, results of these studies have been treated largely as aids to the paleoenvironmental interpretation of cave precipitates, and in relation to the hydrology of particular localities. A more comprehensive and systematic investigation of environmental tracers in cave seeps could add greatly to our understanding of flow and transport in fractured vadose zones.
|
From page 279... ...
In addition, because temperature decreases with increasing elevation, the lighter isotopic composition of higher-altitude precipitation can be used to help determine the source of recharge (Scholl et al., 1996~. The application of stable isotopes to hydrological problems has been reviewed by IAEA (1981)
|
From page 280... ...
The tritium content at the time of recharge can be inferred from the added concentrations of parent and daughter, and the time since recharge from the ratio of the two. This technique is not generally applicable to fractured vadose zones, however, since the 3He can readily escape by volatilization.
|
From page 281... ...
The most notable use of this tracer thus far in fractured vadose zones has been at Yucca Mountain (Fabryka-Martin et al., 1997~. As with tritium, 36C1 is most applicable to longresidence-time vadose zones.
|
From page 282... ...
Strontium isotopes have been applied to reconstructing ancient flow regimes through the fractured vadose zone at Yucca Mountain (Johnson and DePaolo, 1994~. Although the principles are well established, the application of these isotopic techniques to problems of fractured vadose zone hydrology has been very limited.
|
From page 283... ...
In many cases the desired goal is to predict flow and transport through fractured vadose zones under conditions approximating the natural situation. This prediction is usually attempted using numerical models that have been developed and calibrated using a limited number of short-term observations and artificial hydraulic and tracer tests.
|
From page 284... ...
Reproduced with permission from the International Atomic Energy Agency.
|
From page 285... ...
Geothermally stimulated convection of air is undoubtedly a common phenomenon in fractured vadose zones (Weeks, 1987) , and this example illustrates how environmental tracers can identify circumstances in which it exerts a profound influence on vadose-zone hydraulics and porewater chemistry.
|
From page 286... ...
Enriched portion of profile above 30 m overlies a pyroclastic unit that allows air circulation through the fractured vadose zone. Dramatic erosion of the fissure tops is evidence that large amounts of water have been diverted from ephemeral surface drainages or overland flow to depth.
|
From page 288... ...
The rate of imbibition is apparently rapid enough that most fracture flow is depleted within 20 m of the surface, a significant finding with respect to contaminant transport and aquifer recharge. Flow Through Unsaturated Fractured Rhyolite at Yucca Mountain, Nevada Yucca Mountain is at present the nation's only site under investigation for its suitability as a potential high-level nuclear waste repository.
|
From page 289... ...
; 1 _ , ., - e · ~ 1 1 1 1 4 5 6 7 8 '.~ 0 1 2 3 Distance (km) FIGURE 9-9 Distribution of 36Cl/C1 ratios as a function of distance along the Exploratory Studies Facility tunnel beneath Yucca Mountain (Fabryka-Martin et al., 1998.
|
From page 290... ...
Actual applications to classical fractured vadose zones have not been numerous in the published literature, but this situation is more likely to reflect a general under-investigation of this environment rather than a lack of suitable tracers. The most generally applicable tracers would appear to be the stable isotopes of O and H in the water molecule, tritium, halides, and 36C1.
|
From page 291... ...
Evaluation of the use of 3H and 36C1 to estimate groundwater recharge in arid and semi-arid environments. Symposium on Isotopes in Water Resources Management, Vienna, International Atomic Energy Agency, pp.
|
From page 292... ...
Distribution of fast hydrologic paths in the unsaturated zone at Yucca Mountain. 8th Annual International High-Level Radioactive Waste Management Conference, Las Vegas, Nevada.
|
From page 293... ...
Lateral subsurface flow pathways in a semiarid ponderosa pine hillslope. Water Resour.
|
From page 294... ...
International Atomic Energy Agency, Vienna, pp.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.