Tritium Units

Table III. Interlaboratory Comparison Standards Expressed in Tritium Units and Background Counting of Samples...

Tritium 0-10 tritium units 0.01 tritium units Measure ashore Helium-3 ingrowth a>... 

Helium-3 is a decay product of radioactive tritium (3H, half-life = 12.44 years) that has been produced by nuclear bombs as well as naturally by cosmic rays in the upper atmosphere. Because virtually all 3He atoms escape from the surface ocean to the atmosphere, the 3He/tritium ratio in subsurface seawater samples indicates the time since the water s last exposure to the atmosphere. Both 3He and tritium are measured by gas mass spectrometry. Alternatively, tritium may be measured by gas counting with a detection limit of 0.05 to 0.08 tritium unit, where 1 tritium unit represents a 3H/H ratio of lxl0 18. A degassed water sample is sealed and stored for several months to allow the decay product 3He to accumulate in the container. The amount of 3He is then measured by mass spectrometry, yielding a detection limit of 0.001 to 0.003 tritium unit when 400-gram water samples are used. With this technique, the time since a water mass left the surface can be determined within a range from several months to 30 years. 

The counting of tritium in water is a special problem about which much has been written. Current methods for assay of tritium in water have a range of 0.1 -5000 TU, where a tritium unit (TU) has the value of 7.2 dpm/L. The most desirable feature of a tritium measuring system is that it be capable of measuring a large number of samples rapidly, simply, and cheaply as possible with an uncertainty of +10% or better. It is generally more important to assay 100 samples with an uncertainty of +10% than to assay 10 samples with an uncertainty of +3%. 

If the unsaturated zone is composed of relatively fine sediment (silt and fine sands) a quasi-uniform seepage flow can be assumed for the unsaturated zone in humid climate zones over long time spans. Therefore, the transport of infiltration water can be simulated in PHREEQC as a monotonous movement in accordance with the "piston flow model. A constant flow of infiltration water of 0.5 m per year is assumed for the following simulation. Furthermore, it is considered simplistically that the infiltrating precipitation has a tritium activity of 2000 TU (tritium units) over a period of 10 years. Then, it is assumed that the tritium activity decreases to zero again. 

The following example shows how this can be modeled in PHREEQC. First of all, a master- ami a solution species tritium T or T1 have to be defined. Since the input of data for log k und -gamma within the key word SOLUTION SPECIES is required, but unknown, any value can be entered here as a free parameter ( dummy , e g. 0.0). This value is not used for kinetic calculations and thus, does not cause any problems. However, all results based on equilibrium calculations (e.g. the calculation of the saturation index) are nonsense for this species . The tritium values have to be entered in tritium units. However, in order not to have to define or convert them in an extra step, they are entered fictitiously with the unit umol/kgw instead of TU in PHREEQC. As no interactions of tritium with any other species are defined, the unit is eventually irrelevant. After modeling, remember that the result is displayed in mol/kgw as always in PHREEQC and has to be recalculated to the fictitious tritium unit umol/kgw. Entering mol/kgw in the input file, the solution algorithm quits due to problems with too high total ionic strengths. 

The concentrations of tritium in natural waters are expressed by the atomic ratio of T to H a ratio of T/H = 10 is defined as 1 tritium unit (1 TU or 1 TR). ITU in water corresponds to approximately 0.12Bqdm (1 Ci = 3.7 X 10 ° Bq). In the early 1990s, the tritium content of water in Nature was at a low level (under 20 TU), so it becomes necessary to emich the tritium in order to obtain precise measurements. Measurement of tritium in environmental waters by conventional techniques combines emichment and counting, so that precision depends on reducing the errors of both techniques as much as possible. 

Figure 18 Depth profiles of tritium, + ( He)tri (tritium units), and apparent tracer ages from well nests Phi 3 and Ph23 (Ekwurzel et al., 1994) (reproduced by permission of American Geophysical Union from Water Resour. Res.

Tritium Unit 1 Hper lO atoms ofH = 4.012 X lO WSTP He if all decayed... 

Natural tritium concentrations in ocean surface waters and in young groundwater are of the order of 1 tritium atom per 10 hydrogen atoms. This is the reason that tritium concentrations are reported as TU (Tritium Units). One TU stands for a tritium to hydrogen ratio [ H]/[H] of 10. The activity of a water sample with a tritium concentration of 1 TU is equivalent to 3.2 pCi or 0.12 Bq per liter of H2O. The production rate of natural tritium is about 0.5 0.3 atoms cm s (Craig and Lai 1961) leading to natural tritium values in ocean surface waters of about 0.2 TU (Dreisigacker and Roether 1978). 

The northern component reflects the more immediate injection of bomb tritium into the northern hemispheric hydrologic system because virtually all of the major detonations occurred in the Northern Hemisphere. Prior to the bomb tests, the concentration of natural tritium in rainfall was of the order of 5-10 tritium units (1 TU = 1 tritium atom per 10 normal hydrogen atoms). During the mid-1960s, tritium concentrations of more than several thousand TU were recorded in higher latitude, mid-continental locales such as Chicago, USA or Ottawa, Canada. The southern component, on the other hand, is much weaker in amplitude and more smeared out in time... 

Figure 4 Tracer results from a two-dimensional gyre model. The model represents the circulation on a constant density surface (isopycnal) in the main thermocline that outcrops along the northern boundary (shaded region). Thermocline ventilation is indicated by the gradual increase of tritium- He ages around the clockwise flowing gyre circulation. TU, tritium unit (1 TU = 1 atom/10 H...

Tritium half-life is 12.43 years (decay constant 0.0563 year )-Tritium concentration in natural objects is customary to measure in Tritium Units (TU), which describe the content of atoms per 10 atoms of protium. 1 TU corresponds with 7.088 decays per minute in a kilogram... 

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