Geochemical Features of Ultramafic Rocks and Formation of Magnesium–Bicarbonate Groundwaters in the Kraka Massif Area (Southern Urals)

Abstract

The observed shortage of water resources in the western and southern regions of the Russian Federation may soon affect the territory of the Republic of Bashkortostan. An increase in the share of groundwaters can help to solve this problem. To provide the population of the republic with water resources, the groundwater of magnesium–bicarbonate-type from the Kraka ophiolite massifs can be used. The massifs occur on the western slope of the Southers Urals. In this work we studied ultramafic rocks and their influence on the formation of the chemical composition of water. The research area is located in the northern part of the Zilair synclinorium, which occurs within the Central Ural megazone. In terms of hydrogeology, of particular importance to the territory of the synclinorium is the Zilair basin of fracture waters of the second order, which is part of the Uralian hydrogeologic folded zone. The ultramafic rocks from the studied area have consistently high CaO/Al₂O₃ ratios (0.4–1.6), which indicates the widespread development of parageneses with participation of clinopyroxene and a low degree of depletion of the primitive mantle source. Because of the complex geological structure of the area, water samples collected from both water points in the Kraka massifs, and the surrounding Early–Middle Paleozoic rocks were analyzed for major ions using a laboratory method to identify possible hydro-geochemical zoning. A statistical analysis was then conducted based on the obtained anion–cation composition data. From the viewpoint of the hydrolytic concept, the formation of the chemical composition of groundwater takes place due to the removal of Mg²⁺ from the rock-forming minerals of ultramafic rocks (olivine and pyroxene) and the supply of Na⁺, K⁺, Ca²⁺, and SO₄²⁻ Cl⁻ from atmospheric precipitations. The bicarbonate anion has a complex nature, where both biochemical processes in the soil and atmospheric precipitation play a significant role. Magnesium–bicarbonate-type of waters, due to low mineralization (to 1 g/L) and the majority of other geochemical parameters (pH of the medium, and content of Na, K, Ca, SO₄, and Cl), whose values that are within the limits set by the World Health Organization (WHO), can be used as drinking water. The increased values of total hardness (0.20–3.39 mmol/L) in accordance with the regulatory document SanPiN 1.2.3685–21, adopted by the Russian Federation, do not exceed the maximum permissible concentrations (up to 7.00 (10.00) mEq/L or 3.50 (5.00) mmol/L). The high magnesium content, in accordance with GOST (state standard) R 54316–2020, allows the magnesium–bicarbonate waters of the Kraka massifs to be classified as table mineral waters for the treatment of various diseases (including hypomagnesemia).

1. Introduction

Groundwater, being one of the most important natural resources as it serves as a source of drinking water for 2.5 billion people [1], is under threat due to such factors as climate change, natural and anthropogenic pollution, etc. [2,3,4]. Arsenic contamination affects over 200 million people in regions like Bangladesh, India, and Vietnam, and parts of Latin America, causing chronic health issues including cancer, skin lesions, and cardiovascular disease [5]. Groundwater salinization, driven by overexploitation, sea level rise, and irrigation return flows, impacts over 1 billion people in coastal aquifers (e.g., Mediterranean Basin and the North China Plain) and arid inland areas (e.g., the Middle East and Australia), elevating salinity beyond WHO limits (>1 g/L TDS) and rendering water undrinkable [6,7,8]. More than 2 billion people in the world live in the regions with a severe water shortage [9,10]. The problem of providing certain regions with high-quality drinking water is discussed in [11,12]. Magnesium–bicarbonate waters that occur in rocks of ultrabasic and basic composition can provide the population with drinking water resources. This type of groundwater is widely studied by researchers around the world [13,14,15,16,17,18]. The risks of possible contamination of aquifers with heavy metals (Cr, Ni, etc.) contained in the rock-forming and accessory minerals of ultrabasic and basic rocks are reported in [19,20,21,22,23].

These issues highlight the value of uncontaminated groundwater sources, such as magnesium–bicarbonate waters from ultramafic terrains, which offer low-salinity alter-natives with beneficial Mg content for regions facing salinization or metal pollution [13,14,15,16,17,18].

In connection with the impending global water crisis, water resources are becoming a strategic factor in the long-term development of the global economy, including that of Russia. A corresponding member of the Russian Academy of Sciences, V.I. Danilov-Danilyan, mentions that in the western and southern regions (the Republic of Kalmykia, some areas of the Lower Volga region, Stavropol, Krasnodar Territory, Kurgan, Chelyabinsk, Orenburg, Rostov, Moscow regions, etc.) there is a shortage of fresh water [24], and by 2030–2035, the problem will affect the territory of the Republic of Bashkortostan. The study of groundwater quality is one of the main hydrogeological tasks determining the possibility of using these waters for drinking purposes [25]. An increase in the share of groundwaters in the water supply, owing to their consistently high quality compared to surface water, will help solve the water shortage.

