Thermal waters from the Sudetes Mountains (SM) are characterized by lower ionic strength and higher ORP value than most of the waters from the Carpathian Foredeep (CF) Basin (Table 1, Fig. 1). In the analyzed group of mineral waters from the CF, greater ORP dispersion is observed with higher values of ionic strength. The variation of ORP values in CF waters affects the concentration of S(II) and S(VI) compounds (Fig. 2a). With the increase of ORP, the total amount of sulphates and S(II) compounds decreases. Similar dependence can be seen in the case of total iron concentration (Fig. 2b). The absolute difference between concentrations of S(VI) and S(II) decreases with the pH lowering (Fig. 2c). In mineral waters from the CF Basin, the growth of S(II) compounds is also observed while higher concentrations of sulphates occur (Fig. 2d). There is no clear dependency between iron content and sulphur concentrations. The oxidation process is based on increasing concentrations of S(VI) and decreasing the values of Fe(II). However, the research based on the comparison of the EH values measured in the field and estimated in PHREEQC programme with the use of concentration Fe2+/Fe3+ redox couple shows a more reduced state for calculated EH (Hokari et al. 2014) and might suggest lack of equilibrium between Fe2+/Fe3+ and SO42−/HS− redox pairs.
The total effects of chemical interactions in the groundwater-aquifer rock systems can be quantified by saturation index (SI) which shows the relation between real saturation state with respect to the solid and hypothetical equilibrium state:
$$ \mathrm{SI}=\log \frac{\mathrm{IAP}}{K_{\mathrm{T}}} $$
(2)
where IAP is the ion activity products (the product of the activities of forms involved in the reaction) and KT is the equilibrium constant of reaction at the given temperature.
The most common and/or reactive minerals forming studied aquifer rocks were selected for this assessment. The studied groundwater represents various geological settings and aquifer rocks. The common feature of studied waters is chemical equilibrium in relation to calcite (Fig. 3) despite significantly various pH and aqueous chemistries. In sedimentary aquifer rocks of the CF Basin, calcite is a common rock-forming mineral. In crystalline aquifer rocks of thermal waters in the SM, calcite is a very rare secondary mineral. Howbeit, thermal fresh waters are also equilibrated with calcite because of alkaline pH (8.0–9.2). Richer in magnesium, mineral waters of CF (53–1113 mg L−1) are oversaturated with respect to dolomite, whereas SM waters are very poor in this element (from below 0.05 to 0.24 mg L−1 of Mg) and remain undersaturated (Fig. 3).
All analyzed waters are supersaturated with respect to pyrite, whereas CF waters show significantly higher oversaturation due to much higher S(II) concentrations than in SM thermal waters. Mineral waters from the CF are only slightly undersaturated with gypsum, which is a common mineral in these sedimentary aquifers. Gypsum is a very uncommon mineral in SM crystalline aquifer rocks what is likely the main reason for groundwater undersaturation.
Significant differences of pH and S(II) concentrations between both water groups respond for extremely different saturation state with respect to common iron-bearing solids (goethite, haematite) (Fig. 3), despite that, all studied waters are poor in iron. The alkaline and S(II)-poor SM thermal waters are oversaturated with goethite and haematite, whereas near-neutral and S(II)-very rich CF mineral waters are highly undersaturated with both minerals.
Stability field diagrams indicate the predominant (most abundant) aqueous species of chemical element and the solid phases (solid species) with respect to the waters are most saturated (i.e. most stable solids). The EH-pH diagrams have conditionings and limitations. Diagrams are drawn to show equilibrium relations (conditions), but equilibrium is often not attained in aquifer systems, especially of the short turn-over time zone. In studied groundwater, chemical equilibria could be expected because of water old ages, the deepwater circulation systems and/or the presence of reactive minerals in the aquifer rocks. Field boundaries in diagrams are calculated for equilibrium conditions with the assumption that reacting species occur at equal concentrations which is usually not met in natural systems. Diagrams do not take account of slow reactions or metastable forms of chemical elements. Next, uncertainty of thermodynamic data and range and incompatibility of databases also pose restraints of diagrams. However, apprehending conditionings, such diagrams are useful when the occurrence of different forms of selected elements in water analyzed is considered (e.g. Gómez et al. 2006; Hokari et al. 2014; Ioka et al. 2017). Changes of water quality in terms of EH-pH relations can be easily recognized by using such diagrams in monitoring adverse effects, e.g. in therapeutic waters affected by chlorination (Okouchi et al. 2005).
