Gas hazard assessment at the Monticchio crater lakes of Mt. Vulture, a volcano in Southern
Italy
Antonio Caracausi*, P. Mario Nuccio†*, Rocco Favara*, Marco Nicolosi†, Michele Paternoster‡
*Istituto Nazionale di Geofisica e Vulcanologia, sezione di Palermo,
†Dipartimento Chimica e Fisica della Terra e Applicazioni, Università di Palermo,
‡ Dipartimento Scienze Geologiche, Università della Basilicata,
Corresponding author: Antonio Caracausi
Istituto Nazionale di Geofisica e Vulcanologia, sezione di Palermo, via Ugo La Malfa 153, 90146
Palermo (Italy)
Tel. +39 091 6809491
e-mail a.caracausi@pa.ingv.it
Short title: Gas hazard from crater lakes
1
Abstract
Geochemical investigations have shown that there is a considerable inflow of gas into both crater
lakes of Monticchio, Southern Italy. These lakes are located in two maars that formed 140,000
years ago during Mt. Vulture volcano’s last eruptive activity. Isotopic analyses suggest that CO2
and helium are of magmatic origin; the latter displays 3He/4He isotope ratios similar to those
measured in olivines of the maar ejecta. In spite of the fact that the amount of dissolved gases in the
water is less than that found in Lake Nyos (Cameroon), both the results obtained and the historical
reports studied indicate that these crater lakes could be highly hazardous sites, even if they are
located in a region currently considered inactive. This could be of special significance in very
popular tourist areas such as the Monticchio lakes, which are visited by about 30,000 people
throughout the summer, for the most part on Sundays.
Keywords: volcanic gases; gas hazard; crater lakes.
Introduction
A review of recent bibliography has highlighted the fact that only a few crater lakes have ever been
investigated (i.e., Delmelle and Bernard, 2000; Gunkel et al., 2008). The tragedies of Lake
Monoum and Lake Nyos, August 15th 1984 and August 21st 1986 respectively, have given new
insight into the causes of catastrophic events related to volcanic lakes. A lake can act as a reservoir
of deep fluids and can store large amounts of potentially lethal gas in their waters. During the above
mentioned events, massive amounts of CO2 were expelled, causing loss of human life and thereby
turning a volcanic lake into a killer (i.e., Kerr, 1986; Kling, 1987; Sano et al., 1990). Because of the
highly hazardous nature of crater lakes, a lot of new scientific studies have been effected to evaluate
and, possibly, mitigate gas hazard. Furthermore, volcanic lakes situated in volcanic areas that are no
longer active, have so far only been investigated with the aim of evaluating gas flux from the
2
subcontinental mantle (i.e., Igarashi et al., 1992; Aeschbach-Hertig et al., 1999) and/or of making
paleoclimatic reconstructions (i.e., Allen et al., 1999).
Two explosive maar craters, occupied by lakes Lago Piccolo (LPM) and Lago Grande (LGM), are
located on Mt. Vulture (a volcano in Southern Italy), which is an area of intense CO2 degassing
(Gambardella et al., 2004). However, previous geochemical studies focused on the Monticchio
lakes have only recognized the gas-rich nature of LPM’s waters, although they have also considered
the possibility of a limnic eruption in this lake (Chiodini et al., 2000 and references within; Cioni et
al., 2006). In contrast, previous investigations carried out on LGM have mainly focused on
paleoclimatic aspects (i.e., Allen et al., 1999). Our investigations, carried out in July 2007, deal
with the nature and dynamics of the gas recharge in both of these lakes. The carbon isotopic
signature of CH4 and CO2 and the isotopic composition of the noble gases have allowed us to
assess, for the first time, both the origin of the fluids stored in the lakes and the mechanisms
controlling the accumulation of dissolved gases in the waters. With the aim of quantifying the
amount of each gaseous species contained in the lake water, we performed a morpho-bathymetric
survey of the two lakes (Fig. 1). Moreover, to improve our knowledge of these lakes, we also
carefully perused all the relative historical reports available. In detail, our historical investigations
(Tata, 1778; Palmieri and Scacchi, 1852; Ciarallo and Capaldo, 1995) have allowed us to more
completely assess the gas hazard scenario of this area, and we have come to the conclusion that
triggering processes, totally different from a landslide and/or a seismic shock, should be also taken
