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Article

Mineral-Rich Brines from Portuguese Coastal Lagoons: Insights into Their Use in Thalassotherapy and Skin Care

by
Lara Almeida
,
Fernando Rocha
and
Carla Candeias
*
GeoBioTec Research Unit, Department of Geosciences, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal
*
Author to whom correspondence should be addressed.
Water 2025, 17(20), 3021; https://doi.org/10.3390/w17203021
Submission received: 18 September 2025 / Revised: 15 October 2025 / Accepted: 16 October 2025 / Published: 21 October 2025
(This article belongs to the Special Issue Groundwater for Health and Well-Being)
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Abstract

This study characterized saline waters from traditional and semi-industrial saltpans located in the Ria Formosa and Ria de Aveiro Portuguese coastal lagoons, aiming to evaluate their potential for thalassotherapy and dermatological applications. Five saline water samples were collected and analyzed for physicochemical parameters (pH, electrical conductivity, dissolved oxygen, and total dissolved and suspended solids) and chemical composition (major, minor, and trace elements), complemented by SEM-EDS analyses of the suspended solids. All samples exhibited salinities above 70 g/kg, classifying them as mineral-rich brines. Sodium was the dominant element, followed by Mg, K, and Ca, with concentrations significantly higher than those of seawater. Apparent geochemical differences were observed between the two lagoons, with Ria de Aveiro water enriched in Ca, while Ria Formosa showed higher Mg and K contents. Suspended solids were composed mainly of halite, gypsum, K-Mg salts, and biogenic aggregates, reflecting the interaction between evaporitic and microbial processes. These findings highlighted the high therapeutic potential of Portuguese saline waters for skin-related applications, supporting the safe use of natural saline resources in evidence-based wellness and dermatological practices.

1. Introduction

Marine-based wellness applications, including Spa treatments and thalassotherapy, are experiencing significant growth in acceptance and applications [1]. Seas and oceans have long been essential natural resources for Spa and balneotherapy treatments, with the Dead Sea (DS) among the most prominent and widely recognized therapeutic and cosmetic natural environments, often described as “miraculous” and acknowledged for thousands of years [2]. Katz et al. [3] documented that DS has been renowned for its healing properties since ancient times, as evidenced in historical documents, such as one by Aristotle (384–322 BC), who reported these properties. Modern scientific investigation into DS health benefits began with pioneering studies by Dostrovsky and Sagher [4]. The long-time recognition of the therapeutic properties of Dead Sea products demonstrated how saline environments have been valued for health and wellness over the years. Sea water is commonly used in health therapies because of its high density and rich mineral profile, particularly the high NaCl (halite) content, along with significant amounts of Mg, Ca, K, and S [5]. The historical use of saline environments for therapeutic purposes naturally bonds to the broader concept of thalassotherapy, which derives from the Greek word thálassa, meaning sea or ocean [6]. Thalassotherapy refers to the therapeutic and preventive application of sea products, used under medical supervision [7]. This practice is based on a well-defined framework designed to enhance health and address particular health conditions, involving the guidance of qualified healthcare professionals [5]. Widely adopted across Europe, thalassotherapy plays an essential role in both wellness and medical tourism [6]. In addition to traditional marine environments, hypersaline ecosystems, such as coastal lagoons, salt and soda lakes, and artificial salt-production ponds, known as saltpans, have therapeutic potential. These environments often exhibit mineral concentrations exceeding those of seawater, sometimes approaching full salt saturation [8]. Elements like Mg and Ca, commonly found in such brines, are known to improve skin hydration, strengthen the skin barrier, and reduce inflammation [9]. Manoharan and Kaliaperumal [10] investigated the diverse effects of salt on human skin, covering both its therapeutic benefits and possible adverse impacts, while reviewing the application of salt and saltwater in the treatment of various dermatological conditions. Carbajo and Maraver [11] suggested that the therapeutic value of saltwater is derived from the combined action of its mechanical, thermal, and chemical properties, which may explain its benefits in the recovery of rheumatic and dermatological conditions. Studies have demonstrated the therapeutic potential of saltwater baths across a range of health issues. Kim et al. [12] reported that saltwater immersion helped athletes recover faster from fatigue and decreased muscle damage after intense exercise. Emine and Gulbeyaz [13] observed improvements in the long-term quality of life among cancer patients. Beyond these benefits, saltwater baths have also shown promise in dermatological applications. Gambichler et al. [14] demonstrated that concentrated salts increased UV transmission through the epidermis, potentially improving the efficacy of phototherapy treatments. Similarly, saltwater exposure reduced the levels of antimicrobial peptides linked to psoriatic inflammation, suggesting an anti-inflammatory effect. Petersen et al. [15] found that saltwater reduced bathing-related pain, the need for analgesics, and symptoms of infection in children with epidermolysis bullosa, a rare genetic skin disorder. Cegolon et al. [1] highlighted that seawater pools, especially when combined with sunlight exposure, offer superior therapeutic effects compared to freshwater pools in treating inflammatory skin diseases.
Although various saline environments, such as seas, salt lakes, and salterns, have long been associated with health benefits, the scientific literature on their properties and therapeutic potential is still relatively scarce. In particular, there is a lack of systematic studies that explore the composition and assess the suitability of saline waters and salts for dermatological applications. Addressing this gap is essential to support the safe and effective integration of natural saline resources into evidence-based therapeutic practices. Therefore, the present study aims to (a) characterize the physicochemical parameters of five Portuguese saltpan waters, located in two major lagoons; (b) quantify total suspended solids (TSS); (c) compare water samples to the solid content; and (d) evaluate their potential application in thalassotherapy approaches, with a particular emphasis on skin-related health benefits. This assessment aims to support the development of evidence-based therapeutic strategies in dermatology and wellness practices using natural saline resources.

