Natural Ventilation of Karstic Caves

Natural Ventilation of Karstic Caves:
New Data on the Nerja Cave
(Malaga, S of Spain)
C. Liñán and Y. del Rosal
Abstract In the Nerja Cave, there is a natural convective airflow which follows a
seasonal model common in caves known as ‘‘chimney’’ characterized by, at least,
two entrances at different altitudes. To explain this model, contradictory to the
known entrances of the Nerja Cave, located at the same height, some research has
been done in a surrounding cavity, known as the Pintada Cave. The obtained
results confirm the existence of a physical connection between the Nerja Cave and
the Pintada Cave, inaccessible to humans, and describe a very simplified, general
model of airflow circulation between them, which allows for the removal of
anthropogenic impact in the Nerja Cave during the most visited season in the year.
1 Introduction
The Nerja Cave, a good of cultural interest, in the category of Archeological Place
and an internationally recognized heritage sight of Geological Relevance, is one of
the most important tourist caves in Spain, with about 485,541 visitors annually for
the period 1988–2013. The cavity, with a horizontal development and a volume of
300,000 m3 (Fig. 1), has three entrances, which are located at 158, 161, and 162 m
above sea level (SEM 1985). About a third of the cave, the Tourist Galleries, is
C. Liñán (&) Y. del Rosal
Nerja Cave Foundation, Research Institute, Carretera de Maro s/n, 29787 Malaga
Nerja, Spain
e-mail: [email protected]; [email protected]
Y. del Rosal
e-mail: [email protected]
C. Liñán
Centre of Hydrogeology of University of Malaga, 29071 Malaga, Spain
C. Liñán
Faculty of Science, Department of Geology, University of Malaga, 29071 Malaga, Spain
Springer-Verlag Berlin Heidelberg 2015
B. Andreo et al. (eds.), Hydrogeological and Environmental Investigations
in Karst Systems, Environmental Earth Sciences 1,
DOI 10.1007/978-3-642-17435-3_57
505
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C. Liñán and Y. del Rosal
Fig. 1 a The Nerja Cave and The Pintada Cave: location of the airflows measurement stations
(a–e) and of the sensors. b Direction of airflows measured in the Nerja Cave and Pintada Cave
during 2013
open to tourists while the other part, the High and New Galleries, is only visited by
researchers and reduced groups of tourist. The cave has a microclimatic station
comprising of various sensors that measure, with hourly intervals, temperature,
relative humidity, and air concentration of 222Rn and CO2, among other parameters
(Carrasco et al. 2001; Liñán et al. 2009). Furthermore, a weather station measures
the environmental parameters outside the cave (Liñán et al. 2007).
The Pintada Cave is a small cavity near the Nerja Cave (Fig. 1) which has been
explored for some time by the possibility that the two caves were connected
(GEMA 1976). The speleological explorations were unsuccessful so, in 1979,
workers began to drill a well in the Pintada Cave, with the aim of building an
artificial connection with the Nerja Cave. The well, which reached 75–80 m deep,
was finalized in 1982 without reaching the intended aim.
2 Natural Ventilation in the Nerja Cave
In the Nerja Cave, there is a convective airflow due to the difference between the
outside and inside air density (Cañete 1997). The ventilation rate of the cave is
maximum in winter, 3.21 m3/sec, and minimum in summer, 0.04 m3/sec (Dueñas
et al. 1999).
Since 2008, radon concentration in the cave air is measured by a sensor RADIM
5WP (Rosal et al. 2010). From November to April, the outside air is cooler and
denser than the cave air (Fig. 2) so the air easily enters inside the cave, displacing
Natural Ventilation of Karstic Caves: New Data on the Nerja …
507
the indoor air, and decreasing the concentration of 222Rn in the cavity air to values
of the order of 80 Bq/m3. From May to October, the outside air is warmer and less
dense than the cave air, so it reduces the outside air inlet and the concentration of
222
Rn in the air of the cave is higher, to reach the daily average values over the
400 Bq/m3 from July to September.
In some sectors of the Nerja Cave, airflow can be discerned with a certain
intensity (Fig. 1a), its direction can be crudely measured, by dusting silty material.
