The Importance of Salty Groundwater in the Supply of Thalassotherapy: The Case of Portugal

Abstract

Thalassotherapy is a balneotherapy activity with a wellness and therapeutic character, using salty water captured at sea. That activity is very similar to thermalism, which uses natural mineral water (NMW) captured in aquifer systems. In Portugal, thermalism is a well-established activity, and for a medical spa to be in operation, there is a legal requirement to have two specific professionals: a medical doctor—Clinical Director—and a hydrogeologist—Technical Director (TD). The exploitation of the NMW is the responsibility of the TD, a professional with know-how in the field of hydrogeology. Thus, this chapter presents an introduction with some fundamental concepts about thermalism and thalassotherapy, generic aspects of those activities in Portugal, and their objectives. Methodological elements are presented, followed by the main results and interpretations, with the physic-chemical characteristics of Portuguese NMWs; of very salty special groundwaters, not licensed as NMW; and sea and ocean waters. Finally, the main conclusions and several considerations are presented, in the sense that the practice of exploiting salty groundwater, in the proximity of the sea, will be a new field for the activity of hydrogeological professionals, provided that they are classified as NMW, to use in the thermalism activity.

1. Introduction

Portugal, a country in the SW of Europe (Figure 1), has a great tradition in thermalism, which has distant origins in time and predates the Portuguese nation, with many places using special groundwater, namely when they were naturally hot, since the time of the occupation of this territory by the Roman people, about 2000 years ago.

Figure 1.

(a) Geographical setting of Portugal (from ref. [1]); and (b) distinction of three regions under study in the organization of the AMN (from ref. [2]).

Thermalism, in Portugal, corresponds to the use of natural mineral water (NMW) with balneotherapy techniques and/or treatments and other specialized treatments, such as those used in the respiratory tract, among others, for therapeutic and wellness purposes. The official definition of thermalism, published in the medical spa law [3], is as follows: “Thermalism”, corresponds to the use of NMW and other complementary means for prevention, therapy, rehabilitation, or wellness. A notion of the reality of the Portuguese thermalism can be obtained by consulting the Internet site of the Portuguese Medical Spa Association [4]. It should be noted that there are about 50 medical spas in operation in a normal year.

The NMW corresponds to groundwater, which, in order to have that designation, has to be classified by Portuguese law (Decree Law 86/1990 [5] and 54/2015 [6]), which guide the main procedures and studies to be developed, namely of the hydrogeological nature and of the quality of the groundwater under study, for 12 consecutive months, with physical–chemical and microbiological analyses of water samples collected at the head of the abstraction. Examples of studies in this sense can be seen in several published works [7, 8, 9, 10]. NMW has characteristics that distinguish it from other groundwaters, such as the stability levels of their physical–chemical parameters [11]. This specific type of resource does not have recommended or admissible limits for the vast majority of physic-chemical parameters; only for some constituents of NMW (mostly trace chemical elements) concentration limits are established, in accordance with Directive 2003/40/EC, of 16 May [12].

Thalassotherapy in Portugal, generically, corresponds to the use of seawater, in hydrotherapy practices and the like, in a similar way to what is classic in thermalism. The term “Thalassotherapy” comes from the Greek [13]: “thálassa- sea and therapeia—treatment,” being a neologism proposed by La Bonnardiére in 1867 to designate the therapeutic use of seawater, preferably in the form of baths and also taking into account that the treatments are normally carried out close to the sea so as to have a simultaneous action of the maritime climate.

In Portugal, unfortunately, there is no specific legislation in the field of thalassotherapy [14, 15]. Despite this situation, Portugal has some cases of excellence, with extraction of salty water from the sea; the existence of autonomous spas, that is, not included in hotels, deserves mention: (i) the Thalasso Costa da Caparica [16], with group hydrotherapy, massages, physiotherapy, esthetic services, and providing of medical consultations for advice and (ii) Barra Talasso Nazaré [17], with a dynamic swimming pool incorporating several techniques, such as hydromassage, several types of showers, walking circuit, individual hydromassage-application cabins, Vichy shower, Scottish shower, and others. Other spas included in hotels or hotel resorts, almost exclusively for leisure and wellness purposes, also exist in Portugal, such as in the Vilalara Thalasso Resort [18] and the Grande Real Hotel & Spa [19].

Some detailed elements on thalassotherapy in Portugal can be observed in several works [14, 15, 20, 21], where it is evidenced that in thalassotherapy, besides seawater being the basis of such activity, other elements related to the sea can be important in the global activity such as mud, seaweed, sand, and local climatic conditions.

The basic difference between a thalassotherapy center and a medical spa in thermalism is in the base fluid; thalassotherapy uses natural salt water collected from the sea, while a medical spa uses NMW, collected from an aquifer system of a geological formation.

In Portugal, thermalism is a well-established activity. For a medical spa to be operational, there is a legal requirement to have two specific professionals: a doctor—Clinical Director (CD)—and a hydrogeologist—Technical Director (TD). The exploration of NMW is the responsibility of the TD, a professional with know-how in the field of hydrogeology, who makes the NMW available from the aquifer system, at the head of the abstraction, to be used in the medical spa. Thalassotherapy, currently using water captured from the sea, not having the same rules as NMW in the exploration, is also not obliged to comply with the stability in quality that is required for NMW. In Portugal, thalassotherapy is not currently integrated in the thermalism sector, the latter being supervised by the Directorate-General for Energy and Geology (DGEG).

Still, within the scope of this chapter, it is important to emphasize that in these new and current times, in certain regions of the world, such as Portugal, with the current trends in climate change, NMWs tend to decrease their natural availability and, consequently, their production. If the classic hydrological cycle does not occur, with the current trends of lower precipitation and higher evaporation, this will put the sustainable exploitation of aquifer systems at risk, necessarily leading to reductions in exploitation flows and causing potential changes in the quality of the resource. So, as some of the aquifer systems that produce the NMWs are clearly in danger, new alternatives must be sought, and the use of salty groundwater, captured from free (unconfined) aquifer systems, close to the sea, will not pose a problem at least in terms of decreased flows over time, if they are exploited by hydrogeologists who comply with the good rules of the area, namely, being totally rigorous in not causing the advancement of the salt wedge intrusion into the interior of the continent.

Thus, in Portugal, there is a lot of knowledge and experience about thermalism. On the other hand, it is a European country, with an extensive Atlantic coastline, 832 km in continental Portugal, in addition to the 960 km of the archipelagos of Madeira and the Azores [22]. There are therefore conditions, especially in areas where sedimentary geological units occur on the coastline, which facilitate the penetration of the salt wedge to the interior. Based on hydrogeological studies, it will not be difficult to implement abstractions that allow to obtain a resource, natural salt water, with excellent stability in its chemical composition. Therefore, the authors’ understanding is that there are conditions for, in many situations, underground abstraction of saltwater, often in free aquifers (unconfined) or other systems, for extracting water from the earthy massif, coming indirectly from the sea, but with the potential, in certain places, to maintain stability, because the passage of water through the aquifer system ends up purifying certain impurities, and thus, there is potential for physicochemical stability, as happens in natural mineral waters.