2. Materials and Methods

2.1. Areas of Interest

Fuzeta, Olhão, and Tavira saltpans are located in the Algarve basin, specifically in Ria Formosa, a triangular-shaped coastal lagoon in southern Portugal, stretching 55 km in length and 6 km in width along the coastline [16,17]. Geologically, the Algarve basin is primarily composed of Mesozoic sedimentary rocks, including limestones, marls, and sandstones, formed during the Triassic to Cretaceous periods [18]. According to this author, these saltpans are located on recent superficial Quaternary deposits, including alluvial sediments and lagoonal and tidal flat deposits influenced by marine and estuarine environments. Ria Formosa lagoon is separated from the Atlantic Ocean by two peninsulas, Ancão to the west and Cacela to the east, and a series of five barrier islands extending from west to east, Barreta, Culatra, Armona, Tavira, and Cabanas [19]. The lagoon is connected to the Atlantic through six natural and artificial inlets: Ancão, Faro-Olhão, Armona, Fuzeta, Tavira, and Lacèm. This connection facilitates the exchange of water, sediments, chemicals, and nutrients between the ocean and the marshes of the lagoon [20]. Gilão River is the source of the freshwater discharge into the lagoon [16]. Ria Formosa is characterized by vast salt marshes and tidal flats, covering more than 60% of the lagoon’s total area [17]. According to Bebianno [16], this area includes approximately 4000 hectares of salt extraction ponds.
Ria de Aveiro is a shallow coastal lagoon in northwest Portugal, with a length of ~45 km and a width of ~10 km. This lagoon lies within the Aveiro Sedimentary Basin, part of the Lusitanian Basin, dominated by Quaternary deposits, mainly alluvium, beach, and dune sands of varying thickness. Alluvial sediments prevail, consisting of silts (including micaceous and shelly varieties), as well as muddy and coarse sands overlying the substrate [21]. This lagoon receives water from both the Atlantic Ocean and the Vouga and Antuã rivers. It is separated from the ocean by a sandbar, except for an artificial channel that allows water exchange [22]. The lagoon features narrow channels and vast mudflats, where over 270 artificial saltpans were found, with most of them currently abandoned [21].
Both mesotidal lagoons have experienced water quality degradation over time, mainly due to anthropogenic activities related to economic development, such as chemical, fishing, and mariculture industries. This deterioration has contributed to the tidal movement of potentially toxic elements (PTEs) [23]. The lack of effective regulation over these activities in the recent past is evident in the quality of the lagoon waters, which are enriched in PTEs (e.g., Cr, Cu, Pb), even in areas distant from the pollution sources. Consequently, the saltpans and associated materials have been impacted by the contaminated waters.

