Dynamics and distribution of CO2 in karst vadose and epiphreatic zone (CARDIKARST)

Carbonate rocks cover about 15% of the world’s ice-free continental surface and often form karst aquifers used for public water supply. Carbon dioxide (CO2) plays a central part in the weathering of carbonates, giving karst aquifers a potential role of a net global CO2 sink. However, significant part of the karst subsurface consists of the vadose zone, which only recently has been acknowledged as a rich reservoir of CO2. This reservoir is leaky and part of the CO2 is not transferred deeper to the epiphreatic karst zone, but is released to the outer atmosphere through natural ventilation driven by outside meteorological changes. In this project we aim to identify and quantify processes occurring in the karst massif that drive significant, but currently unquantified, exchange of CO2 between the vadose zone, the (epi)phreatic zone and the atmosphere. Our results may give new insights into the global carbon cycle and to the present day speleogenesis. Research will entail cave air monitoring (CO2 concentration, airflow velocity and air temperature), groundwater monitoring and sampling (discharge and water chemistry), and comparisons with outside meteorology (temperature, wind, precipitation, soil conditions). In vadose zone caves, a network of automatic weather stations and data loggers will be installed, providing long-term time series of cave climate parameters. In addition, we will develop and deploy an array of low-cost CO2 and temperature loggers, based on the technological Arduino platform, to achieve higher spatial and temporal data resolution, and also to provide interested students, young researchers and park managers with an affordable tool for geoscientific education on field measurements in karst. Diverse environmental conditions will be ensured by selecting a range of study sites in Slovenia and Croatia. In Slovenia, we will limit to the well known karst system between Postojna in Planina, where measurements in the vadose zone will be based on four existing meteorological stations, and additionally distributed loggers. In the epiphreatic zone, we will deploy continuous long-term CO2 measurements in air and water at several locations in the Postojna–Planina Cave System (PPCS) , following the underground Pivka River. In Croatia, a similar approach will be followed at selected study sites—the Provala Cave (PC), the Upper Barač Cave (UBC) and the Samograd Cave (SC). Vadose zone conditions will be recorded at each site, while epiphreatic conditions will be followed in the cave stream (PC) or in nearby springs (UBC and SC). To date, only one study by the partner group in the USA has shown how cave ventilation can control the dissolution rate in subsurface streams to a greater extent than discharge rate—that promotes erosion in surface waters. Here, by combining timeseries analyses and numerical modelling of subsurface ventilation and CO2 transport, we will develop this by showing how advection driven by natural ventilation affects CO2 concentrations in karst systems, identifying controlling parameters and assessing the mechanisms of CO2 interaction with groundwater flow in the epiphreatic zone. Based on measurements and modelling, we will quantify carbon fluxes along underground stream, from the ponor to the spring, and asses the carbon budget for the karst system and its catchment as a whole. To evaluate our hypotheses and assess the potential application in other karst areas, a meta-analysis of available, worldwide cave monitoring data will be carried out. As most of the study will be performed in show caves and national parks, the results will promote their value and need for protection. The data obtained from the project will be compiled into a publically-available database that will enable karstologists, climate and speleothem scientists to better constrain the functioning of karst systems.