Geochimica delle acque sotterranee del Complesso Carsico di Codula Elune (Sardegna centro-orientale).

The Codula Elune karst system has been recently investigated with the aim to determine the geochemical signature of the main underground river that drains the right side of the homonymous valley. In particular, the water sampling was carried out in the major cave streams of this vast karst system at the relevant segment of the Su Palu cave (the White Nile, stream fed by surface water losses in sub-riverbed, and the Blue Nile, that drains the inland of the karst massif). In addition to the analysis of major elements, a series of periodic measurements of in situ physicochemical parameters (pH, conductivity, total dissolved solids, alkalinity and temperature) have been carried out. Although both streams have confirmed a calcium bicarbonate character, the results show a good dissimilarity between the waters which supply the main collector, with a higher content of calcium and bicarbonate for the Blue Nile stream and a highest concentration values for other mayor ions for the White Nile. Even the monitoring of physicochemical elements shows clearly heterogeneity in the hydro-chemical feature of the drainage network. It is characterized by a high stability for the parameters of the water coming from the innermost area of the aquifer, with temperature and conductivity practically constant during the whole year, while the portion directly feed by the sub-riverbed Codula Elune suffers the seasonality of the external input, which mitigates its effects gradually penetrating towards the zone of confluence. In general, the data show that the White Nile underground water has a composition influenced by weathering of feldspars on which overflows before entering the karst system and by rainfall while the Blue Nile stream extensively interacts with the limestone rock, canceling its original chemical signal.