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Article

Lake Water Composition in Oceanic Islands: Insights from REE Content and 87Sr/86Sr Isotopic Ratio

by
José Virgílio Cruz
1,2,*,
César Andrade
1,
Letícia Ferreira
1 and
Fátima Viveiros
1,2
1
IVAR—Instituto de Investigação em Vulcanologia e Avaliação de Riscos, Universidade dos Açores, 9500-321 Ponta Delgada, Portugal
2
FCT—Faculdade de Ciências e Tecnologia, Universidade dos Açores, 9500-321 Ponta Delgada, Portugal
*
Author to whom correspondence should be addressed.
Water 2025, 17(13), 1849; https://doi.org/10.3390/w17131849 (registering DOI)
Submission received: 15 May 2025 / Revised: 15 June 2025 / Accepted: 17 June 2025 / Published: 21 June 2025
(This article belongs to the Section Hydrology)
Browse Figures
Versions Notes

Abstract

:
A study was carried out with a representative data set of volcanic lakes from the Azores archipelago. A total of 672 samples were collected during four field surveys conducted over the year and along the depth. Following water sampling, temperature, pH, and EC were measured, the dissolved CO2 and alkalinity were determined by titration, and aliquots were taken to perform analysis of major, minor and trace elements, as well as 18O/16O, 2H/1H and 87Sr/86Sr isotopic ratios. Waters are of meteoric origin and from the Na-HCO3 to Na-Cl types. The 87Sr/86Sr ranges between 0.709194 and 0.704294, and most of the lakes depict less radiogenic values than seawater, suggesting a potential contribution from rock dissolution. Along the reciprocal of the Sr vs. 87Sr/86Sr plot, most samples suggest a linear trend between rock values and rainwater. Samples display considerable variability in the ∑REE, ranging from 0.83 µg L−1 to 13.54 µg L−1, and when chondrite normalized, depict a negative slope, showing an enrichment in light REEs compared to heavy REEs. This pattern is consistent with the one from Azores rocks and bottom sediments from some lakes, and most lakes depict Eu anomalies, resulting from interaction between water and sediments or from incongruent mineral dissolution.

1. Introduction

Water composition in lake water bodies results from the interaction of several drivers, such as the morphology of the water bodies and their geological setting, the climatic characteristic of the area where the lake is located, the prevailing biological processes, as well as the several inputs and outputs of water into or draining out of a lake [1,2,3,4]. In active volcanic areas, water composition in lakes may also be influenced by the interaction with magmatic systems [5,6]. Moreover, anthropogenic activities may impact lake water quality [7], and eutrophication can be recognized as a global issue [8].
Research on lake water chemistry has been conducted worldwide, including in volcanic oceanic island environments, with studies focused on major ion composition [9,10,11,12,13], minor and trace element composition [14,15,16,17,18,19], isotopic content [20,21], dissolved organic matter [22], as well as bottom sediment characterization [23].
Rare earth elements (REEs) are lanthanides with similar physicochemical properties (e.g., [24]), due to similar ionic radii, have been typically analyzed as a group. These elements are sensitive tracers of fundamental geochemical processes [25,26], and their broad range of applications include hydrogeochemical processes, such as groundwater flow tracing [27,28,29,30], identification of weathering processes, and environmental studies [31]. Due to the differences in a variety of environmental factors, REEs can fractionate relative to each other resulting in distinct fractionation patterns during geological processes [32]. REEs are strongly controlled directly, or indirectly, by pH, redox conditions, and aqueous complexation, and their concentration and fractionation patterns will vary along groundwater flow paths, as these factors change (e.g., [28,33]). REE content in lake water bodies have been characterized in several studies [34,35,36,37,38,39], some of which in lakes located in volcanic areas [40].
Strontium isotopes have also proven to be an important tool in hydrology, particularly when 87Sr/86Sr ratios are used in combination with other chemical and isotope data to identify water sources (e.g., [41,42,43]. The observed range of dissolved Sr isotopic ratios is controlled by several factors, such as variations in initial (atmospheric) inputs, differences in mineral composition along flow paths, mineral dissolution dynamics, and residence time (e.g., [42]). Strontium isotopes are useful for tracing the origin and evolution of natural waters, as well as for distinguishing mixing processes between sources with distinct isotopic signatures (e.g., [42,44]. The 87Sr/86Sr ratio has also proven to be a useful tool in studies addressing lake water bodies [35,45,46].
The present study was developed in the Azores, an archipelago made of nine volcanic islands, lying in the North Atlantic Ocean between latitudes 36°55′ N to 39°43′ N and longitudes 25°00′ W to 31°16′ W. The main objective of the present study is to present an outline of lake chemistry over a representative dataset made of 13 water bodies spread in the Azores, providing a new insight over main processes that control water composition. For that purpose, the specific objectives of the study are to provide a comprehensive analysis of both the REE content and 87Sr/86Sr isotopic ratio in the Azores lakes (Figure 1), allowing us to evaluate the greater or lesser contribution of rock dissolution to the composition of the water. Despite previous studies on lake water composition in the Azores archipelago, the present paper is the first paper addressing REE and 87Sr/86Sr, as so far, the focus was given mainly to major ion or nutrient (N and P) content.

2. Geological and Hydrological Setting

2.1. Geological Setting

The Azores archipelago lies in the vicinity of the triple junction between the American, Eurasia, and Nubian plates, corresponding to a complex geodynamic setting [47,48]. As a result, the archipelago records a remarkable seismic and volcanic activity, that has been described since historical times following the Azores settlement in the 15th century [49]. Since the settlement, 28 volcanic eruptions took place in the Azores area, both submarine and subaerial events, being the last eruption recorded in 1998–2000 [50,51].
As all the islands are of volcanic nature, despite that in Santa Maria a volcano–sedimentary complex, also known as Touril Complex, is interbedded between the older and younger volcanic complexes in the Island [52], eruptive activity in the islands encompasses effusive to highly explosive eruptions, resulting in diverse volcanic forms across the archipelago: some islands, such as Santa Maria, Pico, and São Jorge, have evolved through Hawaiian and Strombolian eruptions [47,53,54,55,56], associated with basaltic magmas; instead in other islands, like São Miguel, Terceira, Faial, Graciosa, Flores, and Corvo, more evolved volcanic rocks, associated with acid magmas, are also observed, thus resulting in higher explosive eruptions [47,57,58,59,60,61,62,63], being the latter responsible by the large calderas that dominate the top of some central volcanoes in those islands. The oldest subaerial volcanic formations in the archipelago were dated at Santa Maria Island from 8.12 ± 0.85 Ma [64], an age reinterpreted by [52] to 5.70 ± 0.08 Ma. In contrast, Pico Island is the most recent island in the archipelago, with the oldest rocks dating from 300,000 years ago [65].
The studied lakes are associated with a wide range of volcanic forms, encompassing maar craters and large calderas (Table 1; Figure 1). Lakes from Pico Island are not associated with volcanic depressions.
Lake water bodies lying in caldera floors are in São Miguel and Corvo islands. The ones at São Miguel are associated with the three active central volcanoes (Sete Cidades, Fogo, and Furnas) that dominate the geology of the Island, lying in the floor of large Quaternary calderas at the summit of these composite volcanic centers [47,61,63,66,67].
The Sete Cidades caldera is a roughly circular-shaped depression with about 5 km diameter, and walls as high as 300 m, formed following three main phases of collapses, dated from 36,000 years to 16,000 years [68]. Fogo summit caldera is also roughly circular, presenting a maximum diameter of about 3.2 km with walls as high as 370 m [69]. The caldera complex at the summit of Furnas Volcano resulted from several stages of caldera collapse [61], with an older caldera enveloping a younger roughly circular-shaped caldera with a diameter of about 5 km.
The two lakes at Corvo (so-called Caldeirão lakes) are located close to each other in the floor of the 2.3 times 1.8 km across elliptical caldera that dominates the summit of a central volcano [47,70,71,72], locally known as Caldeirão. In the floor of the depression, with a depth of about 300 m, post-caldera scoria and spatter cones are observed [73], among which both lakes occur. The most recent lake sediments are dated at approximately 4.2 k yr cal BP [74].
Amongst lakes lying in maar craters, two are in São Miguel Island: Congro maar corresponds to a roughly circular shape depression, with steep walls and presenting a diameter of about 500 m, formed along an eruptive event from 3800 ± 400 years BP [75]. Instead, the maar of Santiago Lake corresponds to a crater of roughly elliptical shape, 1.1 km times 0.8 km across, limited by steep walls as high as 210 m, formed by a trachytic explosive eruption dated from 2659 ± 55 years [76].
Maar-lying lakes at Flores Island are all located in the so-called Caldeira das Sete Lagoas, a central upland area, above 500 m altitude, about 7 km by 4 km across. In this area, two groups of maar craters are found [77,78]: Comprida Volcanic System, to the north, and Funda Volcanic System, to the south.
The Comprida and Negra lakes are both located along the Comprida Volcanic System, the former corresponding to a 700 m by 300 m across elongated maar with steep-sided walls as high as ~30 m above water level, and the latter corresponding to a roughly 600 m diameter circular maar, being formed following a volcanic eruption dated from 3180 cal yr BP [78]. Instead, Funda Lake lies at the Funda Volcanic System, corresponding to a roughly 1200 m by 800 m across elliptical maar, in which steep-sided walls are as high as 200 m above water level. This structure was formed through an eruption dated from 3250 cal yr BP [77].