Those notions of the previous paragraph were the basis of academic work, essentially around architecture [22], designing a special thalassotherapy spa, for the Cabo Espichel, in Portugal, with this place presenting excellent characteristic potentiators of such a situation. Some elements of that project were presented in several papers [23, 24], presenting in Figure 2 the scheme of principle for the abstraction of salt water, idealized near the sea.

Figure 2.

Sketch of the abstraction system (in profile) of salty groundwater, as a circular and annular well, proposed for Lagosteiros Beach, to supply the thalassotherapy spa proposed for Cabo Espichel (from ref. [24]).

The sketch of the abstraction in Figure 2 is the result of an initial mechanical excavation with the aid of a boom-slew excavator, in order to carry out an excavation in the natural massif, which constitutes the unconfined aquifer. Three tubes are installed vertically, the two outer tubes made of rings of perforated concrete and the interior of stainless steel, grooved in its lower part, to allow the entry of salty water. Between those tubes, there is selected granular material, the outermost with coarse sand and the interior with coarse gravel, so that the underground flow evolves horizontally so as to gradually increase its velocity, until it enters the pumping chamber, inside the stainless-steel tube. In this way, the underground flow regime tends to remain laminar, enhancing not only the proper functioning of the entire system over time but also the quality of the water to be captured, as the filtering system works in a more effective, in case there are fine particles suspended in the water and even pathogenic bacteria. In the abstraction area, in order to minimize the potential for fluids to flow downwards vertically (rainwater and others), in the area most sensitive to contamination, an impermeable geomaterial is installed horizontally, which may be of the sandwich type, consisting of three levels, “geotextile/geomembrane/geotextile,” not only with the mission of fulfilling the water-tightness but also to serve as a “separator,” so that there is no mixing of materials from top to bottom. The strengthening of the water-tightness is provided by the 30 cm layer of compactonite (bentonite in granules). Finally, the whole set is headed with another geomaterial similar to the one already mentioned, in particular to serve as a separator for the local natural materials, so that the abstraction site has no visual impact. The adduction system to the bathhouse, also for reasons of not having a visual impact, must follow in a buried conduit. Regarding the environmental impact of this work, it exists only in the construction phase.

Therefore, the main objective of this chapter is to show that in Portugal, there is already a vast knowledge about thermalism; to present in particular the various types of special groundwater (NMW) that support this activity; to highlight the groundwater that has similarities with seawater; to show in the same language (using graphics and in particular Piper and Stiff diagrams) the main types of seawater (based on elements from the literature); and, finally, to make several considerations in the sense that in the future, thalassotherapy may be supplied by salty groundwater, captured in the proximity of the sea, so that this activity will have the potential for greater stability in its quality, potentially integrating itself in the NMW.

2. Methodological aspects

The present work is organized in two phases:

1st phase—establishment of an inventory of special groundwater occurrences in Portugal and of some ocean and sea waters, with physical–chemical results available in the literature. This phase is limited to the bibliographical research of published works with results of physical–chemical analyses of Portuguese NMW, as well as of other special groundwaters, some of which, in the past, were already classified as NMW but that in the meantime, over time, for various reasons, bureaucratically lost the title but still have emergences (springs) of quality, generically associated to old bathhouses, some in very advanced ruin; there are also some occurrences (springs) that have never been classified but have the potential to be classified. In some of these cases, these waters served therapeutic actions by popular movements but were not integrated within the official rules of thermalism. This phase also included research on sea and ocean waters from various regions of the world, including the results of physical and chemical analyses of their waters. The analytical methods used to perform the physical-chemical analyses are often described in the researched works; however, it is mentioned that they generally follow the standardized techniques and procedures [25, 26].

2nd phase—treatment and interpretation of data from physical–chemical analyses of the studied waters, in order to highlight the chemical types of the occurrences and their classifications. The main classifications used were on the total mineralization—M_T (Table 1), the pH, and the main ionic species in Piper and Stiff diagrams, using adequate software [28].

Designation	Total Mineralization—MT (mg/L)
Hyposaline	< 200
Weakly mineralized	200–1000
Mesosaline	1000–2000
Hypersaline	> 2000

Table 1.

Classification of waters in relation to total mineralization [27].

Besides the classifications presented, there are others, namely those that have to do with some singularities that the groundwaters present, these being the ones that are generally known in the field of thermalism in Portugal. Thus, in that sense, in this chapter, we follow what a recent publication [21] presented, which is in line with Directive 80/778/EEC [25], considering that water acquires its own identity, even with small amounts of certain chemical elements, and therefore, the same author [21] understands that it may also apply to NMW, resulting in the following designations:

• sulfurous waters, when titratable Sulfur (H₂S, HS⁻, S²⁻) ≥ 1 mg/L;

• fluoridated waters—fluorine (F⁻) > 1.0 mg/L,

• ferruginous waters—bivalent iron (Fe²⁺⁾ > 1 mg/L

• silicate waters—colloidal free silica (SiO₂) > 10 mg/L,

• carbonaceous or acidulated (gas-carbonate) waters—free carbon dioxide (CO₂) > 250 mg/L (content at emergence).

The waters may also have other designations if any chemical element occurs singularly and exceptionally.

3. Results and interpretations

3.1 Physical: Chemical aspects of Portuguese NMW

Portugal is a small country but with many and a great variety of special groundwaters, situations that result mainly from the territory being associated with many varieties of geological units. The oldest inventory of the main occurrences of natural springs, which deserves reference, was made by Fonseca Enriques in 1726 [29]. Most of the places where the main medical spas are currently located in Portugal, with the practice of thermalism, were already mentioned in that inventory.

On the chemical composition of Portuguese groundwater, it is worth mentioning the works carried out by Almeida and Almeida, from 1966 to 1988 [30, 31, 32, 33], with the publication of several books for different regions, starting with the Algarve [30], in the south of Portugal; then the region of Trás-Os-Montes e Alto Douro [31]; the region of Beira Alta [32]; and, finally, the region of Minho [33], in the north. Those works present the results of physical–chemical analyses of the NMW that served the medical spas in operation at the time, as well as other waters, corresponding to older NMW, however, deactivated, and still, others, which had never had the NMW classification but with the potential to be. The results of the physical–chemical analyses, obtained at that time, are very much convergent with the results that are still obtained today with more developed physical–chemical research techniques. About the chemical composition of the Portuguese NMW, it is also worth mentioning the work of the DGGM [34], in 1992, with the presentation of detailed physical–chemical analyses of all the medical spas in operation at that time. Mention should be made of the Geothermal Resources Catalog, published by the Geological and Mining Institute, in 1999 [35], with the inclusion of results of physical–chemical analyses of occurrences with geothermal potential, as they have emergence temperatures above 20°C; many of the occurrences in this work also correspond to NMW in use in thermalism, and others, naturally, have the potential to obtain this classification. Finally, it is mentioned that results are also associated in isolated publications [8, 10, 36, 37, 38] of some new NMW, which have recently been legalized and currently serve new medical spas in thermalism.

