Long-term spatio-temporal hydrochemical and 222Rn tracing to investigate groundwater flow and water–rock interaction in the Gran Sasso (central Italy) carbonate aquiferTraçage spatio-temporel hydrochimique longue durée incluant 222Rn pour étudier l’écoulement souterrain et l’interaction eau–roche dans l’aquifère carbonaté du Grand Sasso (Italie centrale)Traza hidroquímica espacio temporal a largo plazo y de 222Rn para investigar el flujo de agua subterránea y la interacción agua–roca en el acuífero carbónático de Gran Sasso (Italia Central)Uso de traçadores hidroquímicos e de 222Rn no espaço-tempo e a longo prazo para investigação do fluxo de água subterrânea e da interação água–rocha no aquífero carbonatado de Gran Sasso (Itália central)

In the Gran Sasso fissured carbonate aquifer (central Italy), a long-term (2001–2007) spatio-temporal hydrochemical and 222Rn tracing survey was performed with the goal to investigate groundwater flow and water–rock interaction. Analyses of the physico-chemical parameters, and comparisons of multichemical and characteristic ratios in space and time, and subsequent statistical analyses, permitted a characterisation of the hydrogeology. At the regional scale, groundwater flows from recharge areas to the springs located at the aquifer boundaries, with a gradual increase of mineralisation and temperature along its flowpaths. However, the parameters of each group of springs may significantly deviate from the regional trend owing to fast flows and to the geological setting of the discharge spring areas, as corroborated by statistical data. Along regional flowpaths, the effects of seasonal recharge and lowering of the water table clearly cause changes in ion concentrations over time. This conceptual model was validated by an analysis of the 222Rn content in groundwater. 222Rn content, for which temporal variability depends on seasonal fluctuations of the water table, local lithology and the fracture network at the spring discharge areas, was considered as a tracer of the final stages of groundwater flowpaths.