CONSIDERATIONS CONCERNING THE ORIGIN OF THE ESTORIL (PORTUGAL) THERMAL WATER

In the urban area of Estoril, a Portuguese tourist village 20 km westward from Lisbon, hot mineral waters (thermal waters) spout out from natural springs which have been known since the 19th century. The thermal waters are of the sodium chloride type, with total dissolved solids and temperatures higher than the ones of the regional waters. Isotopic data are consistent with a meteoric origin for both the regional and thermal waters of the Estoril area: 8180 (%0), --4.16 to -3.52 and 82H (%0), -25.5 to -18.7. The thermal water composition can be derived from the regional water composition assuming the dissolution of evaporite minerals, cation exchange and precipitation of calcite. The thermal water flow system has probably the recharge area somewhere in between Estoril and the Sintra mountain. The elevation difference between the recharge area and the sea provides the driving force for groundwater movement to the Estoril area where the upward movement of the mineralised and warm water is controlled by an impermeable barrier of dykes and open fractures in the pre-existing rocks.


Introduction
In the urban area of Estoril, a Portuguese tourist village 20 km westward from Lisbon (fig. 1),there are two main mineral water springs which waters are of the sodium chloride type, with total dissolved solids (TDS) and temperatures higher 3 to 5 gIL and 10 to 15 oC than the ones of the regional waters, respectively.These thermal springs are known since the 19th century and their water was used for therapeutic purposes in an old spa which building has already been demolished.
According to Portuguese law, mineral waters are State property and their exploitation must be authorized by government since it is a concession.In order to reduce the pollution potencial, to protect the quality and to improve the quantity of mineral water resources, the policy of the Por-

Climate
The Estoril thermal springs are located in a region of great tourist activity which greatly bene-tuguese authorities has.encouraged the const~uc tion of the deep wells m exchange of the spnngs (Carvalho, 2002).According to these guidelines, at present the Estoril thermal springs are not exploited and in the Estoril concession there are two deep wells (TWl and TW2) which thermal water will be exploited for a new spa.However, the plan has yet to be approved by the Portugue~e administration.These wells were constructed m the late 1980's after an hydrogeologic study of the Estoril area.The main conc1usions of this study were presented by Carvalh? et al. (19~2).~h.ey pointed out that the ~ater IS of meteo~lc ongm, infiltrate somewhere m between Estonl and the Sintra mountain, percolate to great depth in the sedimentary basin, and ascend in the low-altitude Estoril area.In order to explain the high TDS they pointed out the mixing of seawater and hot freshwater and the hypothesis of percolation trough the evaporites of the Dagorda formation (Hettangian stage).
The main objectives of this paper are to present new data about the chemistry and the isotope composition and to updat~the conceptual m.odel of the origin and circulatlOn of the Estonl thermal waters.In order to do that, the data on the chemistry of thermal water from the TWl a~d TW2 wells will be considered.Also the chemlcal and isotopic composition of the therma.lwater will be compared with the ones of the regIOnal waters of sorne wells located northward of the TWl and TW2 wells.

Geology
The Estoril thermal springs field is located about 5 km southward of the Sintra subvolcanic massif.The intrusion formed a dome surrounded by a ring sync1ine (Alcabideche sync1ine) which northe~n limb dip increases from the south to the north m direction of the Sintra massif (fig.2).
South of the Alcabideche syncline, a flat monocline structure with sorne second order folds was developed (Costa & KUllberg, 1981).The thermal springs are located at the E l~m? of a s~cond ord~r fold (Abuxarda-Bicesse antlc1me) WhlCh core IS formed by lower Cretaceous terranes.In the thermal springs area the Cenomanian-Albian argil.laceouslimestones dip 5°to 8°to E. Dykes of baslc rocks cut the sedimentary units with the N5°W general trend (fig.3).
At the beginning of the 1980's two 250 ~long boreholes were drilled in the area surroundmg the Estoril Spring using diamond core bits y) investigate the geological and structural c.ondltlons connected with the thermal water dlscharge.The results of the detailed study of the core samples were presented by Carvalho et al (1982).They defined the following stratigraphic units beneath the Cenomanian-Albian argillaceous limestones outcrops: -Lower Albian -Upper Aptian (Upper Almargem Sandstones): fine and coarse sandstones and clays.
. .-Lower aptian -Upper Barremlan (Layers wlth Orbitolina): limestones and argillaceous limestones with sorne intercalated sandstones and clays.fits from a pleasant climate with sorne medite!ranean characteristics.The average rainfall m Monte Estoril is 636 mm/year and the mean annual air temperature is 17 oc.December and January are the coldest months (the mean temperature is about 10 oC) and July and August are the hottest ones (the mean temperature is about 22 OC).Rainfall contribution is 80% during the cold season (October-March) and 20% during the warm season (April-September).The rainy season has two rainfall peaks (January and March) and during July and August there is practically no rain.The annual rainf~ll steadily increases from the south to the north m direction of the Sintra mountain, located about 5 km northward of Estoril (the average rainfall in Sintra-Granja is 840 mm/year).On the other hand, the temperature decreases from the south to the north (the mean annual air temperature is 15 oC in Sintra-Granja).-Upper Barremian -Lower Barremian (Lower A1margem Sandstones): clays, marls and coarse sandstones.
They a1so pointed out that these units are partia1ly obliterated by dykes of basic racks.

