Groundwater chemistry in boreholes
Saito et al. ([2013]) studied the size distribution and elemental compositions of colloids in the MIU groundwater sampled from the 09MI21 borehole located in the −300 m Access/Research Gallery (Figure 2a) using the flow field-flow fractionation (Fl-FFF) technique coupled with an online UV/VIS fluorescence detector and ICP-MS analysis. The Fl-FFF analyzed size-fractionated chemical compositions in colloidal materials trapped on the 1 kDa membrane. The size-fractionated compositions of colloids in the 09MI21 groundwater varied among the samples and sizes as follows: (1) the size distributions of Fe overlapped with samples and sizes with a range of 0.5 to 20 nm; (2) Al and Mg had similar size distributions; (3) changes in REE concentrations were correlated with those in Al, Mg, and organic colloids; and (4) size distributions of U corresponded to those of REEs.
In the present study, size fractionations of groundwater samples with colloids were conducted by ultrafiltration. In general, suspended particles in groundwater were removed by ultrafiltration. However, colloids having particle sizes smaller than the membrane pore size can pass through the filter and mix in the filtrate. The concentrations of colloid-forming elements in the filtrate apparently change according to the colloid penetration.
The size-fractionated concentrations of Fe in the 09MI20 borehole varied with the pore size (Figure 5). The Fe concentration of the 1 kDa filtered groundwater was higher than that of the unfiltered sample. The possible reason for the higher concentration in the filtered groundwater compared to the unfiltered groundwater can be explained by the natural variation of groundwater chemistry during sampling and/or experimental contamination by colloids in the filtrate. For the groundwater sampling from boreholes, anthropogenic precipitation(s) was observed by BTV (Figure 1b). Since the major concern with the mobilization of colloids is rapid groundwater pumping, it is recommended that the pumping rate be kept low, to a maximum of approximately 100 mL/min (Ryan and Elimelech [1996]). In the present case of groundwater sampling using hydrochemical monitoring systems, the groundwater flow rate attained was approximately 1 L/min. Consequently, the deviation in the fractionated concentration of Fe can be attributed to the mobilization of colloids in the borehole.
The concentration of Fe in the filtered groundwater changed with the replacement of groundwater in the sampling-intervals (Figure 6). Moreover, size fractionation by ultrafiltration may cause a change in the quantity of the colloidal material that passed through the 0.2 μm membrane and/or the aqueous concentration. In general, the source of aqueous Fe can be the dissolution of biotite, chlorite, and smectite (Acker and Bricker [1992]). The deeper groundwater is significantly reduced (Table 1). However, Fe hydroxides such as goethite are precipitated on fractures and mineral surfaces in the granite (Iwatsuki and Yoshida [1999]). Sugimori et al. ([2008]) described the weathering processes of biotite and chlorite in the Toki granite. Iron (oxy-)hydroxides such as ferrihydrite and goethite can precipitate depending on the pO2 or Eh.
We calculated saturation indices (SI = log IAP/K
sp; [IAP, ion activity product; Ksp, solubility product]) for Fe-bearing minerals using the program REACT from ‘The Geochemist's Workbench’ (Bethke [1992]) with the thermodynamic database ‘thermo.com.v8.r6 + .dat’. Immediately after groundwater replacement began, the groundwater was supersaturated with respect to high Fe-bearing smectite (SI = 0.03) and low Fe-bearing smectite (SI = 0.03 and 0.52), while they became undersaturated (SI = −0.03 and −0.80) after 20 sampling interval volumes of groundwater were replaced. If Fe-bearing phase(s) passed through the membrane, the filtrate should be in equilibrium and/or oversaturated with respect to corresponding solid phase(s). The high Fe concentration immediately after groundwater purging was the result of Fe-bearing colloids being flushed out and contaminating the filtrate. Based on SEM observations, Fe-bearing aluminosilicates with a diameter of 100 nm were collected on the membrane in the 09MI20 groundwater (Figure 3). Tsubaki et al. ([2012]) reported that aggregations of nanoparticles can be trapped by a membrane. Accordingly, the aggregation of colloids explains the trapping of Fe-bearing aluminosilicates on the membrane. Fe-bearing aluminosilicate in the 09MI20 groundwater (Figure 3) was identified by thermodynamic calculation as Fe-bearing smectite. Such precipitation may be released to groundwater and mobilized by water pressure fluctuations during sampling. The ‘mobilized’ Fe-bearing colloids affect the concentration of Fe by passing through the membrane. The concentrations of Fe in the borehole and fracture seeps were comparable (Figures 6c and 7). Consequently, the Fe-bearing colloids were flushed out with the groundwater from inside the borehole during removal of the groundwater in the borehole. Thus, the variation in the size-fractionated concentration of Fe in 09MI20 can be explained by the contamination by secondary Fe-bearing colloids formed in the borehole after drilling.
