Elsevier

Applied Geochemistry

Volume 14, Issue 3, April 1999, Pages 283-299
Applied Geochemistry

Application of geochemistry and radioactivity in the hydrogeological investigation of carbonate aquifers (Sierras Blanca and Mijas, southern Spain)

https://doi.org/10.1016/S0883-2927(98)00049-3Get rights and content

Abstract

The chemical characteristics, 3H contents and radioactivity of groundwaters from the Sierras Blanca and Mijas (Southern Spain) have been studied in relation to the chemical composition and radioactivity of the aquifer host rocks, and the residence time of the water. The Sierras Blanca and Mijas are made up of calcitic and dolomitic marbles of Triassic age. The groundwaters that drain the calcitic marbles (which outcrop principally in the western Sierra Blanca) have less mineralization, which descreases quickly with recharge (as does the 3H content), and the gross alpha and beta activities are below detection limit. This is due to the short residence time of water inside the aquifers which are conduit flow systems. The waters of the dolomitic marbles (eastern Sierra Blanca and Sierra Mijas) have higher and less variable mineralization and contain greater concentrations of Mg2+, SiO2 and SO2−4 (ions normally associated with slow flows). The 3H contents are more uniform with time (indicating an older age) and there is detectable natural radioactivity, because the waters have a longer residence time in the aquifers, which are diffuse flow systems.

Introduction

The study area of 170 km2 lies in the Blanca and Mijas mountains, situated 100 km E of the Strait of Gibraltar and at an average of 5 km from the Mediterranean coast. These Sierras mark the northern limit of the Costa del Sol to the E of Marbella city, southern Spain (Fig. 1).

Climatically the region has a mean annual temperature between 17–18°C. The average rainfall during the period 1963/64–1993/94 was approximately 700 mm, although it varies from more than 800 mm in the western sector of the Sierra Blanca down to 600 mm in the eastern part of the Sierra Mijas. For this reason, the mineralization of the rainwater in general, and the Cl and Na+ contents in particular, increase from W to E (Andreo, 1997).

In karst hydrogeology, variations in hydrochemical parameters (both major and trace elements) are commonly used to establish the hydrodynamic function of the studied systems (Shuster and White, 1971; Atkinson, 1977; Hess and White, 1993; Blavoux and Mudry, 1993; Bakalowicz, 1994). The 3H content of waters is also useful (Malozewsky and Zuber, 1982; Fontes, 1983; Bradbury, 1991, among others), but natural radioactivity has rarely been considered (Andrews and Wood, 1972; Smart, 1996).

The chemical composition of groundwater is a function of the aquifer rocks, flow conditions and residence time in the aquifer (Schoeller, 1962; Back and Hanshaw, 1971). Groundwater flow and residence time depend on the degree to which karstic conduits are developed. If karst development is limited, water flow is slow and the residence time in the aquifer is long. In this case, mineralization is generally greater, the chemical composition varies slightly with time and it is particularly conditioned by components such as Mg2+ and SiO2 (Drever, 1982; Appelo and Postma, 1993), if they are constituents of the aquifer rocks. A more developed karst drainage implies shorter residence times, thus normally lower mineralization, and a temporally more variable water composition, which mainly comprises Ca2+ and alkalinity derived from rapid dissolution of carbonates (Dreybrodt, 1981) plus additional components introduced by the rain. The residence time can be estimated if the 3H contents of both rainwater and groundwater are known over a representative period (Fontes, 1983).

The objective of this work is to examine the application of hydrochemistry, together with the radioactivity of the waters, to research on carbonate aquifers. The chemical composition, the 3H contents and the natural radioactivity (gross alpha and beta activities) of the groundwater are studied as a function of the chemical and radioactive composition of the aquifer marbles and the water residence time inside the aquifers. The integration of these different research methods should permit a better understanding of aquifer hydrogeology.

