RESULTS FROM STUDIES IN SEVERAL KARST …hydrologie.org/redbooks/a207/iahs_207_0295.pdfthe coastal...

Hydrogeological Processes in Karst Terranes (Proceedings of the Antalya Symposium and Field Seminar, October 1990). IAHS Publ. no. 207, 1993. 295

RESULTS FROM STUDIES IN SEVERAL KARST FORMATIONS IN SOUTHERN CATALONIA (SPAIN)

EMILIO CUSTODIO, ALFONS BAYO, MANEL PASCUAL & XAVIER BOSCH Polytechnic University of Catalonia, Department of Engineering and International Course on Groundwater Hydrology, Gran Capita sin, Môdul D-2, 08034 Barcelona, Spain

ABSTRACT

Catalonia, in northeast Spain, has several important permeable carbonate formations that contain karstic features. They provide important water resources and are subject to intensive exploitation. The paper considers the results of studies carried out during the last 10 years in several areas: the Garraf and Vandellos coastal massifs of Jurassic and Cretaceous age, the southern pre-littoral range of Triassic and Eocene age, and the deep-seated Cretaceous carbonates of the Plana de la Galera area, near the coast. Studies in the coastal formations point to the importance of ion exchange and organic matter oxidation in the generation of carbonate dissolution capacity, that may overcome the effect of fresh-water and salt-water mixing in the enhancement of the corrosiveness. Local tectonic disturbances and possibly the dissolution of gypsum are also important in other areas, and control the development of major flow paths. Sedimentary discontinuities explain the presence of buried karsts that if non-obliterated may constitute major regional aquifers.

INTRODUCTION

Catalonia is situated along the Mediterranean coast in northeast Spain; it has a length of 350 km. Numerous, important carbonate formations are found, corresponding to the Pyrenees, Catalanides and Maestrat mountain ranges (Triassic, Jurassic, Cretaceous and Eocene) and Miocene deposits in the tectonic depressions (Fig. 1). Most formations have a thickness of several hundred metres. The limestones and dolomites are frequently between marls and even gypsum and anhydrite layers (Solé-Sebarfs, 1958; Anadon et al., 1979; AIH, 1989). Some areas present conspicuous karst features and cavities but others are dominated by fracture permeability characteristics. Non-obliterated old sedimentary discontinuities subjected to karst processes under subaerial environment have good water yielding capacity.

Hydrogeological studies have been carried out since 1965 by the Eastern Pyrenees Water Authority and the Public Works Geological Service office in Barcelona, both presently incorporated into the Water Authority of Catalonia, with the cooperation of the Polytechnic University of Catalonia and the International Course on Groundwater Hydrology (REPO, 1970; PHPO, 1985). Most of these studies have been directed to solve real supply problems or to provide the basis for groundwater use planning, but recently more research oriented hydrogeological works have been done by university teams.

The water demands of the relative dense population of the coastal Catalan areas, supporting the large conurbation of Barcelona (about 4 million), important industrial belts, popular tourist resorts and intensively irrigated agricultural areas, have resulted in the over exploitation of aquifers as a source of fresh water. There are numerous examples of large cumulative drawdowns and sea-water intrusion. Also, badly planned, uncontrolled and even criminal pollution activities are common near the large urban, industrial and tourist (mainly weekend) areas. Thus groundwater contamination problems also need to be considered (Custodio, 1982).

The complicated geology and tectonics and the mountainous character of the area, with

296 Emilio Custodio et al.

Fig. 1 - Location map of Catalonia with main areas and mountain ranges. The lower figure is a simplified map indicating the main carbonate formations of hydrogeological interest in southern Catalonia. T = Triassic; J = Jurassic; C = Cretaceous; E = Eocene.

ranges parallel and close to the sea, allows only for small aquifers, thus aggravating local problems. The present paper deals with some examples taken from southern Catalonia, with relatively low rainfall (500-550 mm/year) and moderate recharge (10-50 mm/year under most common circumstances). They do not include the metropolitan area of Barcelona, although the Garraf massif is just at the southwest edge. Carbonate aquifers are mostly of the open type, after the classification of Bayô (Bayô, 1982; Bayô et al., 1986).

Further to more classical studies on hydrogeology and groundwater resources, some research has been done in the southern Garraf and Vandellôs massifs. These studies were in parallel to research carried out on Mallorca Island (Price, 1988; Herman et al., 1985), and all were part of a cooperation between the US Geological Survey (Reston) and the Polytechnic University of Catalonia (Back et al., 1989). One of the goals was to extend studies done in Florida and Yucatan (Back &Hanshaw, 1970a,b, 1982, 1983; Back et al.,

Results from several karst formations in southern Catalonia 297

1979, 1981, 1984, 1986; Hanshaw & Back, 1979, 1980a,b) and elsewhere (Cotecchia, 1977) to other areas in order to understand the role of mixing fresh water and marine water on the enhancement of karstic processes in carbonate coastal aquifers, following the original ideas of ion strength changes (Wigley & Plummer, 1976; Plummer, 1975; Sanford & Konikow, 1989; Herman, 1986).