In the Republic of Bashkortostan (Russian Federation, Volga Federal District), fresh groundwater resources that can be used in economic activities are unevenly distributed [26]. As the population of the republic grows and the environmental situation in the region deteriorates, it is necessary to look for new sources of fresh groundwaters. Groundwater from the Zilair basin, which is part of the Ural complex hydrogeological folded region, can be used for supplying the eastern part of the region. The Zilair basin of fractured waters of the second order is located in the territory of the Zilair synclinorium, which is part of the Central Ural megazone on the western slope of the Southern Urals [27]. The magnesium–bicarbonate-type groundwater is extremely rare in the Southern Urals and typically occurs in the ultrabasic and basic rocks of the Kraka ophiolite massif [27]. The massifs consist of four separate bodies with a total area of more than 900 Km²: Northern, Middle, Uzyansky and Southern Kraka. In order to identify the interaction of the components of the “groundwater–rock” system in the formation of the chemical composition of water [28], as well as to assess its quality for use in economic purposes, during fieldwork in 2016–2024, both rock material and water samples from water points were collected from the territory of the Kraka massifs.

2. History of Geological and Hydrogeologic Studies

The history of the geological study of the Kraka massifs dates back to the second half of the 19th century. The ultramafic rocks of this territory were first mentioned in the works of N.F. Chernyshev, R.I. Murchison, and A. Shtukenberg. In the 1920–30s, when systematic geological survey works and searches for chrome ore and chrysotile asbestos started, the search teams of the “Bashchromite” Trust and other organizations worked in the territory of the massifs. In the 1960s, a geological survey (scale 1:50,000) supervised by A.V. Klochikhin was conducted at the massifs. The material composition of ultramafic rocks was analyzed by S.V. Moskaleva, G.N. Savelyeva, E.A. Denisova, S.G. Kovalev, V.I. Snachev, D.E. Saveliev, and E.N. Savelieva; microstructural studies were carried out by E.A. Denisova, G.N. Savelieva, and D.E. Saveliev [29,30,31,32,33]. Despite the two-century history, the massifs were studied to varying degrees. This is especially true for the Southern Kraka, the western and southwestern parts of which have been best studied. Most of the deposits and ore occurrences of chromitites (Apshakskaya and Malobashartovskaya areas) of the massif occur in these well-studied locations [31,34], while the eastern part has not been sufficiently studied.

The first information about the hydrogeology of the Zilair synclinorium was reported by M.O. Kler in his work in 1928. Later, researchers such as D.I. Itkin and N.N. Tostunov were engaged in hydrogeology. In 1962–1969, the Kagarmanov Party (supervised by A.V. Klochikhin) analyzed the chemical composition of water from water points both in the territory of the Kraka massifs and host rocks. R.F. Abdrakhmanov studied the geochemistry and the conditions of the formation of groundwater in the Southern Urals, including fracture waters of the second order in the Zilair basin [27].

3. Geological and Hydrogeological Characteristics

3.1. Stratigraphy of Host Rocks

The framing of the Kraka massifs is made up of Paleozoic sediments forming two lithologically different sections:

• The carbonate–terrigenous section, represented by sediments from the Nabiullinskaya (O₂–S₁nb) to the Zilair formation (D₃–C₁zl), and located in the western part of the synclinorium;
• The terrigenous–siliceous section composed of rocks from the Sukholyadskaya strata (O_1–3sh) to the Zilair formation (D₃–C₁zl) in the eastern part of the synclinorium (Figure 1).

Virtually all throughout the boundaries of the massifs, sedimentary strata are separated from ultramafic rocks by polymictic melange (mpD₃–C₁), which is made up of blocks of rocks of various ages and compositions. Moreover, the volcanic rocks of the Uzyanbash strata (D₂ub (variolites and hyalobasalts)) are widespread along the eastern border of the massifs (Figure 1d).

3.2. Geologic Structure of the Kraka Massifs

The Kraka massifs cover an area of about 900 km² and consist of four separate bodies of different sizes. The Northern Kraka is an isometric body 20 × 15 km in size and is composed mainly of lherzolites (65–70% olivine, 10–25% orthopyroxene, 5–10% clinopyroxene, and 0.5–3% Cr-spinel), and less often harzburgites, as well as striped, lenticular, or columnar dunite bodies [32,33].

The Central Kraka, covering an area of 180 km², has the most diverse geological structure: ultramafic rocks (peridotites and, less frequently, dunites), which make up 80% of the territory; mafic rocks, forming a meridionally elongated body 0.5–1.5 km wide in the west of the massif, and rocks of the wehrlite–clinopyroxenite transitional complex, occupying an intermediate position. The central and eastern parts of the massif are represented by spinel lherzolites and clinopyroxene-bearing harzburgites. Dunite bodies and spinel-plagioclase peridotites also occur among the rocks [31,32]. The Uzyansky Kraka (45 km²) is composed of 75% rocks of the dunite–harzburgite complex (harzburgites, dunite–harzburgites, and dunites) and 20–25% marginal serpentinites [32,34]. The largest of the four massifs is the Southern Kraka, with an area of 450 km². It is a virtually isometric body separated in the south into eastern and western branches. Spinel peridotites and dunites participate in the structure of the massif. The western part of the territory is composed of rocks of the dunite–harzburgite complex. In addition to harzburgites, spinel lherzolites are also present in the northern and central parts of the massif. Plagioclase-containing lherzolites with a low degree of serpentinization (15 vol.%) are sporadic [42]. Serpentinites are distributed along the periphery of all massifs, where the rock-forming minerals are either preserved as relics or completely replaced by mesh-textured serpentinite. The near-fault zones are dominated by fibrous chrysotile serpentinites, while in the rare outcrops at the Middle and Uzyansky Kraka massifs the presence of stubachites was detected [32].

