DNA and Fluorescein tracer tests to study the recharge, groundwater flowpath and hydraulic contact of aquifers in the Umbria-Marche limestone ridge (central Apennines, Italy)

The purpose of this paper is to highlight the hydrogeological contact among aquifers in karst and fissured systems and study the different flowpaths present in the aquifers; given the complex hydrogeological and tectonics settings, a detailed hydrogeological and geo-structural survey in a selected area of the Umbria-Marche limestone ridges (central Apennines), supported by tracer field tests, was used to achieve this goal. DNA and fluorescent tracers were injected into a sinkhole located in the Montelago karst high-plain (Umbria-Marche ridge) and recovered at different points, thus allowing for the identification of effective hydraulic contact among aquifers belonging to both the same and different hydrogeological complexes, and to identify the main and secondary direction of the groundwater. Moreover, tracer tests, highlighting the role that tectonics play in the groundwater circulation, have shown that the majority of groundwater flow is directed towards the axis of the tectonic structures, emerging in the more depressed areas. In addition, they have demonstrated the influence of karstification on the aquifer recharge and shallow flowpaths, allowing for a more detailed characterisation of water circulation in the unsaturated zone of the Maiolica and the Calcare Massiccio aquifers. In conclusion, the tracer tests confirmed the hydrogeological survey results, while providing additional previously unknown information. Furthermore, the DNA tracer (recently tested in a preceding study) provided good results in the field especially in karst systems and fissured rocks, thus confirming its appropriacy for flowpath investigations and vulnerability analysis of springs.