Geochemical Characterization of Groundwater in a Volcanic System

Abstract

A geochemical investigation was undertaken at Mt. Etna Volcano to better define groundwater characteristics of its aquifers. Results indicate that the Na–Mg ± Ca–HCO₃⁻ ± (SO₄²⁻ or Cl⁻) type accounts for more than 80% of the groundwater composition in the volcano. The remaining 20% is characterized by elevated Ca²⁺. Waters along coastal areas are enriched in SO₄²⁻ or Cl⁻, mainly due to mixing with seawater and anthropogenic effects. The majority of the samples showed values between −4‰ to −9‰ for δ¹⁸O and −19‰ to −53‰ for δ²H, suggesting that precipitation is the predominant source of recharge to the aquifers, especially in the west of the study area. The analysis of δ¹³C and pCO₂ shows values 1 to 3 times higher than those expected for waters in equilibrium with the atmosphere, suggesting a partial gas contribution from deep sources. The diffusion of gasses is likely to be controlled by tectonic structures in the volcano. The ascent of deep brines is also reflected in the CO₂ enrichment (up to 2.2 bars) and enriched δ²H/δ¹⁸O compositions observed in the salt mounts of Paternò.

1. Introduction

Mt. Etna is located on the east coast of Sicily (Italy). It is the tallest active volcano in Europe and one of the most active volcanoes in the world [1]. The volcanic deposits are the most important groundwater reservoir for the entire Sicily, as it is the only drinking water resource for over one million people who live at distances of more than 100 km [2]. The volcano rises over an important regional tectonic system, which causes the crust to break up into an intricate system of fractures and faults that together with geology, paleotopography and geometry of the sedimentary basement are the major factors governing the groundwater flow in the area.

Furthermore, seismic events and volcanic eruptions frequently reshape the morphology of the terrain, potentially altering flow conditions and modifying the groundwater composition. Agriculture and the development of urban and industrial centers such as Catania are largely dependent on the neighboring volcano. In this context, population growth and new economic activities continue to mount pressure on the environment and hence call for a more sustainable use of the region’s groundwater resources. Geochemistry and isotope investigations have been used commonly on Etnean aquifers to study the hydrological processes. For instance [3] determined that groundwater concentrations of minor and trace elements stood out with respect to other Italian aquifers due to the major contribution of volcanic gases and hydrothermal fluids. Later, [4] determined the origin and effects of fluid-rock interaction within Mt Etna by analyzing B, O, H, and Sr concentrations. Oxygen and Cl isotopes were used by [5] to determine groundwater recharge and flow paths along the flanks of Mt. Etna. Findings showed that groundwaters beneath intensely cultivated areas were enriched in ¹⁸O probably as a result of the evaporation of irrigation waters during summer. Finally, [6] investigated the isotopic signature of Etnean waters, and [7] used tritium activities to trace the age and movement of groundwater in the volcano in recent times.

The hydrology of Mt. Etna has been studied over a long period. This work further expands the current understanding by providing an updated snapshot in time of the geochemical and isotopic composition of Mt. Etna’s groundwater.

2. Study Area

2.1. Geological Background

Mt. Etna is located on the eastern coast of Sicily, Italy. The morphology of the volcano is shaped by four summit craters, a caldera of about 18 km perimeter and maximum depth of 1000 m called “Valle del Bove”, and numerous side cones scattered along its flanks (Figure 1). Slopes are gentle (7°–8°) up to an elevation of 1800–2000 m above sea level (A.S.L.), increasing to 20°–25° at higher elevations. The volcanic edifice consists of a lower shield unit overlain by a stratovolcano. The shield rests discordantly on Miocene flysch deposits to the NW, and argillaceous Pleistocene sediments to the SE [8,9] and consists of plateau terraces of submarine lavas derived from fissural emissions generated at an early stage, approximately 500 ka. The basal unit was followed by pyroclastic material and sub-aerial tholeiitic lavas outcropping rather discontinuously in the southern sectors of the volcano approximately 300 ka [10]. Products changed in composition about 200 ka to transitional and later Na-alkaline tholeiites [11].

Figure 1. Outline of the investigation area and hydrogeological basins within Mt. Etna.

Figure 1. Outline of the investigation area and hydrogeological basins within Mt. Etna.

Currently, Mt. Etna releases about 11.66 kTons/day of CO₂ from the summit and as diffuse soil emanations from the upper flanks [12]. This quantity is much larger than other active volcanoes and corresponds to about 10%–15% of the CO₂ produced by all the volcanoes on the planet [13].

The climate of the region follows the pattern of the Mediterranean region, with higher precipitations in autumn and winter. Most rainfall occurs to the east, reaching up to 1200 mm/year. To the south, the average precipitation decreases to about 440 mm/year, with a wide range of intermediate conditions in between. At heights above 2000 m precipitation tends to occur as snow.

2.2. Hydrogeological Setting

Mt. Etna hydrogeological settings are similar to other basaltic volcanoes: fissured and highly permeable lavas are interbedded with discontinuous layers of low permeability pyroclastics. According to [9,14], typical Etnean aquifers can be described as unconfined and hosted by highly permeable volcanites. On the basis of structural, geological and geophysical data, three main hydrogeological basins were defined (Figure 1): (1) the eastern basin, with flow towards the Ionian Sea; (2) the southern and western basins flowing towards the Simeto River; and (3) the northern basin with flow to the Alcantara River [14,15]. Groundwater recharge preferentially occurs at high elevations where rainfall infiltrates through the unsaturated sediments and then follows a radial pattern towards the peripheral sedimentary terrains. Groundwater flow originating at lower heights, where the volcanic cover is much thinner, is mainly controlled by the shape of the impermeable substrate. According to [16], Etna’s volcanites generally have a high intrinsic permeability (2.5 × 10⁻⁷ to 2.9 × 10⁻⁶ cm²). In contrast, the permeability of the basement sediments would average 10⁻¹⁰ cm² [17].

The absence of a developed hydrographic system on the surface suggests an important circulation of groundwater, highlighted by the presence of hundreds of springs and wells of significant flow rates. Yields over 80 L/s have been described by several authors since the 70s [16,18].

Recent hydrogeochemical studies (e.g., [19,20]) indicated that groundwater in Mt. Etna has a general composition of bicarbonate type, with a few samples of chloride-sulphate type. The relative abundance of major elements in solution is generally (Na, Mg) > Ca > K for cations, whilst HCO₃⁻ always prevails over other anions [20].

Findings from [21,22] using δ¹⁸O and δ²H data suggested that Etnean groundwaters are meteoric in origin. Nevertheless, more recent work from [5] showed that the isotopic imprint of groundwater in Mt. Etna might reflect several sources such as evaporation from the Mediterranean Sea to the east, moisture from the Atlantic Ocean on the lower northern flanks, and volcanic vapor affecting precipitation on the upper regions of the cone.

