| Abstract |
The Great Geysir geothermal area in Haukadalur, S-Iceland is one of Icelandís most famous tourist attractions and a treasured historical place. During the last twenty years there has been rather limited geothermal research activity in the area even though exploitation has increased considerably during that time. Considering the future protection of the field it is of utmost importance to know the relationship between the Geysir geothermal field and the reservoir that feeds exploited wells in the neighbourhood. The present project concentrates on the geochemical properties of the field and possible changes over time and also on the environmental effects of nearby production. All available older data were compiled and two hot springs and three wells nearby were sampled and the water and gas analysed. The geothermal waters are sodium chloride and bicarbonate waters with high contents of fluoride and boron indicating reactions with acidic volcanic rocks. The radon concentration in the fluid is found to be exceedingly high, 10-100 times higher than that encountered in most other geothermal fields in Iceland. The calculated quartz geothermometer value for water from the Geysir hot spring indicates minimum reservoir temperatures of 240 ?C. Other hot springs in the area give lower values, probably due to initiation of silica precipitation.The calculated mineral saturation indexes for the waters show that water from Geysir itself is in near equilibrium at 220-240?C, whereas water from the other hot springs shows signs of boiling, condensation and mixing. The Na-K and Na-K-Ca geothermometers give mostly lower values for the hot spring waters than the quartz geothermometer. Only minor changes, probably mainly due to different sampling and analysing methods, are found in the chemistry of the hot spring waters during the last 30 years. However the run-off from the field appears to have declined considerably during that time. The waters in warm springs and wells in the neighbourhood yield carbonate waters with high silica, typical for mixed waters on the borders of high-temperature geothermal fields. The chemistry and isotopic composition indicate that the fluid originates from the mixing of run-off water from the Geysir geothermal field and local cold groundwater. The results of the project stress the need for enhanced reservoir research, increased protection and the monitoring of the effects of nearby exploitation. |