Natural Ventilation of Karstic Caves: New Data on the Nerja Cave (Malaga, S of Spain)

Transcription

1 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. 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 85,5 visitors annually for the period The cavity, with a horizontal development and a volume of,000 m (Fig. ), has three entrances, which are located at 58,, and 2 m above sea level (SEM 985). 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, 298 Malaga Nerja, Spain cbaena@cuevadenerja.es; crilinbae@uma.es Y. del Rosal yolanda@cuevadenerja.es C. Liñán Centre of Hydrogeology of University of Malaga, 290 Malaga, Spain C. Liñán Faculty of Science, Department of Geology, University of Malaga, 290 Malaga, Spain Ó Springer-Verlag Berlin Heidelberg 205 B. Andreo et al. (eds.), Hydrogeological and Environmental Investigations in Karst Systems, Environmental Earth Sciences, DOI.0/ _5 505

2 50 C. Liñán and Y. del Rosal Fig. 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 20 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 2 Rn and CO 2, among other parameters (Carrasco et al. 200; Liñán et al. 2009). Furthermore, a weather station measures the environmental parameters outside the cave (Liñán et al. 200). The Pintada Cave is a small cavity near the Nerja Cave (Fig. ) which has been explored for some time by the possibility that the two caves were connected (GEMA 9). The speleological explorations were unsuccessful so, in 99, 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 5 80 m deep, was finalized in 982 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 99). The ventilation rate of the cave is maximum in winter,.2 m /sec, and minimum in summer, 0.0 m /sec (Dueñas et al. 999). Since 2008, radon concentration in the cave air is measured by a sensor RADIM 5WP (Rosal et al. 20). 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

3 Natural Ventilation of Karstic Caves: New Data on the Nerja 50 the indoor air, and decreasing the concentration of 2 Rn in the cavity air to values of the order of 80 Bq/m. 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 2 Rn in the air of the cave is higher, to reach the daily average values over the 00 Bq/m from July to September. In some sectors of the Nerja Cave, airflow can be discerned with a certain intensity (Fig. a), 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. b). 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 982; Buecher 999), 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 58 and 2 m.a.s.l.), the existence of a possible connection between the Nerja Cave and the Pintada Cave (0 m.a.s.l.) was reconsidered although it was not practicable. To confirm this, in April 20 air flow measures in the Pintada Cave began. Shortly afterward, in June 20, a sensor VAISALA GM0 equipped with probes Text-Tint (ºC) Text-Tint 2 (H-) Rn (H-) Text<Tin Text>Tint Text<Tint Rn (Bq/m ).Nerja Cave (H-) Air 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 Text external temperature; Tint internal temperature

4 508 C. Liñán and Y. del Rosal for the measurements of air temperature, relative humidity, and air CO 2 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. Results and Discussion The results obtained from the airflow measurements made in 20 (Fig. a) 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. b). 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 rd, the CO 2 air concentrations of the Pintada Cave are between 0 and 2,000 ppm, with a mean value 8 ppm (Fig. ). Periodically CO 2 increases are also detected in the air of the Pintada Cave. Since July 5th, CO 2 increases disappear almost completely in the data series of the Pintada Cave and more uniform concentrations of CO 2 in the air appear between 0 and 580 ppm, with a mean value of 8 ppm. Until July rd, CO 2 increases observed in the Pintada Cave would be associated with the air from the Nerja Cave, as a result of the CO 2 increase produced by the visitors. On July 5th, the airflow inversion has already occurred. Thus, the external air, with values of atmospheric CO 2, comes into the Pintada Cave, through the New and High Galleries, arrives at the Tourist Galleries (where the CO 2 values are increased by human impact) and finally goes outside the Nerja Cave. The CO 2 measured in the air of the High Galleries in the Nerja Cave (Fig., H-2) confirms this point as it shows from July a progressive decrease until near to 80 ppm as a result of dwindling supply of anthropogenic CO 2. Before July 5th (from rd to 8th June), a short episode with low CO 2 concentrations in the Pintada Cave is detected, and high concentrations of 2 Rn 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. ). This general air circulation continues until October 9th, when higher values of CO 2 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 th to September 0th) a point reversal airflow is determined due to the occurrence of high concentrations of CO 2 in the Pintada Cave as well as in the High Galleries in the Nerja Cave.

5 Natural Ventilation of Karstic Caves: New Data on the Nerja Air CO2 (ppm) Nerja Cave (H-, Rn) Nerja Cave (H-2, CO2) Pintada Cave (CO2) Air 2 Rn (Bq/m). Nerja Cave (H-2) JUNE JULY AUGUST 2, ,0, ,0 -,0 00-8,0 0 Text-Tint (ºC) 200-2, , ,0 00 2,0-2,0 Text-Tint (ºC) Air CO2 (ppm). Pintada Cave Air CO2 (ppm). Pintada Cave Fig. Temporal evolution of the air 2Rn and air CO2 concentration in the Nerja Cave and air CO2 concentration in the Pintada Cave, from June to August 20 -,0 00 -, Fig. Temporal evolution of the air CO2 concentration in the Pintada Cave (red) and the temperature difference Text-Tint (black) from June st to 0th (top) and September 5th to October th, 20 (below)

6 5 C. Liñán and Y. del Rosal Nerja Cave (H-, 2 Rn) Nerja Cave (H-2, CO2) Pintada Cave (CO2) Air Rn Air CO2 (ppm) (Bq/m ). Nerja Cave (H-2) SEPTEMBER OCTOBER Fig. 5 Temporal evolution of the air 2Rn and air CO2 concentration in the Nerja Cave (H- and H-2 Halls, respectively) and air CO2 concentration in the Pintada Cave, during September and October 20 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.

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