From the previous studies, the results representing the main occurrences of NMW in thermalism, or strong potential to become so, were organized into three groups, presented in Tables 2–4, according to the territory where they occur (Zones I, II, III, Figure 1b). The organization presented in those tables follow the format followed by Frederico Teixeira in 2022 [21], based on Portugal’s systematization in terms of the National Health Service (SNS), since thermalism is framed in the activities of this entity, according to the following:

• Zone I—region under the influence of the ARS of the North;

• Zone II—region under the influence of the ARS of the Centre;

• Zone III—region under the influence of the ARS of Lisbon and Tagus Valley, the ARS of Alentejo, and the ARS of Algarve.

Place	T	pH	MT	ST	SiO2	CO2	Main ionic components (mg/L)	D	Ref.
Amarante	23	9.5	226	20	36	—	F^− = 24, HCO3^− = 48, Cl^− = 20, SO4^2− = 7, Na + =67, Ca^2+ = 2, K^+ = 0.4	S	[37]
Aregos	62	9.2	313	22	54	—	F^− = 19, HCO3^− = 85, Cl^− = 30, SO4^2− = 9, Na^+ = 84, Ca^2+ = 3, K^+ = 2	S	[34]
Caldas da Saúde	27	8.8	564	91	94	—	F^− = 18,HCO3− = 76,Cl^− = 130,SO4^2− = 48, Na + =158, Ca^2+ = 6, K^+ = 7	S	[34]
Caldelas	33	8.3	136	1	23	—	F^− = 2, HCO3^− = 61, Cl^− = 7, SO4^2− = 11, Na^+ = 12, Ca^2+ = 19, K^+ = 1, Mg^2+ = 1	B	[34]
Cana-veses	30	9.4	290	36	39	—	F^− = 24, HCO3^− = 70, Cl^− = 23, SO4^2− = 2, Na^+ = 84, Ca^2+ = 2, K^+ = 1	S	[34]
Carlão	29	8.2	434	20	61		F^− = 16, HCO3^− = 226, Cl^− = 13, SO4^2− = 2, Na^+ = 103, Ca^2+ = 4, K^+ = 4	S	[34]
Carva-lhelhos	20	7.9	249	0	39	—	F^− = 3, HCO3^− = 137, Cl^− = 3, SO4^2− = 6, Na^+ = 50, Ca^2+ = 6, K^+ = 2, Mg^2+ = 1	B	[34]
Chaves	68	6.9	2508	0	74	1150	F^− = 8, HCO3^− = 1665,Cl^− = 41, SO4^2− = 28, Na^+ = 588, Ca^2+ = 24, K^+ = 71	G	[34]
Eirogo	24	8.8	491	51	65	—	F^− = 15, HCO3^− = 96, Cl^− = 87, SO4^2− = 61, Na^+ = 139, Ca^2+ = 6, K^+ = 5	S	[34]
Entre-os-Rios	19	8.7	466	158	43	—	F^− = 20, HCO3^− = 160, Cl^− = 62, SO4^2− = 1, Na^+ = 137, Ca^2+ = 3, K^+ = 3	S	[34]
Gerês	47	9.1	281	1	67	—	F^− = 14, HCO3^− = 81, Cl^− = 15, SO4^2− = 10, Na^+ = 67, Ca^2+ = 3, K^+ = 2	B	[34]
Melgaço	16	6.1	1983	0	68	2310	HCO3^− = 1425, Cl− = 14, SO4^2− = 4, Na^+ = 128, Ca^2+ = 275, Mg^2+ = 50	G	[34]
Moimenta	30	9.0	462	4	32	—	F^− = 9, HCO3^− = 71; Na^+ = 51	S	[21]
Moledo	45	9.2	242	22	36	—	F^− = 17, HCO3^− = 71, Cl^− = 20, SO4^2− = 4, Na^+ = 67, Ca^2+ = 2, K^+ = 1	S	[35]
Monção	49	7.8	559	4	81	—	F^− = 12, HCO3^− = 271, Cl^− = 42, SO4^2− = 8, Na^+ = 125, Ca^2+ = 10, K^+ = 6, Mg^2+ = 3	S	[34]
Pedras Salgadas	17	6.2	2977	—	76	2550	HCO3^− = 2078, Cl^− = 35, SO4^2−=, Na^+ = 528,Ca^2+ = 189,K^+ = 28,Mg^2+ = 26	G	[34]
São Jorge	23	8.6	690	62	62	—	F^− = 17,HCO3^− = 161,Cl^− = 202,SO4^2− = 2, Na^+ = 209, Ca^2+ = 5, K^+ = 9	S	[34]
São Lou-renço	30	8.0	360	2	51	—	F^− = 13, HCO3^− = 185, Cl^− = 13, SO4^2− = 4, Na^+ = 84, Ca^2+ = 6, K^+ = 3	S	[35]
São Vicente	19	8.6	503	164	70	—	F^− = 21, HCO3^− = 162, Cl^− = 61, SO4^2− = 1, Na^+ = 137, Ca^2+ = 4, K^+ = 5	S	[34]
Taipas	31	8.2	256	8	36	—	F^− = 7, HCO3^− = 90, Cl^− = 26, SO4^2− = 26, Na^+ = 48, Ca^2+ = 20, K^+ = 1	S	[34]
Terronha	30	8.8	376	7	27	—	F^− = 17, HCO3^− = 162, Na^+ = 131	S	[21]
Vidago	18	6.5	5604	—	58	1960	F^− = 2, HCO3^− = 3966, Cl^− = 66, SO4^2− = 3, Na^+ = 1180,Ca^2+ = 194,K^+ = 92,Mg^2+ = 34	G	[34]
Vizela	50	9.3	345	30	54	—	F^− = 18, HCO3^− = 83, Cl^− = 39, SO4^2− = 12, Na^+ = 94, Ca^2+ = 3, K^+ = 2	S	[32]

Table 2.

Main physical-chemical parameters of special groundwater in zone I, classified as NMW, in use in thermalism.

T—temperature at emergence (°C), M_T—total mineralization (mg/L), S_T-total sulfuration (in I₂ 0.01 N—mL/L), SiO₂—colloidal free silica (mg/L), CO₂—free carbon dioxide (mg/L), D—typical designation as to chemical composition: B—bicarbonate, G—gas-carbonate, S—sulfurous.

