On the origin and chemical evolution of groundwater at the Olkiluoto site

During the site selection programme for nuclear waste disposal, Teollisuuden Voima Oy has collected groundwater samples from boreholes drilled down to a depth of 1000 m. The former evolutionary interpretation of the Olkiluoto groundwater (e.g. Pitkanen et al. 1994) revealed three different end-member groundwater-types mixing in the bedrock due to the land uplift. These were 1 >modem fresh rainwater in the upper part of the bedrock recharged since the island rose above sea level about 3 000-2 500 BP, 2 >brackish Na-Cl groundwater layer originating from the modem and former Baltic Sea at 100-500 m depth, and 3 >deep-occurring saline Ca-Na-Cl-type groundwater interpreted as a remnant of ancient hydrothermal processes on the basis of the Br/Cl ratio and stable isotopes of water. The latest comprehensive sampling from multipacked boreholes reveals new details of the brackish groundwater layer in particular. Na-Cl water enriched with S04 has been identified at a depth of 100-300 m. The salinity clearly exceeds the present value of the Gulf of Bothnia, whereas the Br/Cl ratio indicates a seawater origin. The 180- 2 H values plot on a mixing line between the samples of the present Baltic (having the highest values at the site) and the group of fresh and other brackish water samples at the site, indicating warmer recharging conditions than today. Below this sulfate-rich layer, firstly Cl (salinity) and 180 are depleting with S04, but the Br/Cl ratio remains stable, reflecting the mixing with colder diluted water in the system. At greater depths, the 180 and Br/Cl ratio begins to increase with Cl towards the values of the most saline groundwaters (Cl > 20 000 mg/1). According to the chemical characteristics of the S04-rich water, it is most likely that it has infiltrated from the Litorina Sea, which was the only more saline stage (7 500-2 500 BP), with an estimated highest Cl content of about 6 500 mg/1 (Kankainen 1986), and also a warmer period than the modem Baltic stage during the Holocene in the Gulf of Bothnia (e.g. Eronen 1990). The stage was about 2 °C warmer than today, and 180 of the Litorina Sea water could have been between -5.2 and -4.7 %o. The 14C data of the S04-rich layer (20 - 35 pM) coupled with the young age of the overlying groundwaters (3-15 TU with 50-60 pM) support the age of the Litorina Sea. The lighter isotope content than in the modem seawater can be explained by the mixing with a colder, dilute pre-Litorina water, probably melt-water from the Weichselian ice sheet. According to the preliminary mixing calculations using Cl as a conservative tracer, the S04-rich layer would contain 55-70% water originated from the Litorina Sea and the rest would be glacial melt-water with 180 varying between -18 and -23 %o. The chemistry suggests that the displacement of melt-water by heavy Litorina water decreases below the S04-rich layer. In the lower part of the brackish groundwater layer the changes in chemistry imply increasing mixing of saline end-member water. The deep location below the cold end-member and high 18 0 content indicate a preglacial origin for saline groundwater. However, the elevated Br/Cl and 2 H/ 180 ratios do not favour seawater origin as the pre-Weichselian Eemian Sea, which may represent about same salinity as oceanwater during the previous interglacial period (Eronen 1990). As a final conclusion, hydrochemistry of Olkiluoto seems to contain a well developed profile of climatic changes from modem time through former Baltic stages in the area to preglacial times.