Revisión de los modelos hidrogeoquímicos de génesis de tobas calcáreas

The calcareous tufa deposits in fresh waters are widely represented in the Iberian Peninsula, mostly related to springs from carbonate aquifers. Most studies on the geochemical modelling of the genesis of calcareous tufa consider the meteoric water percolating through the limestone aquifer is enriched in CO₂ by existing microbiological activity in soil. The new equilibrium with atmospheric CO₂ in springs and/or metabolic activity of microorganisms determines a rich variety of facies of calcareous tufa. The resulting water has a calcium bicarbonate nature. However, the hydrochemical modelling has not completely been addressed and the boundary conditions have not been taken into account in tufa systems in which soils present low CO₂ concentrations or when the nature of waters are Ca-Mg-SO₄-HCO₃. In this paper, hydrochemical results from two important examples of genesis of calcareous tufa are presented: Jucar river (Valdeganga - Presa de la Central Hidroelectrica del Bosque) and Guadiana Alto river (Ruidera pools). These results are also compared with other kart spring waters recently studied in the area. The water nature of Jucar river and Ruidera aquifers shows clearly a Ca-Mg-SO₄-HCO₃ facies. The hydrochemical interpretation cannot exclusively be explained using the CO₂–H₂O– calcite system, which has widely been used in studies of precipitation rates and paleoclimatic records of calcareous tufa or Karstification reactions of limestone aquifers. In this review, we evaluate the dedolomitization and gypsum dissolution, which are independent of partial pressure of CO₂, and kinetics and sustainability of processes in terms of genesis of tufa systems. We also consider other CO₂-dependent reactions such as the calcification gypsum and dissolution and precipitation of calcite by mixing waters. This explains the nature of waters from dolomite-gypsum aquifers and also their great generator potential of tufa systems, and refuses the interpretation based exclusively on the presence of humic acid-rich soils, not very common in arid and semi-arid areas. Finally, we examine mixing waters in multi-layered or compartmentalized seasonally aquifers, which can produce waters that develop calcareous tufa.