Geochemistry of the Albano crater lake

Albano Lake is within the youngest polygenetic crater of Colli Albani, from which several lahar-generating water overflows occurred up to early Roman times. The area has anomalous gas emissions and is affected by seismicity and uplift. The geochemistry of the lake have been systematically investigated since 2003 by measuring physico-chemical parameters along vertical profiles with a multiparametric probe and by collecting water samples for chemical and isotopic analyses. The lake is thermally and chemically stratified, with an anoxic hypolimnion from 270 m to the bottom (2167 m). The isotopic composition of dissolved helium and total carbon is similar to that of the main gas emissions of Colli Albani and of the phenocryst inclusions of the Alban volcanics, suggesting that an endogenous gas of deep provenance is injected into the lake water. The dissolved CO2 content is, however, far from saturation, and no Nyos-type hazardous gas cloud emission may presently occur in the lake. Temperature and chemical time variations indicate that water rollover episodes occur in harsh rainy winters when the surface lake temperature cools below 8.5 8C. Such rollovers tend to homogenize the physico-chemistry of the lake water and reduce the dissolved CO2 content. They may cause an environmental hazard because of related toxic algal blooms.