These studies have been based mainly on chemical equilibria studies and mass balances (Plummer & Back, 1980), but the framework was obtained by more conventional chemical studies and water salinity logging, with the aid of environmental isotopes. The existence of vertical flows and water renovation inside the boreholes has been checked by means of radioactive tracers ( m I ) using known simplified techniques (Baonza et al., 1979; Custodio & Llamas, 1976, Sect. 12; Custodio, 1981; Halevy et al., 1966; Plata, 1982).

Processes in the saturated zone that has been taken into account, further to those leading to ion strength effects, are cation exchange, redox processes and dolomite formation/dissolution (Custodio & Bayô, 1989; Custodio, 1986). Cation exchange is a common process if negative charges exist on mineral surfaces and edges (Sayles & Mangelsdorf, 1977; Mercado, 1985; Magaritz & Luzier, 1985) and dolomite-related processes have also received attention (Hanshaw et al., 1971; Magaritz et al., 1980; Shatkay & Magaritz, 1987; Ward & Halley, 1985).

THE GARRAF MASSIF

The Garraf massif is just to the southwest of Barcelona and constitutes a sparsely populated area due to its abrupt relief and lack of suitable soil, except along the coast when some flat land is available, and along the contact with the Penedés graben. From northeast to southwest carbonate formations (dolomites and limestones) with clay and gypsum layers, dipping to the southeast, are successively covered by Jurassic dolomites and then by Cretaceous limestones and dolomitic limestones, with calcarenite and conglomerate Miocene sediments filling some depressions (IGME, 1982). The massif is the result of a south-north regional overthrust, probably above Paleozoic schists, and consequently is intensively faulted and has horizontal displacements along vertical planes normal to the coastline.

The dominant bare carbonates, especially in the eastern part, present conspicuous karstic features, both in surface and in depth, but sinkholes, some of them more than 100 m deep, generally do not reach sea level. But some submarine caves exist, developed mainly along the horizontal displacement faults, that actually are the main water drains of the massif (Custodio, 1975).

The largest submarine spring is La Falconera (Astier, 1970, 1971) that discharges brackish water with a mean fresh-water content of about 400 1 s"1 (Fig. 2). Sea water enters the La Falconera main duct through the small network of fissures that link the cave with the sea bottom. The greater specific gravity of sea water accounts for the small saltwater flow that converts the fresh water into brackish water (Batista et al., 1983; Custodio & Llamas, 1976, sect. 13; Custodio & Bruggeman, 1986), and precludes the use of these water resources near the coast to solve existing water supply problems. Several attempts have failed.

A large sanitary landfill was constructed in 1973 in the eastern part of the massif, at relatively high elevation (Fig. 3), on Jurassic dolomites (CAPO, 1973). At the start of the operation, a part of the leachates not collected by the bottom drainage system percolated through the fissures and karst-enlarged fractures and moved laterally towards the La Falconera and nearby areas, about 4 km away (Petit, 1981).


Fig. 2 - Cross section through La Falconera submarine spring (modified from Astier, 1970) indicating fresh-water flow and salt-water flow originating offshore due to the higher sea-water specific weight (Custodio & Bruggeman, 1986). Surveys made by speleologists do not show a clear salinity vertical stratification in the water; it may be due to turbulent flow, at least in some sections. The map shows the assumed groundwater flow paths in the active saturated zone (after Soler, 1984).

The first symptoms appeared after eight months in the main spring in the form of a hydrogen sulphide smell, dark colour of the water (probably due to elevated sulphur and tiny iron sulphide particles) and loss of dissolved oxygen, as well as an increase in bicarbonate and hardness in local well water. It was shown that water discharging in La Falconera was depleted in sulphate with respect to the mixture of fresh water and sea water (Fig. 4) as a consequence of the strongly reducing underground environment created by infiltrating leachates (Custodio & Galofré, 1976; Custodio, 1981; Hoyos-Limôn & Palomero, 1975; Cebollada & Urcola, 1978).

Karst caves and sinkholes near the landfill were soon invaded by methane gas, and studies for its recovery for energy purposes were commissioned. Leachate collected by


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the landfill drains showed a total chemical oxygen demand equivalent to about 130 g L1

of 0 2 (Custodio, 1981). The depletion of sulphate in the La Falconera needs an infiltration of about 1 1 s'1 of leachate, in spite of the valley floor on which the landfill is placed being previously lined with cement and a layer of low permeability soily material, a costly and rather ineffective exercise. Observations show that contaminants move at a low angle to groundwater head contours due to anisotropic regional permeability. This is a consequence of the normal (gravity) faulting orientation of the carbonate materials.