Predominant sulphur aqueous species in the SM waters is SO42− (Fig. 4a, b). Both diagrams were constructed for the temperature of 50 °C (average for most of the SM waters). Water from C-1 well (Cieplice) which has a much higher temperature (of around 78–79 °C at the outflow) is not shown in Fig. 4a and b. Nonetheless, the water of C-1 well presents the same feature as other SM thermal waters. CF mineral waters generally present conditions of equilibrium between H2S(aq), HS− and SO42− species. Waters of Kraków region (occurring in Miocene aquifer rocks) which have lower S(VI) concentrations indicate SO42− as the predominant species (Fig. 4c). Chemistry of most of the CF waters shows pyrite as the most stable sulphur-solid (Fig. 4d). The occurrence of H2S plays an essential role in the beneficial effects of sulphurous mineral waters. This form is more available in the environment with a low pH and low oxygen concentrations (Carbajo and Maraver 2017). Even that the impact of pH on the S(II) species is usually discussed in literature due to their behaviour in different parts of the human body (op. cit.), the role of ORP should also be considered when such therapeutic waters are used.
ORP informs about physicochemical properties of water used for treatments and influences the aqueous speciation of chemical elements, what consequently might also affect therapeutic effectiveness. For instance, Takahashi et al. (2007) documented the influence of bathing in waters of different ORP and ionic compositions on the renal system, uric acid excretion and urine pH, and also on the electrical potential of the epidermal cell membranes. It is important to remember that the ORP of water changes during water handling and ageing. Fresh water samples of thermal and therapeutic water have usually low ORP value and its growth during water storage. This results in changes of S(II) and Fe species distribution and, as a consequence, in curative properties of water (Okouchi et al. 2010). That is why the ORP measurements should be performed as well on water wellheads as in the places where treatments are carried out (Ohnami et al. 2008b). ORP research at treatment sites confirmed also usability in the case of assessing the scale of adverse physicochemical changes in thermal water chemically modified by the addition of an artificial additive (Ohnami et al. 2008a).
Sulphur compounds (especially hydrogen sulphide—H2S) in therapeutic/thermal waters show a very well-documented beneficial therapeutic effect (e.g. Nasermoaddeli and Kagamimori 2005; Legwant et al. 2013; Vela-Anero et al. 2017; Gálvez et al. 2018). However, hydrogen sulphide might also create a potential risk for water users. Sporadic tragic incidents (Bassindale and Hosking 2011; Stanhope et al. 2017) show that it is necessary to monitor the hydrogen sulphide in waters and indoor atmosphere in places where H2S-rich waters are used for therapeutic or recreational purposes. One allergic reaction to sulphur was observed during studies conducted by Bender et al. (2014). Varga (2012) proposed a complex strategy for balneoprevention including calculation of toxicological risk of balneological treatment based on a complete chemical analysis (including organics) and specific toxicity test to avoid the negative consequences of the use of therapeutic waters.
