Radiocarbon application in dating “complex” hot and cold CO2-rich mineral water systems: A review of case studies ascribed to the northern Portugal

The use of radioactive isotopes plays a very important role in dating groundwater, providing an apparent age of the systems in the framework of the aquifers conceptual modelling making available important features about the water fluxes, such as recharge, horizontal flow rates and discharge. In this paper, special emphasis has been put on isotopic constraints in the use of δ¹³C and ¹⁴C content as a dating tool in some hot (76 °C) and cold (17 °C) CO₂-rich mineral waters discharging in the Vilarelho da Raia–Pedras Salgadas region (N-Portugal). The radiocarbon content determined in these CO₂-rich mineral waters (¹⁴C activity from 4.3 up to 9.9 pmc) is incompatible with the systematic presence of ³H (from 1.7 to 7.9 TU). The δ¹³C values of the studied CO₂-rich mineral waters indicate that the total C in the recharge waters is being masked by larger quantities of CO₂ (¹⁴C-free) introduced from deep-seated (upper mantle) sources. This paper demonstrates that a good knowledge of mineral water systems is essential to allow hydrologists to make sound conclusions on the use of C isotopic data in each particular situation.     

Metal partitioning in sediments and mineralogical controls on the acid mine drainage in Ribeira da Água Forte (Aljustrel,Iberian Pyrite Belt,Southern Portugal)

This work focuses on the geochemical processes taking place in the acid drainage in the Ribeira da Água Forte, located in the Aljustrel mining area in the Iberian Pyrite Belt. The approach involved water and stream sediment geochemical analyses, as well as other techniques such as sequential extraction, Mössbauer spectroscopy, and X-ray diffraction. Ribeira da Água Forte is a stream that drains the area of the old mine dumps of the Aljustrel mine, which have for decades been a source of acid waters. This stream flows to the north for a little over than 10 km, but mixes with a reduced, organic-rich, high pH waste water from the municipal waste water pools of the village. This water input produces two different results in the chemistry of the stream depending upon the season: (i) in the winter season, effective water mixing takes place, and the flux of acid water from the mine dumps is continuous, resulting in the immediate precipitation of the Fe from the acid waters; (ii) during the summer season, acid drainage is interrupted and only the waste water feeds the stream, resulting in the reductive dissolution of Fe hydroxides and hydroxysulfates in the stream sediments, releasing significant quantities of metals into solution. Throughout the year, water pH stays invariably within 4.0–4.5 for several meters downstream of this mixing zone even when the source waters come from the waste water pools, which have a pH around 8.4. The coupled interplay of dissolution and precipitation of the secondary minerals (hydroxides and sulfates), keeps the system pH between 3.9 and 4.5 all along the stream. In particular, evidence suggests that schwertmannite may be precipitating and later decomposing into Fe hydroxides to sustain the stream water pH at those levels. While Fe content decreases by 50% from solution, the most important trace metals are only slightly attenuated before the solution mixes with the Ribeira do Rôxo stream waters. Concentrations of As are the only ones effectively reduced along the flow path. Partitioning of Cu, Zn and Pb in the contaminated sediments also showed different behavior. Specific/non-specific adsorption is relevant for Cu and Zn in the upstream branch of Ribeira da Água Forte with acid drainage conditions, whereas the mixture with the waste water causes that the association of these metals with oxyhydroxide to be more important. Metals bound to oxyhydroxides are on the order of 60–70% for Pb, 50% for Cu and 30–60% for Zn. Organic matter is only marginally important around the waste water input area showing 2–8% Cu bound to this phase. These results also show that, although the mixing process of both acid and organic-rich waters can suppress and briefly mitigate some adverse effects of acid drainage, the continuing discharge of these waste waters into a dry stream promotes the remobilization of metals fixed in the secondary solid phases in the stream bed back into solution, a situation that can hardly be amended back to its original state.     

Input of Sr/Sr ratios and Sr geochemical signatures to update knowledge on thermal and mineral waters flow paths in fractured rocks (N-Portugal)

Strontium isotopes and other geochemical signatures are used to determine the relationships between CO₂-rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have ⁸⁷Sr/⁸⁶Sr isotopic ratios between 0.716713 and 0.728035. ⁸⁷Sr/⁸⁶Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO₂-rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have ⁸⁷Sr/⁸⁶Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO₂-rich thermal and mineral waters and the granitic rocks. The mean ⁸⁷Sr/⁸⁶Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations.     