2.2. Hydrological Setting

In the Azores archipelago, the mean annual actual evapotranspiration (581 mm yr−1) is well below the mean annual precipitation, estimated as about 1930 mm/a, the latter being in the range from 966 mm yr−1 (Graciosa Island) to 2647 mm yr−1 (Flores Island) [79]. Surface runoff is also highly variable comparing the several islands, being on the range between 1.30 × 107 m3 yr−1 (Corvo Island) and 2.77 × 108 m3 yr−1 (Pico Island), totaling an annual volume of about 3.22 × 108 m3 yr−1 [79]. This runoff is mainly of torrential nature, and permanent rivers are scarce and only observed at the islands of Santa Maria, São Miguel, Faial, and Flores.
Table 1. Geological and hydrological description of the studied lakes based on some selected features (data from [80,81,82,83,84,85,86,87,88]; geological setting from [89]) (Td—tectonic depression; Ud—undifferentiated depression).
Table 1. Geological and hydrological description of the studied lakes based on some selected features (data from [80,81,82,83,84,85,86,87,88]; geological setting from [89]) (Td—tectonic depression; Ud—undifferentiated depression).
IslandLakeAlt.Basin AreaSurf. AreaMax. DepthVolumeResidence TimeGeol.
Setting
Mkm2mm3years
São
Miguel		Congro4200.340.04022.0~5.8 × 1057.3maar
Santiago3640.800.25033.0~5.3 × 10624.5maar
Sete Cidades Azul261 3.58029.5~47.4 × 1062.4Caldera
Sete Cidades Verde 0.81024.5~10.6 × 1063.0Caldera
Fogo5805.041.53031.6~23.4 × 1065.8Caldera
Furnas28012.21.87015~14.0 × 1061.2Caldera
PicoCapitão7810.150.0244.9~5.4 × 1041.9Td
Caiado8020.190.0586.9~1.5 × 1053.8Ud
TerceiraAlgar do Carvão5000.190.00038.2~1.5 × 105n.a.Lava cave
GraciosaFurna do Enxofre920.150.0129.3~5.4 × 104n.a.Lava cave
FloresComprida5300.500.05817.3~4.5 × 1051.7maar
Negra5400.290.130119.0~9.1 × 10617.0maar
Funda3653.140.36935.7~7.9 × 1062.7maar
CorvoCaldeirão East3980.970.0811.9~9.0 × 104n.a.Caldera
Caldeirão West3922.310.1242.8~16.3 × 104n.a.Caldera
About 0.41% of the inland surface area in the archipelago is occupied by 88 lake water bodies, spread at the islands of São Miguel, Terceira, Pico, Flores, and Corvo islands, corresponding to a total water storage of ~1.1 × 108 m3 [79,90,91]. Due to the volcanic nature of the Azores, those lakes are mainly associated with volcanic landforms, as 50% are associated with explosive craters and 16.7% occupy the floor of subsidence calderas [89]. Additionally, two other lakes are located inside volcanic caves, namely in the islands of Terceira and Graciosa [89].
Lake water chemistry has been studied in the last decades allowing to provide a general characterization of the water composition [13,89,92,93]. Due to the volcanic nature of the Azores, particular attention has been given to diffusive CO2 emissions from lakes spread in the archipelago lakes [80,81,82,83,84,85,86], allowing to compute a total flux of about 1.71 × 105 t d−1, from which 42% is of volcanic origin (~7.1 × 104 t d−1; [94]).
Agricultural activities have a negative impact over surface and groundwater quality in the Azores [95,96,97]. As a result, lake water body eutrophication has been a major concern in the Azores since the early studies conducted on the subject [98,99], and since this seminal work where several contributions were made to characterize impacts over quality, as well as monitoring and control measures being taken in the meantime [87,88,100,101,102].
Considering the set of lakes studied, 69.2% are in a eutrophic state (Congro, Santiago, Sete Cidades Verde, Furnas, Capitão, Negra, Funda, Caldeirão East, Caldeirão West), while the remaining are mesotrophic (Sete Cidades Azul, Fogo, Comprida and Caiado), with the exception of the water bodies inside lava caves, to which no data are available [103]. Most of the lakes in the dataset (46%) have a poor status according to the EU Water Framework Directive (WFD) mainly due to eutrophication [103]. Two of the lakes are in moderate status (Caldeirão East and Sete Cidades Azul), while Funda Lake is in bad status. In contrast, 23% of the lakes studied are in good status according to the WFD criteria (Fogo, Caiado, and Comprida).
The studied lakes are located at altitudes between 92 m and 802 m (mean = 450 ± 192 m); nevertheless, most lakes are located at a high altitude, as shown by the median value (409 m). Watersheds that fed the lakes present areas ranging from 0.15 to 12.2 km2 (mean = 2.0 ± 3.4 km2), with the larger values being the ones associated with water bodies lying inside calderas, in the range from 2.3 to 12.2 km2 (mean = 6.5 ± 5.1 km2). Nevertheless, one of the maar lakes has a relatively large watershed (3.14 km2; Funda Lake).
Lake water bodies located inside maar craters have relatively lower surface areas, varying between 0.04 and 0.369 km2 (mean = 1.014 ± 1.205 km2), thus being smaller than areas from lakes lying in caldera floors, that are in the range from 0.124 to 3.580 km2 (mean = 1.583 ± 1.304 km2). Instead, lakes located at Pico Island have relatively small surface areas, with values between 0.024 km2 (Capitão) and 0.058 km2 (Caiado), as well as the water bodies in caves, with values like 0.0003 km2 (Algar do Carvão) and 0.012 km2 (Furna do Enxofre).
Lake depths range from 1.9 to 119 m (mean = 24.1 ± 28.7 m; median = 17.3 m), with the higher values observed in maar crater lakes, ranging from 17.3 to 119 m (mean = 45.4 ± 41.8 m; median = 33 m), and in most of the water bodies, occupying the floor of volcanic calderas. In this latter group, values range between 15 m and 36 m (mean = 25.2 ± 12.9 m; median = 27 m), excluding the very low depths measured in Caldeirão lakes (1.9 and 2.8 m respectively). For the remaining water bodies, values are lower, such as in lakes located inside caves (8.2 and 9.3 m, respectively at Algar do Carvão and Furna do Enxofre), or the ones located at Pico Island (depths between 4.9 and 6.9 m).
Water storage varies by three orders of magnitude, with values in the range from 5.4 × 104 to 4.74 × 107 m3 (mean = 7.96 × 106 ± 1.29 × 107; median = 5.80 × 105 m3). The higher storages are observed at caldera-lying lakes, ranging from 1.63 × 105 and 4.74 × 107 m3 (mean = 1.91 × 107 ± 1.79 × 107 m3; median = 1.4 × 107 m3), and at lakes inside maar craters, in the range from 4.50 × 105 and 9.1 × 106 m3 (mean = 4.67 × 106 ± 4.03 × 106 m3; median = 5.3 × 106 m3). Instead, the lower values are observed at lakes located inside lava caves, with values as high as 1.5 × 105 m3, and at the lakes spread at Pico Island, with values from 5.4 × 104 to 1.5 × 105 m3. The residence time of water in the studied lakes ranges from 1.2 to 24.5 yr, with the higher values estimated for some maar lakes, such as Santiago Lake (24.5 yr) and Congro Lake (7.3 yr).

