Geochemistry of the Albano and Nemi crater lakes in the volcanic district of Alban Hills

Lake Albano, located 20 km to the SE of Rome, is hosted within the most recent crater of the quiescent Alban
Hills volcanic complex that produced hydromagmatic eruptions in Holocene times. Stratigraphic,
archaeological and historical evidence indicates that the lake level underwent important variations in the
Bronze Age. Before the IV century B.C. several lahars were generated by water overflows from the lake and in
the IV century B.C. Romans excavated a drainage tunnel. The lake is located above a buried carbonate horst
that contains a pressurized medium-enthalpy geothermal reservoir from which fluids escape to the surface
to produce many important gas manifestations of mostly CO2. Previous studies recognized the presence of
gas emissions also from the crater bottom. In 1997 the possibility of a Nyos-type event triggered by a lake
rollover was considered very low, because the CO2 water concentration at depth was found to be far from
saturation. However, considering the high population density nearby, the Italian Civil Protection Department
recommended that periodical monitoring be carried out. To this scope we initiated in 2001 a systematic
geochemical study of the lake. Thirteen vertical profiles have been repeatedly carried out in 2001–2006,
especially in the deepest part of the lake (167 m in 2006), measuring T, pH, dissolved O2 and electrical
conductivity. Water samples were collected from various depths and chemically and isotopically analysed.
Two similar profiles have been measured also in the nearby Nemi crater lake. Results indicate that in the
4.5 years of monitoring the pressure of gas dissolved in the Lake Albano deep waters remained much lower
than the hydrostatic pressure. A CO2 soil survey carried out on the borders of the two lakes, indicates the
presence of some zones of anomalous degassing of likely magmatic origin. A water overturn or a heavy
mixing of deep and shallow waters likely occurred in winter 2003–2004, when cold rainfall cooled the
surface water below 8.5 °C. Such overturns cause only a limited gas exsolution from the lake when the deep
water is brought to a few meters depth but can explain the observed decrease with time of dissolved CO2 at
depth and related water pH increase. A gas hazard could occur in the case of a sudden injection through the
lake bottom of a huge quantity of CO2-rich fluids, which might be caused by earthquake induced fracturing of
the rock pile beneath the lake. A limnic gas eruption might also occur should CO2 concentration build up
within the lake for a long time.

Amato, A., Chiarabba, C., Cocco, M., Di Bona, M., Selvaggi, G., 1994. The 1989–1990
seismic swarm in the Alban Hills volcanic area, central Italy. J. Volcanol. Geotherm.
Res. 61, 225–237.
Anzidei, M., Baldi, P., Casula, G., Galvani, A., Riguzzi, F., Zanutta, A., 1998. Evidence of
active crustal deformation of the Colli Albani volcanic area (Central Italy) by gps
surveys. J. Volcanol. Geotherm. Res. 80, 55–65.
Anzidei, M., Esposito, A., De Giosa, F., 2006. The dark side of the Albano crater lake. Ann.
Geophys. 49 (6), 1275–1287.
Barberi, F., Chelini, G., Marinelli, G., Martini, M., 1989. The gas cloud of Lake Nyos
(Cameroon, 1986): results of the Italian technical mission. J. Volcanol. Geotherm.
Res. 39, 125–134.
Beaubien, S.E., Ciotoli, G., Lombardi, S., 2003. Carbon dioxide and radon gas hazard in
the Alban Hills area (Central Italy). J. Volcanol. Geotherm. Res. 123, 63–80.
Caputo, C., Ciccacci, S., D'Alessandro, L., Davoli, L., Fredi, P., La Monica, G.B., Lupia
Palmieri, E., Pugliese, F., Raffi, R., 1986. Progetto “Lago Albano”. Indagini climatiche,
geomorfologiche e sedimentologiche. Provincia di Roma 1–14.
Carapezza, M.L., Badalamenti, B., Cavarra, L., Scalzo, A., 2003. Gas hazard assessment in a
densely inhabited area of Colli Albani volcano (Cava dei Selci, Roma). J. Volcanol.
Geotherm. Res. 123, 81–94.
Carapezza, M.L., Granieri, D., 2004. CO2 soil flux at Vulcano (Italy): comparison between
active and passive methods. Appl. Geochem. 19, 73–88.
Carapezza, M.L., Barberi, F., Tarchini, L., Cavarra, L., Granieri, D., 2005. Le emissioni
gassose dell'area vulcanica dei Colli Albani. In: Carapezza, M.L., et al. (Ed.), Nuovi
dati sull'attività recente del cratere del Lago Albano e sul degassamento dei Colli
Albani. Atti Acc. Naz. Lincei, 218, pp. 229–242.
Carapezza, M.L., Tarchini, L., 2007. Accidental gas emission from shallow pressurized
aquifers at Alban Hills volcano (Rome, Italy): geochemical evidence of magmatic
degassing? J. Volcanol. Geotherm. Res. 165, 5–16.
Chiarabba, C., Amato, A., Delaney, P.T., 1997. Crustal structure, evolution, and volcanic
unrest of the Alban Hills, Central Italy. Bull. Volcanol. 59, 161–170.
