Groundwater Investigations in the Lavia Deep Drilling Hole-Sampling Techniques and Results

The Industrial Power Co. Ltd (TVO) studied the final disposal of spent nuclear fuel in Finnish crystalline bedrock. During the years 1984-85 preparative work for the execution of site investigation was carried out. The tests of investigation methods and equipment have been carried out in a 1000 m deep test borehole at Lavia, Western Finland. Geophysical and hydrological borehole investigations have been carried out and groundwater samples taken.

As a part of TVO's R & D work Imatran Voima Oy (IVO) has planned and built new equipment for groundwater sampling and chemical field measurements in deep boreholes with a minimum diameter of 56 mm and a maximum depth of 1000 m.

The borehole assembly of the groundwater sampling equipment comprises an inflatable double packer system and a membrane pump. It is connected via hoses to a mobile unit consisting of a flow-through-cell system with electrodes and a micro-computer for automatic control and data processing (Fig. 1). Nitrogen gas is used to inflate the packers and to drive the membrane pump. The specific conductivity, amount of dissolved oxygen, pS, pH and Eh of the groundwater sample are measured in a system of four flow-through-cells before the sampled water gets into contact with air.

Using this instrument groundwater samples were taken from the test borehole in three stages, at five depths in the borehole. The concentration of iodide, the tracer of the drilling fluid, was frequently controlled during sampling. Several different physico-chemical analyses was performed comprising e.g. the main cations and anions and trace elements. Also samples for isotopic and gas analyses were taken.

According to field measurements and laboratory analyses, the groundwater seems to have a low ionic strength with no considerable variation in depth. Rather reducing conditions (minimum Eh about −450 mV) were observed below the depth of 400 m. Especially data from isotopic analyses gives evidence of mixing with drilling fluid. In spite of the disturbances in groundwater caused by drilling fluid and vertical flow in the open borehole the samples can be considered representative enough in evaluation of the general ionic composition of the groundwater.

The experience gained from the groundwater sampling at Lavia has had a significant influence on planning the future groundwater sampling during field investigation for the years 1987–92.