Sources of the Estoril thermal water
In the urban area of the Estoril village hot mineral waters (thermal waters) issue fram natural springs as well as from drilled wells.A detailed inventory of springs and dug and drilled wells was presented by Carvalho et al (1982).The water ofthe most important spring ("" 2 L/s), Nascente do Estoril (Estoril Spring), has TDS = 3.4 gIL and temperature = 30 oc.This spring is located at the end of a 44 m length gallery, about 350 m landward fram the Atlantic coastline and 10 m a.s.l., in the contact zone of the Albian argillaceous limestones with the Upper Almargem Sandstones.The Nascente da Por;a (Po<;a Spring), with a diminutive yield, discharges at the Albian argillaceous limestones of the S. Joao do Estoril beach, about 2 m a.s.l.The water has TDS = 3.1 gIL and temperature = 26 oc.Besides these sources, 3 hot water drilled wells located into the spring area are also reported by Carvalho et al. (1982).
The intake part of the Estoril concession wells (TW1 and TW2) was placed in the limestones of the lower Barremian (Limestones with Choffatella and Dasicladaceae).The data on the chemistry of groundwater from the TW1 and TW2 wells include about one hundred analyses fram 1989 until now.The water pumped out of these wells has identical facies to the thermal water springs but the TDS and the temperature are higher.The maximum TDS and temperature values were measured for the TW1 water: 6 gIL and 35.5 oC, respectively.Although the graundwater level in the wells presents sorne seasonal and tidal fluctuations the chemical composition maintains a remarkable constancy (fig.4) -the variation coefficient for the main chemical parameters is lower than 10% (n =60 for the TW1 water).

Methodology
Representative ehemical analysis of the major ions and isotopie data for 18 0, 2H and 3H from the Estoril thermal waters (TWl and TW2 deep wells) and from the assumed regional groundwater (RW13 and RW17 deep wells) were made from samples eolleeted during July 1995.The RW13 and RW17 wells are loeated northward of the thermal area (fig.2).The main eharaeteristics of the wells are presented in Table l.
The Nuclear and Teehnologieal Institute (ITN) in Portugal earried out the isotopic determinations.The 18 0 and 2H of water were measured with a mass speetrometer (SIRA 10 VG ISOGAS) using the methods of Epstein and Mayeda and Friedman, respeetively (Gonfiantini, 1981) expressed in delta notation re1ative to the Vienna-Standard Mean Ocean Water (V-SMOW).The standard deviation is 0.10%0 for 0 18 0 and 1.0%0 for 02H (Paquete, 1998).
Ana1yses for 3H were made using e1ectro1ytic enrichment and subsequent measurement of counting rates by 1iquid scintillation (IAEA, 1976).The 3H are in TU (tritium units) and the standard deviation of the measurements is 0.7 TU (Paquete, 1998).
The Laboratory of Ana1yses of the Department of Geo10gy of the Coimbra University carried out the chemica1 ana1ysis.Alca1inity and pH of these water samples were not measured in the fie1d, so it is possib1e that are sorne minor errors in these parameters.Nevertheless, the alca1inity and pH va1ues are about the same as the ones measured in the fie1d at other time.
The physico-chemica1 and isotopic characteristics of the sam-p1ed water are presented in Table 2.The physico-chemica1 characteristics of Estori1 Spring thermal water and of the water in the meantime abandoned well (RWI9), samp1ed during September 1979 as reported by Carvalho et al. 1982, are also presented.