In the Garraf carbonate formations, after heavy rains - mainly in early autumn - data from the Catalonia Water Authority monitoring groundwater network show fast groundwater head increases, with rapid descending limbs (Fig. 5). Although no detailed studies have been carried out, there is evidence to confirm that this is due to the formation of perched aquifers that also collect in the long screened monitoring boreholes. Small inflows in the upper part can produce large water level increases if the lower section is open to permanently saturated, low permeability carbonates. This is more likely far from the sea, where karstic development is less important below the water table due the high elevation. This also explains the large differences between the monitoring boreholes (Llamas, 1968; Nifierola & Fuster, 1969; PHPO, 1985; REPO, 1970).

The productivity of the wells varies widely as can be expected in fissured and karstified rocks. Most of the wells are in or close to the coastal plains. A plot of well productivity versus position of the well bottom relative to sea level (Fig. 6) shows that most of the wells penetrate just a few metres below sea level in order to get the required yield (generally a few 1 s"1). Some wells unable to get enough water at this penetration were drilled deeper, but generally did not attain the desired results (Pascual, 1990; Pascual et


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Fig. 4 - Chemical composition changes induced by the incorporation of leachate from the Bègues controlled landfill of Barcelona, about 4 km far. The upper figure refers to the Falconera submarine spring water, sampled inside the gallery by divers. The theoretical mixing line is that observed before the landfill and correspond to a simple fresh and marine water mixing. The actual mixing line is a mean position of observed S04

2" vs. CI" contents in 1974 and 1975, after the landfill site was in operation. There is a loss of S04

2" (the intensity varies according to the moment of sampling), equivalent to the introduction of about 6 tons of soluble organic matter. The lower figure shows the corresponding increase in HC03" in some shallow wells at nearby Garraf village. Line A is the distribution before the landfill operation and B afterwards (after Custodio & Galofré, 1976; Hoyos-Limôn &Palomero, 1975).

al., 1986b; PHPO, 1985). This shows a preferential permeability zone around the present sea level. Drillers' well logs frequently mention some karst features at this depth. Below it the carbonate rock seem little affected by general dissolution, in part related to Quaternary sea level changes in the Mediterranean sea (Riba, 1981; Fairbridge, 1972).

Present intensive groundwater exploitation in these coastal areas has resulted in sea-water intrusion (Fig. 7) and consequent groundwater quality degradation (Carbonell et al., 1988; Pascual, 1990; Font & Rafaelli, 1984; Pascualetal., 1986a; Alom&Soler, 1989). Wells obtain a mixture of locally recharged fresh water with inflowing sea water. The landward flow of fresh water and salt water from the coast toward the wells explains why


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Fig. 5 - Hydrograms of two wells in the central Garraf massif, one close to the sea (Sitges) and the other in a valley far from the sea (Olivella). Data from the Catalonia Water Authority groundwater monitoring network. Effective rainfall values (thick bars = recharge; lines = runoff) from daily water balances in the soil (Pascual, 1990) are shown below.

wells with water levels permanently below sea level (Fig. 8) yield brackish water instead of salt water (Pascual & Custodio, 1990). Both fresh water and salt water have water heads below sea level.

Assuming the wells yield a mixture of local and marine water without large chemical changes, it is possible to obtain the chemical composition of the fresh water component if chloride is assumed to be a conservative ion, since the composition of the sea water is known. This shows a reasonably homogeneous fresh water source all over the southern sector (Pascual, 1990) except in the area downstream of the Foix-Castellet dam (Fig. 9), a reservoir which leaks water into the Cretaceous limestones in the Foix River valley (GEOTECNIA, 1980; Ninerola & Fuster, 1969; REPO, 1970; Llamas & Doménech, 1967, 1969). Foix River water is allochthonous to this area and has a lower chloride content and a characteristic high sulphate content. Also the environmental isotope content of the water is lighter and affected by evaporation effects.

CHEMICAL PROCESSES IN THE MIXING ZONE IN SOUTHERN GARRAF CARBONATE FORMATIONS

A detailed hydrochemical study has been carried out in the coastal Cretaceous carbonate formations in the southern part of the Garraf massif to understand the rock-water interactions inside the fresh-water/salt-water mixing zone of the aquifer. This mixing zone is maintained by groundwater abstractions away from the coastline.

A borehole that penetrates the upper half of the mixing zone and placed between the pumping wells and the coast have been periodically sampled, as well as another well inland from the pumping wells, and used as a reference to find the chemical changes in fresh-water recharge with time (Pascual, 1990). This fresh-water well does not represent the actual fresh-water mixing with salt water, but since the area has a rather homogeneous landscape the assumption of representativeness seems acceptable.


