The Cooling of the Selfoss Geothermal Area, S-Iceland

Abstract

The water from the geothermal field at Selfoss has been used for domestic heating since 1948. In the area there are two groundwater systems; one containing cold groundwater and the other geothermal water. Glacial sediments act as cap rock between those two systems. When the pumping of the geothermal water started, leakage of cold groundwater down into the geothermal system occurred. This leakage is both through drill holes with cracked casing and also by seepage of cold water through the rocks. By recasing and cementing the drill holes the cold water flow through them has been eliminated. the cooling of the water pumped from the geothermal area has been measured and so has the cooling in observation drill holes. The two groundwater systems have very different chlorine content, and originally the chlorine content of the thermal water was about ten times higher than that of the cold groundwater. The chlorine content of the thermal water has changed with time. Change in the chlorine content in thermal water can be used to indicated the amount (ratio) of cold water mixed with the thermal water. Hydrological observations show that drawdown is less than expected in an infinite confined aquifer. The explanation is that it is a recharge area, where cold groundwater leaks down.