3. Materials and Methods

A total of 672 samples were collected along the water column at each one of the studied lakes during four field surveys. Samples were collected at a single location in most lakes, except for the ones with larger surface areas (Sete Cidades, Fogo, and Furnas lakes), where three locations were selected, and nevertheless, always ensuring that sample collection took place in the deepest areas of all water bodies.
As most of the studied lakes have a monomictic behavior, field surveys were planned to collect samples at least in summer, when the water column is stratified, and during winter periods. Therefore, samples were collected at Furnas (n = 71), Fogo (n = 96), Capitão (n = 19), Caiado (n = 19), Furna do Enxofre (n = 23), Funda (n = 62), Comprida (n = 30), Negra (n = 78), and Caldeirão (n = 28) lakes during spring (April—May 2023), summer (July 2023), autumn (October—November 2023), and winter (December 2023—February 2024). At Sete Cidades (n = 132), Congro (n = 28), Santiago (n = 56), and Algar do Carvão (n = 23) samples were collected in spring (March—April 2023), summer (twice, in June and September 2023), and winter (December 2023—February 2024).
To proceed to sample collection, a 1 L capacity SEBA water sampler was used. Samples were collected with 2 m depth intervals in most locations, apart from Capitão, Caiado, and Caldeirão lakes, where a 1 m interval was selected due to the lower depth of these water bodies. At the location with lower depth at Sete Cidades Lake, a similar sampling interval was used. Instead, due to the higher depth at Negra Lake, samples were collected using a 3 m interval until reaching a depth of 30 m, and from here, using 10 m intervals until reaching the bottom.
Immediately after sampling, several physico-chemical parameters were measured (pH, temperature, electrical conductivity—EC) using portable equipment (WTW Multi 3620 IDS, Xylem Inc., Washington, United States). Additionally, titrations for the determination of dissolved CO2 and alkalinity were also performed in the field, in both cases following standard procedures [104]. In the laboratory, major ion content was analyzed using atomic absorption spectrometry (AAS) for cations and ionic chromatography for anions, using respectively a GBC 906AA and a Thermo Fisher Dionex Integrion HPIC system (Thermo Fisher Scientific, Waltham, MA, USA). For that purpose, samples were collected in polyethylene flasks, after being filtered in the field with 0.2 µm cellulose filters and were also acidified with suprapur nitric acid in the case of the AAS aliquots. REE content was measured over selected samples by inductively coupled plasma mass spectrometry at the Activation Laboratories (Canada). Moreover, at each location, an AquaTroll 600 probe was used to proceed to continuous measurements of pH, temperature, and EC along the water column.
The determination of the stable isotopic ratios 18O/16O, 2H/1H, 13C/12C, and 87Sr/86Sr was made over samples collected along the 1st survey, at the surface, and at the bottom of the water column. For the δ18O and δ2H determinations, sampling followed the standard procedure [105], and the analytical work was completed at the Stable Isotope Laboratory of Estación Biológica Doñana—CSIC (Spain), through a CRDS (Cavity Ring Down Spectroscopy) laser spectrometer Picarro L2130-i (Picarro Inc., Santa Clara, CA, USA). Maximum error intervals were ±0.3‰ for δ18O and ±3‰ for δ2H.
Regarding sample collection for δ13C-DIC determination, the standard procedure described by the International Atomic Energy Agency [106] was followed. Subsequently, the analytical work was carried out at the stable isotope laboratory from Estación Biológica Doñana—CSIC (Spain), through a continuous flow isotope–ratio mass spectrometry system (Thermo Electron, Thermo Fisher Scientific, Agawam, MA, USA), with a Flash HT Plus elemental analyzer interfaced with a Delta V Advantage mass spectrometer. The maximum error interval was equal to ±0.15‰.
The 87Sr/86Sr ratio determinations were carried out at the ALS environmental laboratories (ALS Scandinavian, Luleå, Sweden) through MC-ICP-MS using internal standardization, and external calibration with bracketing isotope SRMs. The absolute error of the analysis ranged between 0.01818 and 0.00163.

