Soil gas survey in the geothermal area of Bolsena Lake (Vulsini Mts., Central Italy)

Abstract

A soil gas survey has been carried out in the Vulsini Mts. volcanic area, around Bolsena Lake, with the objective of testing the reliability of this geochemical method for geothermal exploration. The thermal gradient is high all over the area; the He/Ne ratio, and He, CO 2, and H 2 concentrations have been determined in 259 samples (2 per km 2, over an area of 130 km 2). Compared with its surroundings, this area exhibits an overall positive CO 2 anomaly, but the distribution of diffusive gases (He and H 2) allows us to distinguish three different sectors around the lake: (1) a northern sector with high CO 2, and H 2/CO 2, He/CO 2, and He/Ne ratios close to the background value; (2) a south-eastern sector, characterized by the presence of cold fumaroles, with high He, H 2, CO 2 and He/Ne, and generally low H 2/CO 2 and He/CO 2; (3) a southern sector with generally atmospheric He and CO 2 and few high H 2 and CO 2 spots. Permeability is assumed to be the main factor controlling the differences between the above three sectors. In sector 1, the flysch (1 km thick) is intruced by a great number of sills and dikes, and the underlying limestones are completely metamorphosed into marbles; CO 2 (thermometamorphic?) is the late stage of a heavy degassing process. Sector 2 includes several volcanic spatter cones along the tectonic trends; the high permeability allows the deep gases to be ducted with minor changes. Sector 3 is an elongated strip with an anti-Apennine trend; diffusion of H 2 only is the result of the thick (>3 km) unaltered flysch cover. In sector 3 the underground outflow of the lake through shallow volcanics entrains large quantities of air and masks any deep gases; the few anomalous spots reproduce situations like that of sector 2. One of these spots is located near a producing well tapping the geothermal reservoir.