For the size-fractionated groundwater sampled from the 10MI26 borehole, Fe and Al were concentrated in unfiltered and 0.2 μm filtered groundwater, whereas the concentrations were very low in 10 and 1 kDa filtered groundwater. The 10 kDa MWCO corresponds to a pore size of 6.3 nm. Consequently, the Fe-bearing and Al-bearing colloids of 0.2 μm to 6.3 nm in size were present in the borehole groundwater. The concentration of Al changed during removal of groundwater from the 10MI26 borehole. Immediately after groundwater removal began, the filtered groundwater was supersaturated with respect to several tens of Al-bearing minerals, kaolinite, low Fe-bearing smectite, pyrophyllite, etc. After purging 20 borehole volumes, the groundwater became undersaturated with respect to low Fe-bearing smectite and pyrophyllite. The high Al concentration immediately after groundwater purging was the result of Al-bearing colloids being flushed out and contaminating the filtrate. Accordingly, the filtered groundwater was supersaturated with respect to the low Fe-bearing smectite and pyrophyllite in the initial sampling. After elimination of the mobilized Al-bearing phase(s), the groundwater became undersaturated with respect to the Al-bearing phase(s) and large portions of the Al-bearing phase(s) were flushed out during the groundwater purging. The colloids and secondary minerals adhering to the borehole walls were mobilized by fluctuating water pressures during borehole sampling. These were unavoidable processes in some uncertainties in the research on colloids in groundwater.
Nevertheless, groundwater sampling from boreholes has the significant advantage of in situ sample collection. At least, the concentrations of Fe, Al, and TOC changed during purging of groundwater from boreholes. The colloids were likely mobilized by fluctuations in water pressure during sampling. For quality control of groundwater and colloid sampling from boreholes, the presence of anthropogenic colloids can be mitigated to some extent by pumping out or flushing the groundwater from a borehole by removing several volumes of groundwater from the packer isolated sampling intervals in the borehole.
Groundwater chemistry of size-fractionated fracture seepage
In the Toki granite, groundwater flow is fracture-controlled (joints and faults) (Illman et al. [2009]). Hence, groundwater from both boreholes and seepage passed through the granitic fractures and/or fracture networks. The concentrations of major constituents in the groundwater seepages in the −300 m gallery were different from the concentrations in the 09MI20 borehole groundwater, even though both sample locations are at the same depth (Table 1). Around the study area, the groundwater chemistry is Na-Ca-HCO3 type at shallow depths, whereas it is Na-(Ca)-Cl type at deep levels (Iwatsuki et al. [2005]). Although the concentration of Cl− in borehole groundwater increased with depth, that in the groundwater seepages were higher than in boreholes at the same depth (−300 m) (Table 1). The sampling locations of boreholes are sandwiched between the main shaft and the ventilation shaft (Figure 2), and thus the underground conditions were depressurized vertically as shaft construction progressed (Mizuno et al. [2013]). In contrast, the fracture seepages are located in the horizontal −300 m access/research gallery (Figure 2). Consequently, the groundwater sources for the boreholes and the fracture seepages on the gallery wall were likely different.