into due consideration. In fact, there have been gas bursts in both the Monticchio lakes up to no
later than 200 years ago, which indicates that they have a recurring-character, even if the temporal
intervals have not been constant (Tata, 1778; Palmieri and Scacchi, 1852; Ciarallo and Capaldo,
1995). A paroxistic activity occurred on June 1st 1810 when a six-meter-high water column formed
in LGM. A new event was observed two months later on July31st, when an intense roar was also
heard; that time the column of water reached a height of about 3 meters. Ten years later, a similar
paroxistic event also occurred in LPM; historical reports state that the water spouts surged upwards
3
out of the lake, reaching a height of five meters. There were no fatalities, although a lot of dead fish
were found on the land, a few meters from the shore, thus bearing witness to the explosive nature
and gas toxicity of the event. It is worth noting that during the subsequent 10 years (before the year
1820), every so often LPM reached similarly dangerous conditions when all the fish living in LPM
were killed (Ciarallo and Capaldo, 1995).
Since this research has also highlighted the fact that catastrophic gas release events have occurred in
the past at both of the lakes, the aim of this study was also to re-evaluate gas hazard both at LPM,
eleven years after the last survey, and at LGM, the waters of which, as far as we know, have never
undergone any geochemical investigations. This has provided new insight into gas hazard from
lakes when they are hosted by inactive volcanoes.
The maar lakes of Monticchio
The two lakes are situated in the maar explosion craters of Monte Vulture (Italy), a complex stratovolcano, the activity of which began in the middle Pleistocene (Stoppa and Principe, 1998). The
most recent explosive activity of Mt. Vulture lead to the formation of the two maar craters ~ 140 ka
ago (Buttner et al., 2006) after a long inter-eruptive period of ~ 350 000 years (Stoppa and Principe,
1998). Both the LPM and LGM maar craters were created by intense explosive volcanic activity
when a relatively small volume of erupted magma sprayed out over a large area, today occupied by
a number of towns (Stoppa and Principe, 1998; Giannandrea et al., 2006). These explosions were
characterized by dry surge mechanisms and triggered by CO2 expansion (Stoppa and Principe,
1998). Buttner et al. (2006) highlighted the fact that Mt. Vulture’s volcanic activity continued over
clusters of ages and was controlled by the activation-deactivation of well-defined regional tectonic
discontinuities. Furthermore, this fact supports the possibility that explosive events of this kind
could occur again in the near future, along the same regional tectonic system faults (Buttner et al.,
2006). Only a few geochemical investigations on the gas and water chemistry have ever been
carried out, and almost exclusively on LPM (Chiodini et al., 2000 and references within; Cioni et
4
al., 2006). These highlighted that LPM is a meromictic lake, permanently stratified owing to the
presence of a vertical (chemical) density gradient, with a stagnant deep water layer, named
monimolimnio, which is not affected by seasonal mixing. The temperature profiles we recorded at
different points of the lake also highlighted a temperature stratification of the LPM (layers from the
surface towards the bottom: epilimnio, termocline, hypolimnio); the temperature increases towards
the bottom in the stagnant deep waters of the hypolimnio, indicating a heat flow from the bottom
into the waters of the lake (Fig. 2).
Methods
The waters were sampled to determine the chemical and isotopic composition of the dissolved
gases, by using stainless steel cylindrical samplers equipped with two pneumatic valves at the two
ends. These were controlled by compressed air, which was supplied by a small air-compressor. The
concentrations of the dissolved gases and the isotopic compositions of He were determined in the
laboratory by using classical methods of gas extraction from waters (Capasso and Inguaggiato,
1998). Chemical abundances of He, H2, O2, CH4 CO2 were measured by a Clarus 500 gaschromatograph, equipped with a 4 m Carboxen column and double detector (HWD and FID), while
using N2 as the gas-carrier. The detection limits were about 5 ppm vol. for O2, and CO2; 2 ppm vol.
for H2 and He, 0.1 ppm vol. for CO and CH4.