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Samples were taken with a grab sampler that was open when lowered to the desired depth and then closed by a sliding weight. The boreholes are cased with PVC. It is assumed that PVC does not induce chemical changes in the water. To know the representativeness of the samples the existence of vertical flows inside the bore was studied by introducing a radioactive tracer (131I) and measuring the dilution rate and also by repeated well logging with a submersible electrical conductivity cell. No bulk vertical flows exist, but between the permeable sections - mainly karstified fissures and layers - there is some flow to adjust the salinity vertical distribution along the water column in order to compensate for hydraulic potential deviations (Custodio et al., 1989).

Permeable sections correspond to the fast changes in water electrical conductivity, and the sections of the salinity log presenting small changes indicate low permeability rock between specially permeable zones (Fig. 10).

To get a time series of samples that can be compared, the low vertical electrical salinity change section was selected - at 23, 40 and 58 m depth - in spite of the possibility of


Fig. 7 - Simplified distribution of groundwater salinity in exploitation wells in the Garraf massif coastal plains (modified from Carbonell et al., 1988; and Soler & Trilla, 1979). The salinity penetrations correspond to the areas with more developed karst features.

chemical changes due to the sluggish renovation of the water (Pascual & Custodio, 1987; Bosch et al., 1990; Custodio et al., 1990).

Conspicuous changes in groundwater temperature have been found in spite of the relative large depth of the samples (Fig. 11). This is explained as the effect of preferential paths for a fraction of rainwater recharge, especially of relatively warm early autumn rainfall, when the soil is less vegetated and dry.

The study of chemical equilibria between water and rock, and also with gas, and the mass balances (Figs 12 and 13) show that (Pascual, 1990; Pascual & Custodio, 1990):

Fig. 8 - Schematic cross section to show groundwater flow in the coastal carbonate aquifers due to well pumping and the mixing of fresh and saline water. Boreholes S.01 and S.02 are those mentioned for the detailed studies. Bedrock is non-karstified carbonates or obliterated karst, the boundary being irregular and transitional.


-Miocene caicareratts = PIio Quaternary cover

Fig. 9 - Chemical analysis of groundwater in the Garraf coastal area. The vertical logarithmic columns at the left-hand side shows the difference between water derived from the Foix River and locally recharged fresh water. The right-hand map shows the modified Stiff diagrams of the fresh water component of pumped groundwater. A large fraction of S0 4 downstream from the Foix Reservoir means there is a possibility of receiving recharge from leakage (Pascual, 1990).

— Ca-Na ion exchange plays a role in the chemical behaviour and is due to the fact that in the area there is a continuous trend toward groundwater salinity increase. Ca2 +

increases in water. Exchange positions are thought to be in clays derived from impure limestones and marls.

— There is calcite precipitation in the zone close to the water table and also some C0 2 exchange with the unsaturated zone gas, mainly dissolution of it.

- Carbonate dissolution occurs in the lower half of the borehole, inside the upper half of the mixing zone.

- Conditions exist for dolomitization and this is probably happening, although it is not easily distinguishable from possible Ca-Mg ion exchange.

Some environmental isotope studies (180, 2H in water; 13C in dissolved inorganic carbon) do not contradict these conclusions but the number of data available is not adequate for conclusions to be drawn from them with confidence. The unsaturated gas isotopic composition is unknown, thus preventing water-gas exchange studies.

Cation exchange processes seem the rule in the Garraf carbonate coastal aquifers due to the continuous trend of increasing salinity (Fig. 14) as shown by PHPO (1985), Carbonell et al. (1988) and Pascual (1990).

SEA-WATER INTRUSION IN THE VANDELLOS CARBONATE MASSIF

The Vandellôs coastal carbonate massif, south of Tarragona, is formed mainly by Jurassic


Fig. 10 - Characteristics of borehole S.01. A - lithology; karstified and highly fissured parts are indicated as indentations of the column and a change in the brick-like symbol for carbonates; below 61 m the borehole is uncased and infilled with debris. B - Gamma-ray log; high radioactivity deflections towards the right. C - Radioactive tracer (1-131) dilution test results (the one made in July 1986) after tracing the whole water column inside the PVC casing; figures show elapsed time after first log (0), immediately after the extraction of the tracer injection hose. D - The two extreme water salinity (electrical conductivity) logs from a series of 10; the shape is conserved in spite of large seasonal salinity variations. E -Explanation.

dolomites and dolomitic limestones (Anadônetal., 1979; Bayé, 1985; Bayôetal., 1976). The Vandellôs Nuclear Site (CNV) is on an alluvial fan at the foot of the mountain range.

The salinization at two deep wells in this carbonate formation (Cadaloques), fairly far from the coast, promoted a study that included the drilling and testing of four deep exploratory wells and several surveys to get chemical and environmental isotopic samples (Custodio et al., 1984). The pumping tests and salinity changes at different well discharges indicate that permeability is higher in the section close to the water level except at one of wells, close to the shoreline, in which permeability was high along the whole penetrated depth.

