Patterns in CO2 exchange to the atmosphere from a borehole located in a Mediterranean karst system (Málaga, southern Spain)

The vadose zone (VZ), found between the surface and groundwater level, can store massive amounts of CO₂, recording values greater than 60,000 ppm to depths of a few tens of meters. The CO₂ is produced mostly in the first meters of soil due to root respiration and microorganisms and, to a lesser extent, to geochemical reactions. Although commonly CO₂ is produced mostly near the surface, the concentration increases with depth, due mainly to transport in two phases: 1) infiltration of CO₂-enriched water followed by precipitation and CO₂ release in deeper layers, and 2) percolation of CO₂-rich air due to its high density. These transport process contribute to natural CO₂ accumulation in the VZ, whose storage capacity depends on its thickness and porosity. All this CO₂ storage can be exchange with the atmosphere mainly determined by differences in the temperature between the internal and external atmosphere.

Here we study a borehole located next to Nerja Cave (Málaga, Spain) developed within fissured and karstified Triassic dolomitic marbles. Our objective is to determine the main drivers involved in subterranean CO₂ exchange with the atmosphere. To do that, CO₂ molar fraction, air temperature, relative humidity, wind speed and direction were monitored in the top of the borehole, and were correlated with external variables as air temperature, relative humidity, atmospheric pressure, rain and sea tides. Results shown that within a few hours, the CO₂ molar fraction can increase ten times more, showing a pattern with two cycles per day. In periods with low CO₂ molar fraction the air penetrates into the borehole, on the other hand, periods with high CO₂ values are due to the borehole CO₂-rich air is moving toward the external atmosphere. We found that the CO₂ emitted to the atmosphere by this borehole is several orders of magnitude than the soil CO₂ fluxes in this area. Therefore, we need to produce accurate long-term estimates of borehole CO₂ fluxes to improve our understanding of its contribution to local carbon balance.