An update on soil gas harvesting technique as a non-depleting source of 222Rn for a large volume calibration chamber for long-term exposure experiments

Abstract

Objective

To demonstrate the versatility of the natural soil gas harvesting technique, coupled with a semi-dynamic injection algorithm, as an efficient and reliable source for continuous ²²²Rn delivery to a large-volume calibration chamber across different soil conditions and climatic conditions.

Methods

Long-term experiments were performed during three seasons of a year in a tropical monsoonal climatic region with high rainfall on the Southwest Coast of India. Soil gas extraction was performed using soil-gas probes, inserted to 1 ​m deep into the ground. Soil gas was harvested at a flow rate of 60 L/min, passed through a moisture trap, a ²²²Rn progeny filter, and a delay volume and pumped into a ²²²Rn calibration chamber of volume 22.7 ​m³. Continuous monitoring of ²²²Rn concentration in the chamber was performed using ionization chamber-based reference monitors. To achieve and maintain the desired ²²²Rn concentration values in the calibration chamber, the semi-dynamic injection method was adopted, in which the ²²²Rn concentration in the soil gas was monitored periodically, and injection into the chamber was controlled using a well-defined pumping algorithm. Three ranges of ²²²Rn concentration values were chosen as the target values to be maintained in the calibration chamber for long-term experiments (15 ​d): low concentration (500–1,000 Bq/m³), medium concentration (1,000–10,000 Bq/m³), and high concentration (10,000–25,000 Bq/m³).

Results

The achieved ²²²Rn concentration values were in good agreement with the target values, with deviations of 12%, 8% and 5% for the low, medium and high concentration exposures, respectively, during the summer season. During the monsoon season, the deviations between the target and the achieved concentration values were 12%, 10% and 5% for low, medium and high concentration exposures, respectively, and the corresponding deviations were 12%, 4%, and 5% for the winter season. These deviations are well within the cumulative uncertainty associated with the measurements.

Conclusions

The results of this study conclusively demonstrate that the soil gas harvesting method, when coupled with the semi-dynamic injection approach, is a reliable method for generating and maintaining the desired ²²²Rn concentration in the large volume calibration chamber during different seasons and environmental conditions.