GNGTS 2013 - Atti del 32° Convegno Nazionale

Scaglia Cinerea Formation, near the core of the anticline, along the right bank of the Tronto River. It has an average discharge of about 180 l/s, and it is sensible to seasonal variations. Some further minor thermal springs are present downstream within the river bed and also at higher altitude, along the right side of the valley. The thermal groundwater can be reached also in the lower sections of the caves in the Rio Garrafo Valley. The groundwater flowpath is heavily influenced by the geologic setting. The prevailing low permeability formations that cap the thermal aquifer reduce its local recharge, and probably only in the Rio Garrafo gorge sinking stream water directly reaches the thermal water inside the caves, creating a fluid mixing and a dilution of the hot water (Galdenzi et al., 2010). The forecast weather conditions have been considered, in order to obtain preliminary information on the potential water recharge for this area. The average annual rainfall is 947 mm/yr over the period 1961-2000; in 2012 rainfall was 905 mm, consistent with the average historical trend, while in 2013 a slight increase was detected in the first eight months, 108 mm more than the historical mean. Geochemical prospection. Chemical and isotopic analyses were carried out since October 2012 on seven selected springs. Four of these are thermal springs and represent almost all the thermal output of the area: T1 is a small outflow located 100 m above the Tronto River bed, T2 and T3 are located along the right side of the river, T4 is the big discharge of the Acquasanta Cave. The other three springs are cold waters, sampled near the hot springs, to be utilized as a comparison: C1 along a slope, C2 in the Rio Garrafo stream, C3 from a fountain at the base of the Laga Formation. First water sam- pling started at the end of October 2012, and was repeated in February 2013. Chemical results show that cold samples are bicarbonate-cal- cic, except for the C3 water that presents much higher quantities of SO 4 and Chlorine. Thermal waters show temperatures ranging from 26°C to 30.3°C, with seasonal var- iations lower than 10°C. They can be classified as chloride-sulphate rich in Na, K, Ca and Mg (Fig. 2a) and containing H 2 S, with small var- iations between the two periods and with differences in electrical con- ductivity. An important amount of Mag- nesium is present, suggesting fur- ther contributions in addition to the common water-clays interac- tion and, looking at minor elements of thermal waters, an anomalous Fig. 2 – a) Piper’s diagram of waters sam- pled in February 2013; b) relation between Oxygen and Deuterium isotopic ratio. 253 GNGTS 2013 S essione 1.3