Unflashed geothermal waters in Iceland are invariably just saturated with calcite. Upon flashing the water always becomes supersaturated, especially during the early stages of flashing. Thus flashing will lead to calcite precipitation. It is believed that the rate of precipitation bears a positive relation to the degree of supersaturation and the water temperature. Accordingly, precipitation will always be most pronounced at the beginning of flashing. The degree of calcite supersaturation, which is produced by flashing, only depends on the temperature and the ionic strength of the unflashed water. Strongest supersaturation is produced at low temperature and at a high ionic strength of the water. Calcite precipitation is known to be troublesome for the exploitation of many geothermal fields in the world. The study of the factors which control the magnitude of this precipitation is therefore of interest to those engaged in the exploitation of geothermal fluids. thermal fields, whether this precipitations occurs in wells or in the aquifers feeding the wells. Cumulation of calcite in flashing zones of geothermal fields may be accompanied with concentration of radium in the rock. From this point of view the process of calcite precipitation is of interest in studies of radon distribution in geothermal gases, but it is well known that radon forms by radioactive decay of radium.
In the present paper an attempt is made to reveal the quantitative effects of those factors, which affect the degree of calcite supersaturation in flashed geothermal waters. The study is based chemical data from selected wet-steam wells in Iceland. The composition of the well discharges is given in Table 1. This composition covers, the observed variation in the chemistry of Icelandic geothermal waters. Aquifer temperatures for individual wells lies in the range 127 to 290~ and the total carbonate content of the total discharge varies from 50 ppm to 1900 ppm. The well locations are shown in Figure 1.