Chemical and isotopic signature of groundwater in the Santa Ninfa karst system and possible inferences on neotectonics

The Santa Ninfa karst system is an area strongly controlled by tectonics, whose intense fracturing gave rise to the formation of a large number of cavities that foster the drainage of water. The hydrogeochemical characters of groundwater circulating in this aquifer, together with its isotopic signature, were investigated in details. The chemistry of groundwater reflects the nature of the rocks hosting the aquifers, constituted by primary and diagenetic selenitic gypsum, salts, and gypsumarenite, whose dissolution is responsible of the geochemical fingerprint of the quasi-totality of the samples. A single site (CAM) is characterised by a different chemical composition, indicating a mixing between Ca-sulphate, Ca-bicarbonate and a NaCl-rich water. From the chemical point of view, no evidence of interaction between shallow groundwater and deep fluids has been detected. Conversely, isotopic fluctuations highlight mixing processes between surficial (evaporated) runoff and groundwater. Different mixing proportion among these endmembers can be reflected in variations of the chemical character of the sampled springs. Changes in mixing proportions can be the effect of differential permeability variations, in turn produced by local stress field changes during seismogenic processes. In this scenario the geochemical monitoring of the Santa Ninfa karst aquifer could be of relevant interest in the study of seismogenic processes in this area, with particular reference to the relationship between seismic and geochemical transients.


Introduction
Western Sicily is an area affected by seismicity, with several strong earthquakes that since historical times have caused relevant damages and fatalities [Guidoboni et al., 2002;Bottari et al., 2009]; the most recent catastrophic earthquake (Mw=5.6) struck the Belice Valley in 1968, destroying 7 towns and killing 330 people [De Panfilis and Marcelli, 1968;Marcelli and Pannocchia, 1971;Bottari, 1973;Anderson and Jackson, 1987;Rovida et al., 2011]. A previous study carried out in the Belice earthquake area [Favara et al., 2001a] suggested a direct connection between local thermal springs (Acqua Pia and Terme Selinuntine) and the main tectonic features, with anomalies of temperature and concentration of dissolved ions (SO 4 , Cl, Na, Total Dissolved Solids) and gases (CO 2 ), in connection with the 1968 seismic sequence.
Other groundwater circulation systems in the area show evident relationships with tectonics; a very peculiar case is the aquifer of the Santa Ninfa gypsum karst system (Figure 1), located within the area struck by the 1968 seismic sequence (the town of Santa Ninfa was completely destroyed). The Santa Ninfa karst system is strongly controlled by tectonics [Madonia et al., 2017], and the intense fracturing gave rise to the formation of a large number of cavities. The largest one, namely "La Grotta", is located at the end of a blind valley and it is a through-cave about 1350 m long and 30 m deep. It consists of at least three main levels of sub-horizontal galleries, the lowest of which is characterized by perennial flowing water, running along the water table [Madonia and Madonia, 2020].
In this study we integrated the results of the geochemical investigation published by Favara et al. [2001b] and Liotta et al. [2013] with unpublished data, with the aim of providing new insights on the hydrogeochemical characterisation of this aquifer. The hydrogeochemical system directly linked to the La Grotta hydrogeological tunnel, involving the mixing with surface runoff, is described in another paper from this volume [Madonia and Madonia, 2020]. Here we report on the general chemical and isotopic character of groundwater, paying attention to the possible inferences on neotectonics.

Rocco Favara et al.
2 Figure 1. a) Geological map of Sicily; b) detailed geological map of the Santa Ninfa area (modified from Agostini and Cucchi, 1989) with locations of sampling sites (see main text for explanation of acronyms).

Study area
The study area is located in western Sicily, along the southern part of the NNW-SSE trending San Vito Lo Capo-Sciacca (SVCS) band [Di Stefano et al., 2015]. This band represents a crustal discontinuity, nearly orthogonal to the main thrust propagation of the Sicilian fold and thrust belt (SFTB), which separates two zones showing a different Permian to Tertiary evolution: thick carbonate platforms in the western sector, facing deep-water successions located in the eastern one. Upper Triassic reefs, huge megabreccia, and submarine volcanism suggest the existence of a long-lasting weakness zone, related to this discontinuity and located at the boundary between its western and eastern sectors. This discontinuity has been episodically reactivated with transpressional and/or transtensional mechanisms [Di Stefano et al., 2015]. The historical and recent seismicity of the area identifies western Sicily as an active deformation zone; the strongest seismic activity was recorded in 1968 (Belice seismic sequence, maximum Mw=5.6). A previously unknown Quaternary fault was identified at Monte Porcello [MPF; Michetti et al., 1995], showing geometry and kinematics consistent with a regional NNW-SSE trending, right-lateral strike-slip zone. The MPF, together with other adjacent faults, crosscuts upper Piacenzian to Gelasian calcarenites and clays, and accommodates the differential shortening of the large-ramp anticline (Gibellina anticline) formed by Tortonian to lower Pliocene sediments [Di Stefano et al., 2015].
The surficial hydrography is mainly constituted by epigeic karst landforms (e.g. dolines and blind valleys) from which precipitations funnel into the underground feeding the aquifer.
Thermometric and pluviometric data recorded from 1951 to 1986 in 3 different local meteorological stations reveal an average annual temperature of 15.9° C, which hottest month is August (average monthly temperature of 25.6° C), whereas January is the coldest one (9.4° C average monthly temperature). The average annual rainfalls account for 584 mm and are distributed in 73 days, with the wettest month in December (87 mm) and the driest in July (6 mm). Following the abovementioned seasonal distributions of air temperature and rainfall, the climate of the Santa Ninfa area can be classified as temperate-semiarid. This area is characterised by well-developed karst phenomena, both at the surface and underground. Their formation is linked to the presence of both tectonic dislocations and impermeable levels that foster the drainage of water through preferential flow pathways [Madonia and Panzica La Manna, 1987]. The hydrogeological setting of the studied area is characterised by an impermeable substrate constituted by clay of Terravecchia Fm, covered by Messinian gypsum that constitutes the main reservoir.

