CHEMICAL EVOLUTION OF GROUNDWATER IN TRIASSIC GYPSUM-BEARING CARBONATE AQUIFERS (LAS-ALPUJARRAS, SOUTHERN SPAIN)

A hydrochemical study employing modelling techniques, was carried out using samples taken at 65 points (springs and wells) in Triassic carbo nate aquifers (Lujar-Gador Unit, Alpujarride Complex, Betic Cordillera ). These aquifers are made up of limestones and dolomites with some gy psum scattered or interbedded. Though the area is semi-arid, recharge is relatively high because of their mountainous nature. The carbonate rocks contain dense microfissuration; the groundwater flow regime is p redominantly diffuse. The karstic forms are in general poorly develope d. Two main hydrochemical processes have been identified in these aqui fers. One is incongruent dissolution of dolomite that determines the c hemical composition of the less mineralised water. The other is dedolo mitisation (dolomite dissolution together with calcite precipitation c aused by dissolution of gypsum), which becomes predominant when the fl ow encounters interbedded gypsum. This reaction is also frequently ass ociated with low temperature thermalism, and can play a part in more i ntense local karstification (cavities, sinkholes, high transmisivity i n wells) observed in the sectors of these aquifers where gypsum is mor e abundant. A reaction path model has been used to simulate the geoche mical processes through a hypothetical aquifer (with similar lithology to the Alpujarride carbonate aquifers). Successive stages of evolutio n through the carbonate sequence, represented by different saturation states with respect to calcite, dolomite gypsum and CO2, have been mod elled and then compared with the field data.