For the groundwater seepage from fractures in galleries, the colloid-forming elements, Fe and Al, are likely to be present as aqueous (complex) ions in the groundwater flowing in fractures (Figure 7). In the Toki granite, montmorillonite and chlorite occur as fracture-filling minerals (Iwatsuki and Yoshida [1999]). The differences in the Al concentrations can be accounted for by the variation of fracture-filling minerals. The groundwater seepage was supersaturated with respect to beidellite in the A-SP-199 fracture, but was undersaturated in the A-SP-198 fracture. The saturation index for Al-bearing minerals in the fractures, the Mg-bearing smectite group, and illite could not be calculated because the Mg concentration is too low, i.e., below the detection limit (Table 3). Since the mobile Fe-bearing and Al-bearing colloids were uncommon in the groundwater from the fracture seepages, the natural Fe-bearing and Al-bearing colloids may be negligible in the groundwater in fractures. The groundwater sampled from seepages was free of contamination by anthropogenic materials. The groundwater sampling from the water-conducting fractures is suitable for water quality control of colloid issues.
Distribution of U on colloids
Changes in the size-fractionated concentrations of U in groundwater from the 09MI20 and 10MI26 boreholes are associated with similar changes in concentration of Fe and Al (Figure 5). For the 09MI21 borehole, the size distributions of U correspond to those of Al and organic matter (Saito et al. [2013]). In the present study, the U association with Al for the 10MI26 is consistent with that for borehole 09MI21. In addition, the U concentration is associated with the Al and Fe in the 10MI26 groundwater. The U concentrations decreased at an approximately constant rate as groundwater replacement proceeded in the boreholes (Figures 5 and 6). Although aqueous U forms strong carbonate complexes (Waite et al. [1994]), alkalinity was constant during the groundwater flushing in the boreholes (1.24 and 1.15 meq/L for the 09MI20 and 10MI26 boreholes, respectively). Saturation indices for U-bearing minerals, such as uraninite (UO2) during flushing of the groundwaters were −1.0 to −1.5 and −1.0 to −1.3 in the 09MI20 and 10MI26 boreholes, respectively. The groundwaters were undersaturated with respect to the U-bearing minerals. Accordingly, U-bearing intrinsic colloids cannot occur in the groundwater. The changes in the U concentrations were similar to those in Fe and TOC in the 09MI20 borehole and Al in the 10MI26 borehole (Figure 6). The association of U with Fe-bearing, Al-bearing, and organic colloids resulted in the contamination of the filtered groundwater. The size distributions of U-bearing colloids correspond to those of Al-bearing colloids and organic colloids (Saito et al. [2013]). The change in U concentration along with the concentrations of Al was consistent with the size fractionation by Fl-FFF. In addition, Fe-bearing and/or organic colloids of diameter <0.2 μm affected the sampling quality of the groundwater.
The size-fractionated chemical compositions of U for the A-SP-198 and A-SP-199 groundwaters are approximately constant, irrespective of filter size. If the U moves with colloids of various particle sizes, the U concentrations should decrease as the pore size decreases. However, the U concentrations were approximately constant regardless of pore size. Thus, the uranium migration by colloidal phases in groundwater fracture flow was not identified in the present study.
Distribution of REEs on colloids
The size-fractionated concentrations of REEs in borehole groundwaters decreased significantly as membrane pore sizes decreased (Figure 8). The REE concentrations of unfiltered groundwater were approximately ten times higher than that of 0.2 μm filtered groundwater. The correlation between size-fractionated concentration of REEs and Al and Fe were not observed in the boreholes (Figures 5 and 8). In contrast, the concentrations of REEs were approximately constant during groundwater replacement in the sampling intervals in the 09MI20 and 10MI26 boreholes (Figure 9). The transport of REEs associated with Fe-bearing and Al-bearing colloids was not observed (Figures 6 and 9). Ultrafiltration was conducted using a 0.2 μm membrane during groundwater flow-through in the sampling intervals in the boreholes. The independence of REE concentrations on groundwater replacement suggested that the >0.2 μm REE-bearing colloids in the groundwater were almost completely trapped on the 0.2 μm membrane. In addition to the borehole groundwater, the concentrations of REEs are significantly higher in unfiltered groundwater from fracture seepages (Figure 10). The REE concentrations decreased with decreasing pore size fractions, which were approximately constant from 200 to 10 kDa. The 200 kDa of MWCO membrane corresponds to a pore size of approximately 100 nm. The mobile Fe-bearing and Al-bearing colloids are rare in the groundwater from fracture seepages (Figure 7). Consequently, the mobility of REEs was scarcely facilitated by Fe-bearing and Al-bearing colloidal materials. The >0.2 μm REE-bearing particles were transported in the MIU groundwater. Iida et al. ([1998]) investigated REE-bearing minerals in the Toki granite, bastnaesite ((LREE)CO3F), and parasite (Ca(LREE)2(CO3)3 F2), which are several hundreds to tens of micrometers in size. Thus, the large REE-bearing mineral(s) were potentially removed from the groundwater by the 0.2 μm membrane. In the present study, REE-bearing mineral(s) suspended in the groundwater cannot be directly observed by SEM observation because of the very low concentration of colloids in groundwater. The REE-bearing particles are mobile in groundwater sampled from both borehole and fracture seepages (Figures 8 and 10).
