The decrease of permeablity caused by the deposition of silica in a porous column (50 cm×5 cm φ) has been studied using the Otake geothermal water at about 90°C and at pH 8. A cylindrical cell was filled with aluminum beads and rock particles both about 2 mm in diameter to make the column. Silica concentrations in the water supplied were 475, 493, and 522 ppm, and their values were constant during each experiment. The initial flow rates were controlled by changing the total hydraulic potentials which were kept constant during the experiment. The hydraulic potentials in the column were measured by eleven manometers in order to calculate the permeability. The silica was deposited chiefly in the first 10 cm of the column. The amount of silica scale decreased rapidly with distance regardless of the material in the column or the silica concentration in the water. The decrease in permeability was inversely proportional to the amount of silica scale, decreasing to 0.5-5 % of the initial value in the uppermost part of the column, whereas it remained about 80 % of the initial value at more distant places. It can be concluded that, when the secondary porosity of the scale ranges from 0.90 to 0.95, the Kozeny-Stein equation can be used to express the relation between the permeablity and the amount of silica scale.