4. Results and Discussion

4.1. General Hydrogeochemistry

The descriptive statistics for the physico-chemical parameters and the major ion content is presented in Table A1 (Appendix A). The relative major ion content shows a diversity of water types (Figure 2): samples from lakes located at São Miguel Island corresponding Na-HCO3 to Na-Cl waters, while at Pico and Corvo, they are mainly from the Na-Cl type. Instead, Algar do Carvão Lake (Terceira Island) is from the Na-HCO3 type waters, while water bodies from Flores present a slight alkali earth enrichment, being from Na-HCO3 to Na-Ca-Cl and Na-Mg-HCO3 waters. In addition to depicting a higher relative enrichment in alkaline earth metals, Furna do Enxofre Lake (Graciosa Island) also shows a higher SO4 content compared to the remaining lakes under study.
According to the dataset, water temperature in the studied lakes range from 11 °C to 27.5 °C (mean = 16.0 °C ± 3.3 °C; median = 15.2 °C). Temperature varies seasonally, with the lower values observed during the fourth sampling survey, carried out during winter (mean = 13.8 °C ± 1.2 °C; median = 13.4 °C), with values at the surface in the range from 12.9 °C to 15.9 °C, respectively at Caldeirão West and Congro lakes, compared to values observed during the summer survey (second survey; mean = 18.1 °C ± 4.1 °C; median = 18.0 °C), where values at the surface range between 12.9 °C (Algar do Carvão) and 27.5 °C (Caldeirão East). At depth, during this latter survey, temperature ranges between 12.2 °C and 26.9 °C, respectively, at the same referred water bodies. Moreover, seasonality is also shown by the monomictic behavior of most of the studied lakes, that present a stratified water column during summer, compared to the remaining seasons (Figure 3). The difference between the temperature at the epilimnion and the hypolimnion during summer (second survey) reaches values as high as 13 °C (Funda Lake), 10.6 °C (Negra Lake) and 8.7 °C (Congro Lake).
The pH in the studied lakes ranged between 6.04 and 10.46 (mean = 8.11 ± 0.78; median = 8.06). The lower values are observed at Furna do Enxofre Lake (mean = 6.57 ± 0.29; median = 6.64), both during summer and winter surveys. Instead, the higher values are observed at Sete Cidades Verde Lake (mean = 9.37 ± 0.65; median = 9.55; sampling point P2). The higher values are usually observed at the surface of the lakes during summer, with pH values of 10.27, 10.23, and 10.22, respectively, at Sete Cidades Verde, Funda, and Negra lakes, a trend that points out to the influence of CO2 removal through the enhanced biological activity [107,108,109]. Along the same period, pH depicts a sharp decreasing trend from the surface to the bottom, also reflecting that CO2, released following organic matter decomposition in sediments, is imprisoned in the hypolimnion. A similar trend was observed in previous studies conducted by [13,94], and is compatible with the measured δ13C-DIC values, as most of the lakes present values in the range from −7.21‰ to −28.1‰ (mean = −17.0 ± 5.7‰; median = −15.0‰), thus clearly revealing a biogenic origin for CO2, being generally well below the atmospheric CO2 value (~−8.3‰; [110]). An exception corresponds to Furna do Enxofre Lake, with values for δ13C-DIC equal to 0.65‰ and −0.21‰, respectively, at the surface and at the bottom of the water column, revealing a magmatic contribution as typical mantellic values range from −3.5‰ to −6‰ [111]. These latter values for Furna do Enxofre Lake are close to the one presented by [84], suggesting the cumulative influence of a mantellic source, besides a biogenic input.
EC measurements range between 22.2 and 628.6 µS cm−1 (mean = 113.8 ± 93.4 µS cm−1; median = 112.7 µS cm−1), with values measured during winter (mean = 110.8 ± 96.1 µS cm−1; median = 112.7 µS cm−1) and summer (mean = 114.8 ± 95.3 µS cm−1; median = 115.3 µS cm−1) being similar. Nevertheless, by plotting major ion content as a function of EC, it is possible to show that Furna do Enxofre Lake presents higher mineralized waters (mean = 572.8 ± 34.4 µS cm−1; median = 567.5 µS cm−1) compared to the remaining lakes, where the highest mean value was observed at Furnas 2 (139.8 ± 4.6 µS cm−1) and the lowest mean was measured at Capitão (25 ± 2 µS cm−1) and Caiado (27 ± 2 µS cm−1) lakes, both in Pico Island (Figure A1—Appendix B and Figure A2—Appendix C). During summer, a slight EC increase along the depth is observed, with the maximum difference observed at Funda Lake, where at the surface, EC reaches a value equal to 85.3 µS cm−1, while at the bottom, it is equal to 135.6 µS cm−1; while during the winter, these values are respectively 117.6 and 120 µS cm−1.
The EC values depict very high positive linear correlations regarding most major ion content (all p-values < 0.001), namely vs. Cl (r = 0.942; Figure A1b), SO4 (r = 0.954; Figure A1c), Na (r = 0.846; Figure A2a), Mg (r = 0.948; Figure A2c), and Ca (r = 0.934; Figure A2d). Instead, weak linear correlations are observed vs. K (r = 0.311; Figure A2b) and HCO3 (r = 0.251; Figure A1a), also depicting p-values lower than 0.001. These relationships highlight the contribution of the main processes that are suggested to control lake water chemistry, namely sea salts deposition and water–rock interaction, the latter responsible for water acidity neutralization [13,89]. These two processes are shown in Figure 4a, where most of the studied lakes fall close to the seawater relative line, despite some lakes depicting a Na enrichment besides what is expected from a single marine source contribution. From the dataset, is also possible to discriminate between the alkaline metal enrichment in lakes from Terceira, São Miguel, and Flores, compared to the alkaline earth metals enrichment in Furna do Enxofre Lake, both associated to rock dissolution (Figure 4b). Despite the δ13C signature in most lakes of the Azores, that suggests that carbon is generally of biogenic origin [13], volcanic CO2-rich gas emissions may enhance water acidity, thus indirectly also influence water chemistry.
The δ18O and δ2H values range between −0.52‰ and −4.22‰ (mean = −2.64 ± 1.08‰) and between 0.4‰ and −18.1‰ (mean = −9.84 ± 5.24‰), respectively. The δ18O vs. δ2H relationship for the selected samples in the studied lakes shows that waters are of meteoric origin, being plotted over or close to the GMWL and the LMWL, despite some samples that present a trend suggesting evaporation or a contribution from seawater, this latter through sea salt spraying (Figure 5; Table A2—Appendix D). This effect is clearly depicted by the Sete Cidades (Verde and Azul) lakes, in which δ18O and δ2H values range from −0.52‰ to −1.61‰, and from 0.4‰ to −5.2‰, respectively.

4.2. 87Sr/86Sr Isotopic Ratio

As previously mentioned, the stable isotopic composition of the lakes shows a typical meteoric water signature. The Sr isotopic signature of rainwater is mainly controlled by the nature of the atmospheric aerosol sources [112], and marine aerosols are characteristic in the North Atlantic area [113,114,115]. The rainwater strontium ratios are assumed to be analogous to those of seawater (87Sr/86Sr = 0.70918 ± 1; [116]), notwithstanding potential variations attributed to continental dust or to the proximity to fumarolic fields [117].
Water 17 01849 g005
Figure 5. Relationship between δ18O (‰) and δ2H (‰) illustrated by a binary plot. Global (GMWL) and regional (RMWL) meteoric water lines from [118] and [119], respectively. Seawater data from [120].
Figure 5. Relationship between δ18O (‰) and δ2H (‰) illustrated by a binary plot. Global (GMWL) and regional (RMWL) meteoric water lines from [118] and [119], respectively. Seawater data from [120].
Water 17 01849 g005
The 87Sr/86Sr ratios of the studied lakes range between 0.709194 ± 0.000084 (Fogo Lake) and 0.704294 ± 0.000050 (Negra Lake) (Table A2—Appendix D), with most of the water bodies presenting less radiogenic values than seawater, suggesting a potential contribution from the rock values (Figure 6a). In the plot, most samples fall between the seawater line and the rock values for each island (Figure 6a), which suggests that the strontium isotopic ratios are the result of a combination of these two endmembers. This is also depicted by the very high positive linear correlation between strontium and calcium (r = 0.956; p-value < 0.001; Figure 6b), elements that can replace each other in mineral structures. Results also show that lake waters did not reach equilibrium with the host rocks, as expected from the overall mineralization of the samples. However, it is highlighted that lakes from Flores Island, namely Funda and Negra, as well as Furna do Enxofre (Graciosa Island), depict lower 87Sr/86Sr ratios compared to the water bodies, been closer to the rock endmember composition. Therefore, the water composition in those lakes has a higher input from rock dissolution, which is also shown by the position of their samples when plotted in the Piper diagram, depicting an alkali earth metal enrichment trend over the cation triangle (Figure 2).
By plotting the reciprocal of Sr vs. the 87Sr/86Sr ratio, it is possible to show that most of the lake water bodies define a linear trend between two endmembers, namely rock values and rainwater, this latter influenced by a marine contribution, as expected in small islands samples (Figure 7). In the same plot, samples from Furna do Enxofre Lake, as well as from Flores water bodies, fall closer to the rock endmember, as well as closer to mineral waters and river waters from the Azores that are hydrothermally influenced, thus suggesting a more pronounced influence of water–rock interaction. Nevertheless, it is expected that rock dissolution is limited, as physico-chemical conditions are not favorable, since lake waters are cold and mainly alkaline. Moreover, the availability of rock surface in lake reservoirs is usually low [121], thus limiting water–rock interaction and, as a result, the interaction between lake water and the bottom sediments is suggested to be more important.
Water 17 01849 g006
Figure 6. (a) 87Sr/86Sr isotopic ratios from the collected samples and the local rocks. (b) Binary diagram displaying Sr vs. Ca. The 87Sr/86Sr isotopic ratio of seawater was extracted from [116]. The rock limits of Flores and Corvo were obtained from [122], Pico from [123], Terceira and Graciosa from [124], and São Miguel from [125].
Figure 6. (a) 87Sr/86Sr isotopic ratios from the collected samples and the local rocks. (b) Binary diagram displaying Sr vs. Ca. The 87Sr/86Sr isotopic ratio of seawater was extracted from [116]. The rock limits of Flores and Corvo were obtained from [122], Pico from [123], Terceira and Graciosa from [124], and São Miguel from [125].
Water 17 01849 g006
Water 17 01849 g007
Figure 7. 87Sr/86Sr isotopic vs. the reciprocal of Sr (data for mineral waters in Furnas Volcano from [117], for rain waters from [126] and for rocks from [122,123,124,125]).
Figure 7. 87Sr/86Sr isotopic vs. the reciprocal of Sr (data for mineral waters in Furnas Volcano from [117], for rain waters from [126] and for rocks from [122,123,124,125]).
Water 17 01849 g007