Place	T	pH	MT	ST	SiO2	CO2	Main ionic components (mg/L)	D	Ref.
Águas	19	7.6	280	9	40	—	F^− = 3, HCO3^− = 147, Cl^− = 12, SO4^2− = 8, Na^+ = 65, Ca^2+ = 1, K^+ = 1	S	[36]
Alcafache	48	8.4	360	1.6	52	—	F^− = 14, HCO3^− = 154, Cl^− = 34, SO4^2− = 3, Na^+ = 92, Ca^2+ = 5, K^+ = 3	S	[34]
Almeida	31	8.3	527	7	30		F^− = 17, HCO3^− = 261, Cl^− = 54, Na^+ = 148, Ca^2+ = 5, K^+ = 3	S	[38]
Azenha*	28	7.3	682	0	13	—	HCO3^− = 239, Cl^− = 193, SO4^2− = 30, Na^+ = 119, Ca^2+ = 56, K^+ = 7, Mg^2+ = 23	C	[34]
Bicanho	28	7.9	721	0	—	—	HCO3^− = 243, Cl^− = 213, SO4^2− = 34, Na^+ = 138, Ca^2+ = 56, Mg^2+ = 24	C	[35]
Carvalhal	23	9.3	336	25	56	—	F^− = 21, HCO3^− = 88, Cl^− = 28, SO4^2− = 5, CO3^2− = 11, Na^+ = 92, Ca^2+ = 2, K^+ = 2	S	[34]
Cavaca	28	8.3	360	9	56	—	F^− = 15, HCO3^− = 164, Cl^− = 21, SO4^2− = 2, CO3^2− = 2, Na^+ = 87, Ca^2+ = 6, K^+ = 3	S	[34]
Cró	23	8.2	395	19	48	—	F^− = 17, HCO3^− = 165, Cl^− = 38, SO4^2− = 11, Na^+ = 104, Ca^2+ = 4, K^+ = 3	S	[35]
Curia	20	7.7	2375	—	10	—	F^− = 1, HCO3^− = 217,Cl^− = 29, SO4^2− = 1454, Na^+ = 20, Ca^2+ = 591, K^+ = 2, Mg^2+ = 51	Su	[34]
Felgueira	36	8.4	414	7	48	—	F^− = 16, HCO3^− = 162, Cl^− = 5, SO4^2− = 12, CO3^2− = 3, Na^+ = 111, Ca^2+ = 5, K^+ = 3, Li^+ = 1	S	[34]
Granjal*	23	9.1	507	47	56	—	F^− = 28, HCO3^− = 149, Cl^− = 74, SO4^2− = 14, CO3^2− = 10, Na^+ = 157, Ca^2+ = 2, K + =3	S	[38]
Longroiva	44	8.9	441	44	58	—	F^− = 23, HCO3^− = 149, Cl^− = 44, SO4^2− = 10, Na^+ = 122, Ca^2+ = 3, K^+ = 5, Li^+ = 1	S	[35]
Luso	22	5.4	40	0	11	—	HCO3^− = 8, Cl^− = 9, SO4^2− = 2, NO3^− = 1, Na^+ = 6, Ca^2+ = 1, Mg^2+ = 1, K^+ = 1	Si	[34]
Man-teigas	48	9.2	179	7	39	—	F^− = 10, HCO3^− = 43, Cl^− = 6, SO4^2− = 13, Na^+ = 43, Ca^2+ = 3, K^+ = 1	S	[35]
Monfor-tinho	27	5.7	49	0	18	—	HCO3− =15, Cl^− = 5, NO3^− = 2, Na^+ = 5, Ca^2+ = 1, Mg^2+ = 1	Si	[34]
Monte Real	18	7.2	2620	20	15	—	HCO3^− = 126, SO4^2− = 1575, Cl^− = 142, Na^+ = 83, Ca^2+ = 626, Mg^2+ = 48	Su	[34]
Sangemil	48	8.4	469	21	82	—	F^− = 17, HCO3^− = 171, Cl^− = 54, SO4^2− = 7, Na^+ = 114, Ca^2+ = 6, K^+ = 5	S	[34]
São Miguel	19	5.9	78	0	25	—	HCO3^− = 23, Cl^− = 7, NO3^− = 6, Na^+ = 9, Ca^2+ = 4, K^+ = 1	B	[8]
São Pedro do Sul	67	8.9	341	23	70	—	F^− = 17, HCO3^− = 104, Cl^− = 29, SO4^2− = 9, CO3^2− = 6, Na^+ = 87, Ca^2+ = 3, K^+ = 4	S	[34]
São Tiago	19	6.7	214	—	52	—	F^− = 1, HCO3^− = 106, Cl^− = 9, SO4^2− = 6, Na^+ = 17, Ca^2+ = 14, Mg^2+ = 8, Fe^2+ = 3	B	[10]
Unhais da Serra	37	8.7	269	13	50	—	F^− = 15, HCO3− = 85, Cl^− = 25, SO4^2− = 7, CO3^2− = 3, Na^+ = 69, Ca^2+ = 4, K^+ = 2	S	[35]
Vale da Mó	15	6.6	165	—	9	—	HCO3^− = 89, Cl^− = 19, SO4^2− = 9, Na^+ = 12, Mg^2+ = 13, Fe^2+ = 9	F	[32]

Table 3.

Main physical-chemical parameters of special groundwater in zone II, classified as NMW, in use in thermalism.

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T—temperature at emergence (°C), M_T—total mineralization (mg/L), S_T—total sulfuration (in I₂ 0.01 N—mL/L), SiO₂—colloidal free silica (mg/L), CO₂—free carbon dioxide (mg/L), D—typical designation as to chemical composition: B—bicarbonate, C—chloride, F—ferruginous, S—sulfurous, Si—silicate, Su—sulfate.

Place	T	pH	MT	ST	SiO2	CO2	Main ionic components (mg/L)	D	Ref.
Caldas Rainha	35	6.9	2990	—	18	—	Cl− = 1007, HCO3− = 315, SO4^2− = 641, Na^+ = 660, Ca^2+ = 270, Mg^2+ = 57	C	[34]
Cucos	35	7.0	3393	—	18	—	F^− = 1, HCO3^− = 378,Cl^− = 1584,SO4^2− = 200, Na^+ = 1004,Ca^2+ = 158, Mg^2+ = 40,K^+ = 9	C	[34]
Enven-dos	22	4.7	26	0	10	—	HCO3^− = 1, Cl^− = 7, SO4^2− = 3, Na^+ = 4, Ca^2+ = 1, Mg^2+ = 1	Si	[34]
Estoril	35	7.1	5743	—	32	—	Cl^− = 2709, HCO3^− = 285, SO4^2− = 358, Na^+ = 1574,Ca^2+ = 273,Mg^2+ = 90,K^+ = 50	C	[35]
Piedade	24	7.0	2444	—	12	—	HCO3^− = 393, Cl^− = 990, SO4^2− = 207, Na^+ = 644, Ca^2+ = 160, Mg^2+ = 34	C	[34]
Sta Mta-Ericeira	17	6.8	3902	—	10	—	HCO3^− = 314, Cl^− = 1395, NO3^− = 500, Na^+ = 993, Ca^2+ = 117, K^+ = 144	C	[21]
Vimeiro (Frades)	26	6.9	3300	—	12	—	HCO3^− = 476, Cl^− = 1441, SO4^2− = 229, Na^+ = 836,Ca^2+ = 171,K^+ = 36,Mg^2+ = 88	C	[34]
Cabeço de Vide	18	11.4	223	—	—	—	Cl^− = 60, OH^− = 30, CO3^2− = 17, SO4^2− = 4, Na^+ = 65, Ca^2+ = 26, K^+ = 5	O	[34]
Castelo de Vide	15	7.2	1010	—	17	—	HCO3^− = 343, Cl^− = 155, NO3^− = 131,SO4^2− =74, Na^+ = 108, Ca^2+ = 78, K^+ = 58, Mg^2+ = 44	N	[34]
Fadago. de Nisa	19	8.3	380	23	30	—	F^− = 10, HCO3^− = 149, Cl^− = 71, SO4^2− = 0.2, Na^+ = 100, Ca^2+ = 11, K^+ = 2	S	[34]
Pizões (Moura)	18	7.0	736	—	24	—	HCO3^− = 388,Cl^− = 102,SO4^2− = 19, NO3^− = 21, Na^+ = 38, Ca^2+ = 110, Mg^2+ = 33, K^+ = 1	B	[34]
Monchi-que	32	9.6	335	—	14	—	F^− = 1, HCO3^− = 105, Cl^− = 33, SO4^2− = 46, Na^+ = 107, Ca^2+ = 1, K^+ = 2	Su	[35]
Malhada Quente*	28	9.0	335	—	11	—	HCO3^− = 181, Cl^− = 24, SO4^2− = 16, Na^+ = 73, Ca^2+ = 20, Mg^2+ = 1	B	[35]
Alferce (Faro)*	27	8.6	607	—	3	—	HCO3^− = 139, Cl^− = 101, SO4^2− = 141, Na^+ = 184, Ca^2+ = 3	Su	[35]
Santo Antó*	25	6.9	745	—	15	—	HCO3^− = 459, Cl^− = 67, SO4^2− = 11,Na^+ = 49, Ca^2+ = 109, Mg2 + =24	B	[35]