2.2. Sampling, Samples Preparation, and Analysis

A total of five saline water samples were collected from artificial saltpans in Ria Formosa and Aveiro lagoons: four in Ria Formosa, one in Fuzeta, two in Tavira, one in Olhão, and one in Ria de Aveiro (Table 1). Sampling sites were selected based on the active salt production. Ria Formosa samples were collected from surface brines in active evaporation ponds. In contrast, the sample from Ria de Aveiro was collected in a more distant area of the crystallization pond. Tavira is the only saltpan with semi-industrial salt production, while the others operated using traditional techniques. Saltpan water samples were collected in dark glass bottles to minimize photodegradation and prevent alterations caused by sunlight during transport and storage.
Physicochemical parameters of the water samples, including pH, electrical conductivity (EC), temperature (T), dissolved oxygen (DO), and total dissolved solids (TDS), were measured in situ using a calibrated Hanna HI 98494 multiparameter probe (Hanna Instruments Inc., Woonsocket, RI, USA). In Fuzeta, these parameters were measured in the laboratory due to technical problems. At the laboratory, one liter of each of the five water samples was subjected to pressure filtration using 47 mm WhatmanTM quartz filters, until the flow ceased, to determine the total suspended solids (TSS). The filters were placed in Petri dishes and dried. TSS was calculated using the formula: TSS = [(CF + SM) − CF]/V, where CF is the mass of the clean filter, SM is the mass of suspended matter retained on the clean filter, and V is the volume of water filtered (1 L). One set of each saline water sample was collected and acidified to pH < 2 by adding ultrapure nitric acid (1% v/v HNO3) for cation analysis, using inductively coupled plasma mass spectrometry (ICP-MS Agilent 7700 Series; Agilent Technologies Inc., Santa Clara, CA, USA). The instrumental detection limits were 10 mg/kg for the major elements (Ca, K, Mg, and Na), 25 μg/kg for As, Ba, Sr, 1000 μg/kg for B, 50 μg/kg for Li, and 10 μg/kg for Mn, Mo, Rb, V and U. Following USEPA [24] and ASTM [25] methods, samples were kept cool at 4 °C, to prevent biological alteration until analysis within the next 24 h. To identify the particles retained on the filters, a scanning electron microscope (SEM), model Vega LMU (TESCAN Orsay Holding, a.s., Brno, Czech Republic), was employed. It operated under both high and low vacuum modes, with image acquisition through secondary and backscattered electron detectors. Morphology and size of individual particles were analyzed, and the semi-quantitative elemental composition was assessed using an energy dispersive spectrometer (EDS). Internal standards, certified reference materials, and quality control blanks were employed to monitor the precision and accuracy of the analyses, with results falling within the 95% confidence limit. All analyses were carried out at the GeoBioTec research unit, Department of Geosciences at the University of Aveiro.