4.3. Rare Earth Elements Composition

Data on the REE content in the studied lakes are shown in Table A3 (Appendix E). The concentrations of rare earth elements in the Azores lakes range from 10−5 to 10−2 times the one from chondrite, being like those observed in rainwaters from São Miguel [126]. The samples display considerable variability in the ∑REE, ranging from 0.83 µg L−1 in Capitão Lake during summer to 13.54 µg L−1 in Furna do Enxofre during winter.
Seasonal variations in REE concentrations are evident, with different trends among lakes: some show higher ∑REE values in summer, while others show increased concentrations in winter. Increased REE concentrations in summer may reflect the effect of concentration processes, which increases the relative contribution of rock weathering, and dilution processes in winter. In contrast, higher winter values may be related to increased sediment input into the lakes, promoting mineral dissolution.
Waters from the lake water bodies under study depict a negative slope when chondrite normalized [127], indicating an enrichment in light REEs (LREEs) in comparison to the heavy REEs (HREEs) (Figure 8a,b), which is consistent with the geochemical signatures of rocks in the Azores (Figure 8c), and sediments from some lakes (Figure 8d). As shown, sediments from São Miguel, Graciosa, and Terceira present a negative europium anomaly, which may suggest weathering of the more unstable minerals.
Water–rock interaction reflects the imbalance between several factors, such as the saturation states regarding primary minerals, the precipitation of secondary minerals, the aqueous chemistry of each component required for soluble and insoluble species, and the acidic character of the environment [128]. Therefore, the REE content and pattern of lake waters is a function of the individual contribution of each mineral in the system, being controlled by the preferential dissolution of one or more of the rock-forming minerals, or instead by fractionation due to newly formed minerals [6]. Of the overall set of primary minerals in magmatic rocks, amphibole and orthoclase, both LREE enriched, and clinopyroxene (HREE enriched), present the highest REE content, while plagioclase depicts a strong positive Eu anomaly [129]. Moreover, dissolved REE content is also influenced by the overall REE content of accessory minerals, that may be much higher, besides the removal of dissolved REE from water when alteration minerals are formed [129].
While the overall decreasing trend in the REE is observed in most lake waters, notable europium (Eu) anomalies are also present (Figure 8a,b; Table A3), which are not observable in the local rocks (Figure 8c). In lakes, such as those in São Miguel, Graciosa, and Terceira, this behavior can be attributed to the interaction between the lake water and the bottom sediments, the latter also presenting an Eu anomaly (Figure 8d). Newly formed minerals, such as alunite–jarosite, may also explain a negative Eu anomaly in some volcanic lakes, but this process is only observed in acid hydrothermally influenced water bodies [129], thus far from the environmental conditions of the lakes of the Azores archipelago. Eu removal through oxides and carbonates may also explain a negative Eu anomaly [130], but again in conditions far from the ones in the present study.
In the other cases, the Eu anomalies observed are likely the result of incongruent mineral dissolution, particularly from minerals such as hornblende and orthopyroxene, which typically exhibit negative Eu anomalies in basaltic rocks [131]. Furthermore, seasonal variability in the Eu anomaly is evident. During winter, lakes from Flores Island display REE patterns that are consistent with the host rocks and sediments, exhibiting no significant Eu anomaly. In contrast, during summer, Comprida and Funda Lakes display a negative Eu anomaly. This is attributed to variations in the volume of rainwater, with higher amounts favoring the transport of sediments to the water, promoting dissolution; while in summer, due to lower sediment transport to the lakes, it is possible to observe signals of preferential mineral dissolution. A comparable behavior is also evident in Furna do Enxofre Lake (Graciosa Island), although it is suggested that the lower europium anomaly during summer is due to concentration effects and less to sediments being transported and mixed into the water. Additionally, Congro Lake depicts a seasonal variation in the spike, with a negative Eu anomaly in winter and a positive one in summer, suggesting an increment in the dissolution of plagioclase or other minerals that incorporate calcium during the summer months.
Water 17 01849 g008
Figure 8. REE patterns normalized to chondrite [127] of the sampled lakes: (a) winter survey; and (b) summer survey. (c) Shows the general behavior of the rocks of each island, Corvo and Flores values were extracted from [132], Pico from [123], Graciosa and Terceira from [124], and São Miguel from [125]. (d) Shows the behavior of the sediments collected in the bottom of the lake along the present study.
Figure 8. REE patterns normalized to chondrite [127] of the sampled lakes: (a) winter survey; and (b) summer survey. (c) Shows the general behavior of the rocks of each island, Corvo and Flores values were extracted from [132], Pico from [123], Graciosa and Terceira from [124], and São Miguel from [125]. (d) Shows the behavior of the sediments collected in the bottom of the lake along the present study.
Water 17 01849 g008

5. Conclusions

Water composition from volcanic lakes in the Azores archipelago have been studied in the last decades in order to address eutrophication, or to characterize the influence of volcanic activity over the water bodies, which was made mainly through major ion and nutrients composition, and in the latter case also by measuring diffusive CO2 emissions, suggesting that two main processes are control water chemistry, namely sea salt deposition and water–rock interaction. In the present study, that comprises a detailed annual-long survey over a representative set of lake water bodies in the archipelago, rare earth element composition and strontium isotopic ratio data were used, in addition to major ion content, to improve knowledge regarding geochemical processes in those lakes.
Data gathered so far suggest that volcanic lakes in the Azores are low-activity lakes according to criteria defined by [133], receiving CO2-dominated fluids from a geogenic source at a relatively low rate in some water bodies. Therefore, in such type of volcanic lakes, rock dissolution is somehow of low magnitude, due to the low water temperature and to the low rock surface available [121]. Instead, other volcanic lakes present rather acid high-TDS brines, resulting from the interaction of acid fluids with volcanic origin and rocks [6]. Considering the actual physico-chemical conditions in most of the water bodies studied during the present study, and despite the active volcanic character of the Azores islands, lake water chemistry is not influenced by hydrothermal fluids, except for Furna do Enxofre. Nevertheless, the dissolution of common minerals in volcanic rocks, or in the bottom sediments that result from their weathering, is influencing water chemistry, as for example, the REE overall content and distribution.
The main findings from the present study demonstrate that besides the influence of sea salt spraying, which is expected as the studied lakes are in small islands, thus close to the sea, the interaction between water and rock sources is also imprinting water geochemistry. The 87Sr/86Sr ratio in the studied lakes ranges between 0.709194 and 0.704294, presenting less radiogenic values than seawater, and most lakes define a trend between a rock endmember and a rainwater endmember, the latter mainly controlled by a marine source. REE data are consistent with findings from 87Sr/86Sr ratios, and when normalized chondrite clearly depicts an enrichment in light REEs compared to the heavy REEs, this is like the geochemical signatures of rocks from the Azores. Moreover, in most waters, an Eu anomaly is also observable, resulting from the interaction with the bottom sediments, that also show a similar anomaly, or from the incongruent mineral dissolution. The influence of those processes varies along the season of the year, with different behaviors being observed in summer and in winter periods.
Building on the present study, a broader investigation into the occurrence and pattern of REEs in lakes in the Azores is underway. This investigation will allow for some gaps in the present study to be overcome, involving a larger number of lakes, to increase representativeness, and a larger number of samples over time, to better evaluate seasonal effects. A geochemical characterization of the host rocks and of the sediments at the bottom of the lakes will also be promoted, which will allow for the distribution of REEs in the water and the geological environment to be related.

Author Contributions

Conceptualization, J.V.C., C.A. and L.F.; methodology, J.V.C. and C.A.; software, L.F.; validation, J.V.C. and C.A.; investigation, J.V.C., C.A., L.F. and F.V.; writing—original draft preparation, J.V.C., C.A. and L.F.; writing—review and editing, J.V.C., C.A., L.F. and F.V.; supervision, J.V.C. and C.A.; funding acquisition, J.V.C. and C.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by FCT—Fundação para a Ciência e Tecnologia, I.P. by project reference 2022.02459.PTDC and DOI identifier 10.54499/2022.02459.PTDC (http://doi.org/10.54499/2022.02459.PTDC).

