Origin and evolution of a salty gypsum/anhydrite karst spring: the case of Poiano (Northern Apennines, Italy)

Poiano is the largest karst spring of the Emilia Romagna region (northern Italy). It drains an aquifer of unique properties composed of anhydrite with halite lenses at depth and gypsum at the surface (both with high NaCl content). Hydrogeological research has been undertaken using automatically recorded hourly data on temperature, electrical conductivity, and water level. Water feeding the Poiano spring is restricted within the gypsum/anhydrite outcrop between the Lucola, Sologno and Secchia rivers. Karstification in the Upper Secchia Valley only concerns the gypsum rocks mainly present along the border and in the shallower parts of the sulfate outcrop and does not appear to occur at depth. Data strongly support the hypothesis that the salt content in the spring water derives from active halokinetic movements. For the first time, the fundamental hydrogeological importance of the anhydrite part inside the sulfate rocks is demonstrated. If gypsum prevails over anhydrite the karst drainage network can extend deep into the rocks following a network of fractures and fissures. Instead, if in the deep parts of the aquifer anhydrite prevails over gypsum, the karst evolution cannot take place at depth and the structure of the underground drainage paths only follows near-surface paths in gypsum.