## 1 Introduction  
Well-developed karst aquifers, characterized by slow infiltration through the rock matrix and fast flow through conduits and fractures, represent highly productive groundwater systems with a high degree of heterogeneity and anisotropy. Therefore, identifying recharge areas coupled with karst aquifer hydrogeological characteristics, which are related to the main kind of groundwater drainage, is important information for sustainable management of these groundwater resources. This is important as groundwater coming from karst aquifers is a key source of fresh water for human supply [1].  
In the study of karst aquifers, the employment of the environmental tracers can be considered as an effective method that allows an integrative investigation in order to perform adequate management of water resources [2]. Natural environmental stable isotopes together with other geochemical parameters are useful tools in cases where artificial tracing is not feasible such as at the scale of a regional aquifer system [3]. In particular, they have been applied to trace water flows inside the hydrological cycle from infiltration, influenced by chemical and isotopic precipitation characteristics, to discharge, due to water-rock interactions and geochemical processes [4; 5]. In addition, radioactive isotopes like tritium (\({}^{3}\)H) provide information on groundwater circulation. This isotope is a useful tracer for determiningthe age of groundwater [6] and it has also been widely used to assess recharge and calculate the groundwater residence time [7].  
The \({}^{18}\)O and \({}^{2}\)H data from groundwater are widely used to obtain information about the functioning of the karst systems [8], to determine important recharge areas of these aquifers, and to define the protection areas for the main springs [9]. The isotopic compositions of O and H in water, modified by meteoric processes are useful tracers for determining groundwater origin and movement because they generally do not change as a result of rock-water interactions at low temperatures [10]. Therefore, oxygen and hydrogen isotopes (\(\delta^{18}\)O and \(\delta^{2}\)H) can be used to determine the origin and path of groundwater and surface waters. In this regard, a similar systematic isotopic fractionation in stable hydrogen and oxygen occurs because their behaviour in the hydrologic cycle is similar [11]. This common behaviour leads to covariance between stable hydrogen and oxygen isotopic concentrations [11] defined by [PERSON] [12] as the Global Meteoric Water Line (GMWL) relation. With the aim of determining groundwater recharge, residence times and water flow paths of oxygen-18 and deuterium have been extensively applied in catchment studies [13].  
The present study focuses on qualitative-quantitative characterization of the karst aquifer feeding the Capodacqua di Spigno Spring, using isotopic tracers. In particular, oxygen-18 and deuterium isotopes have been applied, as environmental tracers of karst spring discharge, to determine the quantitative characteristics of Capodacqua di Spigno Spring. These stable isotopes, which have been already used in several studies to determine the mean altitude of the recharge area of aquifers [10; 14; 1], have also been involved in this study to define how the recharge area of Capodacqua di Spigno Spring varies according to the discharge, supplied by the same spring, in different periods of the year.