Data Availability Statement

The datasets presented in this article are not readily available, but requests to access the datasets should be directed to the corresponding author (jose.vm.cruz@uac.pt).

Acknowledgments

The authors are grateful to Direção Regional do Ambiente e Ação Climática for the support during field work.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Descriptive statistics for the main physico-chemical parameters and major ion content along the four sampling surveys made in the studied lakes.
Table A1. Descriptive statistics for the main physico-chemical parameters and major ion content along the four sampling surveys made in the studied lakes.
IslandLakeStatistical ParameterTpHECCO2HCO3ClSO4NaKMgCa
°C µS cm−1mg L−1
CorvoCaldeirão EastMax27.498.2490.442.604.8825.929.9116.660.601.601.53
Min12.836.8063.181.502.4417.444.718.620.400.900.68
Mean19.007.5475.881.973.7620.766.7312.140.511.161.07
±SD5.560.499.390.370.863.271.323.030.060.250.24
Median17.787.5273.631.953.9719.766.9112.000.521.091.14
Caldeirão WestMax26.568.6788.852.106.1026.968.2717.651.181.601.54
Min12.167.0963.621.401.8316.734.767.620.380.960.57
Mean17.787.8673.901.763.7520.576.5311.580.611.281.02
±SD4.670.469.880.221.173.930.903.620.250.230.36
Median17.207.8070.181.753.9719.126.589.990.521.350.97
FloresFundaMax24.6910.46135.6214.4056.7924.5110.3013.381.913.796.64
Min12.907.5585.320.5036.4816.184.809.881.252.834.96
Mean14.758.48119.394.7143.6620.006.9311.781.493.235.73
±SD2.680.719.444.285.272.511.080.700.160.250.37
Median13.458.25119.292.5043.3119.277.3211.781.523.185.77
NegraMax22.8510.22156.909.6058.5617.3012.5913.951.534.037.71
Min12.277.30115.100.8037.8215.346.699.820.543.045.61
Mean13.968.42127.323.9250.0916.427.7511.671.133.626.81
±SD2.190.685.402.055.130.350.750.780.320.270.35
Median12.998.25125.773.6051.8516.417.6411.581.233.736.78
CompridaMax22.808.7693.478.0021.5319.337.3111.152.322.322.56
Min12.157.0576.520.6013.9115.574.947.740.761.772.03
Mean15.467.8883.091.9916.7817.246.149.521.262.062.25
±SD3.260.456.311.612.131.480.650.900.670.170.16
Median14.737.9379.091.7516.3816.876.199.590.862.032.25
PicoCapitãoMax25.78.76284.76.719.804.026.590.591.531.23
Min11.96.69221.23.054.862.061.660.120.380.38
Mean16.57.70252.34.856.442.573.240.360.730.68
±SD4.70.6120.81.141.450.461.100.160.310.22
Median15.77.40252.24.885.842.483.170.320.600.67
CaiadoMax22.68.06312.310.3712.854.667.290.421.030.83
Min12.06.98240.92.446.211.851.860.070.370.25
Mean15.37.55271.75.077.492.443.130.160.660.47
±SD4.20.3620.42.171.680.691.250.080.230.14
Median12.37.69261.84.276.832.112.960.140.550.45
TerceiraAlgar do CarvãoMax12.869.54162.5053.0057.9516.8012.4226.512.590.651.66
Min11.008.09113.010.3046.9713.428.2820.452.150.070.61
Mean12.068.74129.8628.8851.3214.4810.6523.892.300.331.00
±SD0.300.359.2314.162.870.601.241.610.100.150.25
Median12.078.66130.3427.0050.6314.5010.7924.282.310.370.98
GraciosaFurna do EnxofreMax16.066.97628.62163.1027.3365.24118.1240.656.0528.6051.90
Min14.026.04511.9315.1020.0760.05104.1234.712.7923.8624.80
Mean15.096.57572.7775.6824.2163.25112.9438.364.1126.4336.60
±SD0.690.2934.4242.921.951.353.901.610.831.388.18
Median15.216.64567.4764.2024.8963.14112.9638.204.0326.6438.17
São MiguelFurnas P1Max22.508.72145.4313.3069.5413.5510.3119.069.942.394.03
Min14.127.09116.600.3054.8412.158.2115.204.411.722.75
Mean18.387.81132.823.7763.6112.999.2917.106.782.183.32
±SD2.950.447.172.964.690.430.491.111.800.230.44
Median18.807.91132.982.7065.2713.159.2816.976.732.263.22
Furnas P2Max23.059.44149.919.0073.2013.6610.2925.4611.682.384.22
Min12.756.73131.421.7052.0312.468.2315.064.211.542.72
Mean18.108.37139.753.7963.6413.159.1716.846.662.153.41
±SD3.100.704.651.746.410.380.551.911.680.250.54
Median18.008.46141.653.5565.8813.249.2916.486.692.283.43
Furnas P3Max22.759.65145.793.8070.7614.0313.3017.6813.052.324.02
Min14.137.74121.650.4051.2412.297.7715.503.571.722.73
Mean18.758.67137.381.9962.1412.849.3016.576.882.093.49
±SD3.470.458.300.856.320.411.520.622.310.240.51
Median19.038.64138.301.9064.0512.808.8016.826.652.233.82
CongroMax22.510.3161.013.260.413.19.812.07.62.14.8
Min13.86.987.21.624.011.04.58.64.61.21.5
Mean16.67.9100.44.830.812.37.510.65.61.72.3
±SD2.60.817.32.47.30.41.81.00.80.30.8
Median16.07.892.94.228.812.38.710.95.41.82.1
Fogo P1Max19.488.8051.552.7011.599.705.185.191.670.800.96
Min13.266.9330.660.407.937.800.183.791.190.450.37
Mean15.847.7043.111.549.238.821.314.461.380.580.60
±SD2.490.594.370.510.950.361.630.340.150.100.19
Median16.127.7444.941.459.158.890.744.471.350.560.56
Fogo P2Max19.038.6546.8910.0012.819.554.606.741.320.990.60
Min13.176.2639.550.308.118.272.593.491.130.450.30
Mean15.047.6044.092.049.878.893.474.841.200.750.42
±SD1.880.531.891.501.240.230.400.780.050.170.08
Median13.777.7244.711.709.768.923.404.731.190.780.40
Fogo P3Max19.218.2347.702.6010.989.413.596.421.700.800.54
Min13.367.0741.370.506.718.543.123.921.160.450.32
Mean15.197.6444.851.578.858.863.264.821.300.560.44
±SD2.360.301.450.541.010.250.120.680.140.110.07
Median13.657.6845.251.608.728.813.244.601.270.520.45
SantiagoMax21.89.9139.17.654.916.810.020.73.71.12.1
Min12.97.4105.30.933.612.24.912.91.80.51.0
Mean15.28.2119.63.143.014.47.217.72.20.91.4
±SD2.50.66.71.94.40.71.42.30.30.20.2
Median14.18.1121.02.642.714.57.518.62.10.91.4
Azul P1Max23.229.8294.752.1033.5514.897.3214.852.991.471.97
Min14.587.5772.290.4025.6213.106.0812.122.361.011.64
Mean19.038.3786.361.2329.0613.746.6013.222.671.191.78
±SD3.580.656.050.492.220.550.470.830.160.140.10
Median21.698.4985.801.2028.9813.706.4913.062.651.161.80
Azul P2Max22.639.59122.5116.6057.9514.744.0215.533.271.842.79
Min15.067.4485.350.2024.4012.950.1211.092.400.961.69
Mean17.858.2295.253.2730.6013.650.9612.802.581.441.89
±SD2.680.576.643.226.350.380.950.880.150.270.21
Median17.368.2292.752.4528.6713.750.7812.712.551.401.82
Verde P1Max22.2810.46142.4414.5062.8316.207.4318.863.701.663.03
Min14.807.3453.850.8034.7714.615.4616.172.030.821.96
Mean16.928.52109.813.9240.2015.176.8217.392.321.122.28
±SD2.570.9818.353.146.020.350.460.720.260.310.23
Median15.878.40112.782.7537.7315.206.9817.172.280.972.22
Verde P2Max22.2410.41123.982.2039.6516.778.1618.982.900.992.54
Min14.948.48102.192.0034.1614.616.9616.081.740.742.04
Mean18.719.37114.542.1036.8315.617.4117.662.200.892.19
±SD3.040.655.730.101.950.830.451.090.330.080.16
Median17.719.55114.282.1036.9115.607.2317.872.130.912.15
AzoresAllMax27.4910.46628.62163.1073.2065.24118.1240.6513.0528.6051.90
Min11.006.0422.170.201.834.861.851.660.070.070.25
Mean16.028.11113.806.7233.5915.7110.2213.042.452.553.86
±SD3.280.7893.4017.1918.929.7619.537.032.024.646.70
Median15.168.06112.712.2036.4214.246.9912.191.991.442.08