3. Results and Discussion

The pH of the saline water samples ranged from 6.85 in Tavira (TW1) to 8.34 in Aveiro (AW), indicating neutral to slightly alkaline environments (Table 2). The higher pH observed in Aveiro was possibly related to the influence of sample collection, as it was from a more distant area of crystallization ponds than the other samples. Nevertheless, AW pH was similar to other studies in the Ria de Aveiro [26]. Overall, the samples’ pH was slightly higher than that reported for Dead Sea waters, ranging from 5.9 to 6.5 [27]. This difference is possibly related to the increased alkalinity resulting from the presence of bicarbonate and carbonate ions, which are typical of freshwater and seawater mixtures. Additionally, Kerr et al. [28] highlighted that organic alkalinity from dissolved organic matter can contribute significantly to total alkalinity and raise pH by influencing the carbonate system. Therefore, both inorganic ion composition and organic alkalinity likely explain the higher pH in these saline waters. Although some saline lakes reach extremely high pH levels (>9) [27], the pH values observed in these samples were lower, indicating minor alkaline conditions likely influenced by different geochemical and hydrological settings. All samples showed high EC values, ranging from 145 (AW) to 191.6 mS/cm (TW2). Almeida et al. [26] reported EC values ranging from 44.3 to 200 mS/cm in Ria de Aveiro, with the highest value observed in a sample collected during the summer period, in a saltpan pond with significantly higher salinity. According to the saline water classification proposed by FAO [29], the present study water samples fall into the brine category, once EC exceeds 45 mS/cm, reflecting an advanced stage of evaporation and high ionic concentration, typical of hypersaline environments. Temperature ranged from 23.8 °C (AW) to 37.8 °C (TW1), with the Algarve area known for having the highest temperatures in continental Portugal. According to Sauerheber and Heinz [30], EC exhibits a linear dependence on temperature within the range of 0 to 30 °C. Consistent with this behavior, a general trend of increasing conductivity with higher temperatures was observed: the samples from the Algarve presented the highest EC, while sample AW, corresponding to a lower temperature, showed the lowest EC. Water dissolved oxygen (DO) content is influenced by factors such as salinity, temperature, and eutrophication [31]. It is well established that DO can decrease due to different factors, including rapid fluctuations in temperature and salinity, organic matter respiration, and nutrient inputs [32]. Sample DO ranged from 34.0 (TW1) to 108.7% (FW). The sample collected in Aveiro (AW) exhibited low DO, consistent with Lopes et al. [33], who reported that DO concentrations tend to reach minimum values in late summer. Almeida et al. [26] described a seasonal DO variation, with similar conclusions. All samples revealed very high salinity levels (>70 g/kg), as expected given the saline environment, associated with salt production. Considering this high salinity, these waters can be classified as brines, since salt content exceeded that of seawater (35–100 g/L [34]. Total dissolved solids (TDS) ranged from 72.5 (AW) to 95.8 g/L (TW2), with the highest content found in samples from Algarve saltpans. Total suspended solids (TSS) content ranged from 1.51 (AW) to 4.31 g/L (OW), showing higher and relatively consistent values across Ria Formosa saltpans. TDS and TSS results, obtained with different methods, are consistent, reflecting the sampling conditions: in Ria Formosa, samples were collected from surface brines in active evaporation ponds, influenced by crystallization and salt resuspension, whereas the Ria de Aveiro sample was collected at a location distant from the crystallization pond, leading to lower concentrations. Saltpan water TSS particles were more abundant in sample OW, accounting for 21.7% of the total TSS. Other samples ranged from 7.6 (AW) to 18.2% (TW1), highlighting the higher abundance of suspended particles in the southern saltpans. In both regions, geology is characterized by alluvial deposits that may influence water TSS content, with the loam layer at the bottom of the salt ponds potentially enhancing particle suspension. These results align with findings by Almeida et al. [26] in the Ria de Aveiro, where higher TSS levels were associated with salt-rich waters collected during summer.
Waters Ca concentration ranged from 152 to 1782 mg/kg, K 1794 to 11,073 mg/kg, Mg 5760 to 37,591 mg/kg, and Na 48,791 to 88,875 mg/kg (Table 3). As expected in these types of saline environments, Na was the dominant element, showing the highest concentration. The lowest chemical content was observed in the water sample from Ria de Aveiro, except for Ca, which presented the highest concentration, consistent with the highest pH value observed at that site. Results highlighted a clear distinction between the two sampling areas, reflected in their chemical compositions. The higher Ca concentration and pH observed in Ria de Aveiro can be directly related to the local geological setting. The area is dominated by Quaternary alluvial deposits, including silts with shell fragments and sandy layers, which may enhance carbonate dissolution and contribute to Ca enrichment. Similarly, the occurrence of micaceous silts and feldspathic minerals could explain the presence of K and Mg. At the same time, the overall dominance of Na reflected the saline influence characteristic of the environment. Taking into account the salinity scale, the concentration of major elements in the samples was substantially higher than the reference values reported by UNESCO [35] and Millero et al. [36], of 413 mg/kg Ca, 399 mg/kg K, 1280 mg/kg Mg, and 10,780 mg/kg Na, suggesting a markedly hypersaline and mineral-rich character of these waters.