Chiarucci, P., 1987. Indagini sul Lago di Albano. Documenta Albana, ii serie 9, 19–29.
Chiodini, G., Frondini, F., 2001. Carbon dioxide degassing from the Alban Hills volcanic
region, Central Italy. Chem. Geol. 177, 67–83.
Chiodini, G., Cioni, R., Guidi, M., Raco, B., Marini, L., 1998. Soil CO2 flux measurements in
volcanic and geothermal areas. Appl. Geochem. 13, 543–552.
Cioni, R., Guidi, M., Raco, B., Marini, L., Gambardella, B., 2003. Water chemistry of Lake
Albano (Italy). J. Volcanol. Geotherm. Res. 120, 179–195.
De Rita, D., Narcisi, B.M., 1983. I prodotti dell'attività esplosiva di Nemi (Colli Albani,
Roma). I caratteri deposizionali dell'unità piroclastica intermedia. Rend. Soc. Geol.
It. 6, 17–20.
De Jong, E., Schappert, H.J.V., 1972. Calculation of soil respiration and activity from CO2
profiles in the soil. Soil Sci. 113, 328–333.
Elwood, N.T.W., Albertano, P., 2005. A seasonal survey of the water quality and
phytoplankton dynamics of Lake Albano. In: Carapezza,et al. et al., M.L. (Ed.), Nuovi
dati sull'attività recente del cratere del lago Albano e sul degassamento dei Colli
Albani. Atti Acc. Naz. Lincei, vol. 218, pp. 250–251.
Funiciello, R., Giordano, G., De Rita, D., Carapezza, M.L., Barberi, F., 2002. L'attività
recente del cratere del Lago Albano di Castelgandolfo. Rend. Fis. Acc. Lincei 9–13,
113–143.
Funiciello, R., Giordano, G., De Rita, D., 2003. The Albano Maar Lake (Colli Albani
volcano, Italy): recent volcanic activity and evidence of pre-Roman age catastrophic
lahar events. J. Volcanol. Geotherm. Res. 123, 43–61.
Guilizzoni, P., Oldfield, F., 1996. Palaeoenvironmental analysis of Italian crater lakes and
Adriatic sediments (Paliclas). Mem. Ist. It. Idrobiol. 55.
Longinelli, A., Selmo, E., 2003. Isotopic composition of precipitation in Italy: a first
overall map. J. Hydrol. 270 (270), 75–88.
Martelli, M., Nuccio, P.M., Stuart, F.M., Burgess, R., Ellam, R.M., Italiano, F., 2004. Heliumstrontium
isotope constraints on mantle evolution beneath the Roman comagmatic
province, Italy. Earth Planet. Sci. Lett. 224, 295–308.
Martini, M., Giannini, L., Prati, F., Tassi, F., Capaccioni, B., Iozzelli, P., 1994. Chemical
characters of crater lakes in the Azores and Italy: the anomaly of Lake Albano.
Geoch. Am. J. 28, 173–184.
Oldfield, F.,1996. The Paliclas project: synthesis and overview. In: Guilizzoni, P., Oldfield,
F. (Eds.), Palaeoenvironmental analysis of Italian crater lakes and Adriatic
sediments. Mem. Ist. It. Idrobiol., 55, pp. 329–356.
Peccerillo, A., Panza, G.F., 1999. Upper mantle domains beneath Central-Southern Italy:
petrological, geochemical and geophysical constraints. Pure Appl. Geophys. 156,
421–443.
Pedreschi, L.,1995. I centri lacuali della penisola italiana. III: I centri dei laghi vulcanici di
Castel Gandolfo e di Nemi e quelli dei bacini artificiali. Acc. Lucchese Sci. Lett. Arti,
Studi e testi, Lucca 39 136 pp.
Pizzino, L., Galli, G., Mancini, M., Quattrocchi, F., Scarlato, P., 2002. Natural gas hazard
(CO2, 222Rn) within a quiescent volcanic region and its relation with tectonics: the
case of the Ciampino-Marino area, Alban Hills volcano, Italy. Nat. Haz. 00, 1–35.
Rice, A., 2000. Rollover in volcanic crater lakes: a possible cause for Lake Nyos type
disasters. J. Volcanol. Geotherm. Res. 97, 233–239.
Riguzzi, F., Anzidei, M., Baldi, P., Casula, G., Galbani, A., Pesci, A., Pietrantonio, G., 2002.
Rilievi gps per il controllo delle deformazioni nell'area dei Colli Albani (Roma). Atti
della VI Conferenza Naz. Asita, Perugia 2, 1797–1802 Nov. 2002, vol.
Sigurdsson, H., Devine, J.D., Tchova, F.M., Presser, T.S., Pringle, M.K.W., Evans,W.C., 1987.
Origin of the lethal gas burst from Lake Monoun, Cameroon. J. Volcanol. Geotherm.
Res. 31, 1–16.
Sinclair, A.J., 1974. Selection of threshold values in geochemical data using probability
graphs. J. Geochem. Explor. 3, 129–149.