Loviisa Power Station - Final Disposal of Reactor Waste. On the Age and Origin of Groundwater from the Rapakivi Granite on the Island of Haestholmen

摘要

This study forms a part of the research done to assess the suitability of the rapakivi granitic bedrock of the island of Haestholmen, southern Finland, for the management of reactor waste. The aim is to assess the residence time and the origin of the groundwater. In addition, microfossil analyses and conservative ion data were used in deciphering the origin of the groundwater. Fracture mineral studies were limited to /sup 13/C determinations on two fracture calcites. Groundwater was sampled at several levels of four drill holes, reaching to a depth of some 200 m. The isotopic results were compared with those of water from a percussion drill hole, shallow dug wells, and the Gulf of Finland. The main conclusions are based on /sup 3/H bundances in groundwater, mean residence time of groundwater deduced from /sup 14/C analyses, and stabile isotope content of groundwater, combined with conservative ion data. Additional information was gained from activity ratios of uranium, and sulphur isotope ratios of sulphate. The groundwater of Haestholmen consists of a surface layer of fresh water, and deeper down, of saline water. The fresh water flows and changes rapidly; most of it has precipitated and infiltrated less than 30 years ago. It intermixes with saline water only at the fresh-saline groundwater interface. The saline water underneath the intermediate zone is relatively stagnant. It mainly consists of sea water from the Litorina Sea stage, intermixed with less than 20% glacial melt water. The evolution of the Haestholmen groundwater towards its present stage began during the melting phase of the Weichselian glaciation. Then the groundwater conditions chanced, and infiltration of melt water along open fractures in the bedrock occured. During the Litorian Sea stage heavy saline Litorina sea water slowly infiltrated in the bedrock and displaced the fresh water almost totally. The Haestholmen island rose above the sea level more than 4000 years ago. (ERA citation 13:004568)