Elements B, Li, Rb, and Sr showed concentrations ranging from 11,272 to 71,009 μg/kg, 389 to 2307 μg/kg, 409 to 2740 μg/kg, and 10,432 to 30,499 μg/kg, respectively (Table 4). The lowest B, Li, and Rb content was found in Ria de Aveiro, possibly due to the predominance of alluvial and carbonate-rich deposits, which favor Ca release over trace elements. The relatively higher pH at this site may further promote adsorption or precipitation processes, reducing the mobility and availability of these elements in the water column. Additionally, freshwater input from rivers likely contributed to dilution, contrasting with the more substantial marine influence observed in the southern sites, where higher concentrations were recorded. The Li concentration in the saline waters exceeded the typical values reported for both fresh water (~0.07 to 40 μg/kg) and seawater (~170 to 190 μg/kg) [37], indicating significant enrichment during the evaporation process. The concentration of As, Mn, and Mo was generally lower, with minimum values also recorded in Ria de Aveiro, ranging from 28 to 85 μg/kg, 130 to 659 μg/kg, and 12 to 94 μg/kg, respectively. Other elements, such as Ba, V, and U, were below the detection limit in some samples, with concentrations ranging from 33 to 70 μg/kg, 11 to 14 μg/kg, and 16 to 28 μg/kg, respectively. The As, Mn, U, and V are metal(loid)s commonly found in the environment and diet. While small amounts are reported as essential for maintaining good health, higher concentrations can become toxic or harmful [38]. Zhang et al. [39] reported that in saline waters and brines from China, trace metal concentrations increase with salinity due to intense evaporation in semi-arid to arid environments. This is consistent with observations at Ria de Aveiro, where the lowest EC corresponded to lower trace element concentrations. Although the European Union didn’t establish guideline limits for element content in waters to be used for dermal applications, Health Canada [40] specified the need for the absence of As, considered an impurity in cosmetic products. The presence of Mo in the samples, according to the WHO [41] guidelines for drinking-water quality, is classified as highly soluble in natural waters under alkaline conditions. This emphasizes that natural processes do not readily remove Mo, and therefore, it tends to persist in the water column, particularly in saline and hypersaline environments.
To investigate the composition of total suspended solids (TSS) recovered from saltpan waters in membranes, the four samples collected in Ria Formosa, and one collected in Ria de Aveiro, were analyzed by SEM-EDS. Figure S1 presents a blank quartz filter, analyzed with the same technique, for comparison. In sample FW, collected in Ria Formosa, gypsum (CaSO4·2H2O) was the dominant mineral phase in multiple analyzed particles (Figure 1a; e.g., 1–4, 6–8). Gypsum forms via evaporation of calcium- and sulfate-rich waters, common in saltpan environments, and often precipitates early in the evaporitic sequence [42]. Halite (NaCl) was clearly identified (5 in Figure 1a), with a high concentration of Na (~28%) and Cl (~43%). These nearly pure salt crystals represent the final stage of brine evaporation. Some of the analyzed particles’ chemical content, with relatively high Si and O (i.e., 3, 6, 8), suggested the presence of amorphous Si or diatom frustules, which are common in saltpans and coastal hypersaline wetlands [43,44]. Mg-rich silicates (e.g., kieserite MgSO4·H2O) were also identified in sample FW (1 in Figure 1b), a common secondary mineral in evaporitic environments [45]. At the same time, particle 2, with a whiter color, showed the chemical composition and spectrum of halite. In particle 3, the most representative elements were Si, Mg, and Cl, possibly representing a salt–clay aggregate formed by the incorporation of clay minerals, diatoms, organic residues, and/or weathered debris, reflecting saltpan evaporation and sedimentation processes [46]. Particle 4 (Figure 1b) revealed a K-rich salt phase with a composition typical of sylvite (KCl), a mineral that forms in later stages of brine evaporation, often co-precipitating with halite or gypsum in saltpans. The presence of sylvite suggested advanced evaporitic content, potentially near the halite saturation limit, a common condition in crystallizer ponds [47].
Sample OW, collected in Ria Formosa, showed a mineralogical composition partially similar to FW. Still, there were variations in salt purity, sulphate content, and organic–silicate associations (Figure 2). Halite was the predominant mineral with numerous particles (2, 4–7, 9–10; Figure 2a) exceeding 50% Cl and 30–36% Na, consistent with final-stage brine evaporation and high-purity salt crystallization, typical of salt harvested from crystallizer ponds or natural halite crusts [48]. Other particles (1, 3, 8) showed the co-presence of Ca and S, suggesting gypsum often forms earlier in the evaporitic sequence. Particle 1 presented high Ca and S along with substantial C and O, suggesting a biological influence in gypsum precipitation [49]. Comparatively, Figure 2a TSS showed fewer silica-rich particles than FW, likely due to lower biogenic input or different hydrological or microbial activity. Organic-rich mixed particles (e.g., 3, 8, 9) supported this environmental variation, possibly linked to different basin depths, microbial activity, or sediment dynamics. The distinctive particle 1 (Figure 2b) and inclusion showed a mixture of halite, organic C, and Si, suggesting a salt–organic–silicate aggregate. This aggregate is likely formed through the co-precipitation of brine salts with microbial remains, a common feature in saltpan water TSS material. The presence of minor P and Al supported a potential environmental contamination or detrital incorporation. Particle 2 composition, characterized by high Ca and S, was consistent with gypsum. The significant content of C and O suggests precipitation in shallow zones where biofilm activity was high, consistent with the deep artificial saltpans. The dominant halite in this sample (particle 3) contained more than 55% Cl and more than 35% Na, characteristic of high-purity halite crystals typically formed from late-stage evaporation in hypersaline ponds or crystallizers.
Regarding sample TW1, collected in Ria Formosa, evaporitic minerals found were halite and gypsum. Particles 10 and 16 (Figure 3a) showed high Cl (>47%) and Na (>33%), with typical halite spectra, occasionally hosting organic and/or siliceous inclusions, with organic matter usually becoming entrapped during the mineral formation [50,51]. Gypsum (particles 1, 2, 4, 5, 6, 7, 8, 9, and 11) were identified by dominant Ca and S content, with minor C and Mg/Si, indicating microbial interaction during precipitation, a process observed in other salted areas where gypsum crystals nucleate on microbial mats [50]. Silicate and organic aggregates were identified (e.g., 15), showing high C (>40%) and moderate Si (~10%), typical of diatomaceous or EPS-rich aggregates [52]. Similar biogenic-silicate mixtures were identified in microbial mats in hypersaline ponds [50,53]. Particles 3 and 17, containing ~30% Fe, along with Cl and Si, have similar Fe oxides/hydroxides mixed with salts and silicates, suggesting atmospheric or anthropogenic contamination, which is common in open-air evaporation facilities. Furthermore, points 10, 12, 13, and 14, corresponding to a porous fractured particle, correspond to an aggregate of hydrated K-Mg Cl-sulfate. A loss of water