Final disposal

Groundwater Investigations

Plenty of investigations related to the flow and chemical properties of groundwater are carried out utilising the deep boreholes drilled in Olkiluoto and the ONKALO facility. The purpose of these surveys is to verify that the properties of groundwater are favourable for the final disposal facility and will also remain so long into the future.

Investigations related to the basic state of groundwater flow have been carried out in the Olkiluoto investigation area since the late 1980s. The investigations have consisted of measurements of groundwater table level, hydraulic head at different depths and total rainfall. The results of these investigations have been used, among others, for estimating the amount of rainwater infiltrating into the bedrock and the properties of flow paths inside the rock.

Difference Flow Measurements

The movements of water deep inside the bedrock are investigated through the holes drilled in Olkiluoto using a difference flow meter, an instrument developed for the purpose.

The difference flow meter can be used to measure

  • the water conductivity
  • hydraulic head in individual fractures inside the rock. 
  • groundwater electrical conductivity values and temperatures
  • the pressure profile and air pressure in the boreholes.

The difference flow measurements are carried out for practically all holes drilled in Olkiluoto because the resulting data of groundwater movements is of primary importance not only for deciding on the location of the repository and for its long-term safety, but also for other investigations, such as for planning the groundwater sampling regime.

Transverse Flow Measurements

Another dedicated measurement instrument, transverse flow meter, has been developed for measuring the quantity of groundwater flowing across the borehole. Besides the flow rate, the instrument also registers the direction of flow.

The transverse flow meter was only introduced recently, and the intention is to make more and more transverse flow measurements in the drill holes. The results of transverse flow measurements allow determining the arrival direction of groundwater and the direction where it is heading.

Investigation of the Chemical Properties of Groundwater

The chemical properties of groundwater have also been investigated since the late 1980s. Hundreds of groundwater samples have been taken from the holes drilled in the bedrock of Olkiluoto, and from groundwater observation tubes inserted in the soil, for analysis in different laboratories around the world. The groundwater samples represent different levels up to a depth of 1,000 m.

Taking groundwater samples from deep inside the bedrock requires special equipment and techniques for ensuring the representativeness of samples. The current sampling technique was introduced in 1997. It allows, among others, obtaining representative gas and microbial samples.

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