Table 4.

Main physical-chemical parameters of special groundwater in zone III, classified as NMW, in use in thermalism.

^*

This site is not currently active.

T—temperature at emergence (°C), M_T—total mineralization (mg/L), S_T—total sulfuration (in I₂ 0.01 N—mL/L), SiO₂—colloidal free silica (mg/L), CO₂—free carbon dioxide (mg/L), D—typical designation as to chemical composition: B—Bicarbonate, C—Chloride, F—Ferruginous, N—Nitrate, S—sulfurous, Su—Sulfate.

ARS corresponds to Regional Health Administration authorities.

To better understand the types of waters present, Figures 3–7 are presented to show the variability of M_T, pH, and the types of dominant ions.

Figure 3.

Total mineralization (M_T) of the special groundwaters of mainland Portugal, classified as NMW, in thermalism activity. ^(*) The water of this site, having the potential to be NMW, is not currently in thermalism.

Figure 4.

pH of the special groundwaters of mainland Portugal, classified as NMW, in thermalism activity. ^(*) The water of this site, having the potential to be NMW, is not currently in thermalism.

Figure 5.

Piper (a) and stiff (b) diagrams of special groundwater of zones I, II, and III, classified as NMW, in thermalism activity. ^(*) The water of this site, having the potential to be NMW, is not currently in thermalism.

Figure 6.

Main Portuguese groundwater occurrences with geothermal potential of which most are NMW from ref. [39].

Figure 7.

Singular groundwater occurrences in the Algarve [30].

According to the M_T classification (Table 1): weakly mineralized waters are the most frequent, with M_T between 200 and 1000 mg/L; hyposaline waters (M_T < 200 mg/L) are very few, being only Luso, Monfortinho, Envendos, and Caldelas; mesosaline waters (1000 < M_T < 2000 mg/L) are very rare, with only two, Melgaço and Castelo de Vide; and the hypersaline waters (M_T > 2000 mg/L) have some significance, namely in Zone III, where Estoril is located, which is the most mineralized water of the current NMW, with M_T = 5743 mg/L.

Regarding the pH (Figure 4), most of the waters are basic or alkaline, that is, pH higher than 7, and it should be noted that the two waters with the highest value occur in Zone III, Monchique and Cabeço de Vide, with pH of 9.6 and 11.4, respectively. It should be noted that some acidic waters also occur, being the lowest values in Envendos, and Luso, with 4.7 and 5.4, respectively.

Regarding the main ions (Figure 5): (i) the Zone I waters are mostly of the bicarbonate sodium type (Group V, Figure 5a), and it should be noted that it includes the three most mineralized waters: Chaves, Pedras Salgadas, and Vidago, which is a situation well evidenced in Figure 5b (samples 8, 16 and 22); (ii) in Zone II (Figure 5c), the majority of waters are also of the bicarbonate sodium type, although in this region, a greater diversity is evident than in Zone I; it should be noted that in this zone (II), the two most mineralized waters are Curia and Monte Real, being of the sulfate calcium type (samples 9 and 16, Figure 5d); (iii) in Zone III, the situation is different from the previous ones, since there is only one bicarbonate sodium water (Malhada Quente), and the majority are chloride sodium-type waters (Group II, Figure 5e), with Estoril, Sta Marta (Ericeira), Cucos, and Vimeiro as the most mineralized in this region and even Estoril, the most mineralized, currently in use in the thermalism of mainland Portugal. Mention should be made particularly that the least mineralized NMW in Portugal, Envendos, is also of the chloride sodium type, but this is not possible to show in Figure 5f (sample 3) due to the scale of the diagram imposed in order to show the best relationship between most of the samples.

Regarding the classification of the various Portuguese NMW and, in particular, the classification that each NMW is best known, presented in Tables 2–4, it is now of interest to highlight the same, which in general is a consequence of the most evident components due to their specific singularities. For example, waters that appear as sulfurous in most situations are weakly mineralized, alkaline, and bicarbonate sodium, but the singularities of these waters, because of the various associated forms of sulfur and represented by total sulfuration (S_T), give them a special character, namely the fact that they smell like rotten eggs and present a whitish cream on the surface near the emergences.

Thus, in relation to the totality of the NMW in mainland Portugal, the majority are of the sulfurous type, namely in the northern region (Group I), with the São Vicente NMW having the highest value, with S_T = 162 mg/L. A similar situation to the previous one is considered for fluoride, as all sulfurous waters are also fluoridated. Regarding Fe²⁺, only two Portuguese NMW waters appear in that condition: São Tiago (Fe²⁺ = 3 mg/L) and Vale da Mó (Fe²⁺ = 9 mg/L), although only the latter is known as ferruginous.

About the free SiO₂, the situation in Portugal is amazing, since almost all waters have SiO₂ higher than 10 mg/L, namely almost all sulfurous waters, noteworthy being that the most silicate is Caldas da Saúde, with 94 mg/L. Still on silica, it is interesting to note that in the Portuguese NMW, the commonly known as silicate waters, are hyposaline waters, as in the cases of Luso, Monfortinho, and Envendos (Tables 2 and 3), with SiO₂ between 10 and 18 mg/L, but note that despite those values being low, in relative terms, they have a lot of meaning in relation to M_T, as these waters have M_T between 26 and 49 mg/L, only. Regarding the occurrence of free CO₂, in mainland Portugal, it is not a very common situation; however, it is a special singularity in the NMW of Chaves, Melgaço, Pedras Salgadas, and Vidago, all in the Northern Region (Group I), with CO₂ values between 1150 and 2550 mg/L; these NMWs are commonly called gas-carbonates (Table 2).