3. Sampling and Analytical Methods

An extensive sampling campaign was undertaken in early 2014 at Mt. Etna to determine the chemical characteristics of groundwaters in the volcano. Samples were collected during three stages from 46 boreholes, 14 springs, 2 surface water points, and 6 locations within the so-called “mud volcanoes” (Figure 2). The bores are all being used for water supply or are connected to storage tanks. As a first measure, piped water-supply and taps were disinfected with a 10% hypochlorite solution and water run for a few minutes before sampling commenced. Samples were then directly collected from the source, field-filtered at 0.45 µm, and stored in 500 mL plastic containers for the analysis of dissolved inorganic elements. Bottles were cooled to 4° and dispatched for analysis within 48 h. A number of field blanks and duplicates were also sent to the laboratory for quality control. Standard parameters (pH, EC, temperature) were measured in situ using a Hydrolab Qanta probe, while a colorimetric titration kit was employed to calculate the alkalinity (HCO₃⁻) content in waters. Chemical concentrations were determined at the laboratories of the Geological Survey of Japan in Tsukuba. Major cations were analyzed by an inductively coupled argon plasma atomic spectrophometer (ICP-AES), and anion determinations were carried out by ion-chromatography. Additional samples were collected for the analysis of carbon-13 (δ¹³C), and treated in the field with HgCl₂ to prevent biological fractionation. These samples were stored in glass bottles and analyzed by a mass spectrometer. Results were reported as ‰ deviations (per mil) from the PeeDee Belemnite standard. Oxygen and hydrogen isotopes were also measured by an isotope ratio mass spectrometer. Measurements were referenced to the VSMOW international standard, and reported in the conventional delta notation.

Figure 2. Location of the sampling sites in Mt. Etna.

Figure 2. Location of the sampling sites in Mt. Etna.

4. Results and Discussion

4.1. Groundwater Composition

Table 1 summarizes the groundwater composition of Mt. Etna groundwaters found in this study.

With the exception of SO₄²⁻, ion concentrations increase towards the west, where values are usually two to three times higher than neighboring regions (Figure 3).

There is a general inverse relationship between water mineralization and elevation. The lowest groundwater contents were recorded on the north, and towards the cone summit in the eastern basin. In contrast, maximum concentrations were measured on the southernmost flank of the volcano (western basin). Results also indicate that Etnean waters have low temperatures, with an average of 17 °C. The highest water temperatures were recorded in intense tectonically fractured areas such as Paternò, Biancavilla, and Adrano in the West, and between Pozzillo and Zafferana in the East. This is consistent with [23], who argued that in an active volcanic system such as Mt. Etna, the transfer of deep gases (and the associated heat) toward the surface occurs principally along zones of high permeability in the crust.

Table 1. Average major and trace elements composition of waters sampled in the Mt. Etna.