3.3. Hydrogeological Characteristics of the Northern Part of Zilair Synclinorium

According to the modern concepts, in the territory of the Zilair synclinorium there is a basin of fracture waters of the second order, which is part of the Ural hydrogeological folded system (Figure 2) [27].

The groundwater recharge in this territory comes from atmospheric precipitations. The aquifer rocks in the western part of the territory are represented by the carbonate–terrigenous rocks of the Nabiullinskaya (O₃–S₁nb), Uzyanskaya (S₁uz), Sermenevskaya (S₂sr), and Belskaya formations (D_1–3bl). Clay rocks are confining beds. The Ordovician–Devonian aquifer complex (O–D) occurs as a narrow band in the western part of the Zilair synclinorium. The depth of groundwaters, coming to the surface in the form of springs in the areas of limestone outcrops in the Lower Devonian, frequently forming steep cliffs along the Belaya river, reaches 80–100 m, and within the area of development of other breeds it reaches 50–70 m. Groundwaters in the distributed territory of ultramafic rocks occur at a depth of <10–15 m. They appear in the marginal parts of the massifs in the form of numerous, mainly dispersed sources emerging from the debris of serpentinites. The groundwaters have a magnesium–bicarbonate composition (Figure 2). The rate of spring flows averages 0.1–0.3 L/s and mineralization is 0.1–0.5 g/L [27,38].

4. Materials and Methods

During fieldwork conducted from 2016 to 2023, ultramafic rocks were selected from the Northern and Southern Kraka massifs. Data on the petrochemistry of rocks from the Middle and Uzyansky Kraka massifs from [32,43] were also used. A total of 143 samples were analyzed (Tables S1–S4). To determine the bulk composition of the rocks, tablets (pressed powder pellets) were prepared, which were then analyzed for major oxides and some trace elements (X-ray fluorescence analysis) using a Xenemetrix X-Calibur energy-dispersive X-ray fluorescence spectrometer at the Institute of Geology, Ufa Federal Research Center, Russian Academy of Sciences, Ufa. The detection limit for petrogenic oxides and elements was set as 0.01–0.2 wt. %, and for trace elements it was 5–10 ppm. The samples were ground entirely (ca. 150 g), and then 5 g portions were mixed with binder (poly-vinyl alcohol) and pressed at 25–27 g/cm² onto boric acid substrates. The loss on ignition (LOI) was determined gravimetrically. The standards for ultramafic rocks included SDU-1 (dunite, IGC SB RAS, Irkutsk, Russia), MU-2, MU-3, and MU-4 (serpentinized dunite, hornblendite, and kimberlite, respectively; IGEM RAS, Moscow, Russia).

As ophiolitic ultramafic rocks are prone to regressive serpentinization, chemical analyses were recalculated for anhydrous composition in order to restore their primary nature, and on this basis the normative mineral composition, including three main rock-forming minerals—olivine, enstatite and diopside—was calculated. The calculation method used in the work was developed by N.D. Sobolev (1952) [44] and automated with the use of Excel tools. To determine the amounts of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) in the ultramafic rocks using this method, three equations are solved: Di + En + Ol = MgO + 2Fe₂O₃ + FeO + CaO + MnO; Di + En + 1/2 Ol = SiO₂; and Di = 2CaO.

The water samples were taken in 2024 from the water points of both the territory of the Kraka massifs and their host sedimentary rocks. The work also uses water analyses from the stock report on reserves for 1962–1969, prepared by the Kagarmanov Party under supervision by A.V. Klochikhin (Tables S5–S7). A total of 89 samples were analyzed. The sampling was carried out both from springs and mine workings (holes, pits, and wells), as well as from streams fed by springs emerging from fracture magmatic and sedimentary rocks in the territory. The analysis of water with regard to anions and cations, hardness, mineralization, and pH was conducted using a laboratory ionomer I–160MI (Moscow, Russia) and a capillary electrophorese Kapel–105 M and Kapel–205 (Saint-Petersburg, Russia) according to regulatory documents (state standards (GOST 31867–2012 and GOST 31869–2012); methodology (M 01–58–2018); and environmental regulatory documents of the Russian Federation (ERD F 14.1:2:3:4.121–97, ERD F 14.1:2:3:98–97, ERD F 14.1:2:4.167–2000, and ERD F 14.1:2:4.261–2010)) [45,46,47,48,49,50,51] in the laboratory center of the Hygiene and Epidemiology Center in the Republic of Bashkortostan, Ufa. The obtained data on the chemical composition of water from the weight form (mg/L) were converted into molar (mmol/L) and percentage molar (mol.%). The analyzed and recalculated data on the anion and cation composition of water were shown on the diagrams by A.M. Piper (1944) [52] and G.A. Stiff (1951) [53]. Statistical data processing was carried out using Excel tools.