The measurements show that from October to June, the airflow direction is predominantly from the Tourist Galleries to the deeper galleries of the cave (Fig. 1b).
By contrast, in July, August, and September, the airflow direction goes from the
deeper galleries to the Tourist Galleries. This circulation model is similar to the
one observed in cavities known as ‘‘chimney’’ (Choppy 1982; Buecher 1999),
characterized by, at least, two entrances at different altitudes. During the winter,
the colder and denser outside air enters the cavity through the entrance at bottom,
its temperature gradually increases and decreases its density and exits the cavity
through the entrance located at a higher level. In the summer, the indoor air is
colder and denser than the outside air, so the air comes out of the cave through the
entrance located at the bottom while it is replaced by the warmer outside air, which
enters the cave through the entrance located at a higher altitude.
As the three known Nerja Cave entrances are on a similar altitude (between 158
and 162 m.a.s.l.), the existence of a possible connection between the Nerja Cave
and the Pintada Cave (250 m.a.s.l.) was reconsidered although it was not practicable. To confirm this, in April 2013 air flow measures in the Pintada Cave began.
Shortly afterward, in June 2013, a sensor VAISALA GM70 equipped with probes
10
222Rn
(H-1)
500
6
4
Text-Tint (ºC)
400
Text<Tin
2
Text<Tint
300
0
-2
Text>Tint
200
-4
-6
Air 222Rn (Bq/m3).Nerja Cave (H-1)
8
600
Text-Tint(H-1)
100
-8
-10
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT NOV
DEC
0
Fig. 2 Temporal evolution of the temperature differences Text-Tint versus radon air concentrations measured in the Nerja Cave (Tourist Galleries). Daily average data for the period
2008–2013. Text external temperature; Tint internal temperature
508
C. Liñán and Y. del Rosal
for the measurements of air temperature, relative humidity, and air CO2 concentration was installed in the Pintada Cave. This was done in order to identify the
arrival of air from the Nerja Cave, marked by the antropic impact. At the same
time, in the Nerja Cave measurements of airflow and environmental parameters
continued.
3 Results and Discussion
The results obtained from the 114 airflow measurements made in 2013 (Fig. 1a)
show that from the middle of October to June, there is an airflow incoming in
the Nerja Cave and outgoing through the Pintada Cave (Fig. 1b). From July to
September, the direction of airflows is reversed: the outside air enters through the
Pintada Cave and heads to the Nerja Cave. These results are consistent with
the existence of a possible connection between both cavities.
Furthermore, the environmental parameters analysis shows that since the
beginning of the data series up to July 3rd, the CO2 air concentrations of the
Pintada Cave are between 250 and 2,000 ppm, with a mean value 478 ppm
(Fig. 3). Periodically CO2 increases are also detected in the air of the Pintada
Cave. Since July 5th, CO2 increases disappear almost completely in the data series
of the Pintada Cave and more uniform concentrations of CO2 in the air appear
between 250 and 580 ppm, with a mean value of 381 ppm. Until July 3rd, CO2
increases observed in the Pintada Cave would be associated with the air from the
Nerja Cave, as a result of the CO2 increase produced by the visitors.
On July 5th, the airflow inversion has already occurred. Thus, the external air,
with values of atmospheric CO2, comes into the Pintada Cave, through the New
and High Galleries, arrives at the Tourist Galleries (where the CO2 values are
increased by human impact) and finally goes outside the Nerja Cave. The CO2
measured in the air of the High Galleries in the Nerja Cave (Fig. 3, H-2) confirms
this point as it shows from July a progressive decrease until near to 480–500 ppm
as a result of dwindling supply of anthropogenic CO2. Before July 5th (from 13rd
to 18th June), a short episode with low CO2 concentrations in the Pintada Cave is
detected, and high concentrations of 222Rn in the Nerja Cave which corresponds to
a timely reversal of general ventilation, in response to a temporary rise in temperature differences external–internal (Fig. 4).