Sample	Basin	Temp.	EC	pH	HCO3	F	Cl	NO3	PO4	SO4	Na	K	Ca	Mg	TDS	Hardness	PCO2 at 20°
		°C	µS/cm		meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	mg/L	°fH	bars
B11	E	9.1	219	7.35	2.62	0.02	0.29	0.02	0.02	0.30	1.35	0.21	0.69	1.38	149	10.3	0.007
B12	E	18	1138	7.77	8.16	0.04	3.28	0.92	-	1.70	6.07	0.65	3.50	4.57	736.6	40.3	0.009
B13	E	11.4	289	7.92	1.26	0.01	1.49	0.30	-	0.57	1.33	0.25	0.64	1.87	252.3	12.6	0.001
B14	E	14.7	1123	8.29	3.84	0.03	6.15	1.48	-	2.99	7.73	0.52	1.91	5.95	997.9	39.2	0.001
B15	E	10.6	282	8.37	2.02	0.01	0.96	0.09	0.02	0.79	2.21	0.24	0.57	1.13	191.1	8.5	0.001
B16	E	-	300	0.03	3.00	-	-	1.43	2.75	-	0.34	0.33	1.1	-	-	-	-
B17	E	15.8	1329	6.85	8.86	0.03	4.25	0.73	-	3.76	8.69	0.77	3.73	5.53	912.9	46.2	0.079
B18	E	5.6	424	7.82	4.58	0.03	0.96	0.01	-	1.68	3.79	0.35	1.60	1.46	307.5	15.3	0.004
B20	E	14.1	298	8.39	2.70	0.01	0.51	0.24	0.05	0.52	1.12	0.33	1.92	0.61	203.7	12.6	0.001
B21	E	16.4	967	7.32	8.24	0.06	2.47	0.46	-	1.91	5.07	0.64	2.15	7.67	723.4	49.0	0.025
B22	E	13.9	373	7.87	3.66	0.02	0.70	0.08	-	0.65	2.46	0.28	1.20	1.30	224.7	12.5	0.003
B24	E	17.4	544	7.8	3.90	0.03	1.37	0.31	-	0.69	2.99	0.26	0.70	1.75	268.2	-	0.004
B25	E	16	1056	6.58	7.08	0.02	1.72	0.61	-	2.35	4.15	0.57	2.42	4.60	574.1	-	0.118
B26	E	17.5	1570	6.57	10.67	0.03	2.74	0.28	0.02	3.86	7.07	0.85	3.45	6.75	838.6	-	0.181
B27	E	17	718	6.58	5.03	0.02	1.03	0.85	-	1.42	2.62	0.40	1.78	3.10	410.7	-	0.084
B28	E	16	886	6.4	6.44	0.02	1.35	0.21	-	1.62	3.47	0.54	2.09	3.87	458.8	-	0.162
B29	E	26	944	7.33	8.52	0.01	1.14	0.16	-	0.95	5.15	0.46	1.88	3.59	449.4	-	0.025
B30	E	18.8	588	7.8	3.33	0.03	1.63	0.50	-	0.79	3.08	0.25	0.76	1.90	303.1	-	0.003
B31	E	29.8	1527	7	8.89	0.05	2.86	0.41	-	4.15	5.43	0.76	3.66	6.80	827	-	0.056
B32	E	17.8	996	7.25	6.93	0.03	2.39	0.19	-	1.57	4.82	0.47	1.45	4.64	522.7	-	0.025
B33	E	21.3	1180	6.8	8.54	0.03	1.77	0.61	0.02	2.52	4.35	0.52	2.78	5.71	647.5	-	0.085
S6	E	17.6	2300	7.95	4.37	0.00	17.3	0.77	-	3.59	18.62	0.82	3.74	4.65	1693.5	41.9	0.003
S7	E	16.8	-	7.64	1.52	0.03	2.97	1.77	-	2.78	3.25	0.81	1.44	5.03	638.1	32.3	0.002
S8	E	14.9	330	8.15	1.95	0.02	0.77	0.18	-	1.14	2.14	0.23	0.73	1.34	210.8	10.4	0.001
S9	E	15.5	981	8.44	6.14	0.04	2.88	1.25	0.05	2.52	5.49	0.50	2.68	5.29	729.9	39.8	0.001
S10	E	15.3	676	7.74	5.00	0.04	2.46	0.41	-	0.75	3.77	0.41	2.23	2.39	441.7	23.1	0.006
S11	E	14.3	443	7.79	3.50	0.02	0.86	0.33	-	0.96	2.61	0.30	1.31	1.85	283.6	15.8	0.004
S12	E	19.7	1670	7.5	13.10	0.04	2.84	1.01	-	3.25	7.51	0.66	2.79	8.90	935.2	-	0.026
R1	River E	11.2	490	8.73	5.72	0.03	0.78	0.10	-	1.03	2.11	0.19	1.84	4.61	395.2	32.2	0.001
B1	W	7	1268	7.98	17.51	0.06	3.56	0.10	0.05	1.10	10.04	0.66	6.75	2.84	869.3	47.9	0.012
B2	W	15.9	1335	6.46	16.34	0.02	3.07	0.02	-	0.18	8.81	0.69	3.85	9.48	939.2	66.6	0.355
B3	W	15.1	1004	6.8	12.80	0.03	2.56	0.09	-	1.49	5.10	0.77	2.72	6.46	679.7	45.9	0.128
B4	W	8.8	832	6.2	12.18	0.03	0.91	0.08	-	0.87	4.12	0.48	0.64	6.82	486.9	37.3	0.502
B5	W	15.4	1441	6.71	17.71	0.04	2.17	0.21	-	0.56	6.82	0.78	6.09	6.07	852.2	60.8	0.218
B6	W	15	1153	7.11	12.36	0.04	2.86	0.29	-	0.93	5.65	0.57	5.09	4.83	746.7	49.5	0.061
B7	W	16.7	1013	7.49	7.94	0.03	2.90	0.75	-	1.46	4.89	0.54	3.28	3.58	624.3	34.3	0.016
B8	W	18.1	989	6.48	8.40	0.03	2.50	0.07	0.02	1.46	4.66	0.66	3.44	3.53	574.2	34.8	0.176
B9	W	13.9	384	8.24	2.68	0.04	0.66	0.08	-	0.68	2.26	0.23	0.81	0.94	182.3	8.8	0.001
B10	W	16.4	1951	6.84	23.85	0.06	4.65	0.17	0.02	0.72	6.83	0.59	1.21	18.04	1192	96.2	0.217
B19	W	17.1	1093.6	7.71	7.60	0.01	9.68	2.20	0.02	6.63	13.19	0.82	7.84	3.84	1488.4	58.3	0.009
B23	W	12.1	924	7.42	13.72	0.03	0.90	0.03	-	0.56	4.87	0.54	4.70	3.78	571.2	43.3	0.033
B34	W	18.7	1582	6.51	17.49	0.05	1.87	0.01	-	0.45	6.50	0.52	2.62	10.68	827.3	-	0.341
B35	W	14.9	1182	6.06	12.43	0.04	1.27	0.09	-	0.94	4.54	0.54	2.41	7.68	638.3	-	0.683
B36	W	23	2051	7.1	17.87	0.03	3.63	1.51	-	2.09	10.24	0.95	2.40	10.94	1114.8	-	0.090
B37	W	16.3	1800	7.41	13.33	0.04	4.40	0.93	-	3.09	9.94	0.56	2.02	8.87	995.6	-	0.033
B38	W	14.5	1194	6.51	11.84	0.03	1.44	0.19	0.03	1.15	4.40	0.53	2.41	7.62	648.3	-	0.231
B39	W	17.6	1766	6.98	14.72	0.04	3.16	0.97	-	2.62	8.30	0.68	2.55	9.65	976.8	-	0.097
B40	W	19	2560	7.3	21.90	0.09	5.32	4.80	-	4.42	11.72	0.91	5.83	14.72	1784.3	-	0.069
B41	W	18	1825	7.07	13.64	0.04	3.34	1.36	-	4.19	8.43	0.68	2.95	9.80	1060.1	-	0.073
B42	W	20	1860	6.8	14.57	0.04	3.51	0.97	-	3.15	8.64	0.61	3.17	9.42	1020.3	-	0.146
B43	W	16.4	1228	6.08	14.70	0.04	1.48	0.02	-	0.53	5.29	0.50	2.65	8.78	708.4	-	0.772
B44	W	20	1315	6.18	14.77	0.04	1.47	0.00	-	0.53	5.32	0.51	2.64	8.83	708.9	-	0.616
B45	W	16.2	1212	6.28	12.02	0.03	1.38	0.25	0.02	1.03	4.51	0.53	2.35	7.57	645.4	-	0.398
B46	W	14.6	1250	5.93	13.72	0.04	1.22	-	-	0.63	4.73	0.55	2.51	8.33	662.3	-	1.017
S1	W	14.2	1264	8.48	10.82	0.04	3.05	0.61	0.06	2.70	8.46	0.66	5.13	3.50	822.8	43.1	0.002
S2	W	15	1169	6.85	15.47	0.03	1.75	0.07	-	0.33	7.24	0.62	1.82	9.82	778	58.2	0.138
S2-1	W	13.7	1215	7.08	16.73	0.05	2.23	-	-	0.20	8.64	0.67	3.24	10.22	902.5	67.3	0.086
S2-2	W	15	1240	7	15.85	0.04	2.06	0.02	-	0.25	6.81	0.53	5.30	4.64	717.4	49.7	0.100
S3	W	19.8	1734	6.96	22.34	0.05	1.98	0.06	-	1.09	7.83	0.37	0.86	12.64	866.8	67.5	0.158
S4	W	19.2	1851	6.15	22.56	0.03	2.15	0.04	-	0.69	8.49	0.49	0.48	13.91	917.4	71.9	1.039
S5	W	16	1505	8.33	15.61	0.04	3.85	0.32	-	1.74	9.19	0.66	5.22	4.53	881	48.7	0.005
SM1	Salinella	10.2	67,600	6.1	47.2	1.03	1343.4	-	-	-	1266.2	16.6	14.1	51.7	81,393.9	328.9	2.2
SM2	Salinella	36.5	110,600	6.1	26.0	-	1234.7	1.3	-	14.0	1158.1	19.0	17.0	96.1	75,158.2	565.5	1.3
SM2a	Salinella	12.6	71,700	6.1	28.4	2.03	1286.1	-	-	-	1186.9	19.6	16.3	88.9	77,382.8	525.6	1.4
SM2b	Salinella	11.8	71,600	6.3	35.4	2.36	1317.9	-	-	8.1	1215.7	19.5	17.6	88.5	80,007.4	530.4	1.2
SM2c	Salinella	16.4	72,100	6.3	28.2	1.39	1289.1	-	-	11.6	1186.4	16.5	19.3	76.7	77,444.8	479.6	0.9
SM3	Salinella	18.8	60,100	6.2	30.8	0.91	1138.8	-	-	10.2	1037.3	16.9	33.3	54.5	68,910.1	438.5	1.3
R	River W	9.5	1098	8.54	6.04	0.01	2.91	0.30	-	6.76	8.26	0.53	5.49	4.90	1198.8	51.9	0.001
M1	Seawater	10.0	39,800	7.8	3.3	1.07	626	-	-	58.6	546	12.9	69.8	45.7	40,308	577	0.003

Table 1. Average major and trace elements composition of waters sampled in the Mt. Etna.

