CO2 flux measurements in volcanic areas using the dynamic concentration methods: the influence of the soil permeability

Abstract

In order to evaluate the influence of soil permeability on soil CO2 flux measurements performed with the dynamic concentration method, several tests were carried out using soils characterized by different permeability values and flow rates. A special device was assembled in the laboratory to create a one-dimensional gas flow through a soil of known permeability. Using the advective-diffusion theory, a physical model to predict soil concentration gradients was also developed. The calculated values of CO2 concentrations at different depths were compared with those measured during the tests and a good agreement was found. Four soils with different gas permeability (3.6 10 2 to 1.23 102 mm2) were used. The CO2 flux values were varied from 0.1 kg m 2 d 1 up to 22 kg m 2 d 1. On the basis of these results, a new empirical equation for calculating very accurate soil CO2 flux from dynamic concentration and soil permeability values was proposed. As highlighted by the experimental data, the influence of soil permeability on CO2 flux measurements depends on various factors, of which the flow rate of the suction pump is the most important. Setting low values for the pumping flux (0.4–0.8 L min 1), the mean error due to soil permeability was lower than 5%. Finally, the method was tested by measuring the CO2 flux in a grid of 48 sampling sites on Vulcano (Aeolian Islands, Italy), and the global error, affecting the CO2 flux measurements in a real application, was evaluated.