The iron casings of the wells precluded detailed chemical studies due to the induced pH changes (Bosch & Custodio, 1987a), but electrical conductivity logs (Fig. 15), chloride determinations and some isotope analyses are representative of the groundwater flow conditions. The sluggish flow of groundwater due to the small head gradient explains the slow re-establishment of salinity stratification after completion of drilling or pumping tests. The fresh-water/salt-water mixing zone thickness is fairly well defined up to a suspected fault running parallel to the coast, where permeability apparently decreases (Fig. 16).

Calculation of fresh-water flow towards the sea can be done by means of common approximate formulae, such as (Custodio & Llamas, 1976, sect. 13; Custodio & Bruggeman, 1986):

306 Ermlio Custodio et at

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Fig. 12- Rock-groundwater exchanges in borehole S. 01 at 23 m depth (2 m below the water table). Positive (and negative) values indicate Ca2+

increase (and decrease) by exchange with Na+, calcite dissolution (and precipitation), dolomite formation (and dissolution) and C0 2 inflow from the unsaturated zone (and degassing). After Pascual (1990) and Pascual & Custodio(1990).

which is valid when salt water can be assumed steady (with a head equal to mean sea level), the aquifer reasonably homogeneous, and for sites not too close to the coastal outflow. According to the Badon Ghijben-Herzberg formula, the fresh-water thickness is @ = z + zla = z(l + 1/a). Then:

q0 = [@2 k/2(l + a) X] + (WX/2)

For wells close to the coast the second right-hand term can be neglected. Some approximation for k was derived from pumping tests and also from radioactive tracer ( I ) dilution tests. Results show that q0 values thus obtained are clearly less than those expected by considering that recharge in the massif flows at right angles to the coast line. Also, water environmental isotope data (Bosch & Custodio, 1987b) show that recharge in the more rainy high altitude areas is not found in the wells at lower altitudes and that this water is similar to low elevation recharge (Fig. 17).

High altitude recharge moves westwards at a low angle to the coast, instead of normal to it due to the high permeability anisotropy introduced by the open fractures subparallel to the coast, and discharges in a series of submarine, near-shore springs, well known to the local fishermen. This can explain that coarse, carbonate-dominated alluvial fan

308 Erriïlio Custodio et al.

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Fig. 13 - Rock-groundwater exchanges in borehole S.01 at 40 and 58 m depth. Positive (and negative) values indicate Ca2+ increase (and decrease) by exchange with Na + , calcite dissolution (and precipitation), dolomite formation (and dissolution) and C0 2 inflow from the unsaturated zone (and degassing). After Pascual (1990) and Pascual & Custodio (1990).

sediments in the Vandellôs nuclear site contain diluted marine water and that temperature vertical gradients are reversed, i.e. temperature decrease with depth, due to the cooling effect of deep groundwater flow in the underlying carbonates (CIHS, 1990), recharged at high altitude (Fig. 18).

DETAILED STUDIES IN THE VANDELLOS AREA

To get a better understanding of the hydrogeological processes in the Vandellôs area, two research studies have been carried out, one in the unsaturated zone and another in dominantly carbonate sand deposits near the mouth of a gully. The first was aimed at


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2" is not fully explained (after Custodio et al., 1986). Studies by Carbonell et al. (1988) show the same pattern.

investigating carbonate dissolution in the vadose zone, as suggested by Wood (1985), and the second to evaluate the influence of chemical modifying phenomena, as commented before.

In the carbonates surrounding the Vandellôs nuclear site three places were selected to drill boreholes in the thick unsaturated zone and to install inside them a bundle of small diameter copper tubes. They are tubes open at different depths and isolated ones from the other in order to get point samples from soil gas (Bosch et al., 1990). The maximum depth sampled was 22 m. The C02 content was monitored and once the oxygen content and 13C in C02 . Results (Fig. 19) show that C0 2 partial pressure tends to increase with depth, at the time that 13C becomes heavier and 0 2 partial pressure tends to decrease. In part this is due to the oxidation of down-dragged organic matter and to the effect of carbon isotopic exchange between C02 gas and solid carbonate by means of reactions in the unsaturated zone. This leads to carbonate dissolution in this unsaturated zone. Two of the sites behave this way, with one showing a reverse trend, but this is in an alluvial fan and piedmont carbonate deposits.

The studies in the Cala Jostell sandy sediments were aimed at the understanding of chemical changes inside the fresh-water/salt-water mixing zone under long-term steady-state conditions. These sediments are deposited near the mouth of the Bassa Nova creek and consist mainly of carbonate particles (limestone and dolomite) with some feldspar particles. A series of six shallow bores were fitted with a PVC pipe open only at a given depth. They cover the full thickness of the mixing zone. The area is rich in organic mater and part of the year is under marshy conditions.