Sample	Basin	Temp.	EC	pH	HCO3	F	Cl	NO3	PO4	SO4	Na	K	Ca	Mg	TDS	Hardness	PCO2 at 20°
		°C	µS/cm		meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	meq/L	mg/L	°fH	bars
B11	E	9.1	219	7.35	2.62	0.02	0.29	0.02	0.02	0.30	1.35	0.21	0.69	1.38	149	10.3	0.007
B12	E	18	1138	7.77	8.16	0.04	3.28	0.92	-	1.70	6.07	0.65	3.50	4.57	736.6	40.3	0.009
B13	E	11.4	289	7.92	1.26	0.01	1.49	0.30	-	0.57	1.33	0.25	0.64	1.87	252.3	12.6	0.001
B14	E	14.7	1123	8.29	3.84	0.03	6.15	1.48	-	2.99	7.73	0.52	1.91	5.95	997.9	39.2	0.001
B15	E	10.6	282	8.37	2.02	0.01	0.96	0.09	0.02	0.79	2.21	0.24	0.57	1.13	191.1	8.5	0.001
B16	E	-	300	0.03	3.00	-	-	1.43	2.75	-	0.34	0.33	1.1	-	-	-	-
B17	E	15.8	1329	6.85	8.86	0.03	4.25	0.73	-	3.76	8.69	0.77	3.73	5.53	912.9	46.2	0.079
B18	E	5.6	424	7.82	4.58	0.03	0.96	0.01	-	1.68	3.79	0.35	1.60	1.46	307.5	15.3	0.004
B20	E	14.1	298	8.39	2.70	0.01	0.51	0.24	0.05	0.52	1.12	0.33	1.92	0.61	203.7	12.6	0.001
B21	E	16.4	967	7.32	8.24	0.06	2.47	0.46	-	1.91	5.07	0.64	2.15	7.67	723.4	49.0	0.025
B22	E	13.9	373	7.87	3.66	0.02	0.70	0.08	-	0.65	2.46	0.28	1.20	1.30	224.7	12.5	0.003
B24	E	17.4	544	7.8	3.90	0.03	1.37	0.31	-	0.69	2.99	0.26	0.70	1.75	268.2	-	0.004
B25	E	16	1056	6.58	7.08	0.02	1.72	0.61	-	2.35	4.15	0.57	2.42	4.60	574.1	-	0.118
B26	E	17.5	1570	6.57	10.67	0.03	2.74	0.28	0.02	3.86	7.07	0.85	3.45	6.75	838.6	-	0.181
B27	E	17	718	6.58	5.03	0.02	1.03	0.85	-	1.42	2.62	0.40	1.78	3.10	410.7	-	0.084
B28	E	16	886	6.4	6.44	0.02	1.35	0.21	-	1.62	3.47	0.54	2.09	3.87	458.8	-	0.162
B29	E	26	944	7.33	8.52	0.01	1.14	0.16	-	0.95	5.15	0.46	1.88	3.59	449.4	-	0.025
B30	E	18.8	588	7.8	3.33	0.03	1.63	0.50	-	0.79	3.08	0.25	0.76	1.90	303.1	-	0.003
B31	E	29.8	1527	7	8.89	0.05	2.86	0.41	-	4.15	5.43	0.76	3.66	6.80	827	-	0.056
B32	E	17.8	996	7.25	6.93	0.03	2.39	0.19	-	1.57	4.82	0.47	1.45	4.64	522.7	-	0.025
B33	E	21.3	1180	6.8	8.54	0.03	1.77	0.61	0.02	2.52	4.35	0.52	2.78	5.71	647.5	-	0.085
S6	E	17.6	2300	7.95	4.37	0.00	17.3	0.77	-	3.59	18.62	0.82	3.74	4.65	1693.5	41.9	0.003
S7	E	16.8	-	7.64	1.52	0.03	2.97	1.77	-	2.78	3.25	0.81	1.44	5.03	638.1	32.3	0.002
S8	E	14.9	330	8.15	1.95	0.02	0.77	0.18	-	1.14	2.14	0.23	0.73	1.34	210.8	10.4	0.001
S9	E	15.5	981	8.44	6.14	0.04	2.88	1.25	0.05	2.52	5.49	0.50	2.68	5.29	729.9	39.8	0.001
S10	E	15.3	676	7.74	5.00	0.04	2.46	0.41	-	0.75	3.77	0.41	2.23	2.39	441.7	23.1	0.006
S11	E	14.3	443	7.79	3.50	0.02	0.86	0.33	-	0.96	2.61	0.30	1.31	1.85	283.6	15.8	0.004
S12	E	19.7	1670	7.5	13.10	0.04	2.84	1.01	-	3.25	7.51	0.66	2.79	8.90	935.2	-	0.026
R1	River E	11.2	490	8.73	5.72	0.03	0.78	0.10	-	1.03	2.11	0.19	1.84	4.61	395.2	32.2	0.001
B1	W	7	1268	7.98	17.51	0.06	3.56	0.10	0.05	1.10	10.04	0.66	6.75	2.84	869.3	47.9	0.012
B2	W	15.9	1335	6.46	16.34	0.02	3.07	0.02	-	0.18	8.81	0.69	3.85	9.48	939.2	66.6	0.355
B3	W	15.1	1004	6.8	12.80	0.03	2.56	0.09	-	1.49	5.10	0.77	2.72	6.46	679.7	45.9	0.128
B4	W	8.8	832	6.2	12.18	0.03	0.91	0.08	-	0.87	4.12	0.48	0.64	6.82	486.9	37.3	0.502
B5	W	15.4	1441	6.71	17.71	0.04	2.17	0.21	-	0.56	6.82	0.78	6.09	6.07	852.2	60.8	0.218
B6	W	15	1153	7.11	12.36	0.04	2.86	0.29	-	0.93	5.65	0.57	5.09	4.83	746.7	49.5	0.061
B7	W	16.7	1013	7.49	7.94	0.03	2.90	0.75	-	1.46	4.89	0.54	3.28	3.58	624.3	34.3	0.016
B8	W	18.1	989	6.48	8.40	0.03	2.50	0.07	0.02	1.46	4.66	0.66	3.44	3.53	574.2	34.8	0.176
B9	W	13.9	384	8.24	2.68	0.04	0.66	0.08	-	0.68	2.26	0.23	0.81	0.94	182.3	8.8	0.001
B10	W	16.4	1951	6.84	23.85	0.06	4.65	0.17	0.02	0.72	6.83	0.59	1.21	18.04	1192	96.2	0.217
B19	W	17.1	1093.6	7.71	7.60	0.01	9.68	2.20	0.02	6.63	13.19	0.82	7.84	3.84	1488.4	58.3	0.009
B23	W	12.1	924	7.42	13.72	0.03	0.90	0.03	-	0.56	4.87	0.54	4.70	3.78	571.2	43.3	0.033
B34	W	18.7	1582	6.51	17.49	0.05	1.87	0.01	-	0.45	6.50	0.52	2.62	10.68	827.3	-	0.341
B35	W	14.9	1182	6.06	12.43	0.04	1.27	0.09	-	0.94	4.54	0.54	2.41	7.68	638.3	-	0.683
B36	W	23	2051	7.1	17.87	0.03	3.63	1.51	-	2.09	10.24	0.95	2.40	10.94	1114.8	-	0.090
B37	W	16.3	1800	7.41	13.33	0.04	4.40	0.93	-	3.09	9.94	0.56	2.02	8.87	995.6	-	0.033
B38	W	14.5	1194	6.51	11.84	0.03	1.44	0.19	0.03	1.15	4.40	0.53	2.41	7.62	648.3	-	0.231
B39	W	17.6	1766	6.98	14.72	0.04	3.16	0.97	-	2.62	8.30	0.68	2.55	9.65	976.8	-	0.097
B40	W	19	2560	7.3	21.90	0.09	5.32	4.80	-	4.42	11.72	0.91	5.83	14.72	1784.3	-	0.069
B41	W	18	1825	7.07	13.64	0.04	3.34	1.36	-	4.19	8.43	0.68	2.95	9.80	1060.1	-	0.073
B42	W	20	1860	6.8	14.57	0.04	3.51	0.97	-	3.15	8.64	0.61	3.17	9.42	1020.3	-	0.146
B43	W	16.4	1228	6.08	14.70	0.04	1.48	0.02	-	0.53	5.29	0.50	2.65	8.78	708.4	-	0.772
B44	W	20	1315	6.18	14.77	0.04	1.47	0.00	-	0.53	5.32	0.51	2.64	8.83	708.9	-	0.616
B45	W	16.2	1212	6.28	12.02	0.03	1.38	0.25	0.02	1.03	4.51	0.53	2.35	7.57	645.4	-	0.398
B46	W	14.6	1250	5.93	13.72	0.04	1.22	-	-	0.63	4.73	0.55	2.51	8.33	662.3	-	1.017
S1	W	14.2	1264	8.48	10.82	0.04	3.05	0.61	0.06	2.70	8.46	0.66	5.13	3.50	822.8	43.1	0.002
S2	W	15	1169	6.85	15.47	0.03	1.75	0.07	-	0.33	7.24	0.62	1.82	9.82	778	58.2	0.138
S2-1	W	13.7	1215	7.08	16.73	0.05	2.23	-	-	0.20	8.64	0.67	3.24	10.22	902.5	67.3	0.086
S2-2	W	15	1240	7	15.85	0.04	2.06	0.02	-	0.25	6.81	0.53	5.30	4.64	717.4	49.7	0.100
S3	W	19.8	1734	6.96	22.34	0.05	1.98	0.06	-	1.09	7.83	0.37	0.86	12.64	866.8	67.5	0.158
S4	W	19.2	1851	6.15	22.56	0.03	2.15	0.04	-	0.69	8.49	0.49	0.48	13.91	917.4	71.9	1.039
S5	W	16	1505	8.33	15.61	0.04	3.85	0.32	-	1.74	9.19	0.66	5.22	4.53	881	48.7	0.005
SM1	Salinella	10.2	67,600	6.1	47.2	1.03	1343.4	-	-	-	1266.2	16.6	14.1	51.7	81,393.9	328.9	2.2
SM2	Salinella	36.5	110,600	6.1	26.0	-	1234.7	1.3	-	14.0	1158.1	19.0	17.0	96.1	75,158.2	565.5	1.3
SM2a	Salinella	12.6	71,700	6.1	28.4	2.03	1286.1	-	-	-	1186.9	19.6	16.3	88.9	77,382.8	525.6	1.4
SM2b	Salinella	11.8	71,600	6.3	35.4	2.36	1317.9	-	-	8.1	1215.7	19.5	17.6	88.5	80,007.4	530.4	1.2
SM2c	Salinella	16.4	72,100	6.3	28.2	1.39	1289.1	-	-	11.6	1186.4	16.5	19.3	76.7	77,444.8	479.6	0.9
SM3	Salinella	18.8	60,100	6.2	30.8	0.91	1138.8	-	-	10.2	1037.3	16.9	33.3	54.5	68,910.1	438.5	1.3
R	River W	9.5	1098	8.54	6.04	0.01	2.91	0.30	-	6.76	8.26	0.53	5.49	4.90	1198.8	51.9	0.001
M1	Seawater	10.0	39,800	7.8	3.3	1.07	626	-	-	58.6	546	12.9	69.8	45.7	40,308	577	0.003

