SUMMARY

Carbon dioxide is the main geosphere product affecting the biosphere. Its migration in the ground has several geological constraints whose understanding is essential for the study of its occurrence and behaviour in shallow environments, such as soil and groundwater. In industrialized, urban and rural areas high CO2 concentrations in the ground may be associated with volatile organic compounds whose effects on the biosphere may be remarkable. In these cases any environmental monitoring should consider all the contaminants, both endogenetic and anthropogenetic, occurring in the ground. This paper is an overview of the main geological factors controlling the migration of endogenetic gas to the surface and, in particular, the CO2 occurrence in soil and groundwater. Some guidelines are given for identifying and monitoring leakages of CO2 and other volatile contaminants based upon soil-gas, exhalation and groundwater surveys.

INTRODUCTION

The occurrence of high concentrations of volatile compounds at the Earth's surface, viz. in soil and groundwater, is the object of a wide series of researches and practical applications within the framework of environmental studies, exploration geology and earthquake prediction. In the first case the researches are particularly focused to those gases, both endogenetic and anthropogenetic, which may have toxic relevance or excite modifications in the biosphere (e.g., CO2, VOCs, CH4, CFCs, SO2, H2S, COS, CS2, HC1, HF, Rn). In the second and third case importance is given to those natural volatiles which may be used as “tracers” or “pathfinders” of subsurface energy sources (geothermal reservoirs, hydrocarbon or ore deposits), and which are sensitive to seismic stresses, respectively (e.g., He, Rn, CO2, H2, CH4, Ar, S-compounds).