By the presented, it turns out that there is a considerable variety of types of groundwater. In global terms, the chemical type of groundwater is a consequence of the water-rock interactions, which initially were meteoric, evolve in depth on their path, until they resurface in natural springs or in abstractions strategically well implanted. In some cases, the NMWs, in addition to serving the thermalism, are also used in other applications, such as serving for drinking water, examples being the waters of Luso, Pedras Salgadas, Carvalhelhos, and Monchique [34].

A generic notion on the relationship between the type of groundwater and the regional geology can be observed in Figure 6, which presents on the geological map of Portugal, the location of the main Portuguese groundwaters, mostly in exploration as NMW. From that to Figure 6, it is emphasized that there are some groups of groundwaters to be highlighted: the sulfurous groundwater, which occur in the Central-Iberian zone, whose rock masses are of granitoid type; the chloride groundwaters that occur in the Western Meso–Cenozoic border, whose rock masses are of sedimentary type, ranging from carbonate rocks to detrital rocks; and the hyposaline groundwaters that occur also in the Central-Iberian zone, with the particularity of being associated with rock masses of quartzite type of the Ordovician.

Considering the main goal of this chapter, to enhance the use of thalassotherapy from salty groundwater, indirectly coming from the sea, it is emphasized at this stage that some of the Portuguese NMW, from the previously mentioned, already have a large component of Cl and Na, typical of seawater. This happens as seen in particular in Estoril water (Table 4 and Figure 5e and f), which is the most mineralized water currently used in thermalism, in Portugal. In any case, it is emphasized that there are immense chloride waters in the Western and Southern Meso-Cenozoic border (Figure 6). Also emphasized is the fact that in this region are associated geological formations of the Salt Diapir type and that in certain hydrogeological situations allow occurrences of natural, very salty groundwater; this is the case of Salgadas groundwater (point 35, Figure 6), which is currently not in thermalism use but corresponds to a concession, designated HM-65—Termas Salgadas da Batalha [40] and have an M_T of 32,500 mg/L, corresponding to a hypersaline water, of the Cl-Na type. This water has been used in the past in thermalism for rheumatic lesions, lymphatic disorders, ladies’ diseases, horny skin lesions, consequences of traumatic lesions, and scrofula [41].

3.2 Physical: chemical aspects of salty groundwater in Portugal

This item serves to explore the physical–chemical aspects of salty groundwaters not currently licensed as NMW, namely the salty groundwaters of the Algarve (Figure 7) and the “Salgadas da Batalha” groundwater. The latter had already served in the past in thermalism activities; there are indications that it will soon be reactivated, with new studies not only around chemistry but also on its therapeutic and other, as guided by the recent study on the hydrogenome [40].

Thus, Table 5 presents the main physical–chemical parameters of those waters, available in the literature [30, 41]. It is mentioned that since M_T is not available, and there is a need to compare with the situation of NMW, that parameter is evaluated from [27]:

Place	Type	pH	RD	CO2	SiO2	Main ionic components (mg/L)	Ref.
Sinceira (Vila do Bispo)	Fontain	5.3	289	119	84	Cl^− = 21, HCO3^− = 12, SO4^2− = 8, NO3^− = 38, Na^+ = 66, Ca^2+ = 10, Mg^2+ = 15,	[30]
Salema (Budens)	Hole	6.9	3436	77	12	Cl^− = 1702, HCO3^− = 403, SO4^2− = 296, NO3^− = 40, Na^+ = 970, Ca^2+ = 158, Mg^2+ = 130	[30]
Meia-Praia (Lagos)	Hole	7.2	936	29	10	Cl^− = 355, HCO3^− = 268, SO4^2− = 91, NO3^− = 20, Na^+ = 188, Ca^2+ = 107, Mg^2+ = 25	[30]
Seixosas (Ferragudo-Lagoa)	Hole	7.0	3481	68	24	Cl^− = 1683, HCO3^− = 383, SO4^2− = 240, NO3^− = 2, Na^+ = 916, Ca^2+ = 197, Mg^2+ = 110	[30]
V. dos Pereiros (Ferragudo-Lagoa)	Hole	7.0	2202	68	37	Cl^− = 994, HCO3^− = 395, SO4^2− = 180, NO3^− = 7, Na^+ = 575, Ca^2+ = 146, Mg^2+ = 73	[30]
Olhos de Água (Albufeira)	Spring	7.0	1373	56	5	Cl^− = 557, HCO3^− = 356, SO4^2− = 95, NO3^− = 4, Na^+ = 301, Ca^2+ = 120, Mg^2+ = 55	[30]
Olheiro (Fuseta-Olhão)	Spring	6.8	5577	89	24	Cl^− = 2911, HCO3^− = 400, SO4^2− = 409, NO3^− = 1, Na^+ = 1587,Ca^2+ = 234,Mg^2+ = 199	[30]
Fonte Salgada (Tavira)	Spring	7.3	12,367	24	9	Cl^− = 6135, HCO3^− = 307, SO4^2− = 1417, Na^+ = 4233, Ca^2+ = 44, Mg^2+ = 124, K^+ = 40	[30]
Termas Salgadas da Batalha	Well	7.3	32,000	—	14	Cl^− = 17,333, HCO3^− = 297, SO4^2− = 1783 Na^+ = 11,083, Ca^2+ = 424, Mg^2+ = 58, K^+ = 14	[41]

Table 5.

Results of analyses of chloride sodium groundwater of the Algarve region and the Termas Salgadas da Batalha, Portugal.

R_D—Dry Residue at 180°C (mg/L), CO₂—free carbon dioxide (mg/L), SiO₂—colloidal free silica (mg/L).

$${\mathrm{M}}_{\mathrm{T}}\cong {\mathrm{R}}_{\mathrm{D}}+\mathrm{½}{{\mathrm{HCO}}_3}^{-}$$E1

R_D being the Dry Residue, obtained at 180°C.

At the similar of the graphics presented for the NMW, Figure 8 shows the variability of M_T and pH in the various salty groundwaters, and Figure 9 shows the Piper and Stiff diagrams of the same waters. Regarding M_T, most waters are hypersaline (M_T > 2000 mg/L), only two (Meia Praia and Olhos de Água) are mesosaline (M_T between 1000 and 2000 mg/L), and only one (Sinceira) is weakly mineralized (M_T between 200 and 1000 mg/L). With regard to pH, with the exception of Sinceira, all have a pH of around 7, that is, neutral. Regarding the ionic composition (Figure 9), all these waters fit into the situation where Cl and Na ions are in the majority, integrating exclusively sector II of the Piper diagram, and are therefore chloride sodium-type waters.

Figure 8.

Total mineralization (M_T) and pH of special salty groundwaters in mainland Portugal.

Figure 9.

Piper and stiff diagrams of the special salty groundwaters of mainland Portugal.