Figure 3. Stiff diagrams displaying average ion concentrations in the sampled waters.

Figure 3. Stiff diagrams displaying average ion concentrations in the sampled waters.

Furthermore, high-temperature volatiles released from the magma could interact with descending meteoric waters causing a general increase of the temperature in the rest of the aquifer. This is in line with [24], who suggested a main magmatic signature linked to degassing of an enriched mantle beneath Mt. Etna. [25] characterized Etna’s magma as CO₂-rich while [26], hypothesized that the asthenosphere beneath the volcano rises to a depth that permits the continuous escape of CO₂ from the mantle. Present findings of CO₂ partial pressure °pCO₂), also suggest that Etnean waters might interact with CO₂ of deep origin, arguably magmatic. Measured pCO₂ varies between 5 × 10⁻⁴ and 2.2 bars and more than 90% of the samples exhibit values 1 to 3 orders of magnitude higher than those expected for waters in equilibrium with the atmosphere (pCO₂ = 10^−3.6 bar). These positive emission anomalies could be attributed to the release of CO₂ by fresh magma that intruded into the volcano plumbing system [27,28]. In contrast, the anomalous high pCO₂ values (1.04 bars) in sample S4 could be ascribed to local waters rapidly charged with CO₂ and dissolved elements from the surrounding soil matrix. This would also explain the high conductance values among the sampled springs. Maximum pCO₂ values (up to 2.15 bars) were recorded at the “Salinella” mounts near Paternò, on the southern fringes of Etna. In here, the source of the fluids would be associated with a hydrothermal system enriched in CO₂ with temperatures between 100 °C to 150 °C that extends between Paternò and the central part of the Etna [29].

Direct inputs of deep CO₂ are a key factor in determining the chemistry of Etnean waters. In effect, the interaction between CO₂ and infiltrating rain water lowers the pH to values below 4 [30]. These low-pH waters become highly reactive resulting in chemical weathering of the host basaltic rocks. As a result of this process, HCO₃⁻ (along with H₂CO₃ and CO₃²⁻) is gradually generated, whilst Mg, Ca, K, and Na are released into solution. The general positive relationship between HCO₃⁻ and these cations (especially Na⁺ and Mg²⁺) supports the idea that dissolution from acidic waters is as a major mechanism for the mineralization of Etnean aquifers (Figure 4). Bicarbonate concentrations are normally higher in the western basin, with values up to 23.8 meq/L around the towns of Paternò, S.P. Clarenza, and Biancavilla. In contrast, groundwater in the eastern basin shows a more limited enrichment in HCO₃⁻, with concentration values approximately 1/3 (~5 meq/L) of those observed in the west. This is in line with [31], who argued that chemical weathering in Etna is not spatially uniform since the presence of CO₂ in soils is more abundant in particular areas of the volcano, such as the south-western flank.

Figure 4. HCO₃⁻ against major cations in groundwater.

Figure 4. HCO₃⁻ against major cations in groundwater.