The chemical properties of REEs, especially LREEs, are comparable to those of transuranic elements due to the similarities in ionic radii, oxidation state, and complexation with anions (Krauskopf [1986]). Chondrite-normalized REE patterns of size-fractionated groundwater sampled from boreholes are shown in Figure 11. For the unfiltered 09MI20 groundwater, the REE pattern has a downward convex shape in the middle REE (MREE), which was notable for filtered groundwater (Figure 11a). The REE pattern of unfiltered 10MI26 groundwater was significantly different from those of the filtered groundwaters. The unfiltered 10MI26 groundwater was enriched with LREE (Figure 11b). The REE-bearing materials were suspended in the groundwater and affected the REE pattern of the unfiltered groundwater. In contrast, the REE patterns of filtered groundwater were enriched in heavy REE (HREE), which was comparable to the patterns during groundwater replacement.
Figure 12 shows chondrite-normalized REE patterns of the filtered groundwaters during groundwater replacement in the borehole. Although the concentrations of Fe, Al, and TOC for the filtered groundwater changed, the REE patterns in the groundwaters did not change during the groundwater flow-through in the boreholes and were significantly enriched in HREEs. In CO2-rich and pH-neutral groundwater, HREEs are enriched due to the REE carbonate aqueous complex (Négrel et al. [2000]). In the present study, REE aqueous species were calculated using ‘The Geochemist's Workbench’ (Bethke [1992]), which revealed that the predominant REE species were REE(CO)3
+ in all filtered groundwater. The Takahashi et al. ([2002]) study revealed that the REEs in the groundwater (approximately 150 mbgl) are dissolved carbonate complexes from the Toki granite. The HREE enrichment in the groundwater can be explained by the formation of carbonate complexes.
With respect to borehole sampling, anthropogenic colloids and secondary mineral(s) precipitated on the borehole walls (Figure 1). The anthropogenic colloids formed in the borehole were flushed out during groundwater flow-through (Figure 6). However, the mobility of REEs in the borehole groundwater did not change with the mobilized Fe-bearing, Al-bearing, and organic colloids. Sholkovitz ([1992]) studied the mobility of REEs associated with colloids in river water. The REE patterns of the river water and those of colloids are enriched in HREE and LREE, respectively. The enrichment of LREE in unfiltered groundwater and HREE in filtered groundwater is in agreement with the river colloid samples. In the present study, REE carriers are the <0.2 μm REE bearing particles.
For the groundwater sampled from fractures, chondrite-normalized REE patterns of unfiltered and size-fractionated filtered groundwaters are shown in Figure 13. The REE pattern of the unfiltered groundwater for A-SP-198 was slightly enriched in LREE. In contrast, the REE pattern of the filtrate was almost flat from the lightest REE (La) to the heaviest REE (Lu). For A-SP-199, because of a small change in the concentrations of REEs in the unfiltered and filtered groundwater (Figure 10), changes in the REE patterns by REE-bearing particles were not observed (Figure 13b). The REE-bearing materials are mainly >0.2 μm in size in the groundwater sampled from underground fracture seeps (Figure 10). Accordingly, the REE patterns of unfiltered groundwater probably exhibit a mixture of the groundwater and REE-bearing solid materials. In addition to the borehole sampling, the enrichment of LREEs in the unfiltered groundwater may be accounted for by the contamination of LREE-enriched natural colloidal materials (>0.2 μm) in the groundwater. The mobility of REEs was only slightly facilitated by the Fe-bearing and Al-bearing colloids. Consequently, the REE-bearing materials that originated from the source granite were transported in a continuous groundwater inflow system.