Chemical composition and the origin of solutes
Figure 5 shows the major solute concentrations plotted on a Piper trilinear diagram.Two geochemical groupings are defined: (1) Na-Cl-type waters with TDS values between 2,1 and 5,9 mgIL and (2) Ca-HCOrtype water with TDS value of 0.75 gIL which is consistent with what is expected from the water-rock interaction in carbonated terranes.The Na-Cl-water group includes the Estoril hot mineral waters (T >25 OC) and colder waters (21 and 24 oC) from the RW13 and RW17 wells, respectively.
A t test for the significance of sample correlations, r, is performed (Davis, 1986) t=r -Vl-r2 which has n-2 degrees of freedom.The critical values for t with 4 and 2 degrees of freedom and a 5% level of significance are t =2.776 and t =4.303.
Most of the species in solution are positively correlated (Table 3); the only exception is the negative correlation of HCO r with the other major ions.The correlation of the temperature with the ionic species is also positive with the bicarbonate exception.This suggests the precipitation of carbonates while in meantime the TDS increases and the water percolates deeper.The correlations with pH have not statistical significance.
The saturation indexes of calcite and dolomite, shown in Table 4, reflect supersaturation conditions and suggest precipitation.On the contrary, the negative saturation indexes of the evaporite minerals, like halite and gypsum, reflect subsaturation and dissolution is expected.These facts also support the aboye hypothesis of mineral dissolution with increasing carbonate precipitation.
The value of Na++K+/Cl-(Table 5) for all the water samples is lower than the expected 1: 1 stoichiometric relation of halite+sylvite dissolution, indicating that a fraction of about 15 to 20% chloride is associated with cations different from the sodium and potassium.On the contrary, the value Ca 2 +/S04' is greater than the stoichiometry of gypsum dissolution and the increasing of Ca 2 + with TDS is greater than the one of S04'.So, a supplementary origin for the Ca 2 + is necessary once the reduction of the S04' is unexpected in this geochemistry environment.The exchange of Ca 2 +for Na+ is a possible hypothesis.
The dedolomitization is another geochemical process suggested by the ratios presented in Table 5.In fact, the dissolution of gypsum induces the transformation of dolomite to calcite in the rock and produces waters with increased Mg2+, Ca 2 +, and S04' concentrations, and decreases alkalinity as observed in the studied waters When equilibrium with calcite and dolomite is maintained, the ratio of Mg2+/Ca 2 + must remains around 0.8 and ideal stoichiometric relations for the reaction are Ca 2 +: S04' = 1:1.8and Mg2+: S04' =0.8:1.8.(Appelo & Postma, 1996).These ideal values deeply differ from the ratios presented in table Table 5.So, the dedolomitization does not seem to be a relevant geochemical process in the Estoril aquifer and the responsible for Mg2+ and Ca 2 +dissolution.
According to our interpretation of the data, the excess of solutes of the thermal waters over the ones of the "normal" water is originated from the disso-1ution of evaporite sediments.These ones can occur dispersed in sorne Cretaceous and Jurassic carbonated rocks or like evaporite deposits on the base of the Mesozoic sediments ("Dagorda" formation of the Hettangian stage).These evaporite deposits can also occur at upper levels as a result of the upward intrusion of the salt through faults.Although there For example, TWl water composition can be schematically derived from the RW19 water composition assuming the mass balance approach and the following reactions: The temperature is another peculiar feature of the Estoril thermal water.Assuming a geothermal gradient of 3 oC per 100 m of depth and the mean annual air temperature of 16 oC, a minimum depth of 700 m would be required to obtain the temperature of 35.5 oC (temperature of the TWl well water).In fact the depth and temperature reservoir must be higher because in the upward movement the water loses temperature in contact with the upper rocks -according to temperature measurements in the TWl and TW2 wells the loss estimate is 1 to 1.5 oC per 100 m.So a reservoir depth up 1,500 m seems possible.

Isotopic composition
In the classical plot of 02H as a function of 0 18 0 the samples of the thermal and regional water are O,.-------------------:;;~m ountain ("" 5 km northward of Estoril) probably in the limestone outcrops (upper Jurassic) at the base of this mountain.The elevation difference between the recharge area and the sea provides the driving forces for groundwater movement to the Estoril area.During the deep circulation the groundwater mineralization and temperature increase.The flow system has the discharge in the Estoril area, where the upward movement of the mineralised and warm water is controlled by an impermeable barrier of dykes and open fractures in the pre-existing rocks.

References
grouped close to the meteoric water line (fig.6).So the groundwater is assumed to have originated from the precipitation falling in the recharge area.The 3H concentration of the TWl thermal water is 0.78 ± 0.70 (TU) which means that this groundwater can be dated prior to 1952 in a qualitative manner.

Conclusions
The Estoril thermal water is a mineral water of the sodium chloride type, with TDS and temperature higher than the ones of the regional waters.The maximum TDS and temperature values were measured for the TWl water: 6 gIL and 35.5 oC, respectively.
The thermal water is assumed to have originated from the precipitation falling in the recharge area and the dissolution of evaporite minerals, such as halite, sylvite, magnesium chlorides and gypsum results in an excess of solutes of the thermal waters over the ones of the "normal" water.Gypsum dissolution is mainly responsible for S04= and Ca z +.The cation exchange also contributes to the content of Ca z +.The released calcium precipitates as calcite when the water reaches a critical supersaturation condition.Bicarbonate originated by the COz absorbed from the atmosphere and decomposition of organic matter in the recharge area decreases with the increasing water TDS values.
The thermal water flow system has the recharge area somewhere in between Estoril and the Sintra

Table 1 .
-Characteristics of the selected drilled wells

Table 4 .
-Saturation indexes calculated by the WATEQX program*

Table 5 .
-Ionic ratios of the selected groundwater samples