Hydrogeological, geochemical and structural features of the aquifer feeding the Nadìa spring: an "oasis in the desert" of the Northern Apennines (Italy).

The Nadìa spring is the second largest tapped spring in the Emilia Romagna Region (northern Italy), representing a strategic local source of drinking water, also in the perspective of future global changes. The spring flowrate ranges between 65 l/s in the recharge season and 45 l/s at the end of the low-flow season, when most of the other tapped springs in the region have flowrates lower than 5 l/s. Geological, geomorphological, hydrological and geochemical investigations were carried out in the spring watershed to unravel the factors causing this peculiarly high discharge. The spring arises at the base of a calcarenitic fractured aquifer (Pantano Formation, upper Burdigalian-lower Langhian) underlain by lower permeability units. Karst dissolution along structural discontinuities in the Pantano Formation has been suggested in the past as a possibility to account for the aquifer high permeability resulting in the high spring discharge. A continuous monitoring of the spring flowrate, temperature, electric conductivity and pH was conducted during the 2020-2021 hydrologic year. Hydrographs and chemographs indicated atypical karst flow dynamics. The time to halve the peak-discharge of the spring is between 20 and 50 days, lower than that of most springs of the Northern Apennines (> 50 days). This implies a higher average aquifer flow rate (around 10 m/day) compared to that typical of arenitic or turbiditic aquifers (around 1 m/day). Spring water samples collected once to twice a year since 2011 for the analysis of major ions revealed an obvious calcium-bicarbonate hydrochemistry that is consistent with the hypothesis of karst dissolution. However, the chemical variability over time expected in a karst system due to the drainage of different groundwater fractions (newly infiltrated vs. older groundwater) was not observed. Instead, the water chemical composition was exceptionally constant over time, suggesting that drainage occurs from a large, homogeneous reservoir. Water stable isotopes have been analyzed in 2021 revealing a composition close to that of the local winter precipitations and suggesting exceptional stability of the spring water composition over different seasons. An 80 m deep borehole has been drilled in the Pantano Formation 7 km away from the spring, documenting the occurrence of fractures with decimetric apertures as deep as 60-70 m below ground surface, which may be interpreted as the result of karst dissolution. In addition to the debatable karst aquifer hypothesis, geomorphological observations indicate the occurrence of depressed areas, of likely tectonic origin, in the aquifer overlying the spring, which may provide a favorable setting for concentrated infiltration and groundwater recharge. However, the hypothesis of concentrated recharge is in apparent contrast to the abovementioned stability of spring water chemical composition over time. A volume of the aquifer representing a reasonable reservoir for the spring has been identified based on spring flow recession analysis and a geo-structural model of the Pantano outcrop up-gradient to the spring. The structural-stratigraphic setting of the hypothesized reservoir includes the occurrence of fault-related fractures that cross-cut the low-dipping bedding of the calcarenites, possibly enhancing the local permeability and the drainage towards the Nadìa spring.