Factors which determine the hydrogeochemical behaviour of karstic springs.A case study from the Betic Cordilleras, Spain

The Cabra-Alcaide karstic massif situated in the south of Spain constitutes an important part of the so-called Natural Park of the Subbetic Sierras. T his aquifer system is drained by various springs which supply a population of some 100,000 inhabitants. The feed areas of these springs show very diff erent characteristics with respect to their geological structure, size of t he drainage area, thickness of the vadose zone, elevation and degree of kar stification. In addition, the carbonate rocks lie over a clayey substratum which contains large masses of intercalated evaporites. Due to these condit ions, the hydrochemical composition of the springs is relatively variable. In this study a hydrogeochemical characterization of the aquifer in both sp ace and time is undertaken and the factors that determine it are discussed. To achieve this, 19 monthly samples were taken from the 6 most significant springs of the hydrogeological system. The commencement of sampling coinci ded with the transition from a period of several years of severe drought an d a very wet two-year period, which amplified considerably the hydrochemica l and hydrodynamic response of the springs to the recharge. Identification of hydrogeochemical processes was performed by studying hydrographs, the te mporal evolution of physics-chemical parameters, ionic ratios (mainly Mg/Ca ) and by means of simple and multivariate statistical analyses. The saturat ion status was established using the WATEQF program and the mass transfer w as quantified using PHREEQC. With the exception of the epikarstic subsystem (i.e. the Zarza spring), the majority of the results indicate that the aqu ifer exhibits a diffuse flow model, in which piston flow phenomena are seen , closely linked to the most intense precipitation. Along the direction of flow hydrochemical trends are seen as the water type changes from Ca-HCO3 t o Ca-Mg-HCO3; at the same time enrichment in some ions, derived from the di ssolution of evaporitic minerals of the impermeable substratum, and dedolom itization processes occur. In this way, almost 90% of the water samples wer e oversaturated in calcite, the majority of cases being a consequence of th e dissolution of the gypsum of the substratum. Only in the epikarstic sprin gs can it be considered that the oversaturation in calcite is due to loss o f CO2 from the emerging groundwater. It is concluded that hydrodynamic aspe cts together with hydrogeochemical characteristics need to be taken into ac count to correctly explain the hydrochemical evolution of the karstic sprin gs. Moreover, the use of both approaches permits a more accurate establishm ent of the degree of aquifer karstification, which in turn needs to be know n in order to assess its vulnerability to contamination and to protect rech arge zones. (C) 2001 Elsevier Science Ltd. All rights reserved.