Numerical simulation of heat and mass transfer from hot dry rock to flowing water in a circular fracture was conducted to estimate the concentration of the dissolved silica at the production well. The local mass transfer coefficients between the rock and the fluid, obtained by using the electrochemical method during the laboratory experiments, were used in the calculation. The results of the simulation indicate that the concentration of the silica at the production well increases with increasing distance between the injection and the production wells, increasing fracture diameter and increasing pressure. It decreases with increases in flow rate and porosity in the fracture. The concentration of silica at the production well first increases and then decreases with increasing initial rock temperature due to the decrease in the solubility of silica at high temperatures. Finally, the mass transfer by forced convection of flowing water plays an important role in the variation in the concentration of silica in water, and the assumption that the concentration of dissolved silica is equal to that at the fracture surface is not valid for estimation of the concentration at the production well. (C) 2002 CNR. Published by Elsevier Science Ltd. All rights reserved.