5. Results

5.1. Analysis of the Chemical Composition of Rocks from the Kraka Massifs

The chemical composition of ultramafic rocks from the Kraka massifs is given in

Table 1. Representative analyses of ultramafic rocks from the Kraka massifs (wt.%).

wt.%	NK-78	NK-79	NK-84	CK-64	CK-512	CK-45	UK-1803	SK-2331	SK-2342-2
SiO2	39.60	42.00	39.00	38.90	40.00	36.00	39.58	44.00	34.87
TiO2	0.07	0.07	0.06	0.08	0.30	0.05	0.08	0.15	0.04
Al2O3	0.50	3.60	0.20	0.20	2.40	2.80	2.46	1.89	0.21
Fe2O3	2.40	2.40	8.00	5.80	4.80	5.70	7.80	8.21	8.09
FeO	5.70	5.70	4.30	0.70	3.10	2.80	bdl	3.59	bdl
MnO	0.12	0.14	0.11	0.13	0.13	0.11	0.17	bdl	0.14
CaO	0.20	1.90	0.50	1.50	1.40	1.10	1.83	0.85	0.01
MgO	47.00	42.00	40.00	38.60	40.00	40.50	39.37	40.00	40.55
Na2O	1.30	0.20	0.13	0.07	0.40	0.07	0.20	0.12	0.10
K2O	0.10	0.06	0.06	0.03	0.02	0.06	0.02	bdl	0.01
LOI	2.80	2.00	7.80	13.30	8.00	11.00	8.31	1.20	16.29
Total	99.79	100.07	100.16	99.31	100.55	100.19	99.81	100.01	100.30
S	−15.69	−8.31	−13.47	−6.41	−8.33	−13.16	−7.83	−7.95	−13.94
A	2.1	5.76	0.89	1.8	4.22	4.03	4.51	2.86	0.33

Note: NK—Northern Kraka; CK—Central Kraka; UK—Uzyansky Kraka; and SK—Southern Kraka. The analyses of CK and UK are taken from [32,43]. LOI—Loss of ignition; bdl—below detection limit.

.

The recalculation of chemical analyses of rocks for the normative mineral composition shows that ultramafic massifs are mainly composed of dunites and harzburgites (Uzyansky and Southern Kraka) and to a lesser extent by lherzolites (Northern Kraka) (Figure 3a,b).

The Middle Kraka has a richer petrographic composition: in addition to the mantle section (peridotites and dunites), there are rocks of the transition zone and crustal section (clinopyroxene-bearing dunites, werlites, olivine websterites, and clinopyroxenites) (Figure 3a,b; Table 1 and Tables S1–S4). All studied ultramafic rock samples, to a greater or lesser degree, undergo serpentinization. The degree of serpentinization varies from 15 vol.% for plagioclase-bearing lherzolites up to 80 vol.% for dunites and harzburgites [42]. In all studied thin sections, serpentine is not accompanied by very fine-grained magnetite, which indicates that it belongs to early mesh-textured serpentine.

The content of SiO₂ in peridotites and dunites varies by 30.50–46.00 wt.%, as does TiO₂ by 0.01–0.4 wt.%, Al₂O₃ by 0.07–12.1 wt.%, Fe₂O₃ by 1.00–10.04 wt.%, FeO by 0.30–6.6 wt.%, MnO by 0.02–0.18 wt.%, CaO by 0.01–4.9 wt.%, MgO by 31.00–47.00 wt.%, Na₂O by 0.01–2.51 wt.%, and K₂O by 0.01–0.25 wt.%. The iron content (f) varies in the range of 5.28–15.63%. The increased values of low-melting components and iron content are typical of rocks from the transition zone and the crustal section of the Middle Kraka: SiO₂—34.85–52.00 wt.%; TiO₂—0.01–1.15 wt.%; Al₂O₃—1–16.6 wt.%; Fe₂O₃—1.00–14.81 wt.%; FeO—1.00–9.80 wt.%; MnO—0.07–0.23 wt.%; CaO—0.56–21.28 wt.%; MgO—11.60–42.00 wt.%; Na₂O—0.06–4.05 wt.%; and K₂O—0.01–0.75 wt.%; f = 7.47–37.66%.

The content of the three main rock-forming minerals varies significantly. Most of the studied samples contain diopside (Di) from 0.03(0.67)–1.53% in harzburgites to 10.00–16.97% in lherzolites. The most significant variations in these values are observed for the rocks of the Northern (0.55–16.97%) and Middle Kraka massifs (0.67–9.61%), which are more stable than the Southern (0.03–5.00%) and Uzyansky Kraka (0.84–10.00%). The concentration of normative enstatite (En) in ultramafic rocks ranges from 0.65 to 36.16% and increases from the dunites of the Uzyansky Kraka (0.65%) to the peridotites of the Southern Kraka (30.72–31.84%) and the harzburgites of the Middle Kraka (to 36.16%). Normative olivine (Ol) in all rocks in the massifs increases statistically regularly from lherzolites and harzburgites (62.82–88.89%) to dunites (90.41–99.02%). For clinopyroxene–containing dunites, werlites, and pyroxenites (olivine, clinopyroxenite, and websterite) in the Middle Kraka, the content of rock-forming minerals varies as follows: Di = 1.76–85.10%, En = 0.21–66.03%, and Ol = 7.87–97.92%. The analysis of variations in the CaO/Al₂O₃ ratio, as one of the empirical criteria for the residual origin of spinel peridotites [55], showed that consistently high values of this indicator are typical of peridotites from all four massifs. This indicates the widespread development of parageneses involving clinopyroxene (Figure 3c) and the low degree of depletion of rocks by low-melting components [32]. In the majority of analyses of ultramafic rocks, the CaO/Al₂O₃ ratio varies between 0.4 and 1.6 (Figure 3d). Increased values of this parameter (1.7–8.4 and higher) are typical of dunites and peridotites from the Northern and Middle Kraka, as well as of rocks from the transition zone and crustal section [32].

