Thermodinamíc equations of solution and precipitation carbonates-calcium sulfates let us stablish a mathematical model of calcareous tufa deposits simulation. The conceptual genetic model of calcareous tufa is related with solution of preexisting carbonates in high CO2, presure ground-water. When ground-water reach surface, the fast degassing processes lead to calcite precipitation. Ground-water are considered placed in two different types of acuifer system: dolomitic aquifer (D) and

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... ypsiferous acuifer (DY). Calcium activity in ground-water (aS) may be expressed by the relation: as=10p1 ∙ (pCO2)p2 ∙ (Ts)p3 where, pCO2 is partiel pressure of CO2 in equilibrium with ground-water, and Ts, is absolute temperature of ground-water. Values of p1, p2 and p3, constant are, in the case ofa dolomitic aquicer: p1= 6,71624 p2= 0,32933 p3= 3,64469 However values of p1, p2 and p3 constants in dolomitic-gypsiferous aquifer are: p1= 3,29381 p2= O,03392 p3= -0,40223 In the same way we may calculate the end calcium activity in surface watering (af) after degassing CO2 in equilibrium with a rural atmosphere (pCO2=10-3,503atm). Urban and urban industrial areas may reach pC02= 10-3atm. The rate of precipitation of calcareous tues (Tons per year) may writte: [[(Vt ∙ aB / (VI - SL (eL - pL))] â€" at] ∙ (VI - SL (eL - pL)) ∙ 105 VI=water infiltration in the basin (Hm3/year), sL=m [...]