Methods and procedures applied


V. Determination of tritium activity concentration by T-3He ingrowth method

Tritium is a radioactive isotope of hydrogen, which has a half life of 12.3 years, emitting low-energy beta radiation. T is produced in the atmosphere in small concentration by the influence of cosmic radiation, being oxidized to water and afterthat entering in the water cycle (with a concentration of about 6 TU).

The natural level of tritium in precipitation is as much as 25 TU at high latitudes, decreasing to about 4 TU in the equatorial zone (TU= tritium unit, which is defined as 1 tritium atom per 1018 atoms of hydrogen, which corresponds to 0.118 Bq/l water).

According to the classical model by Kaufman and Libby, the age of the sample can be calculated from the decrease in tritium activity with time using equation

A=A0e-λt

where A refers to the tritium activity of the sample, A0 is the initial tritium activity of the water and l is the decay constant.

The hydrological application of tritium is based mostly on tracing by artificially produced tritium from the thermonuclear test series since 1952. In 1963 the yearly average of the tritium content in precipitation had reached the thousand fold of the natural content and is still high (»100 TU) at the present time.

Tritium labelling has make possible the study of short-term transport, mixing processes and exchange in the groundwater (in particular the 3H labelling allows for determination of mean residence times of unconfined groundwater in fractured and sedimentary aquifers). The qualitative detection of tritium is a certain prove of the presence in a sample of fresh groundwater (recharged after 1964).

Tritium has a special place among the radioactive tracers since water labelled with tritium
(3HHO) is chemically identical with the groundwater investigated and can be expected to represent the flow processes of groundwater very accurately.

Its determination differs from that of most other radioactive tracers, because it requires the measurement of low energy β-radiation. However, tritium as a hydrological tracer can be applied with limitation and with great precaution only in order not to disturb the insights into the hydrological water cycle by measurement of the natural tritium content (including fallout tritium from atomic bomb tests).

The tritium content decreases according to the law of the radioactive decay within the water cycle. Thus, if we know the tritium concentration of the infiltrating precipitation water, we can draw conclusion regarding the residence time of groundwaters. Dating is possible up to about 50 years. Problems are occurring only when mixed waters with different ages are investigated.

Detection of tritium in groundwater provides a definite indication for ongoing groundwater recharge and using different models mean residence times (MRT) can be calculated for groundwater from aquifers receiving diffuse recharge. In the same time, recharge rates or proportion of recent water components can be estimated and the delineation of recharge areas is possible as well.

3H-3He ingrowth method:

Tritium activity is determined via the stable helium-3 produced by the decay of the tritium. The samples are distilled, degassed and then stored at least half a year in a tightly sealed aluminosilicate container under vacuum. The 3He produced during the storage time is measured by a static noble gas mass spectrometer (VG-5400). The tritium concentration (C3H) is calculated from the concentration (C3He) of 3He as follows:

where

C3H tritium concentration in TU
C3He measured tritiogenic 3He in ccSTP
C conversion factor from ccSTP to TU (2.4889*10-15 [(ccSTP/g)/TU])
λ  reciprocal of mean lifetime of tritium (17.93a, [122])
ts, te, tm dates of sampling, extraction and measurement, respectively
W0, W weights of the sample before and after the extraction, respectively
S salinity in ‰
α  correction for the 3H/H fractionation due to loss of (distilled) water during gas extraction. a=1.15 (ratio of tritium concentration in the liquid phase to tritium concentration in the water vapor [122].

The tritium detection limit and the measurement precision is a function of the degassed water volume, the applied ingrowth time and depends on the process blanks to which the sample is exposed. The detection limit achieved in our lab fin case of 2 months ingrowth time and 3 l of sample is 2.6 ± 1.1 mTU.

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