Section snippets

Geological setting

The Sierras Blanca and Mijas form part of the Alpujarride Complex of the Betic Cordillera, and are formed by a lower metapelitic formation with migmatites and gneisses of Paleozoic age overlaying carbonates comprising lower white marbles and upper blue marbles with metapelitic intercalations. The carbonates have a total thickness of 600 m and are of middle and upper Triassic age. The white marbles have a fractured aspect and outcrop mainly in the eastern sector of the Sierra Blanca and Sierra

Hydrogeological characteristics of the Sierras Blanca and Mijas

Marbles outcrop in the studied area constituting the Blanca–Mijas hydrogeological unit (Fig. 2), whose discharge is produced from springs and from pumping the numerous wells which exist all over the Sierra Mijas. In this Sierra the springs have been dry since the seventies and only drained water during 1990 and 1991, in response to an important recharge at the end of 1989 and beginning of 1990.

The elevation of the springs and the height of the piezometric level in the wells are different

Methodology

Samples of both the marbles (blue and white) and the groundwaters from the main water points of Sierras Blanca and Mijas have been collected.

Major (CaO, MgO, Na2O, K2O, SiO2, MnO, Fe2O3) and trace element (Sr, Pb, Zn, Cu and Ni), composition were determined by X-Ray fluorescence at the University of Granada (Spain).

The 226Ra, 232Th and 40K activities of both marbles and peridotites were measured by gamma spectrometry in the Laboratory of Environmental Radioactivity at the University of Malaga.

Chemical composition and radioactivity of the marbles

The most abundant components are CaO and MgO, which make up more than 50% of the rock as would be expected for carbonate aquifers. Silica was found to vary from 0.37% to 30% (Table 1). The most abundant trace element is Sr, which varies from 55 to 1254 ppm. The elements Pb, Zn, Cu and Ni have variable contents depending on local geology, in particular the existence of mineralization which has been exploited in the past (Orueta, 1917).

The natural radioactivity of the marbles is, in general, low

Major components

Representative water samples from the hydrogeological unit were collected (Table 2 and Fig. 2). In all cases, mineralization of water was low, as indicated by the conductivity values (between 314 and 554 μS/cm). The majority of the waters analysed are Ca–HCO3 and Ca–Mg–HCO3 bicarbonate types.

Principal component analysis (PCA) was carried out to summarize the relations between the measured variables. The 3 principal axes of PCA explain 80.9% of the sampling variance, although only the first two

The tritium contents

The nearest rainfall 3H data available for this study are from the Gibraltar station which belongs to International Atomic Energy Agency (IAEA). In a previous study (Garcı́a-López et al., 1994) data from another station located more to the E of the Sierras Blanca and Mijas were used, but these values were similar to those from the station in Gibraltar. Natural processes give an average 3H content of 10 TU in rainwater from the Gibraltar station, with lower values in winter and higher values in

Natural radioactivity of the waters

The gross alpha and beta activities have been determined in sampled waters, although the source isotopes have not been identified at present. The gross alpha activity in the groundwater samples originates from the 238U and its daughter 226Ra (because 222Rn is not included in the measures obtained by the Krieger method), while beta activity is principally from 232Th and 40K (Cothern and Rebers, 1990). All the available data (Table 5) are below legislative limits, as is the case for most of the

Conclusions

This study confirms that the chemical composition of water in this area is a function of the chemical composition of the aquifer rocks. Thus in the calcitic marbles of the western Sierra Blanca Ca–HCO3 waters with higher Sr content occur, while in eastern Sierra Blanca and Sierra Mijas the waters are a Ca–Mg–HCO3 type, because of the dolomitic composition of the marbles that outcrop in this sector.

However, the chemical contents or the mineralization level of the water and its evolution in time

Acknowledgements

This work has been financed by the Investigation Group no. 4021 of the Junta de Andalucia and by the Project PB94-1495 of the DGICYT and it is a contribution to the IGCP 379 of UNESCO. The authors thank Dr T. Atkinson for his interesting suggestions and criticisms and Dr R. Raiswell (University of Leeds) his helpful English corrections.

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