Methodology for Hydrogeochemical Sampling to Characterise Groundwaters in Crystalline Bedrock: Developments Made within the Swedish Radwaste Programme

Figure 2

Schematic drawing of two different situations of the water yielding fractures in an isolated borehole section (modified from [49]). The blue colour intensity illustrates the amount of formation groundwater in the borehole section during pumping. The presence of a single water-bearing fracture in the upper part of the borehole section, close to the outlet of the section (fracture ^① in (a) and (b), on the left part of the figure), is a favourable situation since the rest of the water volume beneath the fracture will stay trapped, regardless of the removed amount of water from the borehole section (i.e., the lower part of the isolated section is a dead volume). Figure (a) illustrates the moment shortly after pumping starts (time 1) when the water from the only fracture has not reached the outlet yet. Figure (b) shows that after a while (time 2) all the water leaving the section is formation groundwater. If there are several fractures in the section ((c) and (d) in the right part of the figure), the section water between the fractures will contribute to the sample until the formation groundwater from the deepest fracture reaches the outlet: shortly after pump starts (c), no formation water has reached the outlet yet; after a certain time (time 2 in (d)), formation groundwater from fracture ^① has reached the outlet and formation groundwater from fracture ^② has passed fracture ^①; however, formation groundwater from fracture ^③ has not yet reached fracture ^② and the section water between fractures ^② and ^③ will still contribute to the sample. In this last case, the water volume that should be extracted prior to collecting a representative sample of the isolated borehole section may become unrealistically long.