HYDROCHEMICAL INVESTIGATIONS AT FINNSJON, SWEDEN

Ground water collected from within and around a highly conductive low-angle fracture zone in crystaline rock has been studied in relation to drilling and sampling methods, water quality and hydrogeochemical interpretation. Air-flush percussion drilling was demonstrated to be efficient down to 200-250 m; at greater depths, large amounts of drilling debris and increasing groundwater contamination from air dissolution become apparent. A stepwise drilling/sampling procedure using water-flush rotary drilling, in combination with water analysis carried out on site using a mobile laboratory, is recommended for future studies. The chemistry of the Finnsjon ground waters shows a sharp contact between saline and non-saline compositions, this contact corresponding to a highly conductive low-angle fracture zone (Zone 2). This zone is therefore a horizon along which ground waters of contrasting age and chemistry come into contact and partially mix. This mixing has produced a highly supersaturated water resulting in the precipitation of calcite. This is most evident in fractures within the more conductive, upper part of Zone 2, thus partly forming a seal between the two groundwater environments. Comparison of the Finnsjon saline waters with other saline environments, particularly in the Fennoscandian Shield, has shown that the ground waters are dominantly marine in origin, but with clear modifications resulting from water/rock interaction. Apparent radiocarbon ages range from 9000 to 15 000 years in those waters containing tritum levels below detection limits. These ages are considered too young and are thought to have resulted from mainly carbon-14 dilution caused by groundwater mixing during the Holocene, when large quantities of glacial melt water were available. This mixing process is also supported by the oxygen-18 data.