Helium isotope analyses were carried out by a static vacuum mass spectrometer (GVI Helix
SFT) double collector in order to detect 3He and 4He ion beams simultaneously; this method
brought the error of the 3He/4He measurements down to very low values. Quantitative and isotopic
analyses (13CTDIC) were carried out by using an AP 2003 IRMS mass spectrometer, on Total
Dissolved Inorganic Carbon (TDIC), after its precipitation, following the technique proposed by
(Gleason et al., 1969). The carbon isotope ratios in the gas phases (CO2 and CH4) were measured
by using a Thermo Delta XP IRMS (external precision ± 0.15) coupled with TRACE GC gaschromatograph., separated using a 30m Restek Q-plot (i.d. 0.32).
5
Origin of the gases dissolved in the Monticchio Lakes
CO2 and CH4 are the major gaseous species dissolved in the waters of both the lakes, while noble
gases are present in trace amounts (Table 1). Almost the entire quantity of CH4 and CO2 is
dissolved in the deep waters of the lakes, the amounts of which are at least four orders of magnitude
above equilibrium with the atmosphere (Table 1). We measured δ13C(CH4) values, ranging between
-61.7‰ and -65.7‰ vs. PDB; these display typical biogenic isotopic signatures and strongly
support Chiodini et al. (2000) and Cioni et al.’s (2006) suggestion that CH4 is of biogenic origin,
although these authors did not have any carbon isotopic data. Previous investigations have also
suggested that CO2 and CH4 have a common biogenic origin, although a possible inorganic
contribution of CO2 has not been ruled out (Cioni et al., 2006). It is worth noting that, normally,
biogenic natural gases are predominantly CH4 (Whiticar et al., 1986), whereas the high CO2/CH4
ratios of the gases dissolved in the waters of the two lakes suggest a significant contribution of CO2
of inorganic origin. Furthermore, the isotopic δ13C values of the total dissolved carbon measured in
our samples from LPM (i.e. between -0.27 and -3.91‰ vs. PDB) do not fall within typical
organogenic values, and instead lie more in the range of those measured in the magmatic fluids of
Italian volcanoes (e.g. Allard et al., 1997), thus suggesting a mainly magmatic origin of the
carbonate species dissolved in the water. Even though analogous δ13C values of total dissolved
carbon in LGM are not available, if we take into due consideration the fact that the two lakes are
very close to each other (i.e. about 150 m. apart) and that the CO2/CH4 (= 12.8) measured at the
bottom of LGM is higher than that measured at LPM (CO2/CH4=3), it would be reasonable to
believe that the CO2 dissolved in LGM is also mainly of magmatic origin.
The helium concentrations in the waters of both the lakes are two orders of magnitude above
equilibrium with the atmosphere (Table 1). Furthermore, below the shallower layers the helium
isotopic ratio is almost constant along each profile in both lakes (Fig. 4). The highest ratios, 6.1 Ra
(Ra is the 3He/4He in atmosphere=1.39x10-6), were measured at LPM in July 2006. They are higher
6
than the previously reported values (Chiodini et al., 2000), 5.5 Ra, and are indistinguishable from
those measured in the fluid inclusions of olivine found in LPM’s explosion ejecta (Martelli et al.,
2007), clearly indicating a magmatic origin of the dissolved helium. Therefore both CO2 and
helium, strongly suggest a still active inflow of magmatic gases.
Hazard assessment
Although CO2 is the most abundant dissolved gaseous species in the lower layers of water in the
Monticchio lakes, its abundance is far below that of saturation. In fact, the highest concentrations
measured were 312.4 and 446.9 ccSTP/liter for LPM and LGM respectively (Table 1). These
figures are only a small fraction of the maximum possible CO2 solubility for these lakes that are
5,700 cc STP/liter for LPM and 6,020 cc STP/liter for LGM (respectively at P= 480 kPa and T=
8.9°C and at P=460 kPa and T=6.1°C), following the model proposed by Duan and Sun (2003).
