Optimization of Water Extraction from Soil Enabling Environmental Measurements of Tritium Concentration in the Soil Using LSC

Tritium is transported in the environment as tritium water (HTO) and tritium gas (HT). Since the evaporation rate of tritium water from soil is low compared to that from vegetation, small amounts of tritium are still likely to be found in soil after there is no longer evidence of tritium in vegetation. This is the main motivation for the development and application of a method for measuring tritium from soil samples.

The optimal method to measure a small amount of tritium in liquids is by measuring its activity in a Liquid Scintillation Counter (LSC). This is due to both the simplicity of the measurement and the short range of β radiation emitted by tritium decay in matter, which lower the counting efficiency. In liquid scintillation counting, the sample is mixed with the liquid scintillation material, increasing the counting efficiency due to the direct contact between the sample and the scintillator.

Liquid should be extracted from soil samples to measure its activity using LSC. The counting efficiency depends on the turbidity level of the sample, which can significantly reduce the light signal if the sample is not turbid. Pre-treatment during sample preparation reduces the level of turbidity, but also reduces the extracted liquid’s volume. Therefore, an optimal extraction method for each type of soil should be chosen for maximal water volume from the soil sample.

This work describes various existing methods to extract water from soil samples, as well as the development of a new method. All methods are implemented and compared. The comparison is made for several types of soil, considering the efficiency of the different extraction methods for each one of them.

The water permeability of soil affects the moisture content in the soil and the extractible amount of water. Hence, three types of soil with different permeability are studied: Loess, Hamra and Terra Rossa, which are representative of 3 different regions: the Negev, the Coastal Plain and the Sharon, and the Galilee and Judea Hills, accordingly.

The efficiency of the different extraction methods is analyzed in two ways: efficiency by weight and "nuclear" efficiency which is based on the activity concentration of tritium in the sample. Each of the three types of soil was examined with three different extraction methods: evaporation, vacuum distillation and pressure squeezing using three different activity concentrations of tritium.

We have proved that the evaporation method and the vacuum distillation method are effective in all types of soil tested. The evaporation method being simpler to implement, is the recommended method, except for cases where the soil is dry (moisture content less than 10%), in which the vacuum distillation method is preferred.