Hazard assessment for gas emission and flank landslides at Albano crater lake (Rome)

Repeated flooding episodes occurred from the crater lake of Albano until 398 B.C. These floods were probably caused by sudden injection of gas and warm waters on the lake bottom, or also by the overturn of the lake which would have brought to the surface the deep water rich in CO₂. Since several years, we have been monitoring the crater lake chemical composition as well as its physico-chemical parameters and dissolved gas content, in order to assess evidences of possible deep fluid input in the lake water. The concentration of dissolved gases, and their isotopic composition (d¹³C_TDIC, ³He/⁴He) suggest the presence of deep gases (CO₂ and CH₄) within the lake bottom layer. However, the total pressure of dissolved gases is presently, at any depth, much lower than the hydrostatic pressure. If, for any reason, a significant volume of deep water should rise to the surface, only limited phenomena of gas exsolution are to be expected. A density variation of shallow lake water due to cooling, in case of heavy rainfall in harsh winters (T<8.5°C), may produce water overturns. Such phenomena, as long as they happen with a certain frequency, would prevent the accumulation of dangerous quantities of CO₂ in the deepest lake water strata. Apart from a volcanic unrest, the most dangerous condition is the occurrence of seismic swarms with hypocentres in the Lake Albano area, which could lead to an increase in the influx of hot gases and fluids in the lake. Currently, the conditions for a rapid release of significant quantities of CO₂ from Lake Albano do not exist. To improve the knowledge of the Lake Albano  water circulation, we investigated also the isotopic ⁸⁷Sr/⁸⁶Sr composition of the lake water, comparing the results with those of the rocks hosting the aquifers. Results indicate that Lake Albano water samples well fit a binary mixing of a high ⁸⁷Sr/⁸⁶Sr ratios end-member (Colli Albani volcanites) and a low ratio end-member (carbonate basement) in a proportion of 75% and 25% respectively. Moreover, the landslide hazard of the internal slopes of Lake Albano has been assessed using Ordinary Least Squares and Empirical Likehood Ratio modelling functions. The presence of numerous dwellings and recreational activities along the internal slopes of the crater lake, makes the area of potentially-high risk, both because of the inner intense slope instability and for possible secondary effects due to tsunami waves that might be generated by the impact of sliding subaerial masses on the lake surface or by sub-aqueous landslides.