According to (Cioni et al., 2006) LPM is stratified and meromictic, as the deep part of its waters
remains unmixed. Furthermore, our data on the dissolved solute indicate that LGM is also
chemically stratified. This is mainly caused by the relatively high amounts of CO2 dissolved in the
deeper waters of the two lakes (Fig. 3), which has a great effect on both the density of the water and
the stability of its stratification. In fact, this situation gives rise to a strong contrast in density,
thereby implying the immiscibility between the monimolimnio (deep high density layer) and the
mixolimnio (low density shallow layer).
Cioni et al., (2006) invoke a landslide and/or a seismic shock as being the potential triggering
process of an eventual future massive gas release from LPM. The driving force could be an overturn
of the lake’s water should dissolved gas pressure exceed total pressure on account of a sudden
upward surge of the deeper gas-rich waters into shallower levels,. Taking into account the quantities
of CH4 and CO2 dissolved in the waters of the two lakes, we calculated a total amount of about 9
tons of CH4 and 386 tons of CO2, dissolved in the LGM, and 83 tons of CH4 and 814 tons of CO2
dissolved in the LPM.
7
We then estimated the equivalent volume of gases (CH4 and CO2) that a hypothetical instantaneous
total release of the dissolved gases could generate. In point of fact, these gases could reach a
maximum volume of 2.3x105 m3 for LGM and of 5.8x105 m3 for LPM, at 1 bar. The thickness of
this lethal gas mixture at LGM would be ~0.6 m and would cover the entire surface of the lake (i.e.
427,000 m2) while at LPM it would be about 3.5 m and would also cover the entire surface of the
lake (i.e. 172,000 m2). Although these volumes are only a fraction of those stored in Lake Nyos, it
is worth noting that the Monticchio lakes are popular tourist places and attract about 30,000 visitors
throughout the summer, for the most part on Sundays (Hansen, 1993), thereby increasing the related
risks on those days.
It is also worth noting that both the lakes are located inside craters. Moreover, the hills bordering
LPM have high, steep flanks which the released gases would probably not be able to rise over and
would therefore remain trapped in the area. These gases could only escape through a narrow sector
of LPM’s shore, close to LGM (see Figure 1), where most of the tourist traffic is concentrated.
Hence, in the absence of wind, the topography of the area could entrap a huge amount of gases,
eventually freed by an instantaneous event of massive release, which could hazardously remain in
the area.
The study of historical documents (Tata, 1778; Palmieri and Scacchi, 1852; Ciarallo and Capaldo,
1995) has highlighted the fact that events of massive gas release from the Monticchio maar lakes
have occurred in the past. This knowledge, together with our results, have allowed us to assess the
gas hazard scenario of this area more in depth. In fact, we have come to the conclusion that
triggering processes, totally dissimilar from a landslide and/or a seismic shock, should be also taken
into due consideration. Therefore, taking into account: (i) the heat flow from the bottom of LPM;
(ii) the amount of magmatic helium having the same isotopic signature of olivine-hosted fluid
inclusions of LPM’s maar ejecta; (iii) the high concentration in the waters of inorganic CO2, having
an isotopic signature in the range of emissions from active regional volcanoes; (vi) the fact that the
two lakes are located in an area of intense CO2 degassing (Gambardella et al., 2004), it is
8
reasonable to assume that analogous dangerous outgassing phenomena could happen again in the
future, whether caused by CO2 accumulation in the crust (Chivas et al., 1987), or by outgassing of
CO2-oversaturated magma, as its presence at depth (in the continental crust or below it) cannot be
fully ruled out.
Finally, we trust that the results obtained by this study will draw attention to all those crater lakes
located in volcanic areas that are apparently no longer active and especially to those situated in
popular tourist areas, where gas risk is probably underestimated or even ignored.
Acknowledgements
The comments of both M. Burton and another anonymous reviewer, together with those of the
Associate Editor have improved this paper. We also thank Dr. F. Pisciotta for his assistance in the
bathymetric reconstruction of the lakes and Mrs. Penelope Dyer for having reviewed the English.