3.3 Physical-chemical aspects of seawater

The temperature of seawater varies from −4°C in the Arctic to 30°C in tropical regions, and the density of seawater varies from 1028 mg/L to 1032 mg/L [15]. In chemical terms, the major constituents in seawater are Cl⁻, Na⁺, Mg²⁺, S0₄²⁻, Ca²⁺, K⁺, Br⁻, and Sr²⁺, and the minor constituents are several, namely: PO₄³⁻, NO₃⁻, and SiO₂ [42]. Trace elements, in the order of μg/L, exist in great variety, and it can be said that seawater has practically all the elements of nature. Figure 10 shows the participation, in relative weight terms, of the six most abundant chemical elements or mineral salts that, in ionic form, are present in 1 kg of seawater [43].

Figure 10.

Major chemical constituents of seawater [43].

The physical–chemical composition may vary depending on the ocean or sea. The most common situation in oceans in terms of total salinity or total mineralization (M_T) is around 33 to 36 g/L; however, as can be seen in Table 6, considering seas and other saltwater masses, M_T can have values from 7 g/L in the Baltic Sea to 350 g/L in the Dead Sea. Seawater, especially ocean water, also contains a large quantity of gases, about 20 to 30 cm³/L, mainly oxygen, hydrogen, and carbon dioxide [21]. The concentrations of chemical species in seawater do not vary much, either horizontally or vertically, except in river-mouth regions where a greater quantity of salts brought by rivers occurs [42].

Sea or Ocean	Salinity / Total mineralization—MT (mg/L)	Ref.
Baltic Sea	7000	[44]
Black Sea	13,000	[44]
Adriatic Sea	25,000	[44]
Pacific Ocean	33,000	[44]
Indian Ocean	33,800	[44]
Atlantic Ocean	36,000	[44]
Mediterranean Sea	39,400	[44]
Arabian Gulf	43,000	[44]
Red Sea	43,000	[44]
Dead Sea	300,000–350,000	[21]

Table 6.

Order of magnitude of total mineralization (M_T) for saline waters from different oceans and seas.

To have a notion of its detailed chemistry, and to compare it with NMW and other Portuguese salty groundwaters, the main physical–chemical parameters of some waters from various oceans and seas are presented in Table 7, based on results available in the literature. Figure 11 shows the Piper and Stiff diagrams of the same waters. From these diagrams, it can be said that, except for the Dead Sea water, all the others fall into Group II as chloride sodium waters in relation to the Piper diagram. The waters of the Dead Sea are in Group VI, as chloride magnesian. The fact that they belong to a different chemical group is also evidenced in the Stiff diagram (Figure 11b), and the proximity of the Mediterranean waters with those of the Atlantic Ocean and even with the water pattern of the Indian Ocean is also emphasized.

Parameter		Atlantic Ocean [43]	Mediterranean Sea [43]	Dead Sea [43]	Dead Sea [45]	Mediterranean [46]	Indian Ocean [47]
pH		—	—	8.05		7.5	7.57
MT* (mg/L)		≈ 35,554	≈ 37,876	≈ 372,874	≈ 322,130	≈ 36,509	24,602
Anions (mg/L)	Cl^−	18,981	20,980	239,287	212,400	19,678	12,623
	HCO3^−	140	155	8211	220	137	2814
	SO4^2−	2470	2720	536	470	2782	1177
	NO2^−	—	—	—		—	0.53
	NO3^−	—	—	16		10	3.0
	CO3^2−	—	—	332		—	—
	F^−	1.3	—			1.5	0.06
	Br-	65	72		5120	6.4
	H3BO3^−	24	27	—		—	—
	H2PO4^−	—	—	—		—	0.07
Cations (mg/L)	Na^+	10,540	11,650	42,090	39,150	11,496	5500
	Mg^2+	1270	1410	47,142	40,650	1396	624
	Ca^2+	400	442	**	16,860	540	1560
	K^+	380	420	18,400	7260	462	300
	Sr^2+	13	—	—		—	—
	Fe^2+	—	—	—		—	0.01
	Li+	—	—	—		0.09	—

Table 7.

Results of physical–chemical analyses of waters from different oceans and seas.

^*

the values presented as approximate (≈) resulted from the sum of the various ionic components presented in this table.

^**

for the purpose of M_T calculation and construction of Piper and Stiff diagrams, it was considered for Ca²⁺, the same value presented in Ref. [45].

Figure 11.

Piper (a) and stiff (b) diagrams of waters from different oceans and seas (from Table 7).

3.4 Relationships of physical-chemical parameters between NMW, special salty groundwater, and seawater

To investigate possible relationships between the various physical–chemical parameters, the relationship between M_T and conductivity (parameters taken from the references mentioned in Tables 2–4) was first studied for all the NMW. The relationship obtained is shown in Figure 12a, with the following expression:

Figure 12.

Relationships between conductivity and total mineralization in the various waters under study: (a) natural mineral waters (NMW), (b) NMW and salty groundwater, and (c) NMW, salty groundwater, and seawater.

$${\mathrm{M}}_{\mathrm{T}}=0.802\mathit{C}$$E2

where C is conductivity in μS/cm, and M_T is total mineralization in mg/L.

Above the trend line (Figure 12a) are clearly located the waters of Zone I, with the gas-carbonate classification (Table 2), namely, Vidago, Pedras Salgadas, Chaves, and Melgaço; in ionic terms, the first three are of the bicarbonate sodium type, and the last one is of the bicarbonate calcium type; all four of these waters are singular in the Stiff diagrams (Figure 5b). The waters classified as sulfate in Zone II (Table 3—Curia and Monte Real) are still above the global trend; these waters are, in ionic terms, of the sulfate calcium type (Figure 5c), and it should be noted that, according to the Stiff diagram, they are unique in relation to the others (Figure 5d). It should also be noted that below that trend (Figure 12a) are the chloride waters, of the Zone III (Table 4), ionically all chloride-sodium (Group II, Piper diagram, Figure 5e), which are also unique in the Stiff diagram (Figure 5f).

In order to have a global notion of the position in that type of graph, of salty groundwater (Table 5), the M_T evaluated from Eq. (1) was used, and Eq. (2) was admitted as valid for this type of groundwater of the reference [30] in Table 5; there is the particularity of the experimental result of the water of reference [41], which is C = 41,100 μS/cm. It results like this Figure 12b, with a trend very close to that obtained for the NMW and which is as follows:

$${\mathrm{M}}_{\mathrm{T}}=0.787\mathit{C}$$E3

with C in μS/cm and M_T in mg/L.

Also, to compare with the seawaters, although only direct results of M_T and C, from two waters (Mediterranean Sea and Dead Sea), are available, Figure 12c is presented. This highlights the exceptionality of the point corresponding to the Dead Sea. Anyway, it is interesting to verify that the Mediterranean Sea water position is very close to the trend of the “NMW + Salty groundwater,” being remarkable, that according to the physical–chemical composition of the other seas waters (Tables 6 and 7), excluding therefore the Dead Sea water, very special indeed, the general trend of M_T versus C, is close to Eq. (3).

