Geochemistry of the Albano crater lake M. L. CARAPEZZA 1 *, M. LELLI 2 & L. TARCHINI 3 1 Istituto Nazionale di Geofisica e Vulcanologia – Sezione Roma 1, Via Vigna Murata 605, 00143 Roma, Italy 2 Istituto di Geoscienze e Georisorse – Consiglio Nazionale delle Ricerche, Via G. Moruzzi 1, 56124 Pisa, Italy 3 Dipartimento di Scienze Geologiche – Universita ` Roma Tre, L.go San L. Murialdo 1, 00146 Roma, Italy *Corresponding author (e-mail: marialuisa.carapezza@ingv.it) Abstract: 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 anoma- lous 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 CO 2 content is, however, far from saturation, and no Nyos-type hazar- dous 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 CO 2 content. They may cause an environmental hazard because of related toxic algal blooms. Albano lake, located 20 km to the SE of Rome, is within the most recent crater of the quiescent Colli Albani volcanic complex (Fig. 1). All the most recent hydromagmatic eruptions of Colli Albani have been generated from this crater, which has also given rise to several episodes of lahar gener- ation through overflow of the lake water, the most recent of which occurred in Roman times in the fourth century BCE (Funiciello et al. 2002, 2003; De Benedetti et al. 2008). Albano is the deepest Italian crater lake (2167 m in November 2005; Anzidei et al. 2008), but its level has been dropping recently because of water over-exploitation from the surrounding aquifer (Capelli & Mazza 2005). The lake has an elliptical shape because of the coale- scence of at least five different craters, the most recent of which is the deepest southern one (Anzidei et al. 2008). Morphological, stratigraphi- cal and archaeological evidence shows that the lake has undergone several important level changes since the Bronze Age (Funiciello et al. 2003; Anzidei et al. 2008). In 394 BCE, Roman engineers dug a tunnel in the crater wall to keep the water level 74 m below the lowest crater rim, with the aim of preventing further hazardous overflows (Funiciello et al. 2003 and references therein). The lake is located above a NW–SE oriented buried carbonate structural high (the Ciam- pino high), which hosts the main regional aquifer from which fluids escape to the surface through faults and fractures and give rise to several cold gas manifestations (Carapezza et al. 2003, 2005, 2007). The presence of gas emissions from the lake bottom has also been recognized (Caputo et al. 1986). In addition to endogenous gas emis- sions, the Albano Lake area is affected by episodic shallow seismic swarms and by an anomalous uplift (Amato & Chiarabba 1995; Chiarabba et al. 1997; Chiarabba et al. 2010). A systematic study of the geochemistry of the lake (Carapezza et al. 2008 and references therein) was conducted to assess whether its present con- dition could produce the hazardous phenomena of water rollover and gas exsolution, as has happened, for example, in the crater lakes of Monoun and Nyos in Cameroon (Sigurdsson et al. 1987; Barberi et al. 1989; Sigvaldason 1989; Rice 2000). Since 2003 the lake has been regularly investigated by physico- chemical vertical profiles and by sampling and analysing its water and dissolved gases. From:Funiciello, R. & Giordano, G. (eds) The Colli Albani Volcano. Special Publications of IAVCEI, 3, 259–267. Geological Society, London. 1750-8207/10/$15.00 # IAVCEI 2010.