310 Emïlio Custodio et al.

WATER ELECTRICAL CONDUCTIVITY, «S/cm

Fig. 15 - Well water electrical conductivity logs from the Cadaloques observation wells, in the Vandellôs massif. Only extreme values are shown. The saturated thickness is in Jurassic dolomites except for the first 20 saturated m in well C, in which a thick Plio-Quaternary valley fill exists. Variations in the fresh and upper mixed zone is due to sluggish renovation of water after well construction and testing. The left-hand boundary is a good representation or undisturbed conditions, except for small vertical flows due to salinity adjustments to get a stable distribution. Black bars indicate sections along which no radioactive tracer dilution has been seen (no flow but not necessarily impermeable) (Custodio et al., 1989). See Fig. 20.

A sand and gravel bar separates this plain from the beach. In this case water-carbonate rock equilibria is influenced by sulphate reducing conditions, with ion exchange processes acting when there are salinity modifications due to hydrodynamic changes. Ion strength effects on activity coefficients play a minor role in this case. There is a clear increase in HC03" with a decrease in S04

2 ' , accompanied by increases in all major cations in diluted water, but in the more saline water Mg2 + , Na+ and K+ decrease in exchange of Ca2+ (Fig. 20). C02 dissolution from organic matter oxidation is a major process that affects carbonate rock-water chemical equilibria.


distanc* to th* shoreline, m

Fig. 16 - Situation map of Cadaloques-Coll de Balaguer area and the Vandellôs Nuclear Site (CNV) in the Vandellôs massif, and schematic cross section with results of sea-water intrusion according to evidence from observation boreholes and pumping wells.

CARBONATE AQUIFERS OF THE SOUTHERN PRE-LITTORAL RANGE

The southern sector of the pre-littoral range, a part of the Catalanides (Catalan Coastal Ranges) is locally formed by a series of folded and fractured Triassic and Eocene sediments with thick carbonate layers, tectonically pushed in a southeast-northwest direction. Schists and granites overthrust the northeast boundary. The southeast boundary is a big fault that constitutes the limit with the deep Penedés graben (Bayô et al., 1979; REPO, 1973). Three main carbonate formations can be distinguished from bottom to top (Fig. 21):


Lower Muschelkalk dolomites, largely a confined aquifer, poorly known, resting on low permeability Bundsanstein clays, sandstones and conglomerates.

Upper Muschelkalk limestones and dolomites, largely outcropping in the southeast part, and resting on clayish formations with anhydrites.

Middle Eocene alveoline limestones, occupying the northwest part, largely unconfined and above the local water table in anticlinal folds. They rest on Keuper and lower Eocene clayish formations, sometimes with thick anhydrite layers, and toward the northwest deep below grey Oligocène marls of the Ebre basin morphological depression (Bayô & Custodio, 1976).

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Fig. 17 - Schematic cross section of the Vandellôs massif through the Cadaloques-Coll de Balaguer area (modified from Pascual et al., 1986a) indicating groundwater recharge. High altitude recharge flows at an angle to the cross section due to anisotropic permeability. The ô2H vs. ô180 plot (using salinity corrected values) shows that well water is isotopically heavier than high altitude recharge, and heavier the closer to the coast. This indicates that recharge flows out of the section towards the west (see Fig. 20). d is the deuterium excess and the lines are parallel to the world meteoric line of slope 8.


Karstic features are frequent towards the west, but tectonically induced fractures, possibly with some dissolution enlargement, dominate in the south and eastern parts, where most of the springs appear. Anhydrite displacement and deformation of its cover due to dissolution surely plays a major role in carbonate rock fracturing and in introducing discontinuities in the aquitard layers.

EC T EC T

Fig. 18 - Two examples of water salinity (EC = electrical conductivity in mS cm"1) and temperature (T in °C) logs in boreholes in the coarse, carbonate-dominated, clastic sediments (S) at the Vandellôs Nuclear Site, in the coastal Vandellôs massif. Heterogeneity and preferentially permeable zones due to dissolution produce the step-like form of the curves. The great thickness of the mixing zone is due to the tidal effect, high dispersivity and some upwards fresh-water flow from the underlying carbonate rocks (J). Temperature decreases with depth indicating the possibility of the upward flow of cooler (higher altitude of recharge), fresh groundwater from the lower carbonates. The two cross sections summarize available information. The depth to the carbonate rock (the top indicated by the irregular line) at the coast is about 130 m according to a deep borehole.


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Fig. 20 - Deviations of actual values with respect to theoretical mixing values for point groundwater samples in Cala Jostell wetland on carbonate materials. End members for mixing calculations are the freshest water (PI sample for the first survey) and the saltiest (P5 sample for the second survey). The depth increases from PI to P5. Carbonate dissolution is the joint result of C0 2 addition by organic matter oxidation in the soil, S04

2" reduction and cation exchange. In this case, cation exchange is zero in the long term (stable mean salinity) but it has a seasonal effect. Between the first and the second survey salinity increased (Bosch et al., 1990).