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Table 1 Helium isotopic signature and chemical composition of the gases dissolved in the waters of
the Monticchio lakes
Site
Depth
LPM1
July 2007
3
He/4He
as R/Ra
He
O2
CH4
CO2
-0,3
-3
-8
-13
-18
-20
-24
-33
-35
-37
1.4E-04
6.4E-05
1.2E-03
1.3E-03
1.4E-03
1.2E-03
8.7E-04
7.0E-04
1.58
1.22
2.33
0.42
0.09
0.07
0.09
0.08
0.08
0.08
0.22
4.97E-03
1.11E-03
0.57
53.54
55.94
65.51
75.46
87.24
102.92
2.2
3.1
13.1
11,1
172.3
200.8
230.7
241.0
297.4
292.0
5.34
5.30
5.79
5.78
5.82
5.80
5.75
5.83
LPM2
July 2007
-16
-21
-24
-28
-32
-36,5
9.7E-04
1.3E-03
1.4E-03
1.9E-03
1.4E-03
8.6E-04
0.11
0.05
0.09
0.08
0.07
0.07
38.09
58.98
63.48
70.97
83.05
115.20
150.9
227.7
234.1
245.2
258.9
312.4
5.76
5.77
5.76
5.74
5.81
5.77
LP
July 2006
-33
-36
LGM
July 2007
-0,5
-5
-10
-15
-20
-23
-26
-29
-32
-35
6.1
6.1
5.3E-05
6.3E-05
2.9E-04
6.9E-04
9.8E-04
1.1E-03
1.2E-03
1.2E-03
1.3E-03
3.41
0.27
0.04
0.22
0.17
0.05
0.09
0.06
0.11
0.02
0.55
1.33
4.38
15.36
24.66
27.93
27.98
28.86
33.05
0.4
7,8
14.8
66.3
149.8
357.4
433.3
460.1
450.6
446.9
2.63
3.71
5.60
5.79
5.78
5.82
5.75
5.76
5.77
The He/Ne ratio is generally higher than 9 and lower than 1 only in those samples taken from a
depth of -0.5 to -15 meters. These ratios highlight a very low atmospheric helium contribution.
12
Figure captions :
Fig. 1 Bathymetric surveys of the Monticchio lakes were carried out by measuring depth and
geographic coordinates of 1,000 and 1,200 points in LPM and LGM respectively. A system made
up of a palmtop computer connected to a GPS Garmin V personal navigator and a Garmin
fishfinder 250C allowed us to speedily and simultaneously measure the depth and the geographical
position of the points. The precision of the Garmin fishfinder and GPS Garmin are ± 0.5 and ± 5.0
meters respectively. The volumes of the lakes were calculated to be 3,98x106 m3 and 3,25x106 m3
for LPM and LGM respectively. These volumes accurately fit previous estimates (Chiodini et al.,
2000 and references within).
Fig. 2 Temperature versus depth for LGM and LPM. The temperature profiles of the Monticchio
lakes were measured by using a thermo-phreatimeter OTR-OG15, the error of which lay within
0.1°C readability of the thermometer. According to previous investigations (Chiodini et al., 2000
and references within; Cioni et al., 2006), our data indicates an increase in temperature from the
bottom of LPM’s monimolimnio which underlines the presence of a homogeneous heat flux from
the bottom of the lake.
Fig. 3 Dissolved solutes: (i) salts [the water was sampled at different depths by using a peristaltic
pump so that chemical analyses could be carried out using the same field-procedures and
instruments described in Federico et al., (2008)]; (ii) salts plus dissolved CO2; (iii) salts plus CO2
and CH4.. The high amounts of dissolved CO2 in the deep water is the main cause of the
stratification of the lakes’ water volumes where shallow waters float on the deeper ones. Our data,
together with those collected during previous investigatios (Cioni et al., 2006 and references
within), underline that this big difference between the dissolved CO2 and salt in the bottom and
shallow waters of LPM is maintained throughout the year.
13
Fig. 4 3He/4He (as R/Ra) vs. depth
14