5.2. Analysis of the Chemical Composition of Groundwaters on the Territory

The groundwaters of the massifs and their host rocks have a diverse chemical composition. Because of the complex geological structure of the northern part of the Zilair synclinorium, composed of igneous and sedimentary rocks, and the observed patterns of changes in the anionic–cationic composition of groundwater, we decided to divide the analyses into three groups (

Table 2. Water samples from water points in the northern part of the Zilair synclinorium.

No Π/Π	Type of Water Point	Quantity of Samples
		Paleozoic Sediments of the Zilair Synclinorium (O2–3 nb–D3–C1zl)	Polymictic Melange (mpD3–C1k)	Ultramafic Rocks of the Kraka Massifs
1	Streams	11	11	31
2	Springs	11	6	16
3	Boreholes	1	0	0
4	Pits	0	1	0
5	Wells	1	0	0
Total quantity		24	18	47

,

Table 3. Representative chemical analyses of groundwater from water points in the northern part of the Zilair synclinorium.

Item No.	TDS g/L	pH	Type of Water Point	Components, mg/L, mmol/L, mol.%									Type of Water
				HCO3−	SO42−	Cl−	Ca2+	Mg2+	Na+	K+	NO3−	CO32−
1	0.2	–	Spring	136.95 2.24 94.4	8.5 0.09 3.7	1.54 0.04 1.8	36.7 0.92 69.4	5.5 0.23 17.1	3.79 0.16 12.5	0.51 0.01 1.0	- - 0.0	- - 0.0	Ca–HCO3
2	0.5	–	Stream	374.05 6.13 98.7	5.2 0.05 0.9	0.96 0.03 0.4	9.3 0.23 7.4	70.1 2.88 91.5	0.5 0.02 0.7	0.5 0.01 0.4	- - 0.0	- - 0.0	Mg–HCO3
3	0.27	8.05	Spring	- - 93.0	- - 4.0	- - 2.0	- - 70.0	- - 26.0	- - 4.0		- - 1.0	- - 0.0	Ca–Mg–HCO3
4	0.12	7.90	Spring	- - 72.0	- - 9.0	- - 16.0	- - 40.0	- - 32.0	- - 28.0		- - 3.0	- - 0.0	Ca–Mg–Na–HCO3
5	0.10	7.30	Spring	- - 40.0	- - 53.0	- - 7.0	- - 40.0	- - 13.0	- - 47.0		- - 0.0	- - 0.0	Na–Ca–SO4– HCO3
6	0.20	7.70	Stream	- - 75.0	- - 7.0	- - 2.0	- - 50.0	- - 32.0	- - 18.0		- - 0.0	- - 16.0	Ca–Mg–HCO3
7	0.4	8.25	Stream	293.23 4.81 93.6	11.9 0.13 2.4	7.28 0.21 4.0	28.9 0.72 26.4	44.0 1.81 66.2	3.32 0.14 5.3	2.28 0.06 2.1	- - 0.0	- - 0.0	Mg–Ca–HCO3
8	0.5	8.29	Stream	401.69 6.58 96.1	11.4 0.12 1.7	5.25 0.15 2.2	34.7 0.87 24.2	61.1 2.51 70.1	3.41 0.15 4.1	2.21 0.06 1.6	- - 0.0	- - 0.0	Mg–Ca–HCO3
9	0.5	8.25	Stream	376.43 6.17 94.9	13.8 0.14 2.2	6.66 0.19 2.9	35.0 0.87 25.6	57.3 2.36 69.1	3.14 0.14 4.0	1.70 0.04 1.3	- - 0.0	- - 0.0	Mg–Ca–HCO3
10	0.4	8.86	Spring	- - 90.0	- - 2.0	- - 1.0	- - 9.0	- - 88.0	- - 3.0		- - 0.0	- - 7.0	Mg–HCO3
11	0.4	8.76	Spring	- - 87.0	- - 2.0	- - 1.0	- - 8.0	- - 86.0	- - 6.0		- - 0.0	- - 10.0	Mg–HCO3
12	0.4	8.85	Spring	- - 89.0	- - 3.0	- - 1.0	- - 10.0	- - 85.0	- - 5.0		- - 0.0	- - 7.0	Mg–HCO3

Note: Analyses of the chemical composition of water samples taken from water points located in the following areas: 1–4—Paleozoic sediments of the Zilair synclinorium; 5–6—polymictic melange; and 7–12—ultramafic rocks of the Kraka massifs. (3–6,10–12) shows the analyses of the chemical composition of water from a stock report for 1962–1969, prepared by the Kagarmanov Party under supervision by A.V. Klochikhin.

and Tables S5–S7): waterpoints in the area of occurrence of the Paleozoic sediments (from Nabiullinskaya to Zilair formations O_2–3nb–D₃–C₁zl), sediments of polymictic melange (mpD₃–C₁k) of the Upper Devonian–Lower Carboniferous, and ultramafic rocks from the Kraka massifs.

