Tracers ideal

Since it is nearly impossible to find an ideal tracer, all measurements made with tracers deviate from measurements made with real contaminants. I he efficiency methods, when using real contaminants, are as near to direct measurement of function as is possible. Use of tracers can also give direct measurements, but the conditions of the tracer may differ so much from the real contaminant that these measurements must be seen as itidirect. 

C1 may be considered as near-ideal tracers for water flow in chaUc. However, and S04 show a more complicated behavior because of ion exclusion, adsorption, desorption, and chemical reactions [186]. 

Pu may be considered as an ideal tracer nuclide for these studies because of its decay characteristics. Its existence or absence in the early solar system can be considered as a crucial test for or against the theories of the synthesis of chemical elements in stars. 

For temporal moments it is best to consider a semi-infinite tube and let c(0, t) = S(t) be the ideal tracer. Initially, c(z, 0) = 0 and c must remain finite as z — 00. These are not the most sophisticated conditions, as we know from Example 3, but they give the swiftest answer. If we take the Laplace transform of Eq. (261), we have... 

IMB labelled with 125I and with nC is an ideal tracer for direct comparison of PET and SPECT (single photon emission computed tomography) for localization and quantification of neuroreceptors in the living human brain. 

Convection, diffusion, and dispersion can only describe part of the processes occurring during transport. Only the transport of species that do not react at all with the solid, liquid or gaseous phase (ideal tracers) can be described adequately by the simplified transport equation (Eq. 94). Tritium as well as chloride and bromide can be called ideal tracers in that sense. Their transport can be modeled by the general transport equation as long as no double-porosity aquifers are modeled. Almost all other species in water somehow react with other species or a solid phase. These reactions can be subdivided into the following groups, some of which have already been considered in the previous part of the book. 

Chloride behaves like an ideal tracer and is only affected by dispersion. Calcium is still not in solution even after a single exchange of all water within the column (shift = 40) as it is exchanged for Na and K. When all sodium has been removed from the exchanger, Ca can only be exchanged for K that leads to a peak in the K-concentration. Only after the water of the column has been exchanged about 2.5 times, the concentration of calcium increases at the outlet. 

Water isotope hydrology focuses on the isotopes that form water molecules the oxygen isotopes (oxygen-16, oxygen-17, and oxygen-18) and the hydrogen isotopes (protium, deuterium, and tritium). These isotopes are ideal tracers of... 

Then the mean residence time and variance of residence times (the same as would be given by the ideal tracer experiment) are... 

It was first observed by Thiemens et al.4 that stratospheric C02 possesses a large and variable mass-independent isotopic composition. This composition was suggested as deriving from isotopic exchange with Of11)), the product of ozone photolysis.5 6 As later confirmed by rocket-borne collection of stratospheric and mesospheric air, this unique isotopic signature provides an ideal tracer of odd oxygen chemistry of the Earth s upper atmosphere, one of the most important upper atmospheric processes. There are, however, several features that require further measurement (laboratory and atmospheric) and theoretical considerations. 

Due to its high solubihty and its strikingly low adsorption behaviour, MTBE migrates in the groimdwater significantly faster than BTEX. MTBE behaves almost like an ideal tracer. The Koc values, as meaningful parameters for the evaluation of the adsorption behaviour for some relevant organic substances, are represented in Fig. 3. 

Suppose a small, sharp pulse of an ideal, nonreactive tracer is injected into a tube at the centerline. An ideal tracer is identical to the bulk fluid in terms of flow properties but is distinguishable in some nonflow aspect that is detectable with suitable... 

As mentioned above H- He age and isolation age unfortunately do not exactly agree in mixed waters. Nevertheless, direct application of H- He age for quantification of water exchange is possible but limited to cases where (1) the concentration is approximately homogeneous, and (2) the water age is small compared to the half-life of tritium. The second condition is required because if H- He ratios are small the water age can be linearized (Eqn. 34). If both conditions are fulfilled, mixing affects the H- He age in the same way as Hetn. Then, the H- He age behaves as a bio-geochemically inert tracer with a constant source term increasing the age by 1 yr per yr and the H- He age can be treated as an ideal tracer of the isolation age. 

In studies where the data are insufficient to calculate the temporal change of e.g., Hetri, water exchange can be estimated from the H- He ages, provided that the H- He ages agree with the isolation age. Then, the H- He age can be treated as ideal tracer for water age i which increases by 1 yr per yr. In case of steady state conditions, volume exchange between box 1 and box 2 can be calculated from the budget method (Eqn. 41) ... 

The two box models discussed above can be extended to a multibox model. In the limit of infinitely small boxes the multibox model corresponds to the continuous model of Jassby and Powell (1975). Using Equation (40), the vertical turbulent diffusivity as function of depth can be obtained from x if t is at steady state and can be treated as ideal tracer with source strength of lyr/yr ... 

Based on the unusual adsorptive and decay characteristics of nicotine, some investigators are turning their attention to 3-ethenylpyridine as a tracer of ETS (Thome et al. 1986 Eatough et al. 1989b Ogden 1991). Second in concentration only to nicotine among the ETS alkaloids, 3-ethenylpyridine more nearly reflects the attributes of an ideal tracer discussed above than do any of the other tobacco alkaloids. 

Fig. 1. Hypothetical equilibrium binding analysis for the ideal tracer, ( H)-ligand showing total, nonspecific, and specific ligand binding. The curves were constructed by incubating a fixed amount of tissue in the presence of increasing concentrations of ( H)-ligand with or without a large excess of unlabeled ligand. The inflection in the total binding curve represents the concentration of tracer required to occupy all available receptors...

---