Environmental issues in urban groundwater systems: a multidisciplinary study of the Paranhos and Salgueiros spring waters,Porto (NW Portugal)

This study presents the results of a multidisciplinary approach, using hydrogeochemical, isotopic and ecotoxicological analyses, performed to assess the nature and suitability for use of Paranhos and Salgueiros spring waters (Porto city, NW Portugal). Based on the surface activities located along the course of the springs, 23 water samples were collected. All the samples were analysed for major element concentrations. The isotopic techniques employed included δ²H, δ¹⁸O and ³H. Standard acute bioassays with Daphnia magna were also performed. The hydrogeochemical analyses showed a nitrate and sulphate-enriched composition for these groundwaters, resulting mainly from urban drainage and sewer leakage. In the ecotoxicological analyses, no significant mortality was observed in any of the tests performed. The results obtained in this study suggest that Porto urban groundwater could be suitable for irrigation uses.     

Tracing salinization processes in coastal aquifers using an isotopic and geochemical approach: comparative studies in western Morocco and southwest Portugal

Environmental stable and radioactive isotopes (δ²H, δ¹³C, δ¹⁸O; ³H and ¹⁴C), together with physical and geochemical data, were used in the determination of the origins of groundwater salinization and geochemical evolution processes in coastal regions. Two case studies on the Atlantic Coast are discussed, one located in the Essaouira sedimentary basin, western Morocco, and the second, in the Lower Tagus–Sado sedimentary basin, southwest Portugal. In both regions, groundwater degradation occurs by salinization increase to different concentrations and in relation to different origins. The main quality issues for the groundwater resources are related to seawater intrusion, dissolution of diapiric structures intruding the aquifer layers, brine dissolution at depth, and/or evaporation of irrigation water. Anthropogenic pollution ascribed to agricultural activities is another source for groundwater degradation, affecting mainly the shallow aquifers. The apparent ¹⁴C age of the analysed samples ranges from 2.9?±?0.3 up to 45.6?±?0.6 pmC in the Miocene groundwater samples from the basin in Portugal; at the Essaouira basin in Morocco, the ¹⁴C content varies from 60 to 86 pmC. In most of the water samples, the ³H concentration is below the detection limit. In both basins, the isotopic results together with the geochemical data provided an effective label for tracing the mineralization origin and groundwater degradation processes. Further, the isotopic signatures were used in the identification of a paleoclimate (colder period), recorded in the stable isotopic composition and corroborated with the ¹⁴C data.     

Mass balance simulation and principal components analysis applied to groundwater resources: Essaouira basin (Morocco)

The water resources of the Essaouira coastal basin (west of Marrakesh) are characteristic of a semi-arid climate and are severely impacted by the climate in terms of quantity and quality. Considering the importance of the Essaouira aquifer in the groundwater supply of a vast region (nearly 1,200 km²), a research study was conducted in order to better understand groundwater evolution in this aquifer system. It is a coastal aquifer located on the Atlantic coastline, southern Morocco, and salinization problems have been reported. Covering the Palaeozoic bedrock, the sedimentary series range from the Triassic to the Quaternary. Besides the possibility of seawater intrusion problems, the geological structures delineate a syncline bordered by the Tidzi diapir (of Triassic age) outcropping to the east and south. This is a recharge area for the aquifer, whereas the main groundwater flow direction is from SE to NW towards the Atlantic Ocean. In spite of the occurrence of calcareous and dolomitic levels, all waters in the Essaouira basin are of the Na–Cl-type. Based on a range of experimental methodologies, combined with PCA and geochemical modelling, it was possible to identify the mineralization processes occurring in the groundwater system, and the importance of the water–rock interaction in the water chemistry. Scenarios were tested using a simple mass balance model through the PHREEQC programme. The reaction path was assumed to be such that waters observed at shallow depths evolved to more mineralized waters. An important contribution of water–rock interaction in groundwater mineralization was found, corroborating the influence of preferential recharge from the Tidzi diapir in the water’s signature. Anthropogenic contamination was also identified and could lead to serious problems with groundwater degradation in the near future, in a country with scarce water resources.     

Assessment of mixing between shallow and thermal waters using geochemical and environmental isotope tracers (N Portugal): a review and reinterpretation

Caldas de Moledo thermal (27–46 °C) spring and borehole waters issue in the region of the famous Port Wine vineyards, in the Douro River valley (Northern Portugal). The most abundant lithotypes are lower Cambrian metasedimentary rocks, Variscan granitoids and aplite-pegmatitic veins. The thermal waters are characterised by pH ≈ 9.0, TDS ranging from 200 to 350 mg/L, and belong to the HCO₃–Na facies indicating that the reservoir rock should be mainly granite. Since the local Spa is strongly dependent on water quality, the effects of mixing between local shallow cold groundwaters and deep thermal waters have been properly investigated. In the SO₄ ^2? (mg/L) versus δ¹⁸O (‰ vs. V-SMOW) diagram we can observe that some of the thermal springs show evidences of mixing (higher SO₄ ^2? concentrations) with local meteoric waters infiltrated at lower altitude sites (enriched δ¹⁸O signatures), showing the “altitude effect” in the isotopic composition of the recharge waters. Similar trends can be found in the K⁺, NO₃ ^?, Ca²⁺ and Na⁺ (mg/L) versus δ¹⁸O (‰ vs. V-SMOW) diagrams. It should be stated that SO₄ ^2?, K⁺ and Ca²⁺ are present in the fertilizers and fungicides used in the vineyards in the northern part of the country. Up to now, the thermal waters from boreholes used in the local Spa do not show evidences of mixing with shallow groundwaters contaminated with agrochemicals. The results obtained so far indicate that in the near future, special attention should be put on the possible occurrence of diffuse agricultural contamination (related to the Port Wine vineyards) in the thermal spring waters.     