In the diagram by A.M. Piper [52,56], the analyses of the territory of the host rocks are localized in the field of calcium/magnesium–bicarbonate-type waters (Figure 4a).

Calcium cations are predominant (Figure 4b), while magnesium cations are less common. Single analyses show a more diverse chemical composition, where sulfate, chloride anions, and sodium cations become more significant. The analyses of the water from the polymictic melange area indicate magnesium–bicarbonate and complex (calcium–sodium–magnesium–bicarbonate–sulfate, calcium–sodium–bicarbonate–sulfate, sodium–calcium–sulfate–bicarbonate) compositions (Figure 5a).

The calculated analyses show approximately equal amounts of calcium and magnesium, which does not imply the predominance of one cation in the chemical composition of water (Figure 5a,b). The water analyses of the Kraka massif demonstrate a more homogeneous chemical composition, with a predominance of bicarbonate anions and magnesium cations (Figure 6a,b).

The mineralization of groundwater on the territory of the host rocks is 0.04–0.50 g/L, the total hardness is 0.20–3.10 mmol/L, and the pH of the medium is 6.80–8.84 (Table S5). For streams and springs flowing through polymictic melange rocks, the water mineralization is 0.04–0.47 g/L, the total hardness is 0.45–1.25 mmol/L, and the pH of the medium is 6.80–8.76 (Table S6). In the analyses of water from water points on the territory of the Kraka massifs, the mineralization is 0.06–0.90 g/L, the total hardness is 1.05–3.39 mmol/L, and the pH of the medium is 7.0–8.88 (Table S7).

6. Discussion

6.1. Hydrogeochemical Zonation and Conditions of Formation of Magnesium–Bicarbonate Waters of the Kraka Massifs

A hydrogeochemical zonation is generally defined as a portion of a groundwater ba-sin (or aquifer) that is relatively homogeneous in water chemistry, within which the hydrogeochemical indicator (or sum of indicators) used to define the zones varies within relatively narrow, conventionally defined boundaries. It is through the identification of correlations that the hydrogeochemical zoning of specific territories can be performed.

The construction of the correlation diagrams of groundwaters from the northern part of the Zilair synclinorium, from the standpoint of the hydrolytic concept of formation of the chemical composition of groundwaters, showed dependencies between individual geochemical parameters. In spite of the virtually homogeneous magnesium–bicarbonate composition of groundwaters in the territory of the Kraka massifs, there are some correlations between ions (SO₄⁻, Cl⁻, Na ⁺ +K⁺, and Ca²⁺), the content of which increases from the Kraka massifs to the host rocks (Tables S5–S7).

A close positive relationship in the groundwater of the Zilair synclinorium is common in sulfate anions and sodium and potassium cations (Figure 7).

A proportional increase in the average ion content from the Kraka massifs (Na + K 6.7%, SO₄ 5.6%) takes place from polymictic melange (Na + K 14.6%, SO₄ 16.8%) to host rocks (Na + K 18.2%, SO₄ 15.8%). Moreover, increased values of sodium, potassium cations (up to 51.0–56.0%), and sulfate anions (up to 51.0–54.0%) are typical of the analyses of water, with mineralization of no more than 0.16–0.17 g/L (less often 0.32–0.4 g/L). Such close relationships between sodium, potassium, and sulfate anions in the analysis of water from the territory of host rocks and polymictic melange can be explained by their joint supply from atmospheric precipitations, which serve as a significant source of dissolved substances. The average annual mineralization of precipitations in the Southern Urals is 20 mg/L. The precipitations have a mixed composition, mainly comprising sulfate (50.0–59.7%), bicarbonate (23.5–30.6%), and chloride anions (up to 26.5%). Among the cations, calcium (37.7–49.9%), magnesium (30.8–39.1%), and sodium (8.1–22.0%) play a significant role. [27].

Medium-strength correlations are observed between magnesium cations and bicarbonate anions, and mineralization (Figure 8 and Figure 9).

An obvious inverse relationship exists between the contents of calcium and magnesium cations, with a gradual predominance of the latter in the host rocks (average values of Ca 41.1%, Mg 40.6%), followed by polymictic melange (Ca 36.2%, Mg 49.3%) and the Kraka massifs (Ca 16.7%, Mg 76.5%). Data on the chemical composition and correlations suggest that the formation of the anionic–cationic composition of the groundwater in the Kraka massifs is related to the removal of magnesium from rock-forming minerals (olivine and pyroxene), and that the supply of bicarbonate anions is associated with biochemical processes in the soil or atmospheric precipitation. The presence of magnesium cations in the analyses of water from the territory of host rocks and polymictic melange (Tables S5 and S6) is the result of its supply from ultramafic massifs.

Less close relationships are observed between calcium cations, and chloride and sulfate anions (Figure 10).

