The discharge rate of HCl gas from the fumarolic area at Kuju volcano had been estimated on the basis of distributions of Cl- concentration in rainwater and precipitation. The rain collectors of open type were set at eleven locations . within 7 km from the main fumarolic area. The rainwater was collected and then analyzed for chemical, compositions at intervals of two weeks from March to December during 1993-1997. The HC1 discharge rate in 1995 was stable at about 2.0 t/d until early in July, and then increased abruptly to 6.7 t/d late in July. This increase of the HCl discharge rate can be a precursor of the phreatic eruption on 11 October 1995. The maximum discharge rates were observed from April to May in 1996. In this period, both seismic activities below the fumarolic area and the energy discharge from the new craters increased. The discharge rate of HCl decreased from July. 1996 with some fluctuations.
Anhydrite scale was recognized in Sumikawa production well SC-1, where it apparently caused a decrease of steam flow rate. To understand the anhydrite scale formation, we studied well fluid composition and flow data with the help of a physico-chemical flow-precipitation model, leading to the following findings.1. A temperature inversion was observed near bottom hole of well SC-1. Geothermal fluids feed from separate high temperature (300°C) and a lower temperature (250°C) zones. Anhydrite precipitates where the two fluids mix.2. The observed concentration, of Sr in anhydrite scale is much higher than predicted from experimentally determined equilibrium partitioning of Sr between aqueous and anhydrite phases. This indicates that the scale formed under non-equilibrium conditions, as in hot spring chimneys on the East Pacific Rise.3. Both the higher and lower temperature fluids that feed SC-1 are exactly saturated with respect to anhydrite. Mixing of the fluids yields supersaturation with anhydrite.4. Using a steady plug flow model, we estimate that the rate constant for anhydrite precipitation is 5.5X10-6 molm-2s-1 and that 10% of the Ca in fluid is precipitated as anhydrite scale.5. The main cause of anhydrite precipitation is the temperature difference between individual fluids fed. to the wellbore. A sensitivity analysis of the amount of precipitation as a function of temperature difference shows that the amount of anhydrite precipitation for a 70°C temperature difference is two times as much as for 50°C, and that a 30°C difference produces half as much as a 50°C difference.