Furthermore, HCO₃⁻ concentrations appear to be influenced by the ground surface elevation. There is a negative trend between HCO₃⁻ and elevation, with the lowest concentrations towards the volcano’s summit. As discussed, more restricted hydrological circuits mean that CO₂-enriched waters near the top of the edifice have fewer opportunities to react with the host basaltic rocks. Lower dissolution rates translate into lower HCO₃⁻ concentrations and a general decrease in the total dissolved solids content (TDS) of the waters.

Electrical conductivity (EC) from 1000 to 2000 µS/cm, and TDS values between 700 mg/L and 1400 mg/L were found to be representative for groundwater in the southernmost flank of Mt. Etna. Exceptional salinities detected at S6 near Catania (EC = 2300 µS/cm; TDS = 1694 mg/L) are attributed to partial mixing with seawater in proximities to the Ionian coast. Longer residence times favoring basalt leaching would be an additional factor explaining the higher salinity in the SW sector of Etna [32]. Conversely, the eastern flank is characterized by EC values between 500 and 1000 µS/cm, and TDS from 300 to 1100 mg/L. This lower TDS might be ascribed to water circulation in more transmissive sediments that facilitate the flow of meteoric recharge and reduce the transit time underground. Elevation would be another factor controlling the TDS distribution in groundwater. Dissolved solids in samples above 600m A.S.L. are usually below 500 mg/L, which can be attributed to the limited water-rock interaction at high elevations. As expected, groundwater at higher elevations receives direct recharge from precipitation, which is unable to interact with the host rocks for long enough to produce major changes in its chemical composition.

Major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) show a similar distribution to TDS. Calcium concentrations average 2.7 meq/L. Higher values were recorded south of Adrano (7–7.8 meq/L), and between Belpasso and Camporotondo (5–7 meq/L) towards the western margin of the volcano. The lowest concentrations were measured south of Paternò and north of Ragalna, on the southwest (<1.5 meq/L). Again, groundwater in the western basin reflects more extensive water-rock interaction due to longer transit and residence times. This may be explained by the rainfall distribution, as maximum precipitation (i.e., groundwater recharge) occurs on the eastern flank of the volcano, contrary to the western flank that receives lower rainfall and consequently shows prolonged interactions between groundwater and the host rock [33].

In particular, Ca²⁺ and K⁺ have a similar distribution, with sharp concentration variations around the region of Paternò. These changes occur over short distances and reflect different aquifers. Higher salinities can be related to contact with alkaline brines discharged by the Paternò mud volcanoes (Salinelle), although groundwater mineralization through permeable faults can still exert some influence.

Magnesium reaches maximum concentrations in the western basin at S.P Clarenza, and over a vast area up to Catania. There exists a close relationship between Mg²⁺ and HCO₃⁻. This suggests that dissolution caused by CO₂-enriched waters on the host rocks is common to both species. In particular, the source of Mg²⁺ can be explained by the leaching of olivines and pyroxenes from the basaltic rocks in the substrate.

Chloride concentrations range from 0.3 to 17 meq/L. In principle, the abundance of Cl⁻ might be attributed to alteration of the volcanic rocks and to the interaction of groundwater with volcanic gases [3]. As in the case of Na⁺, maximum Cl⁻ contents (~17.3 meq/L) were measured at S6, near Catania, likely due to mixing with seawater. Figure 5 shows that waters near the coast exhibit a Na/Cl ratio closer to the 1:1 line, unlike samples from the western basin that are partially depleted of Cl⁻. Seawater intrusion and mixing between shallow groundwater and deep brines could be the reason for these patterns. This is also coincident with [9], who considered both mixing and water-rock interaction to be responsible for the increased salinity of groundwater in Mt. Etna. It is worthy to note that although evaporates do not crop out in the area, their presence beneath the volcanic cover has been hypothesized both from geological and hydrochemical data [21]. These deposits could thus be another contributing factor for Cl⁻ in groundwater.

Maximum concentrations of NO₃⁻ and SO₄²⁻ were observed in the valleys that exist at low elevations in the volcano (e.g., south of Adrano). High concentrations are also visible within the stretch of land south of Giarre to Fiumefreddo, suggesting the leakage of fertilizers into the aquifers. The availability of water resources and the quality of the soils in the lower flanks of the volcano have favored the agricultural exploitation of the region since ancient times. Under conditions of high oxygen, part of the ammonium—sulphate fertilizers applied on the ground would be rapidly converted to NO₃⁻, which is not sorbed by the negatively charged soil colloids and moves readily to the water table [34]. At higher elevations (e.g., Valle del Bove), the SO₄²⁻ inputs into groundwater could be controlled by the ascent of volatiles from vapor-dominated systems such as fumaroles rather than anthropogenic effects.

Figure 5. Relation of samples against the Na:Cl ratio.

Figure 5. Relation of samples against the Na:Cl ratio.

In summary, groundwater in the area of study can be grouped into four main types: (1) Mg–Na ± Ca–HCO₃⁻ (with rare Cl⁻ or SO₄²⁻) (54%); (2) Na–Mg ± Ca–HCO₃⁻ ± (SO₄²⁻ or Cl⁻) (28%); (3) Na–Ca–Mg–HCO₃⁻ (12%); and (4) Na–Mg ± Ca–Cl (with HCO₃⁻ or SO₄²⁻) (6%). The differences between the first and second group are minor and mainly associated with variations in the Na–Mg and SO₄²⁻–Cl⁻ content (with relative concentrations normally higher for the second type). This implies that types “(1)” and “(2)” explain 82% of the waters in the region. Waters of the third group are associated with elevated Ca²⁺ concentrations, while the last type is characterized by high salinity. Samples enriched in Cl⁻ could be the result of mixing with seawater or solute diffusion from marine clay aquitards.

Additionally, carbonate waters are generally predominant in the western basin, probably due to the interaction with hydrothermal CO₂. In contrast, the Cl⁻–SO₄²⁻ type is mainly found along the coast of the eastern basin due to saline influxes. Furthermore, the eastern coast is the area most densely inhabited and some anthropogenic effects are already reflected in the more elevated NO₃⁻ contents, likely derived from agricultural and urban wastewater (e.g., samples B14, S7, S9, S1).

4.2. Isotopic Signature

The isotopic signature of the sampled waters was also used to determine the recharge areas and circulation pathways in Mt. Etna (Table 2). Values for groundwater and spring samples fall between −4‰ to −9‰ for δ¹⁸O and −19‰ to −53‰ for δ²H. This is in close correlation with the Global Meteoric line (δ²H = 8 × δ¹⁸O +10‰, [35]) and the eastern Mediterranean local meteoric water line (EMMWL) defined by [36]: δ²H = 8 × δ¹⁸O + 22‰, suggesting a predominant meteoric origin for the collected samples (Figure 6). Similar trends had been reported by [4], who postulated that most waters in Mt. Etna originated as local precipitation infiltrated at an elevation between 1100 and 1900 m. Minor variations in the δ¹⁸O of the samples also suggest limited effects of rock interaction on the original water composition.

Table 2. Typical isotopic composition of Etnean waters.