The same was observed in the case of sulfate anions and sodium and potassium cations; they were supplied by atmospheric precipitations to the groundwaters of the Kraka and polymictic melange areas. Thus, the formation of the chemical composition of magnesium–bicarbonate groundwater in the Kraka massifs occurred due to the release of magnesium cations from the rock-forming minerals of ultramafic rocks and the introduction of bicarbonate anions from the soil, or, like other ions, from atmospheric precipitation.

6.2. Comparison of the Magnesium–Bicarbonate Waters of the Kraka Massifs with Other Objects

Figure 11 and

Table 4. Comparison of the chemical compositions of groundwater in various ophiolite and intrusive massifs [13,14,15,17,18].

Object	TDS(g/L)	pH	Type of Water	Type of Rock
Kraka massifs	0.06–0.9	7.00–8.88	Mg–HCO3, Mg–Ca–HCO3, Ca–Mg–HCO3–SO4	Ultramafic rock
Ophiolite bodies of the Apennine belt	0.11–0.37	7.63–8.67	Mg–HCO3, Mixed types	Ultramafic rock
Outokumpu assemblage	0.02 9.2–9.5	7.50 9.40–10.20	Ca–Mg–HCO3 Na–Cl	Tremolite-chlorite-actinolite and ultramafic rocks.
Suhanko–Konttijarvi intrusion	0.18–0.20 0.41–0.51	7.10–7.41 9.65–9.81	Ca–Mg–HCO3–(SO4) Na–Ca–Cl	Gabbro, ultramafic rock, quartz diorite
Ylivieska intrusion	0.22–0.28	8.14–8.42 6.97–8.67	Mg–Na–Ca–HCO3 Na–Mg–Ca–Cl	Ultramafic rock gabbro
Samali ophiolite	0.11–1.29 0.46–5.54	7.40–9.60 7.4–11.5	Mg–HCO3 Na–Cl	Ultramafic and mafic rocks
Ronda massif	0.36–0.58 0.16–2.9	8.50–8.90 9.60–12.0	Mg–HCO3, Na-Cl	Ultramafic rock
Mingora-Shangla mélange zone	0.04–0.81	6.95–7.20	Mg–HCO3, Mixed types	Ultramafic rock, green schist, talc-carbonate schist and metabasalts

demonstrate comparison of magnesium–bicarbonate waters of the Kraka massifs, with the analyses of water collected by researchers [13,14,15,17,18] from the ultramafic rocks of ophiolite and intrusive massifs.

As compared to other objects, the Kraka massifs have a more homogeneous water composition, with a predominance of magnesium cations and bicarbonate anions. The analyses of the water points from the Semail ophiolite complex (Oman), the Apennine (Italy), and the Cuban (Cuba) ophiolite belts showed that they are similar in chemical composition, but vary significantly in terms of anion–cation composition (Figure 11).

The ultramafic–mafic intrusive massifs (Suhanko–Konttijarvi, Ylivieska) and the Outokumpu association, which includes serpentinites, tremolite–chlorite–albite rocks, and black shales located in Finland, have calcium-bicarbonate, sodium–chloride, and mixed (two- and three-component cation) compositions (Figure 11 and Table 4).

The Ronda massif (Spain) is characterized by a drastically different highly alkaline water composition compared to the majority of the other studied ophiolite massifs (Figure 11). The composition is determined by low-temperature serpentinization during the infiltration of meteoric magnesium–bicarbonate water into serpentinite aquifers below the aeration zone [57,58,59,60,61,62,63]. An alternative model of formation is the hydration of olivine and pyroxene by fresh water in a closed system with atmospheric CO₂ [61]. Similar waters exist on the territory of the Semail ophiolite complex [15,63]. In terms of the chemical composition, these are sodium chloride waters. The mineralization of these waters reaches 2.9–5.54 g/L and the pH of the medium is 11.5–12.0 (Table 4). This type of highly alkaline water has not been detected within the Kraka massif. Groundwater in the ultramafic zone occurs at depths of less than 10–15 m, making the formation of highly alkaline waters in the aeration zone impossible. Highly mineralized sodium–chloridewaters are generally not common within the Zilair fissure basin (Figure 2).

6.3. Assessment of the Quality of Groundwaters on the Territory of the Kraka Massifs and the Host Rocks for Drinking Purposes

The data on the chemical composition obtained from the analyses of water in the Kraka massifs and the Paleozoic sediments containing them were compared with the standards of the World Health Organization [64] (

Table 5. Assessment of the geochemical parameters of groundwater in the northern part of the Zilair synclinorium with standards established by the World Health Organization [64,65].

Parameters	Kraka Massifs (n = 47)	Paleozoic Sediments of the Zilair Synclinorium (n = 24)	WHO Standards
pH	7.00–8.88	6.80–8.84	6.50–8.50
TDS, g/L	0.06–0.90	0.04–0.50	1.000
Total hardness of water, mmol/L	1.05–3.39	0.20–3.10	<0.40 (soft) 0.40–1.30 (slightly hard) 1.30–2.60 (moderately hard) 2.60–3.90 (hard) >3.90 (very hard)
Ca mg/L	28.90–35.00	9.30–36.70	100.00
Mg mg/L	44.00–57.30	5.50–70.10	50.00
Na mg/L	3.14–3.41	0.50–3.79	200.00
K mg/L	1.70–2.28	0.50–0.51	12.00
HCO3 mg/L	293.00–376.00	136.95–374.05	-
SO4 mg/L	11.40–13.80	0.96–1.54	250.00
Cl mg/L	5.25–7.28	5.20–8.50	250.00

).