CENTRAL M-Q Miocene-Quaternary E-0 Eocene-Oligocene E Eocene limestones T Triassic P Schists & granite X Travertine

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Fig. 21 -Location map, groundwater head contours, main groundwater flow direction and hydrogeological cross sections of the Carme-Capellades-Mediona area of the southern pre-littoral range. The multilayer carbonate aquifer present a tectonic disturbance by overthrusting and faulting that allows deep aquifers to discharge through springs and form large patches of travertines (Custodio & Bayô, 1983).

This is a dominantly rural and forest area, with rainfall higher than in the surrounding plains where the population is concentrated and creates a water demand that clashes with the traditional rights of municipalities in which the springs appear. The main springs have been monitored and sampled over a period of five years to understand changes in


ca** «g** Na* c r 804 COjH*

Fig. 22 - Mean chemical characteristics of water from the main springs showing the effect of turnover time (increase in dissolved salts) and time fluctuations of the major ions due to mixing of different aquifer contributions (Custodio & Bay6, 1983).

major chemistry and water isotopic composition (1 80, 2H, 3H), as well as a few flowing boreholes in the overthrust area (Custodio & Bay<5, 1983).

Chemical results show that sulphate is a dominant anion. Its proportion increases downflow, as also does chloride, in that case at low concentrations. This is the effect of progressively longer flow paths and residence times, with the progressive incorporation of water from deep aquifers that leaks upward by the tectonically disturbed aquitards. But ion concentrations also fluctuate with time (Fig. 22). For the lowest altitude springs this is interpreted as variable contributions from various aquifers, each with different residence times and consequently different water chemistry, although conserving the same chemical facies.

Spring water is saturated with respect to calcite at the relatively high C 0 2 partial pressure found in the aquifer. Near the discharge point, C0 2 degassing to the atmosphere produces calcite oversaturation and the formation of travertines (calcite tufa) if water flows slowly. Actually the area is rich in travertine deposits (REPO, 1973). Tritium concentration in spring-waters shows a decreasing trend in agreement with the decreasing concentration in rain-water (Fig. 23). The large observed fluctuations are the result of mixing of waters of different depths in the springs, as commented before. The deeper the origin of the water the lower the concentration due to a greater component of pre-nuclear recharge.

Trends in tritium concentration can be studied to yield mean turnover times by means of a lumped parameter model of the exponential (well mixed) type (Maloszewski & Zuber, 1982; Zuber, 1985). The use of such a model is justified by the mixing at the outflow


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. path length (springs 1 to 5). à I 80 values and the mean value distribution are also shown. There are variations according to recharge altitude in the south-north direction. Springs in Capellades are a mixture of contributions. Tritium and ô180 time variations reflect changes in the proportion of the different contributions (Custodio & Bayô, 1983).

point. A simplified solution allows for a fast determination of turnover times (Custodio & Bayô, 1983).

The trend of minimum tritium values, assumed to represent the behaviour of the deepest aquifer contribution (with some unavoidable mixing with water from the aquifers above) yield larger turnover times. The mean turnover time multiplied by the spring discharge yield the associated aquifer storage volume. The values thus obtained are larger than the Eocene limestone open space, thus indicating contributions from the Triassic carbonate aquifer, even where they do not outcrop and are deeply seated. Spring water temperature agrees with this conclusion and shows that fissures in the aquitards convey significant quantities of water. 180 and 2H isotope values, although not easy to interpret due to some non-systematic analytical deviations in the laboratory, indicate that the lower altitude springs also discharge lower altitude water.


THE PLANA DE LA GALERA AREA

The Plana de la Galera is a plain that extends from the lower reaches of the Ebre River, in southernmost Catalonia, to the boundary with Valencia County, and occupies a trench between the high Ports de Beceit range (the union of the Maestrat-Iberica ranges with the Catalanides) and the coastal Montsià ranges. It is a northeast-southwest oriented graben (Fig. 24), between the thick carbonate formations of the ranges. The bottom comprises the same materials, buried below continentally dominated sediments and extensive alluvial fans extending from the Ports de Beceit mountains (Bayô et al., 1976; CIHS, 1988; Bayô, 1984; AIH, 1989; SGOPU, 1974).