Appendix B

Water 17 01849 g0a1
Figure A1. Binary plots of the main anions content in the studied lakes as function of EC: (a) EC vs. HCO3 content; (b) EC vs Cl content; (c) EC vs. SO4 content.
Figure A1. Binary plots of the main anions content in the studied lakes as function of EC: (a) EC vs. HCO3 content; (b) EC vs Cl content; (c) EC vs. SO4 content.
Water 17 01849 g0a1

Appendix C

Water 17 01849 g0a2
Figure A2. Binary plots of the main cations content in the studied lakes as function of EC: (a) EC vs. Na content; (b) EC vs K content; (c) EC vs. Mg content; (d) EC vs. Ca content.
Figure A2. Binary plots of the main cations content in the studied lakes as function of EC: (a) EC vs. Na content; (b) EC vs K content; (c) EC vs. Mg content; (d) EC vs. Ca content.
Water 17 01849 g0a2

Appendix D

Table A2. Stable isotopic ratios 18O/16O, 2H/1H and 87Sr/86Sr for selected samples from the studied lakes (measurements for surface and the bottom of the water column; n.d.—not determined).
Table A2. Stable isotopic ratios 18O/16O, 2H/1H and 87Sr/86Sr for selected samples from the studied lakes (measurements for surface and the bottom of the water column; n.d.—not determined).
IslandLakeDepthδ18Oδ2Hδ13C-DIC87Sr/86Sr
‰
CorvoCaldeirão EastSurface−4.22−17.6−22.11n.d.
Bottom−4.17−17.4−21.850.707744
Caldeirão WestSurface−3.89−18.1−22.77n.d.
Bottom−4.03−17.1−23.420.707846
FloresFundaSurface−3.20−11−13.32n.d.
Bottom−3.25−11−13.430.704587
NegraSurface−3.33−12−14.86n.d.
Bottom−1.03−9.2−19.860.704294
CompridaSurface−2.74−12.7−21.05n.d.
Bottom−3.73−13.9−23.610.705787
PicoCapitãoSurface−3.85−16.1−23.66n.d.
Bottom−4.05−17.3−25.020.707846
CaiadoSurface−3.69−14.7−22.04n.d.
Bottom−3.85−15.1−23.200.707459
TerceiraAlgar do CarvãoSurface−3.26−12.4−9.98n.d.
Bottom−3.74−13.7−9.740.705636
GraciosaFurna do EnxofreSurface−3.48−130.65n.d.
Bottom−3.39−12.5−0.210.705058
São MiguelFurnas P1Bottom−2.69−9.7−7.960.705994
Furnas P2Surface−2.60−9.6−9.10n.d.
Bottom−2.66−9.6−12.690.706033
Furnas P3Surface−2.79−10.1−10.74n.d.
Bottom−2.67−10.2−11.180.706023
CongroSurface−2.47−10.5−25.61n.d.
Bottom−2.32−8.1−7.210.706905
Fogo P1Surface−2.13−6.9−14.73n.d.
Bottom−2.31−8.5−14.700.708862
Fogo P2Surface−2.45−9.3−15.00n.d.
Bottom−2.32−8.8−20.570.709194
SantiagoSurface−2.20−7.9−28.10n.d.
Bottom−2.25−7.1−25.530.707443
Azul P1Surface−0.590.9−11.46n.d.
Bottom−0.520.9−17.460.707817
Azul P2Surface−0.630.6−12.88n.d.
Bottom−0.590.4−14.720.705977
Verde P1Bottom−1.47−5.2−12.30n.d.
Surface−1.26−2.3−17.530.705987
Verde P2Surface−1.61−4.6−13.66n.d.
Bottom−1.46−3.5−15.840.706477