in the matrix generated a volumetric contraction, expressed externally as an anastomosing array of shrinkage cracks. The chemical content and morphology suggested that it was probably a kainite (KMgSO4Cl·3H2O) and/or carnallite (KMgCl3·6H2O), with minor gypsum. Water sample TSS TW1 showed a mixture of evaporitic minerals and environmental inclusions, highlighting how biological activities and environmental pollutants can impact saltpan, and that halite can preserve organic materials and environmental data [54]. In Figure 3b, gypsum (1, 2, 5) was clearly identified, particles composed of high concentrations of Ca (16–22 wt%), S (14–18 wt%), and O (37–42 wt%), being one of the earliest minerals to precipitate from the evaporating brine. The carbon (15–20 wt%) content suggested residual organic association or co-precipitation with extracellular polymeric substances [49,50]. Organic–silicate aggregates (3), high carbon (39 wt%), and significant silicon (8.5 wt%) suggest a biogenic silicate aggregate, likely with diatom fragments or microbial debris. The Cl and Na content indicated a halite partial layer. Point 4 analysis revealed higher C content, with minor Cl and Na (halite-related) and Si, indicating a biological matrix associated with salt entrapment. Similar particles were commonly observed in biofilms and microbial mats in saltpan environments, forming cohesive items that trap detrital and mineral phases [53]. Particles (Figure 3b) showed varying degrees of C (15–59%), indicative of crucial biological input, being the association of gypsum or halite with these organic and siliceous phases, suggesting that the saltpan was subjected to active microbial processes during precipitation, allowing both inorganic salts and organic matter to co-crystallize into the particulate record. Results showed that these evaporitic environments are not merely abiotic salt-forming systems; they are dynamic, biologically active systems.
Sample TW2, collected in Ria Formosa, SEM-EDS analysis also identified gypsum as the prevalent evaporite phase on the water TSS (Figure 4a). The organic C (13–16 wt%) suggested precipitation in microbial biofilms, as nucleation for gypsum crystals. Similar microbial mediation was identified in a saltwork in Dhahban, where gypsum formed on cyanobacterial mats and microbial layers [50]. Halite (particle 2) from late-stage brine evaporation was identified, with the absence suggesting a pure, abiotic crystallization, typical of salt from shallow crystallizer ponds. Particle 3 composition suggested an aluminosilicate clay mineral, likely illite or similar, possibly derived from aeolian dust or terrigenous input from the base of the saltpan shallow pond, common detrital minerals in exposed saltpan environments. The presence of Mg together with gypsum (point 4) may indicate Mg-rich silicates in combination with sulfate precipitation, reflecting mixed precipitation dynamics in brine influenced by evaporitic chemistry and microbial or sedimentary inclusions. High C and silica content, within lightly encrusted salt (point 5), of biological-salt co-precipitation is typical of microbially active evaporitic systems. In Figure 3b, gypsum was identified with microbial association (1, 4), by the Ca, S, and O content in association with ~11–13 wt% C, suggesting co-precipitation within or on microbial biofilms, entrapping organic matter [55]. Also, halite was identified (point 2, Cl 47%, Na 29%), which is a composition typical of pure halite in the final stage of brine evaporation. The absence of C indicates abiotic crystallization, characteristic of high-salinity pond environments. In particle 3, Si and Al content suggested an aluminosilicate clay, possibly from aeolian or soil-derived dust, incorporated into the saltpan water column during evaporation processes. The combination of C (44%) and Si (13%) in point 5, with minor Cl, suggested organic–silicate particles, such as diatom aggregates coated by evaporitic minerals, reflecting microbial activity [56,57]. Collectively, the TW2 water TSS sample revealed a biogeochemically diverse saltpan environment with evaporitic mineral precipitation (gypsum, halite), microbial involvement in crystal formation, and external particulate input (clays, detritus), aligning with studies showing that gypsum and halite both share and register environmental and biological information during formation [55,58].
Water TSS recovered from sample AW, collected in Ria de Aveiro, revealed a similar composition to the Ria Formosa water samples. The composition of evaporitic halite crystals (1, 7; Figure 5a) showed that precipitation occurred during the final stages of brine evaporation. Minor C, in particle 7, indicates organic entrapment, usual in microbial-rich saltpan settings [49,59]. Gypsum with microbial influence (2, 5, 8, 10), with high Ca-S-O ratios, suggested formation with embedded C and Mg-silicate, due to microbial or clay-mediated precipitation. Biogenic–organic aggregates (3, 9), with C 43 to 51 wt%, and Si ~13 wt%, suggested diatom or EPS organic aggregates. Clays from sediments (point 4), with Si-Al composition, aligned with clay minerals derived from saltpan pond or wind-blown dust, similar to previous samples. Also, Fe-Si contaminants (point 6) were found, suggesting Fe-oxide particulates of atmospheric or anthropogenic origin. In Figure 5b, the same composition was identified, with halite-dominated particles (1, 3, 8, 13, 17, 22, 26). Detectable C in some of the analyzed points suggested microbial entrapment, as salt can act as a microscale archive for microbial life in saline environments. Gypsum-rich points (2, 7, 9, 14, 18, 23, 27) were also found, with occasional organic or Mg-silicate content, aligning with gypsum crystallization on microbial biofilms [53]. C-rich and Si-bearing particles (5, 10, 15, 20, 24, 28) reflected diatom aggregates entrapped by evaporitic salts [60]. Also, clay and terrigenous inputs were identified (4, 11, 19), suggesting allochthonous phyllosilicate. Additionally, Fe-rich particles (6, 12, 16, 21, 25) suggested atmospheric or anthropogenic dust inclusion, incorporated into evaporitic and particulate matrices, with a possible origin in aerosol transport. Sample AW composition revealed that water TSS comprises a complex assemblage that includes evaporitic minerals (halite and gypsum), organic-enriched gypsum and halite, biogenic aggregates, clay detritus, and anthropogenic Fe particulates. This composition reflected interactions between abiotic evaporation, microbial geochemistry, and environmental deposition. The modifications in salt composition and particulate inclusions have implications for salt quality and, consequently, for pelotherapy applications.

