The Albano Maar Lake high resolution bathymetry and dissolved CO2 budget (Colli Albani volcano, Italy): constrains to hazard evaluation

The Albano Lake is the deepest volcanic lake in Italy (-167 m) and fills the youngest maar of the quiescent Colli Albani volcano. The lake has undergone significant level changes and lahar generating overflows occurred about 5800 yrs B.P. and likely in 398 b.C., when Romans excavated a tunnel drain through the maar wall. Hazardous lake rollovers and CO2 release are still possible because the Albano volcano shows active ground deformation, gas emission and periodic seismic swarms. On November 2005, the first high resolution bathymetric survey of the Albano Lake was performed. Here we present the results provided by a Digital Elevation Model and 2-D and 3-D images of the crater lake floor, which is made by coalescent and partly overlapping craters and wide flat surfaces separated by some evident scarps. Submerged shorelines are identified at depths between -20 m and -41 m and indicate the occurrence of significant lake level changes, likely between 7.1 and 4.1 ka. The current lake volume is ~447.5 x 106 m3 and the total quantity of dissolved CO2 is 6850 tons estimated by chemical analyses of samples collected on May 2006. A decrease of nearly one order of magnitude of the CO2 dissolved in the lake water below -120 m, observed from December 1997 to May 2006 (from 4190 to 465 tons respectively), has been attributed to lake water overturn. The observed oscillations of the dissolved CO2 concentrations justify the efforts of monitoring the chemical and physical charcateristics of the lake. At present the quantity of dissolved CO2 is very far from saturation and Nyos-type events cannot presently occur