Sample No.	Sample Type	Basin	DIC δ13C (PDB ‰)	δ18O (‰ VSMOW)	δ2H (‰ VSMOW)
B1	Bore	W	−0.1	−9.1	−51.1
B2	Bore	W	−1.6	−8.6	−45.3
B3	Bore	W	−2.4	−8.6	−47.0
B4	Bore	W	−1.3	−7.8	−41.2
B5	Bore	W	−1.6	−8.6	−45.3
B6	Bore	W	−2.4	−8.6	−47.0
B7	Bore	W	−1.3	−7.8	−41.2
B8	Bore	W	+0.2	−7.4	−38.8
B9	Bore	W	−1.0	−6.6	−38.9
B10	Bore	W	−1.2	−6.7	−28.6
B19	Bore	W	−10.3	−6.8	−36.7
B23	Bore	W	−7.9	−6.1	−29.0
S1	Spring	W	−1.8	−8.6	−50.2
S2	Spring	W	−1.8	−8.6	−50.2
S2-1	Spring	W	−1.2	−8.7	−46.3
S2-2	Spring	W	−0.6	−8.7	−46.6
S3	Spring	W	−0.1	−7.4	−40.1
S4	Spring	W	−0.7	−8.6	−46.6
S5	Spring	W	−0.1	−7.3	−40.1
R	River	W	−7.5	−8.2	−44.4
B11	Bore	E	−1.0	−7.5	−37.4
B12	Bore	E	−4.6	−6.7	−39.3
B13	Bore	E	−8.6	−7.2	−38.9
B14	Bore	E	−1.5	−7.1	−38.6
B15	Bore	E	−2.0	−7.4	−37.5
B16	Bore	E	-	−6.3	−33.0
B17	Bore	E	−1.8	−7.0	−39.1
B18	Bore	E	−4.6	−6.7	−37.7
B20	Bore	E	−9.4	−4.2	−19.2
B21	Bore	E	−14.5	−6.0	−34.3
B22	Bore	E	−6.9	−7.3	−46.7
B24	Bore	E	−7.7	−7.1	−31.5
B27	Bore	E	−1.8	−6.9	−39.1
B28	Bore	E	−5.0	−7.8	−42.2
B29	Bore	E	−5.8	−7.5	−41.2
B30	Bore	E	−9.4	−4.1	−19.2
B31	Bore	E	−14.5	−5.9	−34.3
B32	Bore	E	−6.9	−7.3	−46.7
B33	Bore	E	+0.8	−9.1	−53.3
S6	Spring	E	−11.2	−6.2	−33.7
S7	Spring	E	−11.2	−6.2	−33.7
S8	Spring	E	−11.8	−6.7	−37.9
S9	Spring	E	−11.4	−6.7	−38.1
S10	Spring	E	−6.1	−6.9	−36.5
S11	Spring	E	−9.1	−7.1	−35.6
S12	Spring	E	−1.2	-	-
R1	River	E	−5.4	−8.0	−44.9
SM1	Salt mount	Paternò	-	+8.8	−14.5
SM2	Salt mount	Paternò	-	+10.4	−12.0
SM3	Salt mount	Paternò	-	+7.5	−13.6
Sm2a	Salt mount	Paternò	+5.3	+9.78	−18.2
Sm2b	Salt mount	Paternò	+3.0	+10.2	−21.5
Sm2c	Salt mount	Paternò	+1.4	+10.1	−19.8
M1	Seawater	-	-	+1.1	+1.3

Table 2. Typical isotopic composition of Etnean waters.

Sample No.	Sample Type	Basin	DIC δ13C (PDB ‰)	δ18O (‰ VSMOW)	δ2H (‰ VSMOW)
B1	Bore	W	−0.1	−9.1	−51.1
B2	Bore	W	−1.6	−8.6	−45.3
B3	Bore	W	−2.4	−8.6	−47.0
B4	Bore	W	−1.3	−7.8	−41.2
B5	Bore	W	−1.6	−8.6	−45.3
B6	Bore	W	−2.4	−8.6	−47.0
B7	Bore	W	−1.3	−7.8	−41.2
B8	Bore	W	+0.2	−7.4	−38.8
B9	Bore	W	−1.0	−6.6	−38.9
B10	Bore	W	−1.2	−6.7	−28.6
B19	Bore	W	−10.3	−6.8	−36.7
B23	Bore	W	−7.9	−6.1	−29.0
S1	Spring	W	−1.8	−8.6	−50.2
S2	Spring	W	−1.8	−8.6	−50.2
S2-1	Spring	W	−1.2	−8.7	−46.3
S2-2	Spring	W	−0.6	−8.7	−46.6
S3	Spring	W	−0.1	−7.4	−40.1
S4	Spring	W	−0.7	−8.6	−46.6
S5	Spring	W	−0.1	−7.3	−40.1
R	River	W	−7.5	−8.2	−44.4
B11	Bore	E	−1.0	−7.5	−37.4
B12	Bore	E	−4.6	−6.7	−39.3
B13	Bore	E	−8.6	−7.2	−38.9
B14	Bore	E	−1.5	−7.1	−38.6
B15	Bore	E	−2.0	−7.4	−37.5
B16	Bore	E	-	−6.3	−33.0
B17	Bore	E	−1.8	−7.0	−39.1
B18	Bore	E	−4.6	−6.7	−37.7
B20	Bore	E	−9.4	−4.2	−19.2
B21	Bore	E	−14.5	−6.0	−34.3
B22	Bore	E	−6.9	−7.3	−46.7
B24	Bore	E	−7.7	−7.1	−31.5
B27	Bore	E	−1.8	−6.9	−39.1
B28	Bore	E	−5.0	−7.8	−42.2
B29	Bore	E	−5.8	−7.5	−41.2
B30	Bore	E	−9.4	−4.1	−19.2
B31	Bore	E	−14.5	−5.9	−34.3
B32	Bore	E	−6.9	−7.3	−46.7
B33	Bore	E	+0.8	−9.1	−53.3
S6	Spring	E	−11.2	−6.2	−33.7
S7	Spring	E	−11.2	−6.2	−33.7
S8	Spring	E	−11.8	−6.7	−37.9
S9	Spring	E	−11.4	−6.7	−38.1
S10	Spring	E	−6.1	−6.9	−36.5
S11	Spring	E	−9.1	−7.1	−35.6
S12	Spring	E	−1.2	-	-
R1	River	E	−5.4	−8.0	−44.9
SM1	Salt mount	Paternò	-	+8.8	−14.5
SM2	Salt mount	Paternò	-	+10.4	−12.0
SM3	Salt mount	Paternò	-	+7.5	−13.6
Sm2a	Salt mount	Paternò	+5.3	+9.78	−18.2
Sm2b	Salt mount	Paternò	+3.0	+10.2	−21.5
Sm2c	Salt mount	Paternò	+1.4	+10.1	−19.8
M1	Seawater	-	-	+1.1	+1.3

A different trend is observed for waters collected in the salt mount brines. The “positive” isotopic ratio of the samples (i.e., δ¹⁸O up to about +10‰, and δ²H ranging from −21.5‰ to a maximum of −12‰) suggests that these waters could be mixed with hydrothermal fluids of deep origin following a more prolonged interaction with the host rocks.