4. Potential Uses in Thalassotherapy and Skin Care

The mineral-rich nature of the studied saline waters, characterized by elevated levels of Na+, Mg2+, K+, and Ca2+, closely resembled the major ion composition of therapeutic waters widely employed in thalassotherapy. Dead Sea brines, for instance, are commonly used in therapeutic spas due to their high concentrations of Mg and K, which contribute to anti-inflammatory, detoxifying, and skin-repairing effects [61]. These studied Portuguese mineral-rich brines, due to their characteristics, can be used in a variety of therapeutic applications, including mineral baths, body wraps, facial treatments, and the formulation of cosmeceuticals such as mineral-enriched creams, exfoliants, and bath salts.
Similar perspectives have been discussed by Almeida et al. [26], who highlighted the potential of Portuguese mineral and saline waters for therapeutic and spa applications, emphasizing their relevance to sustainable wellness tourism and dermocosmetic innovation. Furthermore, a study conducted by Andrade et al. [62] compared the therapeutic effects of aquatic exercises performed in seawater with those performed in a standard chlorinated pool among women diagnosed with fibromyalgia. The study demonstrated that, although both groups experienced improvements in pain, flexibility, and overall quality of life, participants who exercised in seawater showed significantly greater improvements in pain intensity, sleep quality, and emotional well-being. These effects were attributed to the high content of Mg, K, Ca, and Br in seawater, which enhances relaxation, circulation, and anti-inflammatory responses. However, the presence of trace elements requires rigorous monitoring to ensure safety, particularly when these waters are incorporated into dermocosmetic products.

5. Conclusions

The saline waters from the studied coastal lagoons in Portugal were classified as mineral brines, enriched in Na, Mg, K, and Ca, with regional geochemical differences possibly linked to geological settings. High total dissolved and suspended solids confirmed intense evaporitic conditions with suspended matter dominated by evaporitic minerals (halite, gypsum, sylvite, kainite) and biogenic aggregates, indicating coupled physicochemical and microbial processes. The chemical and mineralogical composition of these saline waters suggested their therapeutic potential for thalassotherapy and dermatological applications. The high salinity and mineral richness make these waters highly suitable for different therapeutic and cosmetic applications. Magnesium and potassium, for instance, are known for their anti-inflammatory and skin barrier-repair properties, making the brines potentially effective in treating skin conditions. These waters could also serve as active ingredients in the formulation of mineral-rich cosmetic products, such as therapeutic bath salts and exfoliants. The presence of trace elements, such as As, Mn, and Mo, reinforced the need for careful monitoring to ensure safety when used as dermocosmetic products. This work provided a scientific basis for the sustainable use of Portuguese saline waters in health and wellness strategies, contributing to the valorization of local natural resources and the development of health tourism.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17203021/s1, Figure S1. Blank quartz filter image (SEM), with fibers analyzed by EDS, and SiO2 spectrum, similar to those obtained for all of the fibers. Image for comparison with the filters with TSS deposited.

Author Contributions

L.A.: sampling, formal analysis, writing—original draft; F.R.: funding, supervision, writing—review and editing. C.C.: sampling, conceptualization, methodology, formal analysis, supervision, writing—original draft, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

Lara Almeida is thankful for the FCT PhD funding (2023.01752.BDANA; https://doi.org/10.54499/2023.01752.BDANA). All authors are grateful for the support of GeoBioTec (UIDB/04035), financed by national funds through the FCT/MCTES.

