Tritium isotope effect in precipitation and its distribution in natural water

Tritium, as a part of water molecules, directly participates in the water cycle and undergoes the same evolution process as the hydrogen and oxygen isotopes of water under the same conditions, so it has the same distribution characteristics as the hydrogen and oxygen isotopes of water. The distribution characteristics of tritium in various water bodies in nature are restricted by its source, exchange period, isotope fractionation and various environmental factors. The difference is that it is a radioactive isotope and is bound to be dominated by the law of radioactive decay. Once out of contact with the source region, its content will decrease with time.

1. Latitude effect

Figure 4- 1 1 and Figure 4- 12 list the tritium concentration distribution data in precipitation in China released by IAEA 1963 and Wei Keqin (1978) respectively. It can be seen that the lower the latitude, the lower the tritium concentration, and the higher the tritium concentration with the increase of latitude. The main reasons are as follows: (1) The neutron intensity of cosmic rays is the smallest in the equatorial region and the largest in the polar region, so the generation rate of tritium in nuclear reactions is different in different parts of the stratosphere; ② the influence of atmospheric circulation; ③ Exchange and dilution effects of ocean surface; (4) adding tritium from artificial sources.

Figure 4- Geographical distribution of tritium concentration in atmospheric precipitation 1 1963 (according to IAEA, 1968)

2. Continental effect

In the same latitude zone, the tritium concentration in atmospheric precipitation gradually increases with the distance from the coastline, which is called continental effect (Figure 4- 12). The main reason is the dilution exchange of water and steam on the coastal surface.

Fig. 4- 12 distribution of tritium in precipitation in China (according to Wei keqin, 1978)

3. Height effect

The increase of tritium content in atmospheric precipitation with height is called height effect. Merliva et al. (1973, 1977) measured tritium in snowfall in Greece and Antarctica, and found that its content increased with the increase of sampling height. B blavoux (1978) reached the same conclusion in Switzerland (table 4-9). It shows that the higher the sampling position, the more tritium-rich water vapor comes from the upper atmosphere.

Table 4-9 Tritium Concentration of Snowfall at Different Height in Zermatt, Switzerland

(according to B.Blavoux, 1978)

4. Seasonal effect

Tritium concentration in atmospheric precipitation has obvious seasonal variation characteristics, the highest concentration generally appears in June-July, and the lowest concentration appears in11~ 65438+February. This change is related to the seasonal variation of the leakage effect of the rest layer, and the monsoon may also interfere with this regular seasonal variation (Olive, 1970).

5. Rainfall effect

At the same latitude, the tritium concentration in atmospheric precipitation decreases with the increase of total precipitation.