Analytical methods
were processed using the computer program PHREEQC, version 3.1.2, [Parkhurst & Appelo, 2013], in order to calculate ion activities and PCO 2 (partial pressure of CO 2 ) of the water samples, as well as saturation state with respect to relevant minerals.
The O and H isotopic compositions of groundwater and rain samples were determined using Analytical Precision AP 2003 and FinniganMAT Delta Plus IRMS devices, respectively. The isotopic ratios are expressed as the deviation per mil ( ‰) from the international standard V-SMOW. The uncertainties are ±0.1% for 18 O and ±1% for D.
Contour maps were realised using the kriging algorithm implemented in the contouring options of Golden Software Surfer release 16; we adopted the blanking option for grid points outside the convex hull of data, for avoiding the generation of extrapolated information outside the area covered by measures.

Chemico-physical parameters, concentration of major elements of groundwater
Chemico-physical parameters and the results of the chemical analyses are listed in Table 1. Values of pH, temperature and electrical conductivity range between 5.83 and 7.95, 12.4 and 24.6° C, 776 and 3430 S cm -1 , respectively.
The data are also presented in the ternary diagrams of the main anions and cations (Figure 2a-

Isotopic composition of precipitation and groundwater
The oxygen isotopic composition of rainwater, illustrated in Fig. 5a-b, shows a large variability, with a minimum of -9.54‰ (SNI, December 2005) and a maximum of -2.5‰ (SNI, August 2005); D spans from -60 to -11 ‰ recorded in the same site and months.
The isotopic composition of groundwater (Figure 5a-b) shows a lower range of variability, with 18 O from -6.7 to -5.9‰ and D from -40 to -30‰.

Chemico-physical character of groundwater
The observed geochemical differences between CAM and all other samples can be explained by the distinctive nature of the rocks hosting the aquifers. These are mainly constituted by primary and diagenetic selenitic gypsum, salts, and gypsum-arenite (Gessoso Solfifera Series), whose dissolution is responsible of the geochemical fingerprint of the quasi-totality of the samples. The site CAM is characterised by a different chemical character, indicating a mixing between Ca-sulphate, Ca-bicarbonate and a NaCl-rich water. CAM is located in the Terravecchia Fm, composed of terrigenous and carbonate-clastic sediments, that is sited stratigraphically below the Gessosa Solfifera

Isotope hydrology and reconstruction of hydrogeological circuits
The isotopic composition of rainwater shows a large range of values, mainly due to the high seasonal variability that reflects the climatic features of the Mediterranean area ( Figure 5a). The best fitting line in the diagram D vs 18 O represents the local meteoric water line (LMWL), calculated in this study, described by the equation: D = 6.09 18 O + 5.05. As expected, the most negative values were recorded during the cold months and conversely the less negative during the warm dry-season. The calculated LMWL is comprised between the Global [Craig, 1961] and the Mediterranean (Gat and Carmi, 1970] Meteoric Water Lines. The isotopic vertical gradient can be computed using data from CAC and CAB but excluding SNI, because this site operated in a different period: its value ( 18 O) is -0.12 ‰/100 m, lower than that found by Longinelli and Selmo [2003] for southern Italy (-0.2 ‰/100 m) but equal to that found by Liotta et al. [2013] for western Sicily.
The time distribution of isotopic compositions of groundwater (Figure 5b) presents, as expected, minor fluctuations with respect to rain, with a range of less than 1 ‰ for 18 O. In particular, the most variable is RAM (0.8 ‰) while the most constant is CDS (0.1 ‰); all the others exhibit a range of 0.3-0.5 ‰. The smoothed isotopic signal recorded in groundwater suggests that: i) no direct connections between sinkholes absorbing surface runoff and the main aquifer exist, and ii) the average residence time of groundwater is close to one hydrologic year.
The relationships between the isotopic signal and the hydrogeologic circuit are well evidenced in Figure 6, where the isotopic composition contour map of groundwater is reported. The most variable spring is RAM, which is located