Appendix E

Table A3. REE concentrations in the studied lakes (waters and bottom sediments). Eu/Eu* represents the europium anomaly (Eu/Eu* = EuN/((SmN + GdN)/2).
Table A3. REE concentrations in the studied lakes (waters and bottom sediments). Eu/Eu* represents the europium anomaly (Eu/Eu* = EuN/((SmN + GdN)/2).
LakeLaCePrNdSmEuGdTbDyHoErTmYbLu∑REEEu/Eu*
µg L−1
Summer
Caldeirão West0.2980.5850.0500.2120.0400.0090.0440.0060.0330.0060.0170.0020.0140.0031.320.65
Caldeirão East1.5602.1800.3021.1900.2050.0240.2150.0310.1920.0400.1230.0170.1090.0176.210.35
Comprida0.2790.5420.0480.1800.0360.0090.0370.0050.0250.0050.0140.0020.0130.0021.200.75
Funda2.2003.1800.4651.8700.3360.0300.3650.0500.2990.0610.1700.0230.1430.029.210.26
Negra0.6301.1200.1440.5470.0990.0150.0980.0140.0810.0170.0520.0080.0470.0072.880.46
Caiado2.0803.6500.4501.8500.3340.0580.4070.0550.3160.0640.1780.0230.1400.029.630.48
Capitão0.1630.3740.0340.1400.0310.0070.0300.0040.0220.0040.0110.0020.0090.0020.830.69
F.Enxofre0.3810.5940.0580.2470.0460.0090.0520.0070.0360.0070.0200.0030.0170.0031.480.56
Algar do Carvão1.6204.3500.3661.4400.3050.0400.3530.0520.3210.0690.1940.0270.1640.0249.330.37
Congro1.6502.3100.2660.9600.1910.2900.2060.0210.1110.0220.0610.0080.0510.0076.154.45
Fogo P10.7151.0200.1280.4960.0830.0110.0940.0120.0650.0120.0350.0050.0300.0052.710.38
Fogo P21.1501.4800.2130.8390.1520.0140.1700.0230.1460.0300.0850.0110.0680.0114.390.27
Santiago1.2801.9800.2020.8270.1450.0130.2070.0280.1790.0380.1080.0150.0840.0125.120.23
Furnas P10.4851.0300.1250.5260.1040.0140.0980.0130.0640.0120.0330.0040.0250.0042.540.42
Furnas P21.2102.4500.2981.2700.2550.0280.2460.0320.1830.0330.0850.0110.0660.016.180.34
Azul P10.3870.7160.0800.3610.0740.0160.0780.0100.0570.0110.0290.0040.0250.0051.850.64
Azul P20.6680.9740.1200.4390.0730.0140.0830.0100.0580.0120.0340.0050.0310.0032.530.55
Verde P10.4890.7390.0750.3130.0580.0090.0650.0080.0520.0110.0300.0040.0250.0041.880.45
Verde P22.0302.6700.3471.3300.2290.0240.2700.0390.2340.0500.1520.0200.1370.0197.550.29
Winter
Caldeirão West0.2540.4600.0440.1620.0300.0110.0310.0040.0220.0040.0130.0020.0110.0011.051.09
Caldeirão East0.4570.6670.0840.2900.0520.0070.0630.0080.0460.0090.0270.0030.0220.0031.740.37
Comprida0.7751.2000.1010.3810.0660.0220.0850.0100.0540.0110.0340.0040.0280.0042.780.90
Funda0.4030.5270.0470.1620.0270.0080.0330.0040.0190.0040.0130.0020.0110.0021.260.82
Negra0.2750.4330.0510.2100.0410.0100.0480.0070.0380.0080.0210.0030.0120.0021.160.69
Caiado0.5870.9810.1220.4840.0980.0140.1120.0150.0860.0160.0430.0050.0320.0052.600.41
Capitão1.0401.7600.2230.8540.1500.0180.1550.0200.1130.0210.0590.0070.0450.0074.470.36
F.Enxofre3.2605.3500.5452.2000.3970.0410.5340.0730.4550.0980.2840.0380.2250.03513.540.27
Algar do Carvão1.4103.6100.2921.1200.2410.0410.2930.0430.2770.0580.1650.0230.1370.027.730.47
Congro1.0601.3100.2020.7390.1230.0100.1250.0180.0980.0190.0560.0070.0490.0073.820.24
Fogo P10.7431.1400.1110.4170.0740.0180.0800.0100.0540.0100.0300.0040.0270.0042.720.71
Fogo P21.3301.5900.2451.0400.1920.0170.2350.0330.1850.0380.1030.0130.0730.0115.110.24
Santiago0.9201.1600.1760.6760.1130.0110.1110.0140.0770.0160.0470.0070.0390.0063.370.30
Furnas P10.8901.5000.1650.6980.1130.0110.1550.0220.1320.0280.0810.0100.0600.0093.870.25
Furnas P20.6450.7650.1180.4300.0700.0080.0660.0080.0420.0080.0230.0030.0180.0032.210.35
Azul P10.6701.0900.1140.4280.0690.0100.0750.0100.0520.0110.0300.0040.0260.0042.590.42
Azul P20.5250.8510.0890.3540.0630.0090.0710.0090.0480.0100.0280.0040.0230.0042.090.41
Verde P11.7103.5100.4521.7800.3390.0370.3260.0430.2340.0430.1210.0160.1030.0148.730.34
Verde P20.2030.2700.0370.1540.0310.0080.0360.0050.0340.0090.0280.0040.0240.0040.850.73
Sediments (mg L−1)
Caldeirão West33.5083.407.3230.505.671.794.860.734.130.761.880.261.650.23176.681.02
Caldeirão East35.3094.007.7731.605.931.965.120.794.280.801.980.281.740.23191.781.06
Comprida32.7065.706.8027.704.761.414.160.613.390.671.700.241.480.21151.530.95
Funda25.1054.805.4419.803.591.063.060.432.500.451.250.161.030.16118.830.95
Negra7.2613.801.485.981.230.320.920.120.630.140.370.060.460.0632.840.89
Caiado30.3083.6010.2044.409.503.258.741.317.501.383.520.503.090.44207.731.07
Capitão25.3069.507.6633.807.062.306.480.995.811.102.730.392.440.33165.891.02
F.Enxofre27.5052.606.6327.505.691.785.570.834.430.862.310.312.100.31138.420.96
Algar do Carvão87.60148.0022.3086.8020.502.7621.903.5524.105.4915.702.2414.201.87457.010.40
Congro62.30126.0012.6043.007.030.625.710.835.020.892.430.352.270.32269.360.29
Fogo159.00300.0032.10104.0016.301.0912.602.0311.702.276.040.915.870.88654.790.22
Santiago66.00138.0013.6047.908.131.446.500.955.481.123.070.442.820.41295.860.59
Furnas P1151.00296.0031.50104.0016.101.3812.601.9911.302.215.940.865.540.79641.210.29
Furnas P2144.00284.0028.9096.7016.101.2712.401.8010.502.035.830.785.040.69610.040.27
Azul P1128.00252.0027.0092.0015.202.8612.301.9111.102.075.940.865.560.77557.570.62
Verde P164.60126.0013.1044.607.321.295.550.915.280.982.800.382.570.38275.760.60

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Figure 1. Location of the studied lakes: (a) setting of the Azores archipelago in the North Atlantic Ocean, about 1500 km from Portugal mainland; (b) location of the studied lakes along 6 of the 9 islands in the Azores; (c) from north to south, Negra, Comprida, and Funda (Flores Island); (d) Caldeirão West and Caldeirão East (Corvo); (e) Algar do Carvão (Terceira); (f) Furna do Enxofre (Graciosa); (g) from West to East, Capitão and Caiado (Pico); (h) from West to East, Sete Cidades Azul, Sete Cidades Verde, Santiago, Fogo, Congro, and Furnas (São Miguel). P—Sampling location; (from plot (c–h) UTM coordinates are used; UTM zone 26 N for Flores and Corvo islands, and 25 N for the remaining islands).
Figure 1. Location of the studied lakes: (a) setting of the Azores archipelago in the North Atlantic Ocean, about 1500 km from Portugal mainland; (b) location of the studied lakes along 6 of the 9 islands in the Azores; (c) from north to south, Negra, Comprida, and Funda (Flores Island); (d) Caldeirão West and Caldeirão East (Corvo); (e) Algar do Carvão (Terceira); (f) Furna do Enxofre (Graciosa); (g) from West to East, Capitão and Caiado (Pico); (h) from West to East, Sete Cidades Azul, Sete Cidades Verde, Santiago, Fogo, Congro, and Furnas (São Miguel). P—Sampling location; (from plot (c–h) UTM coordinates are used; UTM zone 26 N for Flores and Corvo islands, and 25 N for the remaining islands).
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Figure 2. Major ion relative composition shown using a Piper-type diagram.
Figure 2. Major ion relative composition shown using a Piper-type diagram.
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Figure 3. Variation of temperature along the water column showing that most lakes have a monomictic behavior, presenting a stratified water column: (a) values measured during summer (2nd sampling survey); (b) values measured during winter (4th sampling survey).
Figure 3. Variation of temperature along the water column showing that most lakes have a monomictic behavior, presenting a stratified water column: (a) values measured during summer (2nd sampling survey); (b) values measured during winter (4th sampling survey).
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Figure 4. Binary plots from some major ions in lake waters: (a) Na vs. Cl plot showing the influence of a marine source over the composition of most of the studied lakes; (b) alkaline vs. alkaline earth metals plot, discriminating between Na + K and Ca + Mg enrichment, to which silicate dissolution is a source.
Figure 4. Binary plots from some major ions in lake waters: (a) Na vs. Cl plot showing the influence of a marine source over the composition of most of the studied lakes; (b) alkaline vs. alkaline earth metals plot, discriminating between Na + K and Ca + Mg enrichment, to which silicate dissolution is a source.
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Cruz, J.V.; Andrade, C.; Ferreira, L.; Viveiros, F. Lake Water Composition in Oceanic Islands: Insights from REE Content and 87Sr/86Sr Isotopic Ratio. Water 2025, 17, 1849. https://doi.org/10.3390/w17131849

AMA Style

Cruz JV, Andrade C, Ferreira L, Viveiros F. Lake Water Composition in Oceanic Islands: Insights from REE Content and 87Sr/86Sr Isotopic Ratio. Water. 2025; 17(13):1849. https://doi.org/10.3390/w17131849

Chicago/Turabian Style

Cruz, José Virgílio, César Andrade, Letícia Ferreira, and Fátima Viveiros. 2025. "Lake Water Composition in Oceanic Islands: Insights from REE Content and 87Sr/86Sr Isotopic Ratio" Water 17, no. 13: 1849. https://doi.org/10.3390/w17131849

APA Style

Cruz, J. V., Andrade, C., Ferreira, L., & Viveiros, F. (2025). Lake Water Composition in Oceanic Islands: Insights from REE Content and 87Sr/86Sr Isotopic Ratio. Water, 17(13), 1849. https://doi.org/10.3390/w17131849

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