Soil gas geochemistry: significance and application in geological prospectings

Abstract

Gas-geochemistry has been proven to be a reliable and simple technique to apply, at different scales, in many geological scenarios. The study of spatial distribution of soil-gas anomalies, at the surface, can give important and interesting information on the origin and processes involving deep and superficial gas species. This information can be applied and studied in different frameworks, for example: I) seismic zonation, examining, at the surface, anomalous concentrations of deep gas species that upraise throughout preferential pathways (faults and/or fractures); II) environmental protection, monitoring naturally occurring toxic gases and highlighting zones with high health risks for humans; III) geomorphological and structural research, detecting the aggressive fluid piping that causes carsic phenomena; IV) radionuclide migration, both in the pollution assessment from abandoned uranium mines and in the study of high-level radioactive-waste isolation systems. Soil-gas distribution could be affected by surface features such as pedological, biogenic and meteorological factors: these are supposed to have only a subordinate effect on gas leakage. However, it is possible to properly interpret soil-gas anomalies and recognize, and avoid, influences of surface features studying the association of different gases (with different origin and physical/chemical behavior), collecting a large number of samples during periods of stable meteorological and soil moisture conditions (e.g., during dry season) and using appropriate statistical treatment of data (i.e., experimental variograms to investigate the spatial dependency of gas concentrations). We will try, in this paper, to give hints for a better comprehension of the main mechanisms ruling soil-gas features both displaying and discussing some data obtained in either prospecting or monitoring case studies. Soil-gas geochemistry involves the study of many gaseous species (i.e., radiogenic, trace and diagenetic gases) each of them can give specific information on the conditions that allow their formation, accumulation and/or migration. In this study, we outline the results from two analyzed soil-gases: radon (222Rn), a radiogenic trace gas, and carbon dioxide (CO2) which generally acts as carrier for trace gases.