A rural and traditionally dry farming area for olive and carub trees, presently is being transformed into irrigated lands for fruit trees. Since water supply from the Ebre River

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Fig. 24 - Schematic hydrogeological behaviour of La Galera Plain. The cross section shows groundwater flow and the effect of low permeability Miocene-Quaternary deposits. The detail shows the role of the palaeokarst below the graben filling as a drain. Shallow wells has a small specific discharge, but the deep ones, if they intersect open palaeokarst, can have a high specific discharge. The block diagram shows the existence of different palaeokarst surfaces in ancient subaerial erosion surfaces (A and B) with possible open spaces for groundwater flow (modified from Bayô et al., 1986).


by means of an almost completed large canal failed due to administrative and social problems, farmers are getting their water from wells that penetrate deeply the carbonate rock base of the depression. Sedimentary discontinuities in the carbonate rock sequence, formerly subjected to subaerial weathering processes, conserve open karst features that constitute water transmitting layers capable of feeding large discharge wells (CIHS, 1988).

Water heads are only slightly above sea level in spite of the relative large distance from the sea and the existence of the Montsià range in between (SGC, 1986; Touris & Custodio, 1983). The situation is one of concern since the large investments in

Fig. 25 - Sulphate and environmental isotopic composition of groundwater from the Plana de la Galera and Baix Ebre (modified from Tourfs & Custodio, 1983; SGC, 1986). Sulphate points to a local source of recharge for the plain except along the La Senia River since all water from the valley springs infiltrates in the plain. Isotopic analysis differentiate allochthonous Ebre River water (lighter and with a smaller deuterium excess due to its high altitude headwaters and Atlantic origin), water from the Ports de Beseit (light) and locally recharged water (heavier). Anomalous S. Carles (5) groundwater is due to sea water contamination. Sample 11 is possibly an analytical error.


agriculture and food industries, and also municipal water supplies, depend on poorly known groundwater resources, their recharge source being unclear. The natural discharge is to the sea and to the large Ebre River (SGOPU, 1975; Baytf et al., 1986), and thus also poorly known.

There are four main sources of recharge:

— Rainwater recharge, directly on the plain. Semiarid conditions make this quantity rather small, about 20 to 50 mm/year from 500 to 550 mm/year mean precipitation. The contribution of return irrigation flows is probably small, except locally.

— Infiltration of runoff from the high elevation areas in the alluvial fans and along the gully channels. Most alluvial fans are now dissected by creeks and then this recharge is significant only along the creeks and locally.

— Infiltration of surface water. The only river is the La Senia, just in the southwest boundary of the area. Most of the normal flow infiltrates the river channel, which displays closed depressions that reflect collapses in the karstified limestones below. But this barely affects the rest of the plain. Some springs in the mountains sustained small flows that fully infiltrated, but now most of them are captured for irrigation purposes.

— Underground transfer of water from the mountain ranges, mainly from the high Ports de Beseit. It seems unlikely that this transfer is important due to the deep trench separating the range from the plain carbonate base, which is filled with low permeability materials. Recharge in the mountains flow laterally to the deep valleys crossing them, mainly the La Senia River, where groundwater discharge in springs and the river conveys the flows outside the ranges, towards the plain, where it infiltrates.

To solve the questions raised by the last point, a preliminary chemical and isotopic survey was carried out (SGC, 1986; Tourfs & Custodio, 1983). The results (Fig. 25) point to the lack of significant underground transfer of water, a rather common circumstance in other systems like this in carbonate formations in southern Catalonia, as the Penedés graben with respect to the southern pre-littoral range before commented or with respect to the southeast boundary of the Garraf massif (PHPO, 1985).

CONCLUSIONS

This study of the hydrogeological behaviour and exploitation problems of the southern Catalonia carbonate aquifers has make successful use of chemical data. Hydrogeochemical studies, both simple surveys, repeated surveys or frequent monitoring of selected points, yield very useful results if the study techniques are selected in order to fit the problem to be solved. Best results are obtained when combined with environmental isotope studies, an extension of hydrogeochemical studies, and also tracer tests in boreholes to know the existence of vertical flows and what the samples represent.

Detailed studies have aimed at a better knowledge of water-carbonate rock interactions in coastal aquifers. Further to the influence of ion strength effect on the activity coefficients to explain calcite dissolution or precipitation in the fresh-water/salt-water mixing zone, other chemical influences play their role and even dominate the chemical processes leading to karstification, karst obliteration, dolomitization and dedolomitization. Such are redox processes and also ion exchange in areas in which salinity evolution with time has a definite trend.


ACKNOWLEDGEMENTS

A large part of the data come from internal reports of the former Eastern Pyrenees Water Authority and the Public Works Geological Service, where three of the authors work or have worked, and are complemented by studies done by the International Course on Groundwater Hydrology and for the doctoral theses of the third and fourth authors. Also the owners of the Electronuclear Plants of Vandellôs (I and II) helped in studies and financed some drilling and surveys. Part of the research studies have been done under project 8309/007 of the extinguished American-Spanish Joint Committee for Science and Technical Co-operation. Dr William Back, of the US Geological Survey co-operated with these studies, as well as Dr M. Ramon Llamas, of the Madrid's Complutense University.

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