Unsaturated flow in a structurally complex karst aquifer

The objective of this doctoral project was to unravel the functioning of the unsaturated zone of a karst aquifer that developed in a complex structural setting. The studied system, Bossea cave (Southern Piedmont, Italy) is located in the Ligurian Alps. A multidisciplinary approach was adopted to investigate the unsaturated flow dynamics of the system. Geological surveys, structural analysis and thin-section microscopy permitted to recognize the main circulation pathways. Bossea karst system developed taking advantage of a peculiar structural setting in which transpression between two left-lateral strike slip faults led to a disharmonic deformation of the Permian basement and the Mesozoic carbonates. Flow circulation developed along the bedding planes of the folded carbonate sequence, but when the water reached the disharmonic surface mechanical erosion of the basement non-carbonate rocks started. The hydrological monitoring of the underground river and several secondary tributaries permitted to recognize a complex and heterogeneous architecture in the unsaturated zone. Drip sites hydrological behavior is controlled by the fracture organization and permeability in the unsaturated zone. The drip sites show a piston flow behavior, but each inflow has peculiar flow characteristics that are related to the drainage of small perched reservoirs in the fractured rocks of the unsaturated zone. The recession analysis, cubic law inversion and fractal analysis confirmed that the fracture network architecture controls the discharge of the vadose inflows. Hydrochemical investigations permitted to recognize the influence of the different lithologies on the inflows discharge. This cave represented a unique opportunity to study in detail the dynamics of an unsaturated zone that is strongly influenced by inputs from different lithologies, in addition to the carbonates. The availability of a huge amount of data collected for more than 15 years permitted to study this system in a wide range of conditions, from drought to extreme flooding events.