Integrating radon and thoron flux data with gamma radiation mapping in radon-prone areas. The case of volcanic outcrops in a highly-urbanized city (Roma, Italy).

An integration of laboratory radon and thoron exhalation data with gamma radiation mapping is applied to assess the geogenic radon and the exposure of people to natural radiation in a highly-urbanized city (Roma, Italy). The study area is a protected territory where ignimbrites from Colli Albani volcano and alluvial sediments largely crop out. A map of total gamma radiation, a gamma transect across Caffarella valley and 9 vertical gamma profiles have been carried out, showing that the main control of gamma levels is, of course, the lithological nature, without neglecting the simultaneous effect of other parameters such as slope morphology, erosion/weathering processes, occurrence of sinkholes or underground tunnels. The surveys allowed to distinguish the medians of ignimbrites (from 816 ±â€¯16 cps to 936 ±â€¯19 cps) from that of alluvial materials (611 ±â€¯14) cps), but showed also that alluvial sediments with anomalously high radioactivity (769 ±â€¯14 cps) can be locally recognized, providing valuable information on the interaction between sedimentation and erosion in fluvial valleys. Total gamma activity was converted into absorbed gamma dose rate ranging from 0.33 to 0.38 µSv/hr. Outdoor Annual Effective Dose Equivalents were also estimated between 0.58 and 0.67 mSv y-1. Laboratory radon and thoron exhalation rates of collected material are positively correlated with gamma radiation. Volcanic and alluvial sediments are well-discriminated. The correlation between the two variables is evident, but not robust because of the variable concentration of 40 K, which is not contributing to radon and thoron exhalation rates. Anomalous data of soil samples located at the foot of a slope can be interpreted as due to reworking and accumulation processes. Similar gamma radiation data documents analogous concentration of radon and thoron parent-nuclides, but coexisting different radon and thoron exhalation rates provides an additional information on different grain size distributions which can be considered as a proxy for soil gas permeability. The integration of gamma mapping and radon and thoron exhalation measurements is a very useful tool to assess people exposure to natural radiation, in terms of dose rates and potential indoor radon. Gamma mapping, which provides data on the radiation source (the bedrock) is fast and not expensive. It allows to obtain very detailed pictures of a study area, but it needs to be combined with laboratory determination of radon and thoron release in order to definitely and correctly interpret variations of gamma signal. Furthermore, laboratory determination of soil radon exhalation gives information on the release of radon and is a good proxy for soil gas permeability. It has the great advantage over in-situ measurements of gas flow not to be influenced by seasonal pedoclimatic parameters and is affected by lower analytical uncertainties. These data are thus reproducible and precise and can be used to estimate potential radon hazard, which is the main source of exposure and thus the most important parameter for human protection from environmental radioactivity.

Using a multi-method approach based on soil radon deficit, resistivity, and induced polarization measurements to monitor non-aqueous phase liquid contamination in two study areas in Italy and India.

Geochemical and geophysical surveys employing radon deficit, resistivity, and induced polarization (IP) measurements were undertaken on soil contaminated with non-aqueous phase liquids (NAPLs) in two different sites in India and in Italy. Radon deficit, validated through the comparison with average soil radon in reference unpolluted areas, shows the extension of contamination in the upper part of the unsaturated aquifers. In site 1 (Italy), the spill is not recent. A residual film of kerosene covers soil grains, inhibiting their chargeability and reducing electrical resistivity difference with background unpolluted areas. No correlation between the two parameters is observed. Soil volatile organic compounds (VOCs) concentration is not linked with radon deficit, supporting the old age of the spillage. NAPL pollution in sites 2a and 2b (India) is more recent and probably still active, as demonstrated by higher values of electrical resistivity. A good correlation with IP values suggests that NAPL is still distributed as droplets or as a continuous phase in the pores, strengthening the scenario of a fresh spill or leakage. Residual fraction of gasoline in the pore space of sites 2a and 2b is respectively 1.5 and 11.8 kg per cubic meter of terrain. This estimation is referred to the shallower portion of the unsaturated aquifer. Electrical resistivity is still very high indicating that the gasoline has not been strongly degraded yet. Temperature and soil water content influence differently radon deficit in the three areas, reducing soil radon concentration and partly masking the deficit in sites 2a and 2b.