Magnesium content and groundwater flow in limestone aquifers: a relationship with potential developments for exploitation management of karst springs.

Karst aquifers are characterized by different types of groundwater flow, related to different types of permeability due to the simultaneous presence of matrix, fractures and conduits. The presence of a well-developed karst conduit system leads to a fast circulation of groundwater, within the aquifer and an impulsive response of the spring flow to the rainfall inputs, with a potential fast transport of contaminants from the hydrogeological basin surface to the output.

In this study, the internal structure of the karst system is not investigated and considered as a black-box model, which modifies the input signal (rainfall) into an output signal (spring discharge). With the help of hydro chemical analyses on spring water samples and single discharge measurements, it is possible to set specific mass balance models correlating ion content to spring flowrates. In particular, Mg²⁺ revealed a reliable application for spring baseflow separation in karst settings. Once the local model has been set, its conservative behaviour, in mostly limestone dominant aquifers, allows using it as a natural tracer of groundwater flow, distinguishing conduit flow and diffuse flow occurrence in the spring outlet, without additional discharge measurements. In karst settings, the difficulty in setting a fixed cross-section for continuously monitoring of spring discharge values makes this application interesting for exploitation management.

This study shows the results obtained for two springs located in Central Italy, confirming that monitoring groundwater quality in karst environments is often the key to successfully characterize springs and assess the total yield when direct measurements are not frequent.