This general air circulation continues until October 9th, when higher values of
CO2 are registered in the Pintada Cave (Fig. 5). The decrease in temperature
differences external–internal produces a new reversal of air circulation and an
increase in the ventilation of the karstic network. Before that (from September
28th to September 30th) a point reversal airflow is determined due to the occurrence of high concentrations of CO2 in the Pintada Cave as well as in the High
Galleries in the Nerja Cave.
Natural Ventilation of Karstic Caves: New Data on the Nerja …
509
2400
700
2000
Air CO2 (ppm)
1800
500
1600
300
1400
1200
222
Nerja Cave (H-1, Rn)
Nerja Cave (H-2, CO2)
Pintada Cave (CO2)
1000
100
-100
800
600
-300
Air 222 Rn (Bq/m3). Nerja Cave (H-2)
900
2200
400
-500
1
4
7
10
13
16
19
22
25
28
1
4
7
10
13
16
19
22
25
28
31
3
6
9
12
15
18
21
24
27
30
200
JUNE
JULY
AUGUST
12,0
2000
8,0
1600
4,0
1200
0,0
800
-4,0
400
-8,0
0
Text-Tint (ºC)
2400
-12,0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
10,0
1000
900
6,0
800
700
2,0
600
-2,0
500
Text-Tint (ºC)
Air CO2 (ppm). Pintada Cave Air CO2 (ppm). Pintada Cave
Fig. 3 Temporal evolution of the air 222Rn and air CO2 concentration in the Nerja Cave and air
CO2 concentration in the Pintada Cave, from June to August 2013
-6,0
400
300
-10,0
15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1
2
3
4
5
6
7
8
9 10 11 12 13 14
Fig. 4 Temporal evolution of the air CO2 concentration in the Pintada Cave (red) and the
temperature difference Text-Tint (black) from June 1st to 30th (top) and September 15th to
October 14th, 2013 (below)
510
C. Liñán and Y. del Rosal
Nerja Cave (H-1, 222 Rn)
Nerja Cave (H-2, CO2)
Pintada Cave (CO2)
2200
2000
700
500
1600
300
1200
100
1000
-100
800
600
222
3
1400
Air Rn
Air CO2 (ppm)
1800
(Bq/m ). Nerja Cave (H-2)
900
2400
-300
400
SEPTEMBER
31
28
22
25
16
19
13
7
10
4
1
28
25
22
16
19
13
7
10
4
-500
1
200
OCTOBER
Fig. 5 Temporal evolution of the air 222Rn and air CO2 concentration in the Nerja Cave (H-1
and H-2 Halls, respectively) and air CO2 concentration in the Pintada Cave, during September
and October 2013
4 Conclusions
The results confirm the existence of a physical connection between the Nerja Cave
and the Pintada Cave, not accessible to humans, and establish a very simplified
general model of airflow circulation. From October to June, outside air enters the
Tourist Galleries of the Nerja Cave, which is enriched in CO2 as a result of
the contribution of visitors. This anthropogenic CO2 goes to the High and New
Galleries of the Nerja Cave and finally arrives at the Pintada Cave and exits. From
July to September, the airflow direction is reversed: the outside air, with atmospheric CO2 values, enters the Pintada Cave, through the High and New Galleries
of the Nerja Cave, arrives at the Tourist Galleries (where it is enriched with
anthropogenic CO2), and finally exits. This airflow model contributes to the
elimination of anthropogenic impact in the Nerja Cave during the summer, when
the cave receives the highest number of visitors. The existence of a new access to
the karstic network of the Nerja Cave (the Pintada Cave) is a topic that must
necessarily be considered for appropriate management and conservation.
Acknowledgments We thank Jose Manuel Cabezas Cabello for his collaboration in the translation of this paper.
Natural Ventilation of Karstic Caves: New Data on the Nerja …
511
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