GEOCHEMICAL AND ISOTOPIC TOOLS TO ASSESS CALDAS DA RAINHA THERMOMINERAL WATER SYSTEM ASCRIBED TO A KARST/FISSURED-POROUS ENVIRONMENT (PORTUGAL): A REVIEW J.M. Marques a, C. Matos b, P.M. Carreira c and M.O. Neves a a Centro de Recursos Naturais e Ambientais (CERENA), Instituto Superior Técnico, Universidade de Lisboa, Portugal b Instituto D. Luiz, Faculdade de Ciências, Universidade de Lisboa, Portugal c Centro de Ciências e Tecnologias Nucleares (C 2 TN), Instituto Superior Técnico, Universidade de Lisboa, Portugal
1. OBJECTIVES The aim of this study was to update the conceptual hydrogeological model of Caldas da Rainha thermomineral waters (used in Spas, Central Portugal). Special emphasis was put on: - the characterization of local/regional flow paths and - the interaction between shallow cold ( 17 ºC) dilute groundwaters with warm ( 33 ºC) thermomineral waters.
2. GEOLOGICAL SETTING The thermomineral waters discharge from springs located close to a locally N-S oriented oblique fault (60º E). This fault places loamy detritic Upper Jurassic rocks and Hetangian- Retian marls (and evaporitic deposits) in contact. The thermomineral system is dominated by deep fissured-porous Jurassic limestones containing slow flowing groundwater in the fissures resulting in a high storage capacity. Geological map of the research region. Location of the Caldas da Rainha Spa. Adapted from Carta Geológica de Portugal Folha 26 -D Caldas da Rainha. Serviços Geológicos de Portugal.
2. GEOLOGICAL SETTING (cont.) Schematic geological and structural cross-section of the studied area. (A) J 1 ab; (B) J 2 abc; (C) J 3 c / J 3 b; (D) J 3-4 ; (E) P. (1) Caldas da Rainha diapir; (2) Candeeiros Mountain; (3) Fonte da Bica diapir. The arrows indicate the movement directions. (i) thermomineral spring at Caldas da Rainha Spa; (ii) exploitation borehole of thermomineral water. Adapted from Zbyszewski (1959). (A) (B) (C) (D) (E) J 1 ab - marls - Hetangian-Retian J 2 abc limestones LowerJurassic J 3 c / J 3 b - limestones Middle Jurassic J 3-4 - loamy detritic Upper Jurassic rocks P Sedimentary deposits - Pliocen
Some sampling sites in the study region: Casal dos Chãos (dug well); Quinta das Janelas (thermomineral spring 31 º C); Tornada stream (the water flows under the bridge; Moitas (dug well).
3. GEOCHEMICAL SIGNATURES OF CALDAS DA RAINHA THERMOMINERAL WATERS 1) Caldas da Rainha thermomineral waters belong to the Cl-Na sulphurous-type. 2) The presence of reduced species of sulphur is likely related to bacterial sulphate reduction. 3) The presence of HCO 3-, Ca 2+ (and even Mg 2+ ) are ascribed to water-limestone interactions. 4) The Na +, Cl - and SO 4 2- concentrations are associated with the dissolution of halite and gypsum lenses founded along the regional syncline. 5) The issue temperature is 33 ºC. 6) Dry residuum 2800 mg/l.
4. STABLE ISOTOPES ( 18 O; 2 H) Schematic geological and structural cross-section of the studied area. Adapted from Zbyszewski (1959) δ 2 H vs δ 18 O diagram for groundwaters ascribed to different geological formations. (A) (B) (C) (D) (E) J 1 ab - marls - Hetangian-Retian J 2 abc limestones Jurassic J 3 c / J 3 b - limestones Upper Jurassic J 3-4 - loamy detritic Upper Jurassic rocks P Sedimentary deposits - Pliocen Caldas da Rainha thermomineral aquifer is depleted in heavy isotopes, when compared to the shallow cold groundwater sampled in the J 3-4 loamy and detritic formations, indicating the presence of distinct aquifer systems.
5. AGE" OF THE THERMOMINERAL WATERS The thermomineral spring waters with no 3 H and high mineralization should be faced as the most representative of the deep thermomineral aquifer system. The presence of 3 H (from 1.1 to 2.8 TU) in AC1-B, AC2 and JK1 Caldas da Rainha borehole waters should be linked to the existence of a shallow thermomineral groundwater flow system. In the case of the thermomineral waters 13 C values range from - 9.9 to - 6.6 vs. V-PDB, which are typical for carbonate dissolution enhanced by soil derived H 2 CO 3 (mainly from the recharge areas). The results of 14 C age determination (with 13 C correction) indicate a considerable apparent age for the Caldas da Rainha thermomineral waters: from 2.5 ka in borehole waters, up to 8.8 ka BP in spring waters data from Marques et al. 2013).
6. ENVIRONMENTAL ISSUES Diagrams SO 4 vs. 3 H and NO 3 vs. 3 H for groundwaters ascribed to different geological formations. Data from Marques et al. (2013). - The shallow cold groundwaters (J 3-4 - loamy detritic Upper Jurassic rocks) present elevated NO 3 and SO 4 concentrations, due to intense agriculture practices. - This trend was not detected in the thermomineral waters, which seem to be isolated from anthropogenic contamination.
6. ENVIRONMENTAL ISSUES (cont.) Environmental impact on the thermomineral waters, derived from mixing with shallow contaminated groundwaters, was evaluated based on δ 18 O and geochemical data. NO 3 - vs. δ 18 O for groundwaters sampled in different geological formations.
7. 34 S (SO4) e 18 O (SO4) ISOTOPIC SIGNATURES IN GROUNDWATERS Refª. 34 S (SO4) o / oo 18 O (SO4) o / oo SO 4 (mg/l) Quinta das Janelas 19.1 16.2 802 Piscina da Rainha Group 1 (thermomineral waters) 17.1 13.3 600 AC2 17.8 11.1 604 Variation of 34 S (SO4) and 18 O (SO4) values observed in atmospheric, pedogenic and geogenic sulphates. Adapted from Krouse and Mayer (2001). Data from Marques et al. (2013). AC3 14.9 14.8 600 Moitas Casal da Marinha Group 2 (J 3-4 ) 4.1 9.3 56 1.5 8.6 48
8. CONCEPTUAL MODEL - The infiltrated meteoric waters percolate along the syncline (c.a. 15 km length) promoting water-rock interaction. - The presence of HCO 3-, Ca 2+ (and even Mg 2+ ) seems to be ascribed to the water-limestone interaction. - The Na +, Cl - and SO 4 2- concentrations should be mainly associated with the dissolution of halite, gypsum and anhydrite lenses founded along the deeper parts of the regional syncline. //////////// ////////////
Acknowledgements CERENA/IST acknowledges the FCT support through the UID/ECI/04028/2013 Project and C2TN/IST author gratefully acknowledge the FCT support through the UID/Multi/04349/2013 Project.
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