In order to analyze the situation between the various types of waters in ionic terms, the pH situation with HCO₃⁻ and Na⁺ is shown in Figure 13 as an example. The graphs in that figure show that there is no pH trend with those ions. A similar situation occurs with the other major ions (Cl⁻, SO₄²⁻, Ca²⁺).

Figure 13.

Relationships between pH and ions are often very important in the various waters under study.

The relationships between M_T and the most important ions in the various waters under study were investigated, resulting in some interesting relationships on a bi-log scale, as shown in the graphs in Figure 14. There is a clear contribution of Na⁺, Cl⁻, Ca²⁺, and Cl⁻ in the increase of M_T in all the studied waters (NMW, salty groundwater, and seawater). The case of HCO₃⁻ clearly contributes to the increase of M_T for most of the NMW, but this does not happen for the salty groundwater and also for the seawater.

Figure 14.

Relationships between M_T and ions are often very important in the various waters under study.

In relation to the main ions, Figure 15 shows some graphs of those considered most important. The increasing of Na⁺ with increasing Cl⁻ is a very evident situation for most of all waters under study; the situation of increasing Na⁺ and Ca²⁺ with increasing HCO₃⁻ in global terms is also true, but there are many exceptions, namely for the case of Na⁺ in salty groundwater and seawater and even more exceptions for the case of Ca²⁺. In the case of the relationship between HCO₃⁻ and Cl⁻, the situation is very clear that these ions are not related to each other (Figure 15d); independently of the Cl⁻ value, HCO₃⁻ in most situations occurs with values between 100 and 1000 mg/L.

Figure 15.

Relationships between various ions are often very important in the various waters under study.

4. Conclusions and final notes

In Portugal, there is a wide variety of natural mineral waters (NMWs) in chemical terms (Figure 6), including some relatively salty waters (rich in Cl and Na), with Estoril water being the most mineralized water in Portugal (Figure 3), currently in operation for thermalism, with a total mineralization (M_T) of 5743 mg/L (Table 4).

It is emphasized that NMW, according to Portuguese legislation, is currently a groundwater, which, among other factors, presents a high physical–chemical stability over time, regardless of the M_T value.

There are some very salty groundwaters (rich in Cl⁻ and Na⁺) in Portugal, namely in the Algarve region (Figure 7) and also in the Western Meso-Cenozoic Border region (Figure 6), associated in particular with geological formations of the salt diapir type, as is the case of the Salgadas da Batalha water, with an M_T of about 32148.5 mg/L, and it should be noted that this has already been used in thermalism.

On saline waters of various oceans and seas, they are mostly composed of Cl⁻ and Na⁺ ions (Figure 10), with M_T varying between orders of magnitude from 7000 mg/L in the Baltic Sea to 43,000 mg/L in the Arabian Gulf and Red Sea (Table 6).

In the Atlantic Ocean, which coexists with western mainland Portugal, the M_T takes on values of the order of 36,000 mg/L, and in the Mediterranean Sea, which coexists with Portugal, to the south, in the Algarve, the M_T takes on the order of magnitude of 39,400 mg/L. The Dead Sea water reaches very high values of M_T, from 300,000 to 350,000 mg/L (Tables 6 and 7), and the two major ions are Cl⁻ and Mg²⁺ (Table 7, Figure 11).

The sea water, in Portugal, for use in thalassotherapy, is captured directly from the Atlantic Ocean or from the Mediterranean Sea. Some thalassotherapy spas already have the supervision of a doctor with a specialization in Hydrological Medicine and therefore, in some situations, have activities for users very similar to those of thermalism.

Thermalism, which has very specific and very clear legislation, uses the NMW, highlighting that these are special groundwaters, namely for presenting high physical–chemical stability over time, and Portuguese law requires that their exploitation, from the geological environment, is the responsibility of a professional in the field of hydrogeology.

Thalassotherapy in Portugal has no specific legislation. Its activity is guided in accordance with general laws applied to the sector regulating the exploration of municipal swimming pools, private swimming pools, spas in hotels, private spas, and tourist complexes, among others.

Thus, Portugal, having already in use some NMW of the chloride sodium type, if it has, in the future, salty groundwaters, captured from the aquifer system in contact with the sea, and if, after all the studies, legally imposed for the NMW, come to demonstrate adequate quality, it is admitted that without any doubt in the future the Portuguese thalassotherapy network can be integrated in the network and rules of thermalism in Portugal.

The thermalism sector will grow, and the thalassotherapy sector, with great probability, will have salty waters of greater stability in quality. Thalassotherapy water would no longer be abstracted in the liquid wedge of the sea, where it normally has great oscillations, namely of vestigial chemical species and remains of hydrocarbons and oils, among others, but be abstracted as classic groundwater. In this way, the water to be exploited, before entering the well (groundwater abstraction), passes through the geological environment, which acts with its natural purifying power, to eliminate potential harmful elements that destabilize its quality.

The construction of groundwater abstractions must follow the good rules of hydrogeology, with a particularity of having the notion of not causing the development of the saline wedge to the interior of the territory, as shown in many works in this scientific field [48, 49].

In zones of free (unconfined) aquifers, in loose sands, an abstraction such as the one shown in Figure 2 can be built, which minimizes this problem. However, it is acceptable to build boreholes or classic wells with the aid of roto percussion, percussion, or other drilling machines but with due care, both in drilling and in pumping operations during flow tests, especially in the prospecting phase.

The groundwater abstractions of the thalassotherapies, after their official classification as NMW, would have the compulsory implementation of a protection perimeter, with three zones surrounding the abstraction (Immediate, Intermediate, and Extended Zone) to be subject to continuous “supervision” over the occupation of the territory and anthropic actions in it, besides having the obligation to include a hydrogeologist, the Technical Director, as responsible for the exploration of that resource. Such situations will be guarantees for the problem of the control of the geometric stability of the wedge-saline, namely in the eventual prohibition of new abstractions of groundwater without rules and control for third parties, whether in tourism, agriculture, or other activities.

A major advantage when evolving towards this type of solution is the possibility of obtaining stable exploration flows over time, from salty groundwater abstractions, close to the sea. It should be noted that the stability of exploitation flows of the NMW is starting to be in danger due to climate change, since it is common sense that if normal hydrological cycles do not occur, that is, with the current trends of less precipitation and more evaporation, the sustainable exploitation of aquifer systems will be put in danger, leading necessarily to a decrease in exploitation flows and causing consequent potential changes in the quality of the resource.

Thalassotherapies supplied by salty groundwater, coming indirectly from the sea, can be an excellent way for the future of thermalism, which already has a millenary tradition, and it is necessary to guarantee its continuity and consolidation in this new phase of humanity that faces the fact of climate change in many regions of the world.

Acknowledgments

FCT—Foundation for Science and Technology supported this work with Portuguese funds within the GeoBioTec Centre (Project UIDB/04035/2020).

Conflict of interest

The data sets generated during and/or analyzed during the current study are available from the corresponding author on request after the publication of work.