In terms of mineralization and the content of basic ions, the groundwater in the northern part of the Zilair synclinorium (both the territories of the Kraka massifs and their host rocks) does not exceed the maximum permissible values [64]. At the same time, the pH value of the medium and the Mg content for both groups of analyses exceed the established upper limit.

According to the regulatory document SanPiN (sanitary rules and regulations) 1.2.3685–21 [66] approved in the Russian Federation, the values of mineralization, the pH of the medium, and the total hardness do not exceed the maximum permissible concentrations (

Table 6. Comparison of geochemical parameters of groundwater analyses in the northern part of the Zilair synclinorium with standards given in SanPiN 1.2.3685–21 [66].

Parameters	Sanitary Rules and Regulations 1.2.3685–21	Kraka Massifs (n = 47)	Polymictic Melange (mpD3–C1k) (n = 28)	Paleozoic Sediments Zilair Synclinorium(O2-3nb–D3-C1zl) (n = 24)
pH	6.00–9.00	7.00–8.88	6.80–8.84	6.80–8.76
TDS, g/L	1.00 (1.50)	0.06–0.90	0.04–0.50	0.04–0.50
Total hardness of water, mEq/L, mmol/L	7.00 (10.00) 3.50 (5.00)	2.10–6.80 1.05–3.39	0.90–2.50 0.45–1.25	0.40–6.20 0.20–3.10

).

Magnesium–bicarbonate waters in the territory of the Kraka massifs and their host rocks, in terms of the Mg content and mineralization, with respect to GOST R 54316–2020 [67] standards (

Table 7. Representatives of groups of hydrochemical types of table mineral waters according to GOST R 54316-2020 [67].

Type of Water	Name of the Mineral Water Type and Its Location	Dominant Ions, %-Eq/L	TDS, g/L	mg/L
				HCO3−	SO42−	Cl−	Ca2+	Mg2+	(Na + + K+)
Ca–Mg–HCO3, Mg–Ca–HCO3	Senezh (boreholes 1/GVK-46240620, ZGVK-46247919). Senezh field, Moscow region	HCO3 > 70 Ca 40–75, Mg 20–55	0.3–0.7	250–450	<15	<10	40–90	10–50	10–40
Ca–Mg–HCO3, Mg–Ca–HCO3	Sineborie (boreholes 55-T). Vladimir Region	HCO3 > 80, Ca 50–80, Mg 30–50	0.2–0.5	100–400	<25	<15	20–80	10–40	<15
Ca–Mg–HCO3, Ca–Na–Mg–HCO3, Ca–Mg–Na–HCO3	The Legend of the Arkhyz Mountains (boreholes No. 3). Nizhne-Arkhyzskoye field, Karachay-Cherkess Republic	HCO3 > 80, Ca 45–70, Mg 20–40, (Na + K) 15–30	0.1–0.25	50–200	5–15	5–15	5–50	2–20	5–50

), can be used as table mineral waters.

Magnesium–bicarbonate waters found in the territory of the Russian Federation with a mineralization of up to 1 g/L are actively used as table minerals to strengthen bone tissue, teeth, normalize heart function, restore water balance, improve the digestive tract, and strengthen the nervous system. The deficiency of magnesium in the body leads to heart failure, the slowing of the conduction of nerve tissues, and the development of hypomagnesemia.

7. Conclusions

The comparative study of the compositional features of ultramafic rock samples from the Kraka massifs showed that the rocks are characterized by a varying degree of depletion, increasing from thespinel–plagioclase peridotites of the Northern Kraka to the spinel peridotites of the Southern Kraka.

The observed correlation between the chemical compositions of water from the water points and underlying ultramafic rocks indicates the interaction of the components of the groundwater–rock system during the formation of the anionic–cationic composition. This is confirmed in the correlation diagrams. There is also a distinct regularity in the change in the groundwater composition from the territory of Paleozoic sediments to the Kraka massifs. An inverse correlation is observed between the contents of calcium and magnesium cations, with a gradual increase in the latter closer to ultramafic rocks.

The comparison of the groundwater from the Kraka massifs with that from other ophiolitic and intrusive massifs showed that the magnesium–bicarbonate waters of these objects are low-mineralized, rarely exceeding 1 g/L, with the pH of the medium being lower than nine. The exception is the highly alkaline groundwater found by the researchers in Spain (Ronda massif) and Oman (Samali ophiolite), with a mineralization of 5.54 g/L and a pH to 11–12.

The comparison of the obtained values of mineralization, the pH of the medium, the total hardness and the anionic–cationic composition of groundwater in the territory of the Zilair synclinorium with the standards approved by the World Health Organization (WHO) and the regulatory documents SanPiN 1.2.3685–21 and GOST R 54316–2020 showed that these waters can be used as table mineral waters.