Molecular and isotopic characterization of the particulate organic matter from an eutrophic coastal bay in SE Brazil

The present work aimed at studying the origin of particulate organic matter in Guanabara Bay and in some rivers of the Guanabara basin by using elemental composition, isotopic ratios (δ¹³C and δ¹⁵N) and molecular markers (sterols) in samples collected in two periods (winter and summer). Elemental and isotopic compositions were determined by dry combustion and mass spectrometry, respectively, while sterols were investigated by GC–FID and GC–MS. Higher sterol concentrations were present in the north-western part of the bay in winter (5.10–23.5 μg L^–1). The high abundance of algal sterols (26–57% of total sterols), the elemental composition (C/N=6–8) and the isotopic signatures (δ¹³C=−21.3‰ to −15.1‰ and δ¹⁵N=+7.3‰ to +11.1‰) suggested the predominance of autochthonous organic matter, as expected for an eutrophic bay, although seasonal variation in phytoplankton activity was observed. Coprostanol concentration (fecal sterol) was at least one order of magnitude higher in the particulate material from fluvial samples (4.65–55.98 μg L^–1) than in the bay waters (<0.33 μg L^–1). This could be ascribed to a combination of factors including efficient particle removal to sediments in the estuarine transition zone, dilution with bay water and bacterial degradation during particle transport in the water column.     

Carbon isotopes and geochemical processes in CO2-rich cold mineral water,N-Portugal

This paper summarizes a new outlook on the conceptual model of Melgaço–Messegães CO₂-rich cold (≈18 °C) mineral water systems, issuing in N of Portugal, based on their isotopic (²H, ³H, ¹³C, ¹⁴C and ¹⁸O) and geochemical features. Stable isotopes indicate the meteoric origin of these CO₂-rich mineral waters. Based on the isotopic fractionation with the altitude, a recharge altitude between 513 up to 740 m a.s.l. was estimated, corroborating the tritium results. The lowest ³H content (0 TU) is found in the groundwater samples with the highest mineralization. The mineral waters circulation are mainly related to a granitic and granodioritic environment inducing two different groundwater types (Ca/Na–HCO₃ and Na/Ca–HCO₃), indicating different underground flow paths. Calcium dissolution is controlled by hydrolysis of rock-matrix silicate minerals (e.g. Ca-plagioclases) and not associated to anthropogenic sources. The shallow dilute groundwaters exhibit signatures of anthropogenic origins (e.g. NO₃) and higher Na/Ca ratios. The stable isotopes together with the geochemistry provided no indication of mixing between the regional shallow cold dilute groundwater and mineral water systems. The heavy isotopic signatures identified in the δ¹³C data (δ¹³C = 4.7 ‰, performed on the total dissolved inorganic carbon (TDIC) of CO₂-rich mineral waters) could be derived from a deep-seated (upper mantle) source or associated to methanogenesis (CH₄ source). The negligible ¹⁴C content (≈2 pmC) determined in the TDIC of the mineral waters, corroborates the hypothesis of a mantle-derived carbon source to the mineral groundwater systems or dissolution of carbonate layers at depth.     

Poor housing and stroke mortality in population over 64 years old at the parish level,in mainland Portugal

Housing conditions can impact physical and mental health. In 2013, Portugal was still the fourth European Union country with the highest percentage of population without an adequately heated home in winter. Other adverse conditions are, for instance, overcrowding and living in older buildings. Some studies stress the relationship between stroke and poor living conditions in the elderly population, especially cold homes. Univariate and multivariate spatial cluster analysis were used to explore the relationship between excess stroke death risk, from 1998 to 2004, measured by the standardized mortality ratio (SMR) for several cohorts (including persons over 64 years old), and poor housing variables from the 2001 census, at the parish level in continental Portugal. A multivariate cluster of parishes, with population without any form of heating their homes as dominant condition, was detected in northwest Portugal. Mean and median SMR values across all cohorts were consistently higher within this cluster. This strengthens the hypothesis that cold homes deserve more attention in stroke prevention and mitigation amongst elderly persons, especially in northwestern continental Portugal.