Data Availability Statement

Data used is available within the manuscript.

Conflicts of Interest

The authors declare no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Water 17 03021 g001
Figure 1. SEM-EDS image of sample FW, with spectra of the main minerals identified with numbers: (a) mainly gypsum particles; and (b) kieserite and sylvite particles.
Figure 1. SEM-EDS image of sample FW, with spectra of the main minerals identified with numbers: (a) mainly gypsum particles; and (b) kieserite and sylvite particles.
Water 17 03021 g001
Water 17 03021 g002
Figure 2. SEM-EDS image of sample OW, with spectra of the main minerals identified with numbers: (a) mainly halite particles, and (b) detail of halite and gypsum particles.
Figure 2. SEM-EDS image of sample OW, with spectra of the main minerals identified with numbers: (a) mainly halite particles, and (b) detail of halite and gypsum particles.
Water 17 03021 g002
Water 17 03021 g003
Figure 3. SEM-EDS image of sample TW1, with spectra of the main minerals identified with numbers.
Figure 3. SEM-EDS image of sample TW1, with spectra of the main minerals identified with numbers.
Water 17 03021 g003
Water 17 03021 g004
Figure 4. SEM-EDS image of sample TW2, with spectra of the main minerals identified.
Figure 4. SEM-EDS image of sample TW2, with spectra of the main minerals identified.
Water 17 03021 g004
Water 17 03021 g005
Figure 5. SEM-EDS image of sample AW, with spectra of the main minerals identified with numbers.
Figure 5. SEM-EDS image of sample AW, with spectra of the main minerals identified with numbers.
Water 17 03021 g005
Table 1. Saltpan water sample location and description.
Table 1. Saltpan water sample location and description.
IDLocationSaltpanSalt ProductionCollection
FWRia FormosaFuzetaTraditionalJuly 2024
OWOlhãoTraditional
TW1TaviraSemi-industrial
TW2
AWRia de AveiroAveiroTraditionalSeptember 2024
Table 2. Physicochemical parameters measured, namely pH, electrical conductivity (EC; in mS/m), temperature (T; in °C), dissolved oxygen (DO; in %), total dissolved solids (TDS; in g/L), and total suspended solids (TSS; in g/L).
Table 2. Physicochemical parameters measured, namely pH, electrical conductivity (EC; in mS/m), temperature (T; in °C), dissolved oxygen (DO; in %), total dissolved solids (TDS; in g/L), and total suspended solids (TSS; in g/L).
IDpHECTDOTDSTSS
FW7.28189.627.1108.794.83.55
OW6.94155.329.963.377.74.31
TW16.85165.637.834.082.83.63
TW27.19191.632.169.395.83.20
AW8.34145.023.851.172.51.51
Table 3. Major elements of saline water samples (mg/kg).
Table 3. Major elements of saline water samples (mg/kg).
IDCaKMgNa
FW254825326,74773,397
OW15211,07337,59151,490
T1W18510,92334,32659,967
T2W334646118,72788,875
AW17821794576048,791
Table 4. Minor and trace elements of saline water samples (μg/kg).
Table 4. Minor and trace elements of saline water samples (μg/kg).
IDAsBBaLiMnMoRbSrVU
FW2850,114bdl153813034180321,1111116
OW8571,009bdl230762653274010,4321428
TW17366,250bdl210765940256013,9841222
TW23240,63433113320394148130,4991127
AWbdl11,272703891841240926,181bdlbdl
Note: bdl—below detection limit.
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Almeida, L.; Rocha, F.; Candeias, C. Mineral-Rich Brines from Portuguese Coastal Lagoons: Insights into Their Use in Thalassotherapy and Skin Care. Water 2025, 17, 3021. https://doi.org/10.3390/w17203021

AMA Style

Almeida L, Rocha F, Candeias C. Mineral-Rich Brines from Portuguese Coastal Lagoons: Insights into Their Use in Thalassotherapy and Skin Care. Water. 2025; 17(20):3021. https://doi.org/10.3390/w17203021

Chicago/Turabian Style

Almeida, Lara, Fernando Rocha, and Carla Candeias. 2025. "Mineral-Rich Brines from Portuguese Coastal Lagoons: Insights into Their Use in Thalassotherapy and Skin Care" Water 17, no. 20: 3021. https://doi.org/10.3390/w17203021

APA Style

Almeida, L., Rocha, F., & Candeias, C. (2025). Mineral-Rich Brines from Portuguese Coastal Lagoons: Insights into Their Use in Thalassotherapy and Skin Care. Water, 17(20), 3021. https://doi.org/10.3390/w17203021

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