In terms of iron species, all SM thermal waters (including C-1 well water in Cieplice) demonstrate Fe(SO4)22− as the predominant Fe aqueous species (Fig. 5a, b), whereas the chemistry of the CF mineral waters reveals Fe2+ and FeSO40 as the predominant ones (Fig. 5c), wherein the latter form mainly in waters of the Kraków region (which have relatively higher ORP and lower S(II) concentrations than waters of the Busko region). All CF waters clearly express the stability of pyrite in the aquifer systems (Fig. 5d). The knowledge about the concentration of selected species of specific components is important because adequate speciation can affect the action of the therapeutic water. Researches indicate that the most effective in some treatments are mineral sulphide waters with the sulphur occurred in the form of hydrogen sulphide. This form presents a good pharmacological effect under low pH and low oxygen concentration conditions (Carbajo and Maraver 2017). Ferrous waters are more bioavailable when iron exists in a form of Fe2+ ions (Li et al. 2009; Ems and Huecker 2019). Iron-rich therapeutic waters (from Levico and Vetriolo spas, Trento, Italy) showed effectiveness and durability of therapy effects in the treatment of osteoarthritis and fibromyalgia (Cantarini et al. 2007; Fioravanti et al. 2013, 2018). Waters with high sulphates content have a good physiological action when SO42− is above 1200 mg L−1 (Gutenbrunner and Hildebrandt 1994; pp. 28, 177).
Various thermal waters of unchanged chemical composition and originally low ORP, containing increased sulphur or iron solutes, showed beneficial therapeutic effects, in contrast to the same waters affected by ageing (e.g. in open-air conditions) during water transfer and handling (Okouchi et al. 2010).
Groundwater differ each other in terms of many features of the occurrence environment, like depth of their presence, degree of isolation from the influence of surface conditions (i.e. recharging by precipitation water related to contemporary hydrological cycle, inflow of pollution), water flow velocity, time and rate of water turn-over in an aquifer system. The volume of water resources available for exploitation, water quality and vulnerability to possible contamination are closely associated with the abovementioned features. Part of therapeutic water resources might be regarded as a renewable one. This applies to groundwater in the active turn-over zone, i.e. in aquifers which are directly or indirectly recharged by water related to the modern hydrological cycle. Therapeutic waters, which occur in retarded water turn-over or stagnant water zones, are regarded as non-renewable. Nonetheless, one should be emphasized strongly that both types of therapeutic waters have a limited resource understood as the available flow rate (debit) or available total volume of water, respectively. This means the necessity to monitor their quantity and quality for proper managing and protecting the therapeutic water resources.
The exploitation of therapeutic waters always violates the natural physical and chemical status quo of the system and can lead to fluctuations and/or deterioration of water quantity and quality. Rationalizing the efforts and the costs of therapeutic water monitoring, the most informative, indicative and cheap-in-study parameters, like ORP, should be implemented and routinely tested. The ORP characterizes the resultant effects of all redox reactions, i.e. reactions in which electrons are transferred between aqueous (and solid) species, and it is one of few indicative parameters which quantitatively inform about the physicochemical status of the solution.
Chemistry of groundwater of the active turn-over zone commonly does not reach the full chemical equilibrium with aquifer rocks. Our results show that most of the studied waters reached equilibrium with aquifer rocks, which mainly results from their occurrence in a retarded water turn-over or stagnant water zones. Such kinds of waters present chemical status far from conditions in near-surface or surface environments. During groundwater exploitation (e.g. by pumping) and its further handling on the surface, inevitable changes in water quality might be expected, which reflect in ORP value.
Because the ORP results from and affects aqueous species of electro-active chemical elements, it helps understand the behaviour of the beneficial, balneologically appreciated components, like sulphur and iron solutes, in therapeutic waters. For the reasons summarized above, it should be recommended to monitor ORP in therapeutic water both at intake site and at treatment places, as it is for example required by Begriffsbestimmungen (2016).
Understanding oxidation-reduction conditions can be useful in various ways. For instance, a serious weakness of balneochemical research is too little attention paid to the presence and role of organic compounds in therapeutic waters (Varga 2011; Szabó and Varga 2019). In this field, ORP can provide important data about distribution and behaviour of organic species (e.g. Fekete et al. 2012). In the context of ORP studies, another example of an interesting proposition is assessing a reductive ability of therapeutic thermal waters by the volumetric method with povidone-iodine (Okouchi et al. 2011). This method provides complementary information, regarding the “classical” ORP measurement, and allows for estimating the chemical oxygen demand and the vitamin C equivalence with respect to the reductive ability of therapeutic waters.