Abstract
Colloids and their association with analogue elements, uranium, and rare earth elements (REEs), in deep granitic groundwater were investigated at the Mizunami Underground Research Laboratory (MIU). Groundwater was sampled from underground boreholes and gallery walls, and the colloids were separated by size-fractionated ultrafiltration (pore sizes, 0.2 μm, 10 kDa, and 1 kDa). For the groundwater sampled from fractures in excavation walls, the size-fractionated concentrations of the colloid-forming elements were approximately constant relative to different size fractions (0.2 μm, 200 kDa, 50 kDa, and 10 kDa). The contamination of Fe- and Al-bearing materials was insignificant in the filtered groundwater from fracture seepages. Changes in the concentrations of U in the groundwater sampled from boreholes and excavation walls were associated with the Al-bearing colloids, Fe-bearing colloids, and organic matter. The REE-bearing material(s) that were >0.2 μm in size were mobile in the deep granitic groundwater, rather than occurring in association with Al-bearing, Fe-bearing colloids, and organic matter. It is suggested that sampling from water-conducting fractures in host rock and colloid elimination in borehole are important components of water quality control in geochemical investigations.
Highlights
Colloid sampling procedures in groundwater are an important aspect of groundwater research
scanning electron microscopy (SEM) observations of colloids in borehole groundwater The backscattered electron image of the colloids collected in the 09MI20 borehole indicates that they are spherical in shape and have a diameter of approximately 100 nm (Figure 3a)
Groundwater chemistry in boreholes Saito et al (2013) studied the size distribution and elemental compositions of colloids in the Mizunami Underground Research Laboratory (MIU) groundwater sampled from the 09MI21 borehole located in the −300 m Access/Research Gallery (Figure 2a) using the flow fieldflow fractionation (Fl-FFF) technique coupled with an online UV/VIS fluorescence detector and ICP-MS analysis
Summary
Colloid sampling procedures in groundwater are an important aspect of groundwater research. The artesian pressure in groundwater may decrease to atmospheric pressure, and the in situ oxidation-reduction condition of the groundwater may change during sampling. In situ groundwater and colloid sampling techniques that are capable of maintaining artesian pressures and anaerobic conditions have been developed at the Mizunami Underground Research Laboratory (MIU) in Mizunami, Gifu Prefecture, Japan (Aosai et al 2010). We have observed anthropogenic secondary minerals on borehole walls using a borehole televiewer (BTV) in boreholes drilled into granitic rock. The granite is fresh immediately after drilling, BTV photographs taken in different boreholes [09MI20 (Figure 1a) and 10MI26 (Figure 1b) details in the Sampling from boreholes section] show that a b an anthropogenic blackish precipitate has developed on the borehole wall several days after drilling (Figure 1b)
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