Determination of 222Rn in water by absorption in polydimethylsiloxane mixed with activated carbon and gamma-ray spectrometry: an example application in the radon budget of Paterno submerged sinkhole (Central Italy).

Passive gas accumulators made of polydimethylsiloxane (PDMS) mixed with activated C (AC) were studied to measure their efficiency for sampling Rn in water. In this composite the water-impermeable properties of PDMS act synergistically with adsorptive properties of AC, even when the accumulators are immersed in water for many days. A series of tests where cylindrical shaped PDMS-AC disks were exposed to different 222Rn-enriched waters showed that measured 222Rn specific activity matches the theoretical results coming from the equation that describes the process of internal diffusion integrated with the Rn decay term. The linear relationship between 222Rn in water and the accumulation process in PDMS-AC, the influence of temperature and the different sensitivity of the composite and its components were also studied and discussed. The high Rn volumetric enrichment factor in PDMS-AC disks respect to water resulted in about 206: 1, so lowering detection limits for 222Rn in water to 20 Bq m-3 when the total activity of Rn progeny in disks is measured by high resolution gamma-ray spectrometry. The use of PDMS-AC accumulators was tested at the Paterno submerged sinkhole, in central Italy. This study allowed the production of a detailed synchronous vertical profile of the Rn content in the middle of the lake and to define the Rn balance by assessing the discharge rate of submerged springs and the average residence time of the lake water.