Heat as a tool for hydrodynamic characterization of fractured karst systems in Italy’s Northern Apennines

Carbonate aquifers represent the main reservoir in the Apennines. The degree of their karstification controls the heterogeneity and anisotropy of the system at different scales. Bulk permeability of the karstified rock mass is controlled by a dual behavior, in which one end member is the conduit network with high hydraulic conductivity and short water-residence time, and the other is the microfractured blocks with longer water-residence times. The geometry and the location of these flow paths in the study area are poorly constrained. An analysis of hydrographs integrated with the chemical or thermal responses can provide useful information on the geometries of the conduit drainages. A comparison of the thermal response of the karst springs with the record of cave stream temperatures will allow to distinguish between the conduit drainage and that of the fracture network. The spring hydrograph response is mainly controlled by the geometry of karst conduits and by the variation of the recharge area. Convective heat exchange in the channels and non-conservative thermal exchange between the water and the porous matrix characterize the flow in the whole karst system, as recorded by the heat output signals. The temperature signal in the conduits tends to be augmented by the high flow velocities, while being dampened by heat conduction resulting from the low flow velocities in fracture matrix. Using this information, the relationship between the system geometry and recharge area can be better defined. The Scirca Spring is located at the foot of the Monte Cucco karstic massif in Italy’s Northern Apennines, where outcrops of Meso-Cenozoic carbonates are observable. The average discharge of the spring is about 0.2 m3/sec. It has a catchment area of about 3.4 km2 characterized by 30 km of karst conduits extending to a depth of about 1km. Several caves, in which underground streams feed the phreatic zone, allow to access the vadose zone of the drainage system. The karst aquifer was characterized by comparing the thermal behavior of the spring with that of the different cave streams as well as with the tracer test results and other physical-chemical data. The thermographs indicate a minimum of three fluctuation patterns, with hourly, daily and seasonal. The seasonal variations are out of phase with respect to the surface temperatures, showing a lag of many months. This data suggest a dampening effect exerted by the phreatic zone, in which the fracture network controls the main drainage and local inputs can modify the short term temperature signal.