Inferring threshold behavior of the Supramonte karst aquifer from tracer test and its implication for groundwater protection (Sardinia, Italy)

For an efficient protection of karst water against contamination it is essential to understand the characteristics of the solute transport within the aquifer in different hydrological conditions. This can be investigated performing an artificial tracer test as simulation of a contamination event. This approach was applied to the Supramonte massif (Sardinia, Italy), a karst aquifer hosted in a remote area unfavourable for dense human settlement. At the end of July 2014, the fluorescein was diluted in the sinking stream just below the waterfall within the Dorgheddie Cave in the fluviokarstic canyon of Gorropu at the eastern side of the Supramonte aquifer, while 3-bags of charcoal were placed into the water of Gorrupu and Su Gologone springs, the main outflows of the karst aquifer. six months later from dye injection and after few days of intense rainstorms, the fluorescein was detected at the Su Gologone spring and even after several weeks of sampling no fluorescent tracer was detected in Gorropu spring. The tracer test showed that at low water condition there is a geological threshold that avoid water from the eastern drainage network to reach the main outlet in the northern side. It was also highlighted the groundwater vulnerability of this karst system to a hypothetic contaminant transport that could be detect at the spring with long delay.