Soil CO2 Flux Measurements in Volcanic and Seismic Areas: Laboratory Experiments and Field Applications

Abstract

The measurement of the CO2 flux exhaled from the soils is a delicate operation because of unavoidable errors caused by the measuring apparatus that disturbs both the soil and the gases circulation. Several methods have been developed in order to perform accurate measurements of soil CO2 flux. The methods used most widely to measure the emission of CO2 from the soil to the atmosphere in volcanic and geothermal areas are the dynamic method Gurrieri and Valenza 1988) and the accumulation chamber method (Baubron et al., 1990;Tonani and Miele., 1991). The flux measurements performed using the dynamic method can be influenced by soil permeability and by the rate of the sampling pump. The accumulation chamber measurements can also be affected by several problems such as wind speed, pumping flux, valuation of tangent at t 0 of the CO2 plot, etc. A laboratory apparatus able to simulate different flux regimens, under known conditions, has been developed and was used to test the performance and reliability of these two methods. The investigated fluxes fell within the range of values close to soil respiration up to those normally measured in active volcanic and geothermal areas. The correct functioning of the laboratory apparatus was checked by comparing the experimental steady state concentration profiles with those predicted by the advective-diffusion model. As can be inferred from the data obtained, the flux measurements performed using the dynamic method are significantly influenced by soil permeability especially if the measurements are taken at high pumping flux. An empirical equation for performing careful soil CO2 flux measurements as a function of the soil permeability was obtained by fitting experimental data to a model that explained the functioning of the system. In order to measure in situ soil permeability, a new method based on the theory of radial gas advection through an isotropic porous medium was developed. The method was tested in the laboratory and at several locations on the island of Vulcano (Aeolian Islands, Italy). Tests performed on the accumulation chamber method have highlighted several sources of errors in measuring CO2 flux with this method. The magnitude and sign of the obtained errors depend on the imposed flux, on soil permeability and on the rate used to induce air circulation in the close loop of the system. Permeability measurements were performed with the radial gas advection method over a large sector of the island of Vulcano (Aeolian Islands, Italy) and the results compared with soil CO2 fluxes measured at the same sites using the dynamic method. Based on the results, the influence of soil permeability on the flux measurements and on their spatial distribution was assessed. Finally, the dynamic method was also applied to a seismic area of Sicily (Capo Calava) in order to study the relationships between soil degassing and tectonics.