Multi-disciplinary approach for assessing the impact of a flood event in a shallow karst cavity (Pindal Cave, Spain)

Pindal Cave (Asturias, Spain) and its Paleolithic art have been part of the UNESCO World Heritage List since 2008. The importance of this Paleolithic art led to a research project that deals with deciphering the relationships between environmental conditions and microbial activity in natural underground ecosystems and its application to the design of conservation strategies.

The Pindal karstic system develops in a calcareous massif (Carboniferous) modeled in the form of an erosional marine terrace (rasa) by coastal morphogenetic processes. This marine terrace level is located at an elevation of 30-68 meters above current sea level and constitutes the preferred catchment area for runoff water from another higher level (140-170 m) developed on quartzite layers with very low permeability (Ordovician). The cave is the main endokarstic feature of the system. On the surface of the 30-68 m rasa there are numerous exokarstic structures of sinkhole and polje type. On one of the sinkholes, located almost vertically to the cave, a cattle farm was installed in 1995.

Between October 19 and 23, 2019, an extreme episode of rainfall occurred in the area with a cumulative total of 209 l/m². This event caused a strong accumulation of water in the aforementioned sinkhole that finally collapsed, flooding the cave for several days. Immediately after the cave had been drained, environmental measurements and sediment samplings were carried out at various points in order to determine the changes caused in the underground ecosystem. In the most affected area by the flood, changes in humidity and temperature of air and sediments were recorded for several months. Biogeochemical data indicated that the sediments deposited as a result of the flooding presented high values of available organic matter, nitrogen, phosphorus and potassium, much higher than those of the innermost areas did not directly affect by the flooding. The comparative microbiological study of sediment samples revealed that the flood produced very significant changes in the microbial composition of sediments: the appearance of the bacterial phyla Bacillota and Bacteroidota, including groups of opportunistic bacterial pathogens (Corynebacterium, Thauera, Clostridiales) and the almost complete disappearance of Rokubacteriales and Nitrospirota. Bacillota and Bacteroidota are common in the intestinal tract of mammals and are dominant in liquid and solid samples of manure from dairy farms. Overall, the results conclude that the sediments dragged into the cave were accompanied by residues from livestock farming and indicate the high degree of vulnerability of this type of cave. Although livestock activity finally ceased in 2021, we continue analyzing environmental parameters, waters, sediments and microbial populations to evaluate their evolution in the medium-long term.

Research funded by PID2019-110603RB-I00 – SUBSYST project and PID2020-114978GB-I00