Figure 6 also evidences the contrast between the mainly meteoric groundwater against Cl-enriched fluids from the salt mounts. As postulated by [4], the anomalous enrichment of Cl⁻ might be related to the proximity of the sampled bodies to areas of intense magmatic outgassing, where Cl-rich gases are likely to interact with shallow aquifers. Considering that the ratio Cl/δ¹⁸O in the salt mounts is higher than seawater (sample M1), the direct mixing between these two fluids would be improbable.

Figure 6. Relation between oxygen and hydrogen isotopic composition in Etnean groundwater.

Figure 6. Relation between oxygen and hydrogen isotopic composition in Etnean groundwater.

Additional information may be inferred by assessing the geographical distribution of the stable isotope ratios in the volcano. Waters from the southern flanks of Mt. Etna are relatively homogeneous and generally fit with the EMMWL. In contrast, waters in the eastern sector deviate from the meteoric line suggesting that more complex processes could take place there. These processes might be associated to heterogeneous recharge and irregular circulation patterns, especially on the alluvial lowlands of the eastern basin, where surface runoff and upward flows could also affect the chemical signature of the waters. These observations are consistent with [4,15]), who described considerably more diversity for waters in the eastern flanks of the volcano.

The isotopic composition of the waters might also be influenced by the terrain elevation, although the relationship is generally limited and not always distinguished (Figure 7). In effect, samples with a lighter isotope composition often coincide with higher elevations towards the cone summit, but the groundwater character still varies considerably, likely in response to additional underlying factors. As clouds rise up the volcano, the heavy isotopes are depleted and the residual precipitation gets isotopically lighter [37]. The most depleted samples would locate in the southern basin where the δ²H composition approaches −55‰. These differences between the isotopic compositions in the south and other regions of the volcano could be another indication of variations in meteoric inputs and the heterogeneity of the hydrological circuits within Etna. A second trend suggests that waters become lighter from East to West, which is coincident with the main wind direction and the geographical distribution of precipitations. In effect, rainfall amounts peak on the eastern flanks of the Etna, mainly in relation to cooling sea breezes and clouds from the neighbor Mediterranean Sea [8]. The water vapor from the sea and the subsequent clouds and precipitation, are characterized by a high ²H excess which in turn, is reflected in the composition of the waters [38]. In its migration across the volcano, falling precipitation undergoes fractionation and becomes increasingly lighter.

Figure 7. Stable isotopes in groundwater against elevation in the volcano.

Figure 7. Stable isotopes in groundwater against elevation in the volcano.

Values for δ¹³C vary from as low as −14‰ in proximities to the town of Giarre (B31), up to about 5‰ in waters nearby the mud volcanoes of Paternò. Measured values plot above the line of pCO₂ in the atmosphere and largely outside the typical range of groundwater suggesting a contribution of external CO₂ (Figure 8). Given that groundwater in Etna does not come in contact with outcrops or superficial carbonate rocks [14], the prevailing pCO₂ source would be at depth.

Figure 8. Partial pressure of CO₂ vs. δ¹³C showing the possible contribution of magmatic gasses. Typical groundwater composition within the dashed area.

Figure 8. Partial pressure of CO₂ vs. δ¹³C showing the possible contribution of magmatic gasses. Typical groundwater composition within the dashed area.

Under low HCO₃⁻ concentrations, δ¹³C values range broadly from −2‰ to −15‰ (Figure 9). An increase in HCO₃⁻ (>10 meq/L) is coincident with δ¹³C in the range of −2.4‰ and +2.3‰, with most samples clustering around −1.5‰. In such conditions, the equilibrium between HCO₃⁻ and CO₂ causes the isotopic composition of the latter species to become more positive. Therefore, the Etnean magmatic CO₂ becomes isotopically heavier [39]. This pattern suggests some additional inputs of CO₂, possibly related to hydrothermal fluids that stripped the gasses from a magmatic reservoir and transported them into the shallow aquifers. In such conditions, the rise of CO₂ would lower the pH of the circulating waters and result in higher concentration of dissolved HCO₃⁻ after weathering the host rocks. Thus, many waters in the studied area might be partially influenced by this secondary CO₂ despite their overall meteoric origin.

Figure 9. Change in groundwater composition in relation to the HCO₃⁻ and δ¹³C concentrations.

Figure 9. Change in groundwater composition in relation to the HCO₃⁻ and δ¹³C concentrations.

In short, two major groundwater groups can be discriminated on the basis of δ¹³C: (1) waters with compositions below −4.6‰, mainly recharged by direct percolation of rainfall; (2) waters with δ¹³C > −2.4‰ in a stretch of land between Adrano and Misterbianco in the south, and around Pozillo to the east, that include a component of external dissolved gas. This trend suggests that the diffusion of CO₂ gasses in Mt. Etna is unevenly distributed, and essentially controlled by the main tectonic structures of the volcano. During their ascent to the surface, these gases interact in different ways with shallow water-bearing strata changing their concentrations as they cross the aquifers [23].

It is worthy to note that a more particular isotopic signature was recorded at the “Salinelle di Paternò”. In these mud volcanoes, the δ¹³C compositions ranged from 1.4‰ to 5.3‰ while the pCO₂ values varied from 1.3 to 2.2 bars. The enrichment in CO₂ and the high “positives” stable isotopic ratio observed (δ²H ~ −12‰ and δ¹⁸O ~ 10.4‰) seem to indicate a very deep origin of these fluids.

5. Summary and Conclusions

A new survey dataset of major ion concentrations and isotope ratios was used to update the knowledge of geochemical characteristics of groundwaters and to better understand the flow system around Mt. Etna. The Etnean groundwaters possess a marked bicarbonate-alkaline chemistry, which is consistent with an abundance of dissolved CO₂ gas and the composition of the volcanic host rocks. Chloride-sulphate and nitrate dominated waters can locally prevail along the Ionian Sea coast, largely due to urban contamination and the leakage of agricultural fertilizers. Other distinctive characteristics of the sampled waters include low temperatures, high conductance, and elevated hardness. The salinity of the waters decreases with elevation due to the proximity to recharge areas and shorter travel paths. Oxygen-deuterium isotopes showed that waters are essentially recharged by infiltrating rainfall. Especially to the south, most of the samples display a good correlation with the eastern Mediterranean meteoric water line. To the east, collected samples deviate from the meteoric line, suggesting more heterogeneous circulation paths, and variable degrees of interaction between meteoric waters and the aquifer rocks.

Furthermore, the isotope composition is influenced by the provenance of wet air masses from the Mediterranean Sea. In this regard, waters become isotopically lighter to the west, following the distribution of precipitation on the volcano. Similarly, the liquid-vapor fractionation of waters results in lighter waters along with an increase in elevation or in proximity to the cone summit.

The analysis of δ¹³C indicates that at least a proportion of the waters, mainly in the southern region of the volcano, would be affected by external CO₂ contributions, possibly of hydrothermal origin. This is also supported by pCO₂ values 1 to 3 orders of magnitude higher than those expected for waters in equilibrium with the atmosphere. Furthermore, a high enrichment in CO₂ along with high positive values for δ²H/δ¹⁸O suggest that waters in the salt mounts around Paternò would be influenced by brines originating at depth within the system.